light bulb noise reduction issues

[guntherbuffalo]

Hi there,

THere are a couple of other threads about this sort of thing. THD in particular use a 13v 4.4w light bulb in parallel with the speaker out on their Univalve to create a noise reduction system, as well as a cool effect where the bulb lights up in sympathy with your guitar playing

I have been trying a similar thing on a 4 watt champ style amp I have built. I have linked up a bulb to a similar spec (4w 12v to be precise). However when I play guitar, background hum does not seem to be reduced, and the bulb does not seem to begin to light up until the amp is cranked, and certainly not to the brightness of the bulb in the univalve. I have tried this with a 12v 1w bulb to similar effect.

Does this mean I need a lower voltage bulb? I believe this would make the light brighter earlier, would it also reduce noise more? I imagine with this sort of thing involves trying various bulbs to see which works best, any idea which way to go in terms of bulb voltage/wattage and how an increase/decrease in each bulbs rating affects the lighting up of the bulb, as well as noise reduction.

This is my first amp build so I am fairly new to this, any input would be appreciated! :D

[sluckey]

Does this mean I need a lower voltage bulb? I believe this would make the light brighter earlier, would it also reduce noise more?

Yes, use a lower voltage lower wattage build. I can't say about noise reduction though.

[HotBluePlates]

... THD in particular use a 13v 4.4w light bulb in parallel with the speaker out on their Univalve to create a noise reduction system ...

That's not "noise reduction" it is a power attenuator. Some of the output power is used to light up the bulb instead of move the speaker.

... I have been trying a similar thing on a 4 watt champ style amp I have built. ... background hum does not seem to be reduced...

If you have background hum, you'll need to fix the thing that's actually causing them hum, whatever that might be.

[jjasilli]

Ohm's Law Pie Chart: Watts = Voltage² / Resistance.  Let's say you have 20W into an 8Ω speaker:

20 = V² / 8; V = √160 = about 13V.

So a 13V bulb has enough voltage to glow brightly only at full volume on a 20W amp.  (This ignores the resistance of the bulb in parallel with the speaker's impedance, but that's the general point.)

At 4W:  4 = V² / 4Ω (?); V = √16 = 4V; it's surprising a 12v bulb glows at all.

[guntherbuffalo]

Awesome! Very helpful! Cheers!

[HotBluePlates]

... the resistance of the bulb in parallel with the speaker's impedance ...

I'm pretty sure the lightbulb is in series with the speaker, just like our friend the lightbulb limiter. 

[guntherbuffalo]

This was the thread earlier calling it noise reduction:

http://www.el34world.com/Forum/index.php?topic=15325.0

[guntherbuffalo]

In the THD Univalve it is in parallel I believe. From what they say in parallel it has a sort of "expansion" effect, in series they call it a compressor. I have been using bulbs in parallel. :D

[alerich]

For what it's worth I use a THD Hot Plate and I can't tell a difference between having the lamp switched on or off when I use it on the Hot Plate, either. I leave it on simply for the cool effect you alluded to.

[kagliostro]

Some time ago I had the same curiosity

here you can read some other explanation on the matter

http://www.el34world.com/Forum/index.php?topic=14404.0

K

[HotBluePlates]

Yeah, just found a Univalve schematic, which (though hard to follow) shows the bulb in parallel.

This was the thread earlier calling it noise reduction

... it has a sort of "expansion" effect ...

Compression is a reduction of dynamic range (the difference between LOUD and soft). Usually, you reduce compress, then amplify what remains for a louder average signal (radio stations compress the snot out of your tunes to seem loud on the dial and easy to tune in at distance).

Expansion is the opposite: an "expansion" of dynamic range. One of the historical uses of expansion was to offset compression required in the recording process of early records, so the result had a dynamic range similar to the live (orchestra) sound source that was recorded. I've used expanders (with a gate) in a recording studio on drums to "turn the volume down" faster than the natural decay of a note, mainly to hide the action of a noise gate closing.

The original poster may have noticed that noise in the amp was reduced, along with everything else.

But that's not why THD did it. You bought your Univalve to get some good output tube distortion, but your spouse/girlfriend/neighboring apartment doesn't appreciate a good 6L6 or EL34 like you & me. So THD gave a handy way to knock down volume. They tell you as much in the product info on the THD website (though they don't say the lightbulb is doing it.

So... do you have noise problems? Or did you use the bulb to reduce output power? For power reduction, jjasilli is right, and you'll need a ~4v bulb. For hum reduction, you need to fix the cause of hum in the amp (Champ, probably full-range speaker bumping into the lousy filtering of the original circuit, so you probably need another 20uF and ~100-200Ω between the rectifier and your present 1st filter cap).

[jjasilli]

... the resistance of the bulb in parallel with the speaker's impedance ...

I'm pretty sure the lightbulb is in series with the speaker, just like our friend the lightbulb limiter. 

I was wondering the same thing; thanks for clearing that up.  The series connection would: i) increase impedance; and ii) the voltage "demand" of the bulb + speaker would add-up.  Each of these factors would contribute to signal attenuation.  Also, the bulb would be more dim.

[jjasilli]

Awesome! Very helpful! Cheers!

The Ohm's Law pie chart is at the heart of everything (we do here).  It fully covers all 4 factors:  watts, resistance, voltage and current.  Usually you know 2 of these factors and need to find another one.  E.g., in this case we know Watts and nominal speaker Impedance (R), and need to find Voltage.  It's simply a matter of finding the applicable formula in the pie chart.

[kagliostro]

http://www.system-maintenance.com/maint/ohms-law-calculator-pg-25.html

K

[Willabe]

Light bulb compressor pedal. Plugs in between the amp and speaker.

He's got a second 1 now, Jr. I'm not sure about the light bulb in the new one.

http://swartamps.com/night_light_attenuator.htm


               Brad     

[guntherbuffalo]

"So... do you have noise problems? Or did you use the bulb to reduce output power? For power reduction, jjasilli is right, and you'll need a ~4v bulb. For hum reduction, you need to fix the cause of hum in the amp (Champ, probably full-range speaker bumping into the lousy filtering of the original circuit, so you probably need another 20uF and ~100-200Ω between the rectifier and your present 1st filter cap)."

Well the main reason is that I have built my champ into the box and chassis of an old 50's reel to reel (that was completely broken) that I found in a skip. The original chassis had a "magic eye" valve on its front panel. I figured the light show provided by this light bulb trick would be a good replacement for the magic eye, as it would be easy to wire up, and fills the space in the chassis with an equally cool effect. Mainly a cosmetic thing, but also as an attenuator for bedroom use.

 As it happens there is a low level hum, which I think is to do with my layout. (that's what the guy at Ampmaker tells me who sold me the kit and has been providing tech support). We have tried moving a couple of things, checking for ground loops, all we know is that the hum increases with volume, so must be before the Volume pot. I figured if this bulb also reduced noise it may kill 2 birds with one stone, but was planning on having the bulb regardless.

[jjasilli]

Light bulb compressor pedal. Plugs in between the amp and speaker. He's got a second 1 now, Jr. I'm not sure about the light bulb in the new one. http://swartamps.com/night_light_attenuator.htm
Brad     

Intariguing!  But how to build one?  Hint here:  http://www.freestompboxes.org/viewtopic.php?f=2&t=9463

I'm guessing there's a fixed 8Ω 50W power R in parallel with the speaker.  The bulb is in series with the +feed line, after the top of the resistor and the +speaker lug.  But how to make it variable?  Series resistors? L-pad?

[HotBluePlates]

Search for "lightbulb attenuator schematic" and you'll find the answer. Especially a Google image search.

You'll also see why I originally thought the bulb was in series.

[PRR]

> The original chassis had a "magic eye" valve

If you kept the tube and socket, these are easy to re-purpose as dancing-audio. Young kids who have never seen a Magic Eye will think it's a lot of CPUs and OLEDs.

You may be able to use the Standard Circuit (look-up the tube-type for a datasheet), but connect the input to the speaker output.
_________________

The hash-reduction "can" happen because a cold lamp is 10X lower resistance than a hot lamp. If scaled for 16 Ohms hot (loud), that's 1.6 ohms cold ("silent"). Across a 8 Ohm tap on a no-NFB amp, that's a significant drag which goes-away when you push it. But both voltage and current are critical.

This is an "expander". The series connection gives compression.

[kagliostro]

The Selmer Zodiac used one of those Magic Eye, an EM84 tube 

http://www.el34world.com/charts/Schematics/files/selmer/selmer_zodiac_twin30_.pdf

K

[jjasilli]

Search for "lightbulb attenuator schematic" and you'll find the answer. Especially a Google image search.

You'll also see why I originally thought the bulb was in series.

Found lot's of great stuff, but not how to to make a light bulb attenuator variable; except for an LDR circuit.  This is a fascinating topic: compression vs. raw attenuation.

The Selmer amp is also intriguing, but it looks to me that the magic eye tube is part of the tremolo circuit, instead of vol reduction.

[HotBluePlates]

Search for "lightbulb attenuator schematic" and you'll find the answer. Especially a Google image search.

You'll also see why I originally thought the bulb was in series.

Found lot's of great stuff, but not how to to make a light bulb attenuator variable ...

First row of results for me. Move the lightbulb to a point after the 8Ω resistor (so it's now always in parallel), add a pot in series with the bulb, wiper to the speaker jack.

[jjasilli]

Not getting it.  It seems there is no point after the 8 ohm R, which seems to act as a dummy load in order to use the line-out alone.  The 8 Ohm R is automatically SW'd out of the circuit when a phono plug is inserted in the speaker jack, thus defeating the dummy load.  There is no protection for the OT if the bulb blows.  Or am I reading this wrong?

[Willabe]

That's what I think too.

What about putting a 270ohm (?) (KOC forum/books and others) from the OT + to ground like a number of guys do and recommend?



                    Brad    

[HotBluePlates]

Not getting it. 

You're right... the drawing has no protection for an open bulb.

But you're a smart guy; you can mentally move the parts to be where they need to be to make it work.

The "5kΩ" pot below isn't necessarily 5k. I don't know what a good value would be but maybe 50-100Ω or perhaps higher. It would ba an easy experiment.

[jjasilli]

OK, thanks. Have been snowed under at work, not to mention the actual blizzard.  So, I'm thinking:  make your 8Ω fixed, shunt resistor an 8Ω 50W (or more) L-pad rheostat wired as a vol pot.  Entirely ditch the pot in your drwaing, as that function is now done by the L-pad.  Put the bulb in series with the L-pad output to the +Speaker lug.

The L-pad (lugs 1 - 3) will function as an 8 ohm dummy load if the bulb blows. The bulb will further compress signal, which is subject to variable attenuation by the L-pad.  I just built an L-pad attenuator, and agree with web postings that it has very little functional sweep.  The bulb should improve the sweep of the L-pad, and the whole thing will be variable.

It's just a matter of breadboarding a light bulb with the attenuator I just built.  But on top of a heavy workload, today's my 60th birthday, and family has already arrived for the weekend.  Should get to this soon.

[Willabe]

today's my 60th birthday, and family has already arrived for the weekend. 

 

 

[kagliostro]

Buon Compleanno Jjasilli

K