So, 2deaf, speaker attenuators

[John]

   Whattaya think?


I've never played through one. Never have played through a 100 watt Marshall, or Twin Reverb, or or or any of the other amps you guys talk about a lot. (I'm so virginal) So, my only opinion is it seems as though you'd maybe be making it hard on the OT by the time you got real volume reduction, but kept that "cranked sound".

[HotBluePlates]

... is it seems as though you'd maybe be making it hard on the OT by the time you got real volume reduction ...

Nah, the OT is (or should be) made for the full output power level. The only thing working hard are the resistors (or whatever mechanism) in the attenuator turning most of that output power into heat.

Of course resistors aren't the reactive load that speakers are, and so the output stage feels different to the tubes (again, should feel easier, because resistive loads aren't bouncing all over the place like reactive loads).

But the speakrs aren't flapping the way they would with big watts, and it doesn't feel the same as big watts. So if nothing else, that'll make it sound/feel different.

[2deaf]

I've overdriven big amps for decades and never burned-up an OT. Burned-up just about everything else, but never an OT.  I can tell you what happens and what to do about it, but right now let's talk attenuation.

First off, let me state my position:  The only way to get the sound of a tube full stack at full bore is to use a tube full stack at full bore.

Anybody that is using an attenuator must agree with this or at least a corollary of this (e.g. The only way to get the sound of a medium-sized tube amp at "8" is to use a medium-sized tube amp at "8"). 

Tom Scholz put out an attenuator called Power Soak somewhere in the deep dark past.  It is the resistive load type and it performs precisely as HBP noted above.  I use one as a dummy-load on my bench and it works great for that, but it would defeat the purpose if used as an attenuator for an overdriven amp.

The best attenuator that I've ever heard is the one I built for my studio that I call a "dead box".  Basically, it is seven speakers in a plywood box that sits outside the main recording room with the eighth speaker inside the room.  The eighth speaker is actually a 4-12 bottom and the microphone is placed in front of only one of those four. There is more to it than that, but this is the skinny.

A guy from Sony Records came by and told me I was nuts.  I think he meant about the dead box, but it could have been my general demeanor.  Anyways, he said that they just use THD Hot Plates.  So I bought one.  It works way, way better than the Power Soak and is actually usable, but I got it beat.  I like the light that runs off the speaker current.  I have other designs that work pretty good, but they don't have a light like that.     

 

[Willabe]

I like the light that runs off the speaker current.  I have other designs that work pretty good, but they don't have a light like that.

Which is?


                  Brad   

[SILVERGUN]

I'll throw this out there just for discussion sake.....and because it will be simple and cheap for you to try.

I tried something different last week when bench testing an amp that has 2 parallel speaker jacks that are labeled "total load 4 ohms".

I plugged an 8 ohm 12" speaker into one jack, and an 8 ohm dummy resistor in the other and got a major reduction in output volume without the perceived loss of highs that I would normally get when going 'through' the attenuator (with some series resistance) to get to the speaker.

So, in my mind, the resistor became the second speaker in my imaginary 2x12 cab,, in my accidental experiment.

It sounded better to me than my homemade attenuator (which has one resistor in series, and one in parallel, in order to keep the load resistance constant) but I was busy with a long to-do list and didn't spend much time confirming my beliefs. A light bulb lit dimly over my head for a few seconds, and I thought,,,I'll get back to that.

Here's an article that contains the project plans for the attenuator that I built:
http://www.hearditontheweb.com/images/pdf/attenuator.pdf

[2deaf]

HBP pointed out the two main factors to consider with attenuation:  resistance and reactivity.  Resistance is the easiest factor to deal with.  Speakers are coils, so they change their impedance with frequency.  The main load and the series load also need to change impedance in order for the speaker and transformer to see what they usually see.  So I plotted out impedance vs. frequency for a Celestion G-12T-75 and imitated it with a collection of coils, resistors, and capacitors.  When I used this instead of straight resistors, I got a quantum improvement in the sound.  It sounds the same as a THD Hot Plate, so THD probably does something with the same tact.

[Ritchie200]

Ill put my old man two cents in.  Way, way, way back in the day before the Scholz Power Soak there was the Altair.  I used to have two plugged into the back of the beast as they were only rated for 100 watts max.  They got VERY hot but sounded pretty darn good unless you really choked it down.  Looking inside they used what looked like stove elements and a few other parts - maybe for an impedance tweak.  Bought two Power Soaks when it came out and was disappointed.  The Altair was head and tails better.  I've used the Hot Plate on a few amps at my buddy's music store and was not that impressed either, but probably better than the Scholz.  Without a side-by-side, it would be a tough call.  So, long story short, maybe the old stove elements had a reactive quality to them as well as resistive?    WAY WAY back in the day I used a headphone volume control (big 2" diameter fine wire wound rheostat - say that fast three times!) on a 6V6 amp with absolutely no change in freq response until almost choked completely down - until it burned up.  I have no idea what horrid torture it was doing to my poor amp - this was late 60's.  Again, maybe it was wound big enough for some reactive qualities?

On a somewhat similar but somewhat off topic note...  It kills me to read posts from these guys who choke a dimed 100watt Marshall to bedroom levels and complain about how their Power Soak or (fill in the blank) is defective because it is killing their output tubes.  I know I am preaching to the choir, but I changed a LOT of tubes when I was playing out and playing loud four or five times a week.  If you drive it like you stole it on a daily basis, your gonna have to change an engine or two!

Jim

[2deaf]

Reactivity is a more complicated factor than resistance.  Here you have a dynamic coil with more than one spring acting on it.  Not only does everything change with frequency, but also with amplitude.  On top of everything else, a dynamic coil in a magnetic field is also a generator that puts out a current which interacts with the output tubes.  I don't think anything in a little box is going to imitate all this.

So I hooked the speakers up in pure parallel with the inductor-resistor-capacitor network instead of series/parallel.  Since most of the current goes through the network, the reactivity from the speakers is diminished somewhat, but then again, so is the SPL.  It isn't as good as a dead box, but it is a lot better than THD.

[sluckey]

Any opinions on or experience with Weber's speaker motor based attenuators?

http://www.tedweber.com/atten.htm

[2deaf]

I plugged an 8 ohm 12" speaker into one jack, and an 8 ohm dummy resistor in the other

So then you did the same thing except I use a 64 ohm speaker and a 4.3 ohm I-R-C network.

they used what looked like stove elements

You mean like electric heater elements?

[2deaf]

Any opinions on or experience with Weber's speaker motor based attenuators?

I've only tried the Scholz and the THD, but I did use speaker motors in a couple of designs.  I cut-up some Jensen Mod5-30's and got some real good results, but the wires to the voice coils kept breaking.  I gave up on it because of the lack of reliability.

[Ritchie200]

they used what looked like stove elements

You mean like electric heater elements?

It looked like the kinked nichrome elements you would see in a toaster.  I had a few connections come unsoldered due to the heat so I silver soldered them, no problems after that.  I've read all these "expert user" reviews on the net saying it blew up anything you plugged into it and it sounded like crap.  It all sounds grandiose and cool for internet lore but I'd bet none of those clowns were out of diapers in 1978 much less ever been closer to one than an online pic.  It was the best sounding one I've ever used.  I understand Joe Bonamassa is still using an Altair on his dumbles.  I don't think Joe's tone is suffering either....  Again, just my humble .02

Jim

[Ritchie200]

Reactivity is a more complicated factor than resistance.  Here you have a dynamic coil with more than one spring acting on it.  Not only does everything change with frequency, but also with amplitude.  On top of everything else, a dynamic coil in a magnetic field is also a generator that puts out a current which interacts with the output tubes.  I don't think anything in a little box is going to imitate all this.

So I hooked the speakers up in pure parallel with the inductor-resistor-capacitor network instead of series/parallel.  Since most of the current goes through the network, the reactivity from the speakers is diminished somewhat, but then again, so is the SPL.  It isn't as good as a dead box, but it is a lot better than THD.

Isn't that essentially what you did?  Or are you talking about a line out application?  Also, not sure what you mean about "On top of everything else, a dynamic coil in a magnetic field is also a generator that puts out a current which interacts with the output tubes"?  The electromotive action responsible for the movement of that coil is from the amp.  The reaction is the impedance the output tranny is seeing from the speaker coil and anything else we are putting in between.

Jim

[Ritchie200]

Sorry, reactance, not reaction.  It's early....need coffee.....

Jim

[2deaf]

Isn't that essentially what you did?

My dead box is the size of a 4-12 bottom.  Even the Mod5-30's were in a box 21" wide.  I meant a little box like the THD Hot Plate.

a dynamic coil in a magnetic field is also a generator

If you are doubting that speakers are generators, it is very easy to demonstrate.  Speakers just love to flap at their resonant frequency and the current generated there becomes a real problem for the amp.  I have a Fender that actually has a damping factor switch on it that is supposed to compensate for this effect to varying degrees.

The pictures I posted on the Crazy Idea thread were with a resistive load.  Had I used a speaker load, the waves would have had a wild character to them and the clarity of my point would have been reduced.  The wildness is caused by the interaction between the tube's current and the speaker's current.  The impedance of a speaker at any given frequency is constant.

[sluckey]

If you are doubting that speakers are generators, it is very easy to demonstrate.

A perfect example that utilizes that characteristic of a speaker is the old 2-way intercoms. No mics involved. When you press to talk, the speaker is switched around to act as a mic.

[Ritchie200]

I understand that, as the intercom is then amplifying the crude reproduction - microphone 101.  However, my point is speaker resonance (the point where damping is critical) is all over the place depending on power, cabinet design, and many other factors.  Amplifier design (negative feedback) and even cords can have an impact on damping.  Running full bore reproducing various freqs this can certainly be a factor, but in this case we are very much modifying what the speaker is finally seeing - at a much lower volume.  I'm not saying it does not exist, I just have never found it to be an influence or issue with the guitar amps I've played at major volume (but then I am hearing challenged...)? My thoughts are the interaction would be negligible in a attenuator application (sorry, I should have been clearer)?  I would think it would be much more noticeable for sound reinforcement or hifi than it would be for guitar due to the wider range of freqs being reproduced.   

 You talked about a inductor/resistor/capacitor network (in a little box? ) running pure parallel.  How are the Scholz, THD and all the others wired?  Are you sure they are series/parallel? I've read a few articles claiming everything that has come after the Scholz are identical to it circuit wise (including the THD) except the Weber.  Other than a few notch filter tweaks along the way (like you did), maybe Tom did have the right idea to begin with.  I've never been interested enough to try and gather all the schematics to prove or disprove.  I'll stick with my toaster design and heat up some samiches!

Jim

[2deaf]

My thoughts are the interaction would be negligible in a attenuator application

Once you record it, process it, mix it and burn it, the difference between moving a little and moving a lot is not apparent.  The difference between moving a little and not moving at all is easily detectable.  Weber obviously agrees with me because he is advertising moving speaker motors in his.

in a little box?

Nope.  The heat sink alone is 23" long.  I was talking about getting speakers into a stomp box, anyways.

How are the Scholz, THD and all the others wired?

I dunno.  They didn't do what I wanted, so I never checked it out.  I can take them apart if you really want.

Are you sure they are series/parallel?

No.  I just presumed they were because they present 4 ohms to the amp and something is messing with the speaker.  THD is definitely different than Scholz, but in the same vein.  If you read the Farnsworth article at Weber (see the link above, otherwise you get BBQ's), he has what I consider to be pre-made excuses for why the device isn't perfect.  When he blames treble loss on reduced volume, it tips me off because this is a characteristic of resisters in the circuit.  Besides, they couldn't get enough motors in such a little box to do what it does.  If he is using moving speaker motors, his device will blow Scholz and THD away.  Even with some resistors mixed in, it will be better.

[2deaf]

I'll stick with my toaster design

I wonder if the resistance of a red-hot toaster element changes with frequency.  It would be a tricky test, especially getting an element out of the toaster without my wife noticing.

You can see everything in the Scholz one without removing the cover and all it has is 17 power resistors.  I opened the THD and it has one stationary coil, power resistors that are molded onto the heat sink, some capacitors, and a twisted two-sided board that light doesn't penetrate.  There will be no circuit tracing here.  Attached is a picture of the THD.
 

[Ritchie200]

Cool beans, I've never seen the THD naked.


No, what I'm trying to say is I think the speaker generating current will have little effect on what we are wanting to achieve or what we are hearing since we are still plugged into speaker(s) anyway.  Also, the weber motors would have no resonance - or if they did, it probably would not be in our range of hearing!


Resistors sounds like crap, we all know that.  A resistor has very little or no reactance.  A big wire wound headphone volume control probably has quite a bit (which may be why I did not notice any degradation until almost shut down).  Maybe the toaster coils wrapped on a asbestos card do too?  Something good is going on there that was immediately noticeable to me when I went to pure resistive and it sounded like crap.  Inductors, like our moving speaker coil, may sound better because we know the impedance of the inductor changes with freq.  So I wonder if having a big coil of wire is the secret to a limited success?  A compromise, but if we are just taking a 100watt Super Lead down to bar room levels, it might not be noticeable?  Plus we are still plugged into a speaker(s) for any feedback affect that might have.


Jim

[2deaf]

No, what I'm trying to say

I think I know what you are trying to say.  Neither one of us is denying the existence of inductive reactance (X_L) or reactivity (VOO_doo) and I think both of us are using the term "reactive load" to mean a load that produces a back-current.  I'm not clear about your stance on inductive reactance, but I'm pretty sure you feel that reactivity is of little or no consequence.  On the other hand, I feel that reactivity is the difference between a THD Hot Plate and a full stack.

toaster coils wrapped on a asbestos card

The toaster element was wound into a coil?  That changes everything. 

[HotBluePlates]

Also, not sure what you mean about "On top of everything else, a dynamic coil in a magnetic field is also a generator that puts out a current which interacts with the output tubes"?  The electromotive action responsible for the movement of that coil is from the amp.

I typed a beautiful long post explaining what this meant, then my 3yr-old grabbed the mouse & deleted it. 

Read the first 2 chapters of NEETS Module 2. They explain the issue about as simply & read-ably as it gets.

After reading that, you'll better see what I mean when I say:
The voice coil's motion generates a back-EMF that tend to counter the amp's output current. Additionally, the speaker's surround and spider play a role... A brief sharp impulse from your amp, which drives the voice coil forward then stops, is not the end of the story. Now the voice coil is displaced and amp current stops, the surround and spider physically pull the voice coil back to its starting position. The return motion of the voice coil again creates a back-EMF, and the resulting current can be relayed through the OT to the tubes (transformers work both directions).

Current resulting from the back-EMF of speaker motion won't be great enough to completely oppose the amp's output power (otherwise, we'd never get audio output). But it does make a speaker feel different to an amp than a simple stationary choke coil (which has  fewer sources of back-EMF). That makes a speaker feel different to an amp than simple caps or inductors, so a moving speaker will always be a special case that is difficult to mimic with non-speaker load devices.

Mental image:
A rubber band is stretched between 2 posts. You push in the middle of the rubber band to displace it (amp's output power). At the same time, it is pushing back against your hand (voice coil back-EMF and surround/spider pulling the voice coil back to starting position). Rubber band still moves, but pressure against your hand wouldn't exist if the band wasn't fixed to the posts (voice coil moving in a magnetic field vs. a stationary inductor with no surrounding magnet or physical action of surround/spider).

Bottom-line: Plain chokes create some back-EMF, but not as much or in as many ways as a speaker.

[2deaf]

We're saying the exact same thing, you say it very eloquently and I say it all awkward and disjointed.

The back-current from a stationary coil is a function of the AC voltage applied to the thing.  It is always the same function and, as such, is very boring and it goes unnoticed.  My only use for a stationary coil in this application is to imitate the impedance of a speaker at different frequencies.

Unlike stationary coils, speakers are real exciting.  You got the spring effects from the spider, paper cone, and the compression / decompression of air.  The momentum of the whole structure is opposing the current applied, and of course, there's the rotation of the planet and the alignment of the stars.  This ain't no simple function here.

And then we drop into the realm of opinion.  I think that the only good attenuator has to have a path through the speakers that is only speakers of some sort.  As soon as you put inductors and/or resistors in the path, you only have a fair attenuator.

I also think that the air spring is the least important of all the springs.  A half stack has speakers that are experiencing twice the air spring as a speaker in a full stack, but you're going to be real hard-pressed to pick out the difference on a recording.  Also, if you replace the cone with a silicon cross thereby eliminating the air spring, it only has a minimal impact on the amp/speaker relationship.

I also think that it doesn't take very much back-current from the speaker path to get "that sound" going.  It is one of those exponential things.

[Ritchie200]

No, what I am trying to say is

In the end, we are still connecting to a speaker.  That is why I originally asked if you were talking about a line out.  Granted, bedroom level speaker excursion is not blazing stadium level speaker excursion.  However, bar level is still pretty significant or, I would imagine, studio levels will move that speaker as well.  This is a constant, unless we are going line out - so in my mind this was a non-issue with our attenuator design....until now....

HBP, I do have a question.  A typical audio waveform is a sine wave - complex, but still a sine wave - ultimately returning to zero state.  An amplifier is driving a speaker to reproduce this form - again, ultimately returning to zero state.  I would disagree with the NEETS author in that the amplifier pulls the cone back to the starting position in reproducing that sine wave, not the speaker itself.  Obviously amplifier design with damping characteristics plays a part. 

Ok, what about this....  I am assuming the current generated by a speaker that you guys feel is an important feedback component might be from the slightly out of phase mechanical lag inherent with the cone and spider, otherwise would it not be completely overwhelmed by the amplifier's signal?  Again, I don't deny that it exists, but the term, "peeing on a forest fire" comes to mind.  If that is the case, could a network be built to recreate this feedback with a slight phase shift and a slight pitch shift due to the speaker characteristics?  Similar to how a chorus effect gives the original signal a richer quality.  So if we combine this with a proper inductor (what I think is most important ), we might have something.  If this effect could be recreated, maybe a unit could be built to take a 100watt Super Lead down to bedroom levels without loss of "character".  Just thinking out loud.

Jim

[HotBluePlates]

HBP, I do have a question.  A typical audio waveform is a sine wave - complex, but still a sine wave - ultimately returning to zero state.  An amplifier is driving a speaker to reproduce this form - again, ultimately returning to zero state.  I would disagree with the NEETS author in that the amplifier pulls the cone back to the starting position in reproducing that sine wave, not the speaker itself. 

NEETS doesn't have anything about speakers, it's writing up basic electronics so new Navy personnel can understand. Application to a speaker is my doing (NEETS does talk about generators and resulting back-EMF; their example typically shows rotating motor generators, where a speaker does the same but is a linear motor).

"Typical audio waveform" can be anything. We talk about sine waves because it makes math and measurement easier. But musicologists classify the guitar as a percussion instrument because of the means of generating notes, and a digital recording program will show as much: tall initial percussive impulses, and much lower level sustained ringing.

You are not wrong in your description of an amp outputting a sine wave. What happens when the output is not a sine, but an impulse (like your pick attack, muted strumming, non-compressed raw guitar, etc)?

Folks testing speakers & headphones have a test for it: Impulse Response. In the image below, only the initial positive peak is actually supplied by the amplifier (in the form of a very fast, very short positive square wave impulse). The initial extreme low level squiggly-ness is the speaker overcoming inertia to move. The whole negative half of the first spike isn't supplied by the amp, it's the speaker components pulling the cone back to its starting position and the effect of inertia when the cone reaches and flies past the starting point. The rest of the waveform is the cone bouncing back n forth while settling back to the starting point, and may include the effect of amplifier negative feedback and damping to shorten the oscillations. Otherwise, the speaker would tend to ring at either its resonant frequency or that at which the cabinet is tuned.

Absolutely right that amps with a lot of negative feedback tend to absorb the effective of physical springiness and back-EMF, and have the speaker on a choke-chain compared to no-NFB amps.

I am assuming the current generated by a speaker that you guys feel is an important feedback component ...

I'm not saying it's important or desirable. I'm saying if you want to replicate amp-speaker interaction while attenuating output power by placing something between amp & speaker, it's important to acknowledge its existence and consider whether/how you'll mimic it. It also helps you understand why simple approaches seem to alter the sound (we hadn't even addressed how high resistance probably zaps your treble response because its interacting with capacitance to form a low-pass to the speaker (or a high-pass to ground).

[2deaf]

zaps your treble response because its interacting with capacitance to form a low-pass to the speaker (or a high-pass to ground).

I was just talking about the application of Ohm's law.  More impedance in the speaker with constant resistance in the load means less current through the speaker at higher frequencies.

[jojokeo]

Once you record it, process it, mix it and burn it, the difference between moving a little and moving a lot is not apparent.  The difference between moving a little and not moving at all is easily detectable.  Weber obviously agrees with me because he is advertising moving speaker motors in his.

No, Weber has a "speaker motor" as it's advertised but it doesn't actually move? Here's a photo of one. I also have a HO Attneuator which is more like a re-amplifier. And I made a (Gerald) Weber headphone/line-out using a "speaker motor" but you just glue the coil in place inside the magnet w/out the spider assy, & making a "live motor" removed out of a speaker which does move

[2deaf]

He called his motor "reactive" from which I inferred, apparently erroneously, that the thing moved. 

[jojokeo]

Yes, and giving it a cool dynamic name called a speaker "motor" helps with that. Great marketing there. Gerald infers a similar reactive nature with his gizmo. Their thinking is that you have a voice coil in a magnetic field housing yielding a load close to what you get with a speaker. Yes, there's no movement but being in a magnetic field would or could there be any reactance anyway???
I have another attenuator that's interesting as I remember it being a variable inducter similar to rheostat or inductors on/in a rheostat? It's been a while since I last messed with it. I'll get pictures inside of it tonight. The Ultimate Attenuator has a bunch of stuff inside of them but they plug in like the Ho does so I'm assuming it's something similar w/ heck of a lot more stuff going on. I'm trying to find all of my info & photos that I thought I had - but that was all in an old computer (hopefully it's not lost?). As you might guess this subject has had my attention from a while back. In the end I haven't used any of them on any gigs to date. Seems like just more stuff to carry, set-up, and have possible issues with and the last thing you need on stage is more stuff to worry about.

[2deaf]

Yes, there's no movement but being in a magnetic field would or could there be any reactance anyway?

Reactance is a characteristic of inductors and capacitors.  Both are defined by a function with frequency and impedance in them so reactance is the measure of the impedance of these devices at any given frequency.  I think that "reactance" is being used here to mean the EMF effect of coils.

Reactivity is in my Chemistry book instead of my Physics book.  I don't know if this term has a formal definition in electronics, but the resident authorities here would know.  I use it to mean the current generated by an intact speaker. 

   

[jojokeo]

Reactance is of two types, inductive and capacitive. Inductive reactance is associated with the varying magnetic field that surrounds a wire or a coil carrying a current, ie - the speaker motor. Capacitive reactance is associated with the changing electric field between two conducting surfaces separated from each other by an insulating medium.

[HotBluePlates]

Reactance is of two types, inductive and capacitive. Inductive reactance is ... Capacitive reactance is ...

All we need to know about those terms is for inductive reactance, it gets bigger with increasing frequency; for capacitive reactance, it gets smaller with increasing frequency.

Maybe there's a proper term for variable back-EMF generated in speaker motors, but I don't know what it is. Instead of saying "reactance" or "reactive" which are too close to established terms with a precise meaning that is not speaker-dependent, we could just say a "responsive load" or a "varying dynamic load."

[2deaf]

Yeah, I was thinking I would avoid both of those terms in future posts.

[PRR]

A live speaker is:

Resistive zero to 10 Hz
Inductive 20Hz to 80Hz
Resonant at 80Hz
*Capacitive* 80Hz to 300Hz
roughly resistive 300Hz 1KHz
roughly inductive 1KHz on up.

The action 20Hz-300Hz is all about the coil, stiffness, and mass moving in the magnetic field. If you could block the coil *solid* (it aint easy), the resonance and reactances here would vanish.

The reactances also weaken when the coil is driven outside the magnetic gap. That never happens in "good reproduction" but is very common on guitar amps playing heavy bass.

There's no simple way to fake this with loose parts.

[2deaf]

There's no simple way to fake this with loose parts.

Yep.  That's what I've been saying all along.

jojokeo,
Those are some really cool pictures.  If the Weber motor doesn't move, that tricky spider may be used to dissipate voice coil heat.  It's glued to the chassis which may be a heat sink for it. You don't happen to have a schematic for it do you?   

[Ritchie200]

Again, I go back to this is not a line out application - we are still connecting to a speaker(s).  I think our goal is to make our soak as transparent as possible.

HBP, you posted a graph showing cone bounce after a "thunk" of 20 millivolts.  Is this because a 100watt "thunk" of many, many volts would still show a <1 volt bounce and not be noticeable in a graph with a scale of 20v?  I find it hard to believe that, other than a problem at resonance, there is any appreciable feedback when your pants are flapping in the wind and the first few rows of the stadium are wearing earplugs. A relatively small coil of wire in a magnet will not give you that much - I don't care how fast you stroke it!  AGAIN, I do not deny it exists, I just question the sonic impact - especially at high volume.  I go back to my peeing on a forest fire analogy.  And it is this high volume "response" that we are trying to recreate.

PRR, air-core coils have parasitic capacitance and vary reactance with freq (textbook tank circuit) and resonance can be adjusted.  Hmmm, sounds like a speaker.  A coil with a slug tends to attenuate lower freqs so a fixed coil in a magnet (Weber motor), would seem to not be the best solution?  Unless, they are employing some sort of high pass or notch filter to restore the freq spread?  I do not see the advantage...

I'm sorry, I am probably just not getting it.  I have more questions than answers and some of the answers just do not jive with the real world high dB experiences I've had.

Jim

[PRR]

> air-core coils have parasitic capacitance and vary reactance with freq (textbook tank circuit) and resonance can be adjusted.  Hmmm, sounds like a speaker.

Put some numbers on that. Hundreds of pFd? It would be highly unusual to find an air-coil which is resonant in the audio band. IIRC, a Tesla coil (big air job) needs added C (glass cap) to get resonance down in the low radio band.

My dubious sim is showing that when a speaker is driven by an amp of low damping factor (typical of many g-amps), at Fs, and then the drive is artificially cut-off, the first half-cycle of ringing (booming) touches 0.4 of drive, fading to 0.06 at the fifth cycle, 60mS later. A pure resistor would. of course, stop the instant the drive is removed.

The bass resonant reactances, for a speaker of nominal 8 Ohms, are around 300uFd and 10mH. You don't get these values in a small air-core coil, especially the 300uFd (I'd expect a million times smaller). The 10mH is possible in a BIG air-coil; but the actual inductance of the blocked coil is more like 1mH. These huge reactances are the motional impedance of the speaker mass and stiffness, reflected (back EMF) to the electrical side.

[Ritchie200]

So you are saying the first reflected EMF is .4 of the original spike?  So going back to my HBP graph, if I have a 20v "thunk" the first reflected spike will be 8v?  I just do not see how a voice coil can produce that with a single linear movement...  It's not like a field collapse in a transformer - how can that be?

So, lets say we accept that as the absolute truth...  I go back to a few posts ago.  We need a slight phase shift of the original signal. Each speaker will voice the signal differently so we also need a slight pitch shift.  How about a single spring reverb tank?  That and a toaster element and we are good to go!!

Jim

[HotBluePlates]

I just do not see how a voice coil can produce that with a single linear movement...  It's not like a field collapse in a transformer - how can that be?

A physical speaker is not just a coil of wire. It also has a cone (mass, with inertia) and 2 springs (surround, spider). Displace the coil & cone forward, then let go. The mechanical springs of spider & surround pull the cone & coil back to where they started, inertia keeps it moving past the zero point. Meanwhile you have a physical coil moving through a magnetic field, and a current is induced in the coil.

You're hung up on debating the magnitude, I'm only talking about whether the phenomenon exists. I'll take PRR's numbers as representative, cause I don't know what the magnitude of the effect is.

I only know these side-issue don't exist in resistors.

[jojokeo]

I have one more attenuator called "the cube" which is different than the others. See photo. All of the attenuators work well doing their job but they all have a certain subtle quirkiness in a certain manner and it really depends upon how much signal you're trying to minimize. I think they work & function the best adjusting your amp/set-up total power within reason while going easy on how much attenuation you use. This retains most of the sound and feel as it would be under "normal" conditions without one. Nobody should just turn all knobs to e11even and crank down the attenuator. No attenuator will overcome that, plus it's damn hard on your amp and running it needlessy high & and just begging for something to give way. An attenuator needs time to get used how you control it along with amp settings and style to get the most out of them and they compress things, which is why I feel they should be used like you use salt on your food. Just a little goes a long way and it's very easy to use too much and ruin an otherwise great meal/amp tone! I will say that after playing all three in a comparison test just now (never did this before), the HO attenuator is definitely the "best". The others worked pretty well & the only one (Weber mini Mass) with a bright switch is the one that needed it most. They all gave very useable sound that would be more than fine for any gig situation - recording...maybe not so much? But then again, if you're combining with a bunch of other instruments & a recording was played back - and you never knew what went into the recording - you'd never say, "I bet he was using an attenuator, I can hear it". Sorry, I'd bet anything nobody would be able to tell this. In the end attenuators take a worse rap than they deserve mostly because most people don't know how to use them properly and "over salt" their sound.

All that being said above, I don't feel the need or desire to have to have one in my rig or in my room. If I want super cool harmonically rich overdrive at a low vol level, I'll simply use one of my germanium based fuzz pedals like the FuzzBurner or Fuzz Shway (which people can still get by PM'ing me BTW) 

[jojokeo]

jojokeo,
Those are some really cool pictures.  If the Weber motor doesn't move, that tricky spider may be used to dissipate voice coil heat.  It's glued to the chassis which may be a heat sink for it. You don't happen to have a schematic for it do you?

I don't have a schem of the mini Mass but it's not hard to figure out. I opened it up again and guess what? The good news; The motor assy does have a moving coil which is connected to the spider with the coil running on the outside of the magnet instead of being on the inside as in a speaker. The magnet and outside part of the spider are both anchored down, only the coil can move around the magnet. The bad news; It doesn't move whether I turn the amp up full and whether attenuation is on full or minimum setting. If it does actually move it's so little that any perceived vibration could easily be attributed to everything else vibrating also due to how loud things were. It should've definitely been seen or felt even moving a little bit. However in analyzing this like HBP does, the coil can't move because there's no cone for cone excursion to take place?! So just because there's a coil & a magnet with some energy behind it doesn't mean it will move right? Or maybe the spider is too rigid to be moved? (ha, now I'm thinking the attenuator isn't "broken in" yet, lol  )

[Ed_Chambley]

I have tried a couple of different ways to trim power.  All the attenuators work fine, but not what is usually wanted.  Most times players are simply using too much power and too many speakers.  The best attenuation I have found is have a couple of different cabinets with different speaker efficiencies.

However, I have a friend who owns a Samamp.  It has a light bulb that works somehow.  I do not know how.  I just do not understand why my buddies with the booteek amps will not let me take them apart.

Anyway, the design works very well on what is basically a Deluxe Reverb with a tweed flair.  I cannot remember, but it has 3 or 4 different settings.  Even on the lowest setting it is fairly loud, but it has to be to sound good.  In playing all the different types for me his design is the best.  I remember the lowest setting is 7 watts and it keeps the same feel and still has enough to move some air.  I was not fond of the speaker, but I plugged it into a 1, 12 Celestion Blue on the 7 watt setting and it sounded and felt great.

Anyone know how to do this?  OOPS, just found it.  samamp.com

[2deaf]

I think they work & function the best adjusting your amp/set-up total power within reason while going easy on how much attenuation you use.

I totally agree with you.  Not only on this thread but on other ones I've seen you on.  My studio dead box just knocks it down enough so that you can hear the music in the headphones while still being loud enough to shake the strings.  My speaker / network ones will knock it down to bedroom levels, but I'm not going to pull a 100 watt rig into my bedroom.  I use a small amp with four-stage preamp overdrive for home use just as you use the pedals.

[2deaf]

So just because there's a coil & a magnet with some energy behind it doesn't mean it will move right?

Right.  The magnet has to have a specific magnetic field in relation to the coil.

However, I have a friend who owns a Samamp.

I'll betcha using a light bulb to reduce the power to the output tubes works real good.  Probably got the idea from that light bulb limiter you guys always use here.  However, I'm wanting the sound of all four big bottles at full voltage.  Besides, I'd probably stub my toe in the dark because I put my nightlight bulb in the amp.

[PRR]

> I just do not see how a voice coil can produce that with a single linear movement...

If a closed conductor moves in a magnetic field it makes a current.

And the other way: current in a closed conductor in a magnetic field wants to move.

The "linear" isn't at issue. The conductor doesn't really know the whole picture, rotary or linear.

Take the motor out of your car heater (many toy motors will work similar). Put DC on it, it spins. Spin it, you get DC. Yes, DC motors ARE generators, and will generate fine. (For "best" generator action you do some small mods.)

The speed of a motor is proportional to voltage.

The voltage of a generator is proportional to speed.

If there was no friction, air-drag, or copper loss, the motor/generator would be perfectly reversible. 12V in makes 1,200RPM. 1,200RPM makes 12V out.

In fact we always have losses. Say that 12V in only spun up to 1,100RPM. And that a 1,200RPM spin only made 11V.

Then if we spin-up with 12V to 1,100RPM, and then cut the voltage supply, and watch the generator output, we will get around 10V at the moment of cut-over (decreasing as RPM falls).

In rotary motors, coil mass is not a critical thing. It costs money and wants to fly off the armature, but we don't have to pinch grams. We want good efficiency and use enough copper to get near 90% efficient (10% loss). In a vibratory motor working very fast, mass hurts. We minimize the amount of copper, which means high copper resistance. Efficiency won't be as high as 30%, losses are huge. So instead of the maybe 80% we got from a rotary motor/generator we might get 40% (as in my example). And if we also want wide bandwidth (and damn the efficiency) we might get 10% or 5% as in HBP's plot.
________________________________

> magnet has to have a specific magnetic field in relation to the coil.

For "best" balance of efficiency, bandwidth, and cost, there's an "optimum" field strength.

However ANY field strength will cause SOME coil force, and presumably motion.

Is Weber's dingus really a magnet?

Is that spider really allowing the coil to move?

I dunno. Maybe it is just an odd resistor, perhaps using-up stray parts from a speaker builder.

If it truly moves, the motion *may* be invisible. You can make heaps of treble with 0.001" motion. Speakers (and speaker motors) move mostly with deep bass. Any good guitar speaker motor should move 0.050" (two matchbook covers) with heavy bass, and that's usually visible.

[2deaf]

However ANY field strength will cause SOME coil force, and presumably motion.

I don't want to drop into a discussion about magnetic lines and speaker theory here, but I can tell you from experience that most arrangements have no discernible motive force on the coil.

Is Weber's dingus really a magnet?

It's a dead ringer for the center pole of a cheap speaker magnet.

You can make heaps of treble with 0.001" motion

This relates to my earlier statement that it doesn't take very much speaker-generated current to get "that sound" going from your amp. Those M5-30's that I cut up were some damn loud little suckers, illustrating that it doesn't take much cone to make heaps of treble, either.

[jojokeo]

"This relates to my earlier statement that it doesn't take very much speaker-generated current to get "that sound" going from your amp. Those M5-30's that I cut up were some damn loud little suckers, illustrating that it doesn't take much cone to make heaps of treble, either."
Maybe the slight vibrations I felt with my finger were indeed movement and not caused by my cab?
 
But still - the mini mass had the most treble loss of all my attenuators which had to have bypass caps put across the rheostat (see earlier photo) and they worked well like when we put a treble cap across a simple vol control. The sizes of those were much larger than what we use.

[Ed_Chambley]

So just because there's a coil & a magnet with some energy behind it doesn't mean it will move right?

Right.  The magnet has to have a specific magnetic field in relation to the coil.

However, I have a friend who owns a Samamp.

I'll betcha using a light bulb to reduce the power to the output tubes works real good.  Probably got the idea from that light bulb limiter you guys always use here.  However, I'm wanting the sound of all four big bottles at full voltage.  Besides, I'd probably stub my toe in the dark because I put my nightlight bulb in the amp.

I prefer the sound of not limiting power myself.  Just so happens the topic is not "how to get full output and not stub your toe".

I have a soundproof amp room.  Soundproof!  I control my volume by how far I open the door, but this presents a problem.  In a small to medium size room while playing live, I just cannot seem to carry it with me.

I have never owned an attenuator and am not really crazy about Master Volumes.  I have used the LarMar with some pleasing results.  That is why I have tons of amps with varying wattage and many different speaker cabinets.

A nice guy sent me the patent papers on the sansamp.  It is US Patent number US20060140419A1 for anyone interested.  So if anyone who is interested in another method, I hope this helps.  It seems it would be like a VVR, but it doesn't "feel" the same way.  It has a very strong and punchy feel.  It is just preference.

Maybe one of you very knowledgeable guys could elaborate on why this would feel different than a VVR/Power Scaling?

[2deaf]

Maybe one of you very knowledgeable guys could elaborate on why this would feel different than a VVR/Power Scaling?

While you're waiting for one of the very knowledgible guys (they really are) to reply, maybe you could consider that a VVR reduces the power to the preamp and output tubes and the light bulb only reduces the power to the output tubes.

Just so happens the topic is not "how to get full output and not stub your toe".

I didn't know that being snide was acceptable on this forum.

[jojokeo]

Ed, there's too many things I want to comment on but your humor is great as always. Wish I had that soundproof room with the door 
 
You said Sansamp, that's different than Samamp. I take it that you meant the VAC control?