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turning a vol pedal into a stereo vol pedal


 
12/15/2002 8:14 PM
Bolero
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turning a vol pedal into a stereo vol pedal
anyone know how hard/possible it would be to turn a run-of-the-mill volume pedal into a stereo volume pedal?  
 
ie: pedal back =100% volume to #1, no output to #2  
 
pedal middle = 100% volume to #1 & 2  
 
pedal forward = 100% volume to #2, no output to #1  
 
O know ernie ball makes a stereo volume pedal, but they're damned expensive  
 
thx!
 
12/16/2002 6:34 PM
Mark Hammer
That's a tall order to do passively. What you're essentially asking for requires a custom pot.  
 
Why? Well, consider this. First, for smooth volume sweeps, the pots have to logarithmic. For the volume to sweep from one output to the other, the two pot sections have to be opposite in taper, so that one is log and the other anti-log.  
 
The additional component is that you essentially don't want any attenuation until the other side of the midpoint (100%+100% in the middle, right?), which requires a VERY special pot.  
 
You CAN do this other ways, however. And if you don't mind having each channel be at half-volume at the midpoint, it's even easier.  
 
One way us to buffer the ins and outs actively, and have the pot be a balance control that selectively bleeds off signal to ground from each channel. It is possible to wire up a single linear pot so that it has a quasi-log-/anti-log taper (i.e., linear towards the middle, log-ish at the ends) or a dual linear in the same manner. See the docs on the secret life of pots at GEOFEX.COM.
 
 
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12/16/2002 10:20 PM
Bolero
wow, I never thought of any of that!!  
 
how do you think the Ernie Ball stereo vol/pan pedal works? it is passive, no batteries.  
 
I guess what I want to do is really "pan" between both amps.  
 
thx for the input, that's sure is useful to know!!
 
12/17/2002 1:58 AM
Mark Hammer
As far as I know, yes the Ernie Ball pedal uses some sort of custom dual-ganged reverse-taper pot.  
 
If what you want to do is pan, then I suggest you do the following:  
 
1) Pick yourself up either a dual ganged 500k or 1M pot - LINEAR taper. Higher value pots will result in less loss of high end. Normally, log pots are used for volume control, as noted earlier, but you want to be able to *produce* the taper of the pot and make each one the opposite of the other. For that you need to start out with linear taper so they are both identical.  
 
2) Read the documents on GEOFEX and at AMZ regarding pot taper.  
 
3) Pick out a suitable value of fixed resistors to adjust the taper of the pots (they will go between the wiper and one of the legs on the relevent pot, opposite leg for the other ganged pot).  
 
This should produce the desired result, where the midpoint of the pot will result in equal level outputs for each channel, and moving it more in one direction will simultaneously increase one channel and attenuate the other.  
 
I need to point out that the midpoint does not *really* produce equal level outputs. What it does is produce equal level *attenuation* for each channel. If they weren't even volume to start with they won't be when the ganged pot is at the middle either.  
 
There are several solutions to this:  
 
One is to have a second individual volume control (1M also) for each channel mounted on the chassis between the ganged pot and output jack, so that you can adjust the output level to be even when the ganged pot is at its midpoint. This is a passive circuit and while it won't eat or need batteries and will get the job done, it will also potentially load down the next device in line and lose treble.  
 
A second is the stick in a second linear pot in conjunction with the ganged pot. The wiper of this additonal pot goes to ground, and the outside lugs are tied to the *ground* end of each of the ganged pots. Varying the additional pot essentially adds (or subtracts) a bit more or less additional resistance to the wiper-to-ground legs of the ganged pots. This is essentially a balance fine-tuning control. It will also do the job without batteries, etc., but it may throw the pot taper off a bit. A 100k pot will probably be sufficient for this. Set to its midpoint, it will only add 50k to a 1meg pot section, which isn't much. Rotated all the way to one side, it will add 100k to that pot half.  
 
The third soultion is an active one and requires batteries and a chip. Build a simple op-amp gain stage (inverting or non-inverting, whatever your taste is) and stick a variable resistor in the feedback loop so that you can adjust the gain of the stage. The dual-ganged pot will now attenuate the output of that gain stage. Having the gain stage as a buffer, means you can also use a lower value dual-ganged pot (e.g., 10k or 50k) without eroding the tone.  
 
There are probably more ways than these 3, but they are all useful and practical and easy to do.  
 
In the big picture, they are all probably overkill unless you have some very special circumstances. Why do I say this? Well, when was the last time you were able to accurately identify the midpoint of pot rotation with your foot?
 
12/20/2002 7:21 AM
Ben N
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The additional component is that you essentially don't want any attenuation until the other side of the midpoint (100%+100% in the middle, right?), which requires a VERY special pot.  
 
Actually, Mark, IMO having attenuation of both signals across the entire range works well for a pan pedal, because it keeps the volume even across the entire range (assuming the volume levels of the two channels or amps or whatever are in balance in the first place). If you had 100% + 100% in the middle, that point would be a good bit louder than either end of the travel.
 
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