Mini Mixer with tone?

[darkseid]

Mini Mixer with tone?

This schematic is based on GGG Mini Mixer



Only two input jacks, R9 is lowered for more gain and  R3, R4 changes the impedance, I would think R3 and R4 changes the tone as well, maybe adding more bass to it.  My question is.... Is this tone section right?    

Thanks in advanced

[aron]

It looks weird to me and it looks like it would roll off bass if anything.

[Mark Hammer]

Depends what you want it to do.  Aron is correct in that it, in principle, it rolls off low end . The leg of the tone pot that is tied to the wiper of the input level pot provides a bypass of sorts for the full spectrum.  If that resistance is high enough, then only contetn for which C1/C2 provide a low-impedance path will ge through.

That's in theory.  In practice, the low-end rolloff of C1/R5 and C2/R6 is about 16 hz, so providing a "bypass" is not exactly going to be audible.  A bass-cut control you will never hear.

There are a zillion ways to shaope tone simply and easily.  What is it you want to do with this mini-mixer?  That will provide some clues about what to recommend.

[darkseid]

Mark...Shape tone is what I want to do. To be able to run two guitars into the mixer, one is bass the other a regular 6 string guitar.... I read on this board that the mini mixer lacks bass and gain, so  C1,R4 and C2,R3 is the only easy way I can think of for controlling the tone...   

I'm more than willing to learn more ways of shaping the tone in this circuit...

[aron]

I read on this board that the mini mixer lacks bass and gain

I don't see why it would lack bass (without the tone controls). That's strange. R7 and R9 could be altered to give more of a bosst (within voltage limits).

[darkseid]

Aron my memory is bad   in this thread I read lacks bass http://www.diystompboxes.com/smfforum/index.php?topic=16484.0 But that is because he left out the 100k input Resistors lol...  This circuit probably doesn't lack bass at all, me bad lol..   Though I still need a tone control for this circuit.

[Mark Hammer]

Still not clear what tone-shaping is needed.  For instance, is there a desire to tame the bass so that a subsequent stage is not overdriven?  Is there a desire to make things more crisp, so as to compensate for lousy connector cable?  Is there a desire to "re-voice" an instrument the way you would on an amp?  What I'm missing is a listing of purposes that would tell me what is needed.  Not trying to be picky or a pest here.  Just trying to get to the most elegant solution to the problem...whatever that problem may be.

[darkseid]

Mark: Yes Yes Yes lol

is there a desire to tame the bass so that a subsequent stage is not overdriven? ..........Yes
Is there a desire to make things more crisp, so as to compensate for lousy connector cable? ...........Yes
Is there a desire to "re-voice" an instrument the way you would on an amp? .............Yes

But

I'm just looking for something simple, hmm.. maybe making R5 and R6 variable would make a good tone control.....
I just realized that in my last post lol... the guy took out the input resistors and had less bass:)

I thought maybe making C1,C2 variable might do the trick...But now I'm thinking R5,R6 may do it. The question is could R5,R6 be variable and be beefed up for more bass.

Basically I want to be able to turn the bass all the way down and to be able to add more bass if need be.

[Mark Hammer]

Well, then I would seriously recommend that you look at the various parametric EQ articles at GEOFEX, and that you look at the Tonemender circuit/project at runoffgroove.  While the Tonemender has a board layout available and the GEOFEX articles do not, the GEOFEX design offers a great deal of flexibility.

Here's what I would suggest for an overall approach to the design:

1) There are some things you might want for all channels and some things you might want in common to all.  A 3-position toggle to select between input cap values is a simple way of providing a couple of preset bass rolloffs.  Simple, compact, and no current requirements.  You can do the math by calculating F for 1/(2*pi*C*R) whwere R is 0.1meg (100k) and C is the value of the caps you may want to use in uf.  The simplest, most noise-free method would be to place two selected caps in series, connect the common of the toggle to their junction, and let each of the side positions in the toggle shunt one of the caps.  This will provide 3 different capacitance values, and 3 different rolloffs.

2) Best S/N ratio will be achieved if gain is not all concentrated in one spot, so place some gain in that first stage.  Let's say a gain of 5-10.  Given that this will result in some highish resistance values, let's drop the input resistor down to 10k (adjusting the formula noted in #1) and use a 100k feedback resistor.  You don't want hiss accumulating so stick a cap in the feedback loop as well.  Let's make it 150pf, so that we get a rolloff around 10khz (it's for guitar, right?).

3) You can stick either the tone stack of the Tonemender, or a couple of resonant EQ sections of the GEOFEX type between the two op-amps.  You may THINK that sharing a set of controls between two inputs forfeits tonal control, but if you cut the bass on the input of one, and turn a common bass control up, you are restoring normal bass to one input while boosting bass for the other, so there is more selectivity than you think.

4) Since a single common set of tone controls allows you some complexity of control without having to duplicate the circuit and build something too complex, you stil get your revoicing options.

5) Give the second op-amp some gain as well, but don't go much above 3.  That will give a cumulative gain of 30 (10 x 3).  This is not huge gain, but will allow for enough *compensatory* gain, that you can crank the volume for one weak input signal, and turn it way down for a hot input on the other, and still get a reasonable signal amplitude from the output, once the gain is factored in. 

6) While you're in the neighbourhood, stick a feedback cap between pins 6 and 7 as well.  Using the formula above aim for the same rolloff point or as close to it as you can get.  If your intent is mixing signals coming from magnetic pickups, and eventually feeding it to a guitar speaker, you really don't need more than 10khz bandwidth.

7) Make C5 bigger; it is your "reserve supply".  Go with 100uf .

Use an NE5532 instead of a TL072.  You're not using hi-z inputs and whomping resistors or pots, so take advantage of the lower noise of the 5532 under those conditions.

[seanm]

Mark...Shape tone is what I want to do. To be able to run two guitars into the mixer, one is bass the other a regular 6 string guitar....

Just some random thoughts.

What is the output going in to? Bass and guitar have very different "amplification" needs for lack of a better term.

Is the mixer going to be a pedal? If not, I would recommend 12V or higher to give the bass more headroom. Or maybe 18V with 2 batteries?

Do you have a favorite bass amp? Favorite guitar amp? Steal the tone stacks from amps you like  Notice the plural, you probably want different tone stacks for the two inputs.

Take a look at the concepts behind Radial Bassbone. The Bassbone is ment to match up two different basses. Ignoring the useless EQ switch, channel 1 is flat. All you can do is attenuate the signal. Channel 2 has a full EQ: high, mid and low.  The idea is that you setup the *amp* to be correct for channel 1 and use the Bassbone EQ to compensate for the second bass. You might be able to do the same and save a tone stack.

I would also take Mark's advice and give the first stage some gain. Then stick the tone stack *between* the stages.

[darkseid]

seanm the mixer is not going to be a pedal, I'm thinking about using it for my computer, running it into my guitarport.  I get tired of switching between guitar and bass, just thought it would be cool to make a mixer for this.

Headroom does sound good;) I may go with 18volts, I like The idea that you setup the *amp* to be corrected for a channel.

Mark thanks for the advice, the number 1 and 5 procedures did cross my mind,   GEOFEX articles are a great read..... It may take me awhile to grasp some of this stuff.  Though sticking the tone stack *between* the stages would not give me tone control over each channel separately. So I would think the tone stacks would need to be near the inputs?

Ah the Tone Stack... I have a little bit of understanding of basic electronics, man I was thinking a simple R/C circuit would do the trick for tone lol.... so much for simple ha........ Last night I spent at least 4 hours reading about Tone Stacks, EQ and I dont think I remember much I read lol..... Hopefully it will sink in. Ahhh so much to learn....... I did download two Tone Stack calculators last night....  Now it is a matter of taste, for some reason I'm liking the Baxandall stack, only because I see how it works... It is nice to know there is plenty of ways to building tone.   I have been enlighten ... As if I gain some spiritual insight hahaha....... Thanks Guys 

Now it is getting addicting,  I never would of thought doing a tone control could be so much fun to learn.......... It is one thing being able to read schematics and it is another knowing how it works lol.  Give me another ten years and I will have this down to a T..... lol 

I may have bit off more than I can chew, but I can swallow some of it lol......

[darkseid]

delete.....