Balanced stereo level control / potentiometer / fader or knob or whatever

[TyphoidHippo]

I'm trying to get up to speed with all this stuff, and I think I understand that if I wire a "quad potentiometer" inline between a balanced, stereo, line level source and an amp that it will work as a simple level controller.

Something like this: Slide Quad Potentiometer Alps 10K Lin 60mm Travel Qty 2 | eBay (and maybe wire it to some TRS or XLR ins and outs if I feel fancy when I'm actually doing this). Will this actually work? Seems like surely there's got to be some reason I can't have a 9 dollar solution to 2 broken mixing boards (which I was only using as level controllers) on my hands... Thanks for any input, guys

 

[ecc83]

I'm trying to get up to speed with all this stuff, and I think I understand that if I wire a "quad potentiometer" inline between a balanced, stereo, line level source and an amp that it will work as a simple level controller.

Something like this: Slide Quad Potentiometer Alps 10K Lin 60mm Travel Qty 2 | eBay (and maybe wire it to some TRS or XLR ins and outs if I feel fancy when I'm actually doing this). Will this actually work? Seems like surely there's got to be some reason I can't have a 9 dollar solution to 2 broken mixing boards (which I was only using as level controllers) on my hands... Thanks for any input, guys

No. That is a linear potentiometer and totally unsuited to audio level control.

You could look for a log version but don't hold your breath and unless you pay a great deal for one the stereo tracking will be poor. I make no excuse for once again saying "Get Duggy Selfs book!"

The technically best option is to uses 1/2 an NE 5532 as an "unbal" amp per channel. A twin linear pot with a "log" loading resistor and another 5532 as a balanced op amp.

Such a setup will give virtually no more noise and be free of loading/response problems.

$10 should cover it!

Dave.

 

[TyphoidHippo]

No. That is a linear potentiometer and totally unsuited to audio level control.

You could look for a log version but don't hold your breath and unless you pay a great deal for one the stereo tracking will be poor. I make no excuse for once again saying "Get Duggy Selfs book!"

The technically best option is to uses 1/2 an NE 5532 as an "unbal" amp per channel. A twin linear pot with a "log" loading resistor and another 5532 as a balanced op amp.

Such a setup will give virtually no more noise and be free of loading/response problems.

$10 should cover it!

Dave.

Oh, sweet, thanks dude

Didn't notice (or think about) the linear/log thing. Ok - instead of ordering and waiting for pots, I just de-soldered a ton of them out of these (otherwise broken) mixing boards. I have quite a few that were for stereo levels/returns/etc that I'm assuming are the twin ones you speak of. Watching the resistance on the various posts while turning/sliding them looks like the will do exactly what I need, since they have almost no resistance at full level/setting/whatever you call it when they're wide open. I'm having trouble understanding what the op-amp would be for since the signal only needs to be attenuated when the slider goes down, which it appears it already is. What am I missing there? Something kinda fundamental, I'm sure.... Is the idea for the pot to be like... a controller only for the op-amp and not actually supposed to be wired into the signal? Or something like that? And don't those things need some kind of power to work?

Oh - I looked for that guy's book, but Amazon has never heard of him....and Google wasn't much help either - other than a forum post mentioning "Small signal amplifiers". Is that it? If so - any idea where that can be purchased? Thanks again

 

[ecc83]

See my crude drawing of the block diagram of what is needed.

IC 1a/b unbalances each feed and gives a very low impedance drive to the stereo fader (this can be a log pot or a linear track with law distorting resistors for better stereo matching).

ICs 2 and 3 are crosscoupled pairs that deliver back a balanced signal. Or you could use a single 5532 section and have an "impedance" balanced output.

N.B. This is just a representation, the actual circuit is a little more complex but not a lot!
But! Since you ask if these chips need power (they do!) perhaps you are not quite ready for this level of DIY?

(BTW HR. H ave you mucked about with the attachmment arrangements please? I could not attach a .PDF nor a teeeensy monochrome .bmp! I do not recall such arbitrary limits a few weeks ago?)

Dave.

 

[bouldersoundguy]

The Level Pilot by T.C. Electronics does a fine job with passive components. Maybe you can copy that circuit. I tried searching for a schematic or block diagram but all I got was a bunch of gas heater and avionics stuff.

 

[kip4]

group diy or tapersection forums maybe able to help you out.
Op amps are not that complex. Its worth the effort to buff up and learn some before embarking on such a project IMO.
Good luck and If i can be of assitance just shout.
Rich

 

[TyphoidHippo]

But! Since you ask if these chips need power (they do!) perhaps you are not quite ready for this level of DIY?

Yea, not ready and not really interested, I guess. I was thinking much simpler - just a single passive component between source and amp. After looking at, and almost buying, the level pilot bouldersoundguy mentioned and tinkering with all kinds of other stuff in my free time over the last week or so (there's a pile of mutilated pots and attenuators in the corner of this room, lol): it occurred to me that I could do what I wanted with software, since the source for both setups in question is a computer, and both have control surfaces of some sort already attached - the problem being that the faders and sliders and stuff only work in DAW software - not for just general Windows volume control... So this hardware DIY idea was for something really just to turn the volume up and down on youtube videos and whatnot. Anyway... I just wrote a C# program that listens to the midi messages from the surfaces *all the time* and sends a message to Windows to change the system volume when you move the slider. Problem solved with no extra hardware, heh.... kinda weird I never thought of this before, I guess...it was surprisingly simple to write. I suppose I'll keep tinkering with it and make panning and stuff work, too - maybe even make the motorized ones move when you change the system volume in Windows. might as well...

Thanks anyway for the help, guys - looks like I'm taking a software DIY route instead (which I'm uhh... significantly more familiar with, lol)

 

[ecc83]

The "problem" if you will allow, with software control is that when the control goes ape***t it will be at 2am and Mr Sueyourassoff next door will get on the case!

I want a knob connected to a physical resistor to dive for. You mentioned "motorized pots"? Could you have a physical button that told them to go to infinite attenuation PDQ?
But with MY luck the "bits" would get it wrong and slam the pots to max!

Dave.

 

[kip4]

The "problem" if you will allow, with software control is that when the control goes ape***t it will be at 2am and Mr Sueyourassoff next door will get on the case!

I want a knob connected to a physical resistor to dive for. You mentioned "motorized pots"? Could you have a physical button that told them to go to infinite attenuation PDQ?
But with MY luck the "bits" would get it wrong and slam the pots to max!

Dave.

 

[drtechno]

well it seems like you need to build an attenuator. H bridge or T type balanced type use a shorting rotary switch to select the center resistor:


also a working example:
http://ep.yimg.com/ca/I/glass-ware_2271_1340292


linear taper is generally used when you vary the gain of the stage via negative feedback.

log taper is for vary the signal on the signal path itself.

for proper balance operation and higher reliability, I recommend using a rotary switch than a potentiometer because the control is marginally accurate in balance circuits.