1 knob volume cut or boost circuit

[chefboyardee]

Hi all!

my friend uses a switcher for his array of pedals and tells me he would like a transparent volume boost or cut so he can adjust certain pedals volume

i know how to solder so building the pedal is no problem.. my knowledge stops here.

anyone have a design i can build for my friends needs?

thanks

[garcho]

AMZ mosfet booster

Throw a volume pot on the rear end and you have attenuation.

EDIT: DOH! I just realized that makes for 2 pots.

There's this thing: tonepad lm13700 simple vca

[marcelomd]

I'm not at home. But look for "active volume control"
It's basically a clever use of an inverting opamp

[MrStab]

+1 on active volume control, though may need an input buffer with high impedance, necessitating 3 op-amps (a quad package could do nicely and allow for buffered vref).

Simpler, perhaps noisier approach is just a non-inverting, fixed gain stage with a volume pot placed after it. Somewhere on the travel of that pot would bring you back to unity, with "boost" on one side of that point and "cut" on the other. Gary's suggestion but with a fixed resistor in the gain stage instead of a pot would suffice.

[FiveseveN]

There's one bit of missing information that has a big impact on how fancy the gain staging needs to be: how much boost?
There's also a zero-cost solution: set his quietest pedal (on max volume) as the target level and attenuate everything else to fit. These pedals have volume controls, right? Or is there more to this story?

[Ben N]

Some of us may be overthinking this. Wouldn't "a transparent volume boost or cut" be easily and simply achievable by either (a) a non-inverting opamp, preferably running on a higher voltage than the FX,  with a volume control following, or (b) an inverting op amp with gain control? Opamp, charge pump, pot, and a few diodes, caps & resistors, and done.

[FiveseveN]

Of course it's trivial if you know how op-amps work but I get the impression that @chefboyardee is looking for a ready-to-solder project. Otherwise it's a good opportunity to get one's feet wet if they're willing.

[GibsonGM]

All the above, plus having your friend really think about what's going on, what he wants to do and why. 

A boost can do a lot of things...it can be used on its own to juice the front end of an amp to get more distortion from its early stages, sometimes that adds warmth and punch, primarily for a tube amp.  Maybe it's only on for leads to give some volume, depending if the amp has the headroom. Then it will usually comes LAST in the chain just before the input. 

It can be used to increase signal level to a following pedal...slamming a distortion pedal with a higher signal.  Resulting in mayhem - some ppl love that.

It can be (as you said) 'transparent' and simply make a clean level a little louder - but if taken too far, it will cause the following 'things' to distort, which may not be desirable (say in a chorus, flange or something like that).  That is a 'try it and see' situation.

If someone likes their pedal chain but is running something like a chorus, and they turn it down (or it's kind of quiet itself), you may feel you're 'missing something' - I know I do.  Sound is really good, but it's just not slamming you in the @ss the way it should.  In that circumstance, maybe 'just your basic boost' like a mosfet boost, coming last in the chain, will give you what you want.  Might require playing with the low end, too, since bright can make 'bite'.   "Time domain" effects like chorus cause me that issue.

Taking a boost like that to the pedal chain and amp, and placing it in different positions, might further clarify what is missing for this dude.  Might be as simple as 9V generic gain stage like discussed above, with an opamp or what have you.   As always, for more b@lls, I'd try the ol' mosfet boost

[PRR]

I don't know what "transparent" means, except no deliberate signal damage. Yes, how much boost/cut?? Is it coming from a low-Z box or right from guitar pickup? Is it driving a telephone line or just a stage cable? LOW-hiss? HIGH headroom?

Three opamps seems extravagant. Two is trivial but tends to require thinking about overload vs hiss.

PLAGIARIZE!!  There is a one opamp plan found on many pedals (I can not remember which) which has non-LOW output impedance but lower than a lot of stage toys. Here is the essence. You probably want to add DC-block caps, bias resistor and bias voltage.

[chefboyardee]

Hi all and thanks for the many replies

i will talk with him and get back to you with all the missing info.

i should be back in 24hrs

thanks again!

[chefboyardee]

Hi i'm back and i have read all your posts ..many thanks to everyone

my friend plays  electric and acoustic guitar and his switcher is a Voodoo Lab GCX with Ground Control

not all effects have a volume control. he needs to boost some and cut others so it can be 2 different circuits
no huge boost needed here just a slight volume increase / decrease.

i would like to be able to fit the circuit in a Hammond 1590A

the AMZ mosfet booster suggested by Garcho looks like what i need to increase the volume and has avail boards that fit the 1590a

should i take Garcho's advice and add a secondary pot and will it fit the 1590a

or make another circuit just for the pedals that need their volumes cut?

much appreciated. thank you

[Ben N]

Just thinking a little out of the box, if the switcher does MIDI and your pal does presets for his fx, one could probably fairly easily put together a simple variable boost/cut (any of the above circuit ideas might do) with the volume controlled by an Arduino with MIDI input (PC or CC) through a digipot or vactrol. You could probably even rig up some kind of LED string as an indicator of what setting the boost/cut is currently on. The whole thing would probably go in a 1590B with 2 1/4" jacks for I/O, a MIDI-in jack, and a power jack, and you only need one for the whole rig. (Dissipating heat from regulating 9v to 5v is something that would require a little thought.) No pot required!

[GibsonGM]

Actually, if all you want is as you say - to either add volume or decrease it - then a simple opamp gain stage would probably fit the requirement better than a 'boost'.  The opamp will not color the sound very much at all. And the gain required is going to be quite small, just a few dB.   

The gain could be set via an internal trimmer, leaving you with a single volume knob on the output and a very small PCB...there are MANY single opamp designs you can plagiarize as Paul suggested above (Blues Breaker, Tube Screamer, data sheet for whatever opamp such as TL071...).  Try searching "opamp volume control", and find a basic one - which is all you need.   

Build something on breadboard, try it out, find out what the gain should be (the feedback resistor, usually shown as a pot....use one on the breadboard for adjustment) by trial & error....replace it w/fixed resistor, now you only have the 1 volume pot on the output...or include a small trimmer for potential future adjustment.   It really IS that simple