Stereo boost?

[therealfindo]

So, a mate asked me about a stereo boost, with a single control knob.

I'm thinking that I could basically run two simple clean boost circuits in parallel, with a stereo pot to control the volume / gain?

Or is there a more technical way to do it with a single circuit?

Cheers.

[PRR]

> is there a more technical way to do it

No.

Stereo is mono, twice, with two controls on one shaft/knob.

Well, in this modern age, there ARE chips with two amps and electronic volume control which can be worked with one control. But most modern ones expect a computer.

LM1036 is a "simple" straightforward path from the 1970s. http://www.ti.com/product/lm1036

You probably want the LM1036N (DIP, with legs) option. DigiKey and Mouser show hundreds in stock for a few bucks each.

But it sucks quite a lot of battery current, input impedance is too low for good guitar tone, you gotta gimmic the control inputs for Balance, Bass, and Treble.

TL072 and a gang-pot is the way to go.

[J0K3RX]

Not sure what the benefit would be with running a boost this way, especially with a singular control knob? Why not just run it mono with 2 output jacks or a splitter of some sort? I could see maybe if you wanted to control the boosts independently, maybe, but with single controls... Why?

[cloudscapes]

Not sure what the benefit would be with running a boost this way, especially with a singular control knob? Why not just run it mono with 2 output jacks or a splitter of some sort? I could see maybe if you wanted to control the boosts independently, maybe, but with single controls... Why?

If I'm putting a boost after a ping-pong delay, stereo chorus or other kind of stereo "widener", I'd want the boost to be full stereo as well.

[J0K3RX]

Not sure what the benefit would be with running a boost this way, especially with a singular control knob? Why not just run it mono with 2 output jacks or a splitter of some sort? I could see maybe if you wanted to control the boosts independently, maybe, but with single controls... Why?

If I'm putting a boost after a ping-pong delay, stereo chorus or other kind of stereo "widener", I'd want the boost to be full stereo as well.

If that's the case, then I would build 2 boost pedals, not much to them... You could have them both in one enclosure with independent volume/gain controls and one foot switch.

[alanp]

Minor issue, you might want to put a trimmer in series with the volume/gain so that the left and right channel have the same loudness at the same points across the knobs' travel.

[therealfindo]

If that's the case, then I would build 2 boost pedals, not much to them... You could have them both in one enclosure with independent volume/gain controls and one foot switch.

Yep.. that's what I was going for in the OP

[therealfindo]

Minor issue, you might want to put a trimmer in series with the volume/gain so that the left and right channel have the same loudness at the same points across the knobs' travel.

Hi, can you excplain that in a little more detail for this newbie?
The signal will be coming in from a stereo delay / reverb - if the two parallel circuits are identical, what would cause the levels to be different

[alanp]

If you use a FET boost, it's not common for two random FETs to have the same gain, meaning if the volume knob is at 12 o'clock for both, it's not likely that the actual volume will be the same.

[therealfindo]

If you use a FET boost, it's not common for two random FETs to have the same gain, meaning if the volume knob is at 12 o'clock for both, it's not likely that the actual volume will be the same.

Ah, OK! So I need to use a trimpot of some value to adjust it? Could I measure them, or is the likelihood of finding a pair still low?

[samhay]

Why not do as PRR suggested and use a dual op-amp?

[therealfindo]

Why not do as PRR suggested and use a dual op-amp?

I have some TL072s on order, so I might still... not sure of the circuit yet.. just trying to figure out the options. 

[PRR]

Sorry, LM1036 was a bum steer. As-designed, it has no voltage-gain (no "boost"). So now we need a buffer in front *and* a boost somewhere... all you really gain is one-pot control of stereo Volume (and bass/treb if you like!). Though it seems to me that a hack on the "bass" circuit could give overall boost. And if it is truly coming out of a box (stereo deverb) then the 30K input impedance may be a non-issue. CanaKits has a kit for this chip (or its twin LM1035). "CK146 - Advanced Stereo Tone Control". Worth a look. May not be the tool of choice.

Since it's coming out of a box, a dual-gang pot and a modest-gain amp seems the obvious path.

Check the stompbox vendors first, but few stomps use dual pots. Radio Shack used to stock a 100K dual which was always very good to me. (Ignore the 4th lug.) PartsExpress is a reliable stereo-stuff supplier, http://www.parts-express.com/pe/showdetl.cfm?partnumber=023-650 may be suitable.

[therealfindo]

Sorry, LM1036 was a bum steer. As-designed, it has no voltage-gain (no "boost"). So now we need a buffer in front *and* a boost somewhere... all you really gain is one-pot control of stereo Volume (and bass/treb if you like!). Though it seems to me that a hack on the "bass" circuit could give overall boost. And if it is truly coming out of a box (stereo deverb) then the 30K input impedance may be a non-issue. Worth a look. May not be the tool of choice. CanaKits has a kit for this chip (or its twin LM1035).

Since it's coming out of a box, a dual-gang pot and a modest-gain amp seems the obvious path.

Check the stompbox vendors first, but few stomps use dual pots. Radio Shack used to stock a 100K dual which was always very good to me. (Ignore the 4th lug.) PartsExpress is a reliable stereo-stuff supplier, http://www.parts-express.com/pe/showdetl.cfm?partnumber=023-650 may be suitable.

Thanks..

sourcing a dual-gang pot is not a problem.. I'm just rather ignorant when it comes to dual op-amps (amongst many things). I've built the AMZ mini-booster circuit before, so that's why I thought a parallel version might be ok.. but the mismatched fets is a problem. So.. now would be a good time to learn how the TL072 works, right?

[PRR]

I'm not over-concerned about channel matching. It is important in sweet-spot uses (home hi-fi, mastering room); so important that you must trim anyway. But in paying gigs, 90% of the crowd is so far off the room's acoustic center that electric balance doesn't matter much.

Still basic hygiene to boost both channels the same. FETs vary 20%. Opamps do what their resistors tell them to do. 10% tolerance resistors are almost good enough. You can't buy worse than 5% today, and that's almost always plenty good. 2% resistors are cheap; in one-off you can pick 2% pairs from a 5% batch with your meter. Of course not a lot more work to rig two FETs, feed signal, and pick offsetting resistors to get matching gains.

[therealfindo]

Ok, so I've been looking at the TL072 datasheet and some stereo amp schematics.. (still trying to learn all this stuff)

The op-amp basically looks like this, right?


One input would go to 3 and another 5 and then 1 & 7 would go to the dual-gang pot. But there are caps and resistors in play too.. this is where I'm ignorant again - how do I know where they need to be and which values to use?

Also, how would I wire the bypass switch?  Would this be problematic?


sorry about the rough drawings...
Thanks for the help!!

[therealfindo]

Ok..

so, I took a schematic for a (mono) TL071 boost circuit, and doubled it to use a TL072:



Will this work?

[PRR]

Yes.

[therealfindo]

Yes.

sweet! Thanks.
Any ideas on the footswitch?

[therealfindo]

Ok..

so, I took a schematic for a (mono) TL071 boost circuit, and doubled it to use a TL072:



Will this work?

hmm.. finally got the bits to start breadboarding this.. have tried one side (the top part of this schematic) and I'm only getting unity gain with the pot at 100%...