GGG Mini Mixer - Sucking Tone

[fixr1984]

I see what you mean now, I will have to correct that.

[fixr1984]

cool!

a couple corrections:  R12 should go to the inverting input of IC2B rather than the output.

Also the Vref supply coming off the output of IC2A is not connected to anything, should be going to the bottom of all the pots and one side of R1 through R4 to bias the inputs to 1/2 the supply.

I think a TL074 would work much better than LM324 because it is low noise and also has JFET inputs.  Actually, thinking about it, a modern rail to rail opamp would work best for the dual opamp for the most headroom on the output. 

If 9v isn't enough headroom, the power supply voltage could be as high as the opamps can take.

I updated the layout and transfer. Had to have R13 stand on end to fit.
I put an LM324 as a choice simply cause RS has them and if you need one in a pinch it will still work.
I tried both and didnt really notice much of a difference. I would still say the the TLO74 is first choice.

[Wounded Paw]

So would this be a good solution to mix guitars with keyboards?
I built a ggg mini-mixer with an LPB on the guitar channel to boost it a little and buffer it but ended up with distortion.  So I lowered the amount of boost and padded the lpb output but now it sucks high end pretty badly.

I'm gonna try this design with buffers on all channels but is there any easy way to bring the keyboard levels down a bit so that they're closer in level to the guitar signal?  I mean so that with all pots at midway the guitar level would be close to the keyboards levels.  with lowest part count and pcb size?

[fixr1984]

So would this be a good solution to mix guitars with keyboards?
I built a ggg mini-mixer with an LPB on the guitar channel to boost it a little and buffer it but ended up with distortion.  So I lowered the amount of boost and padded the lpb output but now it sucks high end pretty badly.

I'm gonna try this design with buffers on all channels but is there any easy way to bring the keyboard levels down a bit so that they're closer in level to the guitar signal?  I mean so that with all pots at midway the guitar level would be close to the keyboards levels.  with lowest part count and pcb size?

Having a buffered input is supposed to solve the tone suck problems associated with the GGG mixer.  I have yet to make
Version 2 but I would guess with increassing gain by 10x you should have some pretty good flexibility in pot adjustment.
I built my version 1 into a 4" x 6" RS box.

[Ben N]

With an impedance matching transformer and xlr input, and jacks for post-buffer pre-mixer line-outs, that could make a nice Tunstallish mixer for a looper input.

http://www.musicplayer.com/article/kt-tunstall/Jun-06/21172
http://www.youtube.com/watch?v=Z-RM3IAZ9_s

[rasco22862]

What about 4 VU Meters ?
Like these:
http://www.circuit-projects.com/10-led-vu-meter-project-by-lm3915-and-lm324.html
http://sound.westhost.com/project60.htm

[rasco22862]

Heres the updated version. unverified but should still work. Per the schematic this version will give a gain of 10x.
The first version will be unity.

So , is this layout verified?

[fixr1984]

The only change I made was adding R12, R13 and C7. That will give you gain of 10x.
For some reason if there is a problem with those and I dont believe there is just omit those parts
and jumper R13 and it will be unity gain.

[Wounded Paw]

So what difference would it make to replace one of the volume pots with say a 100kA or even 250kA instead of 10kA?

[Processaurus]

So what difference would it make to replace one of the volume pots with say a 100kA or even 250kA instead of 10kA?

It might not make a big deal, you could use higher pots and scale the summing resistors (R5 to R8) up accordingly.  It is nice to keep them lower because lower impedance = lower noise, but its also nice not to have to buy parts, if you have some 100K's laying around.

[Wounded Paw]

Okay so  I built the 4 Channel Buffered Mixer for a band that was mixing keyboards and guitars together after the GGG mini-mixer didn't work out for them.  I originally built it without the boost on the summing op-amp ie pins 6 and 7 jumpered together.  It worked fine, no distortion, no tone sucking but the output was significantly lower.  So I added in the R12, R13 and C7 section to boost the output but now the guitar signal is noticeably distorted.  I've rechecked my work and don't see anything wrong.  What might be going on here?

[Processaurus]

hmmm, does it get less distorted when you turn its volume down?  That would tell if the distortion is happening in the last stage or not.  If it is, you could make R13 larger, 2K would lower the gain to 5x.

Might be good to double check the opamp's appropriate supply pins are indeed getting 9v, are grounded, all the non inverting inputs are getting the 4.5v bias they deserve.

no tone sucking but the output was significantly lower.

I see a problem with the design without gain compensation, summing the channels to the non-inverting input means the loudest one signal can get with all the other channels turned down is about 1/4 volume!  Imagine that the 100K summing resistors are actually a voltage divider, if one is tied to the signal, and the other 3 are tied to Vref (which to an AC signal will behave the same as ground), that means the three 100Ks in parallel from the non inverting input to Vref look like a 33K resistor. 

Hadn't seen it that way.  The traditional inverting summing stage would be more appropriate.  Or the gain compensation will work.  Mr. Wounded Paw, if you can post your voltages I'd have a better idea what's up with your mixer, I can't see anything in the design that would clip.

[kriista]

Sorry to bump such an old thread, but I built a mini mixer and wasn't happy with the results.

Is this layout/schematic here verified as working as per the corrections mentioned in the thread?

I'd like to build a 4-1 buffered mixer (using instrument level inputs) and I'm not really tech savy enough to mod/troubleshoot something that isn't working right from jump.

[Processaurus]

Hi, I was looking this over, trying to see what the problem was, the schematic will work but there is an error in the V2 layout, the buffered 4.5v supply (Vref) that comes out of pin 1 of IC2 didn't get connected to anything it was supposed to.  It should go to all points on the schematic marked Vref.

Also a FET input opamp (TL074 the most common) should be used for both IC1, an lm 324 would be inferior (though it might sound OK).  It's the chevy nova of opamps.  And a schottkey diode is better than a 1n914 for D1, but that's a little thing.

With R12, R13, and C7 in, the gain is only about 2.5x, not 10x, so I would up R13 to 47K, that will give gain up to ~12x.

It is a simple circuit, you could build it on perfboard, and if you had problems we could help solve them.

[kriista]

Does 2.5x gain mean 2.5 times louder than unity gain?
Is having 12x better sound wise? I'm only going to use this as a summing device, not to boost anything beyond it's regular output levels).

Not sure I follow on the Vref thing. So there's basically a trace missing from pin1 of IC2 going a bunch of places?

Lastly, is there some automated way to turn a schematic (specifically that one) into a perfboard layout?

[Processaurus]

Does 2.5x gain mean 2.5 times louder than unity gain?

Yes.

Is having 12x better sound wise? I'm only going to use this as a summing device, not to boost anything beyond it's regular output levels).

Soundwise they would sound the same, the gain is just nice insurance, in case something is real quiet, like some piezo equipped instruments or mic or something.  You could make R13 4.7K and have just a little more than unity gain if you don't want extra gain available.

Not sure I follow on the Vref thing. So there's basically a trace missing from pin1 of IC2 going a bunch of places?

the 4.5v Vref biases all the opamps to half the supply voltage, so the negative cycle on the audio doesn't get cut off by the amplifiers not being able to produce a negative (lower than 0v) voltage.  Yes, it is supposed to go a bunch of places, everywhere on the schematic that has VREF in a diamond shaped border tied to it.

Lastly, is there some automated way to turn a schematic (specifically that one) into a perfboard layout?

No, but luckily elbow grease is the original automation.

I don't bother with a layout, just start by making the board the size it needs to be, drilling holes in it so it can be mounted later, then stick the ICs on  in a roomy location with plenty of pads around them for the discrete components, then start from the inputs and work through the circuit, one connection at a time.  You're making a little copper road system, it doesn't matter the shape of the road, as long as it ends up in the right place and makes the right stops.  With a roomy board, there's a million ways to lay it out right.  Some are more clever than others, but the only thing that matters is if it works.

One could keep track of connections made on the schematic with a highlighter, if inclined.  I just kind of triple check everything as I go and continually review the little area I'm doing, checking the connections are right, polarity of components is right, no unintended shorts, that kind of thing.

[Processaurus]

It can help, with perfboarding, to run a bare wire for a ground bus around some edges of the board, to always have a handy, nearby place to ground stuff.  Same goes for power and vref, if there's a bunch of connections on the schematic, otherwise just fanning them out with wires from the source is fine.

[Processaurus]

Oh, I just read back a little, the Vref is connected on the P'n'P, just not on the layout, so fixr1984's layout should work fine.

[kriista]

Is the PnP the thing under the layout? So V2 of that thing should work fine?

That makes sense with the 12x gain thing. I would probably run some piezo stuff every now and again, so I'll probably do that.


Is it correct to assume that PnP is 'press and peel' etching?

[Processaurus]

Yes, that funny blue stuff for etching pcbs.  I've actually never etched a pcb before, but have some kicking around...  Sure, go ahead and try it out, even if there's a little error with the pcb it'd be easy to kludge it right.