GGG Mini Mixer - Sucking Tone

[tatems]

Hey all,

I just built the GGG Mini Mixer on a pcb board i etched (using the GGG design). One problem i face is that it sucks the high end of my tone. I was wondering if this is normal or something is wrong with my mixer.

Cheers

Tatems

[Processaurus]

Is that when your guitar is plugged straight into a channel?  If so its the same loading from that channels vol pot as when you turn your volume knob down on your guitar, and would be a weakness of the simplicity of the design (like passive vol pedals).  The easiest solution is to put a buffered pedal between the guitar and the mixer, or build a buffer inside the mixer for one of the channels, to make that a high impedance input.  I like the GGG JFET buffer for things like that.  I'd only do that for one channel, if you're doing parallel effects or electronic instruments in the other channels.  If you're using it to mix multiple guitars (or other instruments with passive electronics, like mics, Wurlitzers, etc), buffer each of those channels before the volume pots.

To buffer all 4 channels you could save some parts and use a quad opamp like a TL074

[tatems]

Great, thanks for that.

I'm actually building it for a friend so he can play with other guitarists or bassists at home. I think it'll be fine without the buffers. So just to clarify when i plug the guitar into one of the channels i turn the guitar volume and the mixer channel volume fully up. I then play and the high end is not a prominet as when i play without the mixer. So as you said i would need a buffer to match my impendence then.

Cheers

Tatems

[Processaurus]

Without some sort of high impedance buffer, he'll get the tone suck with the existing mixer design.  The impedance of the mixer is only 10K, the value of the pots.  For guitar you'd want an impedance at least 10 times that.  You could try a stiffer pot, 100K would be a good start.  You'd then want to make the summing resistors (now 100K) 1M then to make it work right and not have the channels interact together.

The buffers followed by the stock mixer would sound better, in my opinion.  Depends how much time you feel like spending.

[Mark Hammer]

First, the mini-mixer uses an inverting op-amp, which I gather has a lower input impedance than a non-inverting one.  For guitar, that shouldn't, in theory, pose too big a problem.  My sense, however, is that the brunt of such simple mixer circuits anticipate low-impedance sources, such as line outputs or microphones.  If you feed the mixer with an effect pedal, usually the output impedance of the pedal will be low-enough that it falls in the desired range.  If the mixer is fed directly from the guitar, though, especially if it's through a long-ish cable, you're asking for trouble.

Certainly the use of higher value pots (e.g., 100k or greater) is an excellent suggestion.  Maybe the best insurance, though, would be to leave the pots as is, and use a quad op-amp, like a TL074, and use the two additional op-amps as high input-impedance non-inverting unity-gain buffers, just ahead of the mixer pot. 

What does such a stage look like?

• 1M (or greater) resistor from input jack to ground
• output and inverting input pins (pins 1+2, 7+6, 14+13, or 9+8) shorted together
• 470k or greater resistor from noninverting input pin (3, 5, 10, or 12) to Vref (in this case, the junction of R11/R12)
• 0.1uf (100n) plastic cap between input jack and noninverting pin
• polarized cap of 1uf or greater between output of buffer and input lug of pot (+ side to op-amp output pin)

This is essentially the circuit you see on the right side of page 15 here: http://www.ortodoxism.ro/datasheets/texasinstruments/tl074.pdf or at the top of the screen here: http://europa.spaceports.com/~fishbake/mixer/mixr.jpg
but with the necessary additional components to make it work right using a single-ended power supply.  The .1uf cap prevents any stray DC feeding the buffer, the 470k to Vref provides a 4.5Vdc bias, and the 1uf on the output removes the DC before feeding the mixer pot.  The 1M resistor on the input prevents the input cap from popping when you plug in.

What does this get you?  There will be no direct impact on levels from each of the inputs, however, with less signal loss from high-impedance sources, the two buffered inputs will appear to be a bit louder and brighter.  Not enough that substantial level-pot setting changes are required.  The two unbuffered inputs will be perfectly fine for lower-impedance sources, like CD/tape outputs, buffered effects pedals, etc.  The two bufefred inputs will now be able to directly accept things like guitar or bass without any tone sucking.  Of course the downside is that the existing PCB layout won't accommodate these changes.  On the other hand, this is not a particularly complicated circuit, so perfing or vero-boarding it should not be too painful or time-consuming.

[bluesdevil]

Wow, thanks a lot for that Mark! I'm just now getting into learning about applying buffer circuits and still trying to wrap my head around the impedance thing, but it's starting to make sense to me after reading threads like this one.

[Processaurus]

Excellent, Mark, thats what I'd do if it were my project.  You could even skip coupling caps between the buffers and the channel volume if the volume pot is tied to Vref rather than ground.


Here's something I put together to foot the bill, because the tone sucking problem with the GGG mixer interested me:  here's an opamp buffered 4 channel mixer. This uses a tl074 and tl072, low parts count being the goal, no bells nor whistles.


I didn't use the inverting summing stage, but I doubt the channels will interact anyway and if they do, it will only be a little bit.  Anyway, this ain't no Neve.  Its set for unity gain, that could be changed by adding two resistors and a cap to making the final stage a non inverting amplifier, with a set amount of gain.  Higher power supply voltage would give it more headroom, as would rail to rail opamps, if it sounds like it clips on transients.

[fixr1984]

Its been almost a year but what the heck if someone can help.

I'm most likely missing something here but on your schem pin 11 is 9V and pin 4 is ground but
if you look at the Data Sheet they are reversed.
Misprint? Supposed to be that way?

Thanks

[R.G.]

Trust the datasheet.

[fixr1984]

thank you

[Processaurus]

Good eye! I think the quad opamp in my schematic program is backwards, someone else pointed out the same error in another thing I did.  If you already made a circuit for it, the nice thing about quad opamps is you can just flip the chip 180 degrees and everything is the same except the + and - supply pins are swapped.

I fixed it in case anyone wants to make it.  Thinking about it now, the last opamp stage would be better to have a fixed gain of about 10, rather than a unity gain buffer, because then you can get quiet signals up to a normal level, and amount of noise this would add is negligible, and headroom is preserved because you control the signal volume before the gainstage.  With 10x gain the channel volume knobs in the 12 o'clock position would be approximately unity.

[fixr1984]

I just noticed that pin 1 and 2 in the first stage should be connected.
I was going nuts trying to figure out why input 2-3-and 4 worked great but input 1
was dead.

[Processaurus]

Wha? it's always been connected   I drew in the option of kicking the gain up as well, with R12, R13, and C7.

[fixr1984]

  
If anyone is interested i have a verified pcb layout for this one.
Just doing some finishing touches in paint shop.

[rasco22862]

 
If anyone is interested i have a verified pcb layout for this one.
Just doing some finishing touches in paint shop.

I´m very interested

[fixr1984]

Here you go. Not the smallest layout but it does work. I printed mine at 3.5'' x 1.75''
For the pot connections they are labeled as p (pot) and L (lug)

[fixr1984]

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.

[Processaurus]

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.

[fixr1984]

Vref is connected on the pnp, i did that in paint shop cause i had forgotten to do it on the layout.
Do you mead r12 on the layout or the schematic? I have 12 and 13 numbered backwards to the schem.

[Processaurus]

Ah, I just looked at the layout.  The 1K, that is connected to C7, its other end should go to pin 6 of IC2, the inverting input, rather than pin 7, the output.  The way it's on there now, it'll work, but will be unity gain.