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Old 18th December 2011, 01:56 AM   #1
dfeweer is offline dfeweer  United States
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Default mixer circuits (looking for simple, but very high quality)...

I am guitar player and I am in need of a mixer to mix my effects.

All I need is a simple 2-channel mixer so that I can mix in a couple of rack effects, an Eventide DSP4000 and a Lexicon PCM91.

I am sure that all are aware that these are high quality processors and I do not want to degrade their sound by sending them through a cheap mixer.

I have some electronics background so I figured that I could build my own and I should add that size is a factor (in other words, really I don't want a rack unit).

I started to look at a bunch of mixer schematics that are available on the web. Everything from home grown mixers to mixers that are put out by highly-rated/high-quality manufacturers.

What I have found is that there are many mixer topologies (is that the right term?) meaning there are many different solutions to get the mixing done (did that make any sense?).

What I am wondering is what is the "best" mixer topology that will degrade my tone the least?

Thanks in advance.

dfe

Last edited by dfeweer; 18th December 2011 at 02:12 AM.
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Old 18th December 2011, 07:50 AM   #2
Mooly is online now Mooly  United Kingdom
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A mixer need be nothing more than a "virtual earth" opamp configuration.

Just because it's simple doesn't mean it's no good. The only downside to a single opamp stage is that it inverts the signal so needs another inverting buffer to put it back again (if absolute phase is important to you).

Look at this,
Universal Preamp/ Mixer

Figure 2 is all you need for a mixer.
Figure 1 adds an input buffer, tone control and phase inversion (to counter the effect of phase inversion in the mixer).

The opamp (TL072)... well although you can substitute it for something else, don't discount it until you have at least tried one such a circuit.
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Old 18th December 2011, 06:46 PM   #3
dfeweer is offline dfeweer  United States
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Mooly,

Thanks for the reply.

Quote:
Originally Posted by Mooly View Post
Just because it's simple doesn't mean it's no good.
I agree.

I probably should have been a little more specific in the info I was looking for.

What I was looking for are what are good design practices, what should I avoid, etc.

For example, referring to the schematics in the link you posted...

Click the image to open in full size.

Click the image to open in full size.

In the first schematic (ignoring the tone circuit, since I don't need one), the volume pot is before the opamp.

In the second schematic, the volume pot is after the opamp.

I have seen bunch of schematics with both configurations but I don't know which one would be the better design (i.e. is it better to put the volume pot before or after the opamp?)

I have also seen schematics where the volume pot in the feedback loop of the opamp but I think I read somewhere that that configuration is a bad idea since it can introduce additional noise.

Any input/help/tips would be appreciated.

Thanks in advance.

dfe
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Old 18th December 2011, 07:14 PM   #4
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If you are building a mixer with mono channels you might want to read up on panning laws so you can design the pan pot the way you prefer it.

Just sayin…
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Old 18th December 2011, 07:34 PM   #5
Mooly is online now Mooly  United Kingdom
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From a purely theoretical point it has to be better to put the volume control after the opamp because then you are attenuating all the noise/hum/hash as the volume is reduced.

Putting the volume control in the feedback loop is an absolutely valid method and can form the basis of a very high quality active volume control but it has to be done correctly for the best results. Choice of opamp is important and so to is ensuring that no DC current flows in the wiper of the pot. Doug Self used that approach in his precision preamp using two opamps per channel. You won't find a better example I suspect.

From a practical point, noise (as in circuit noise... not hum etc due to poor build) is the least of your worries.
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Old 18th December 2011, 07:39 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

Any decent effects unit can be used as effects only, for a desk effects loop,
or 2 channel mixing within itself, with mixed direct and effects outputs to
go into the effects loop of any decent guitar amplifier. I simply can't see
why you'd need an additional 2 channel mixer.

Unless you want to use the effects in parallel, that is a different ball game.

rgds, sreten.
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Last edited by sreten; 18th December 2011 at 07:41 PM.
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Old 21st December 2011, 10:58 PM   #7
PRR is offline PRR  United States
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> there are many mixer topologies....

.....because there are many different situations where "a mixer" is wanted.

What are your levels? 1V line or 0.05V guitar?

If you have good hot levels, you can fade, mix, and then make-up gain.

If you are at guitar level, you can't afford to lose level, you must gain, fade, mix.

For two inputs in music creation, passive mixing is fine. Active mixing becomes essential to pass BBC Acceptance Tests and generally when large groups of inputs must be switched on/off with "NO!" change in other channels... in music creation with just a few inputs the interaction is mild and often favorable.

I've installed lots of systems where two line-level sources were mixed (without faders) in just two resistors. The system main volume had to be advanced from 4 to 4.5, not a problem.

Start simple. Two 10K pots, two 10K mix resistors, and on to your next box. This adds loss, you have to turn-up somewhere or add makeup gain. Listen, Has the loss put a weak signal down in the hiss? Then you may need makeup in front of fade and mix.
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