L-Pad REACTIVE Attenuator?

[jbefumo]

I've been playing around with reactive attenuators based on the Weber Mass design for several  years now, and have generally been pretty happy with the results, but it seems to me that, as designed, a lot of the load is still purely resistive.  I've also looked at several attenuation circuits using L-pads, which, of course, are completely resistive.
Was thinking that by using a dual-ganged potentiometer, with the two units wired in reverse, that is, turning one way would direct more signal to the mass motor, and less to the speaker, and vice versa. Since there are, to my knowledge, no dual  potentiometers offering the required power handling, and no high-power rheostats in dual-ganged configuration, I'm wondering whether I can use a dual L-pad to accomplish the same thing. Since internally the L-pad isn't the same as a potentiometer, and by my measurements the two sides of each mono l-pad are not the same, I came up with the attached circuit that seems like it should do the job.
Would hate to fry my Mercury Magnetics OT, thought I'd ask here: a) is it safe, and b) is it likely to work as envisioned?
 
Thanks.
 
Joe

[PRR]

> is it likely to work as envisioned?

You get only slight attenuation of the speaker. 8r in series with speaker isn't even half volume.

Look again at the top, factory, plan. Strip the frills. The WM unit does not need to be grounded. He put it in series with the speaker, then a 50 Ohm pot to divert power to the WM or the speaker. It looks good on paper, it apparently works OK as a commercial product, it is ripe for stealing.

[kagliostro]

Don't know if this can be of interest

Reading this post remembered me an old article at Duncan's Amp related with Dummy Load

http://www.duncanamps.com/technical/dummyload.html



May be the circuit proposed can be used instead of the MassMotor (?)

Franco

[jbefumo]

Yeah, I've seen that and played with it a bit in MultiSim -- with a bit of tweaking I got it to where the frequency curve is pretty close to a Celestion G12H.


What I'm trying to work out now is getting to signal to alternate between the dummy load (either the attached or the Mass Motor) with the reactive elements representing all or most of the load.  (Actually, I have in mind using this as part of a larger solution, which I will most momentarily, otherwise I would just buy the unit from Weber.)

Don't know if this can be of interest

Reading this post remembered me an old article at Duncan's Amp related with Dummy Load

http://www.duncanamps.com/technical/dummyload.html



May be the circuit proposed can be used instead of the MassMotor (?)

Franco

[jbefumo]

Yeah, after posting I played around on the bench and realized my mistake.  However, by taking that line to ground on the dummy load and running it instead to the input, as shown in attachment, it does appear to work.


Again, what I'm trying to accomplish is to essentially create a balance control. The details of the dummy load will probably be changed.

> is it likely to work as envisioned?

You get only slight attenuation of the speaker. 8r in series with speaker isn't even half volume.

Look again at the top, factory, plan. Strip the frills. The WM unit does not need to be grounded. He put it in series with the speaker, then a 50 Ohm pot to divert power to the WM or the speaker. It looks good on paper, it apparently works OK as a commercial product, it is ripe for stealing.

[jbefumo]

I actually have the attached up and working.  The grand scheme is that I plug into the device, which then has two outputs, one going to the Vibrato channel of my Super Reverb, and the other to the Normal Channel.  In the default state, the attenuator is completely out of the circuit, and the guitar signal is routed to the Vibrato channel -- nice clean rhythm sound.  When I activate the single footswitch, it simultaneously engages the attenuator, and switches the input to the Normal channel, which I have cranked to about 8, with the attenuator then used to reduce the output so it's more or less balanced with the rhythm setting -- maybe just a little boost.  I can also add any other effects I might want for one or the other in the appropriate chains.  At this point what I'm mainly trying to do is  improve on the actual attenuation circuit, or at least make it less of a rip-off of Weber's design.

[jbefumo]

Oh yeah -- I KNOW the Super Reverb wants a 2-ohm load -- right now it's in a 2X12 cabinet with 8-ohm and a 4-ohm speakers in parallel.  I plan on replacing the current Mercury Tone-Clone OT with a 4-8-16 equivalent and replace the current 4-ohm speaker with an 8 for a net 4-ohm or 16-ohm load - which ever works best with the attenuation device I ultimately cobble up.

[kagliostro]

In an old thread at DIYStompbox there was this schematic, have you it ?


Franco

[PRR]

> reactive elements representing all or most of the load.

But, as you say, a real speaker is MOSTLY resistive across most of the audio band.

If you setup the 50mH 100uFd bass resonance, without a 50-100 Ohm damp resistor across it, if you thump a 71Hz note the amplifier is essentially UN-loaded, and may bust its OT insulation.

Note that Randall's design posted at Duncan's is scaled for 16 Ohm nominal.

[jbefumo]

I do, thank you!


joe

In an old thread at DIYStompbox there was this schematic, have you it ?


Franco

[Glennjeff]

Where does one purchase those Mass Motors, can't find them as a separate item on Weber site.

[jbefumo]

hmmm -- used to be under Amp Tech Tools but I just looked and it appears they no longer sell them. Glad I grabbed two while I still could.  Was planning on replacing them with the capacitor-inductor-resistor array someone posted above (which, I believe, originated at Aiken Amps.)

Where does one purchase those Mass Motors, can't find them as a separate item on Weber site.

[jbefumo]

Before I discovered the Weber units, what I used to employ as a dummy load on the bench was something like this:


http://www.mcmelectronics.com/product/STAGELINE-SP-100-8-/55-5110


I just squeezed an entire tube of flexible silicone sealant over the cone and mounted it flat against a piece of board.  Not sure whether restricting the cone movement like that would result in damage to the coil if used continuously, though.  Kind of a shame that Weber stopped supplying them.  I seriously doubt that the few people with the skills and inclination to use them were offering much competition to their assembled units. As I said earlier, I would prefer to just buy their finished units if they were flexible enough for my intended application.


Oh well. 


Maybe Warehouse Guitar Speakers (https://wgs4.com/) could be interested in producing them if a few of us got together to generate a quantity worth their while.

hmmm -- used to be under Amp Tech Tools but I just looked and it appears they no longer sell them. Glad I grabbed two while I still could.  Was planning on replacing them with the capacitor-inductor-resistor array someone posted above (which, I believe, originated at Aiken Amps.)

Where does one purchase those Mass Motors, can't find them as a separate item on Weber site.