2 Channel Mixer Circuit

I found this attached circuit I am going to try. I think I have a Dual Bifet I could use. I think I will just mess with pots until the levels are set where I want and then just use its equivalent voltage divider. Thanks for references!
 

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cabirio

Member
2015-09-30 6:43 pm
Spain
Load impedance is key for an electric guitar signal (I'm assuming a passive one) and it should be high, the de-facto standard being ~1M. The frequency response is shaped by the pickup characteristics, volume and tone controls, cable capacitance and amplifier / pedal / mixer input impedance. All those together result in a low-pass response with a peak whose frequency and level defines the particular "voice" of the guitar. Plug it into a normal line input with some 10s of k of input impedance and it you will see what I mean.

As for level it varies a lot, from a low output Tele neck pickup to a hot ceramic humbucker there can be a factor of ~5 or more. I've seen sensitivities all the way from ~10mV up to ~500mV for amps and so called "instrument" inputs in some interfaces where you can plug in a guitar or bass directly (and those are always high impedance too).

I would definitely buffer the guitar signal with a FET opamp and an input impedance of 1M and I would also provide for some gain adjustment as well as the main fader, maybe an internal trim pot that you can adjust and forget or a temporary pot that you adjust, measure and replace with a fixed resistor.

Cheers,

Cabirio

Edit: the second opamp in that schematic simply reverts the phase back to in-phase after the inverting mixer. Phase isn't that important if you're going straight into a guitar amp so you could use that second opamp section (if the one you have is dual) for the guitar input buffer.
 
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Edit: the second opamp in that schematic simply reverts the phase back to in-phase after the inverting mixer. Phase isn't that important if you're going straight into a guitar amp so you could use that second opamp section (if the one you have is dual) for the guitar input buffer.

Thats what I was thinking! I have a 1 M shunt impedance resistor already on my input jack (this circuit is actually going inside an amp). My only concern is choosing the correct values for the pot and series resistor for the uC signal, considering it is much larger that the guitar signal. If I remember correct Rfeedback/Rseries sets gain? little lost when there is two inputs as far as trying to use equations go. My goal is to have it go from almost all guitar input to all effects input using one pot.

I have my circuit attached
 

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cabirio

Member
2015-09-30 6:43 pm
Spain
My goal is to have it go from almost all guitar input to all effects input using one pot.
Do you mean like a balance / pan pot? In that case you could use something like the circuit below. R7-R9 represents a 10k linear pot with the wiper to ground. The main caveat with this good old balance setup is poor channel separation, but given that this is a mixer, that shouldn't be a problem here. With those values and the pot fully towards the guitar side, the overall gain is 1, which should work well going directly into the amp input, and towards the DAC side it's ~0.33. You can adjust this by increasing / decreasing R10 for less / more gain, respectively. Again you can use a trim / temporary pot here to find the value that works best and replace with a fixed resistor. To give you an idea, 100k would give you ~0.05 and 1k ~0.8, so you have a wide range to play with.

In the middle possition the attenuation of both signals with respect to the extremes is about 2.5dB. Pan pot laws is a whole subject and the best one depends on many things, like whether it's a pan or a balance pot and the correlation between the two signals (which in this case will vary a lot depending on the type of effect coming from the DAC) and in any case it's always a compromise. I think that for this application we don't need to split hairs and it will probably work just fine as it is.

C3 and C4 may or may not be needed for stability depending on the opamp used but it's always good practice to use them just in case. The same goes for R12 which isolates the output of the opamp from any capacitive load that may destabilise it. Anything between 47 and 100R works well for this. The impedance of the DAC input is 10k minimum which should be safe enough.

As you can see I show a 1M resistor at the guitar input and another one at the output of the whole setup. The one in the amp now not only sets the input impedance but may also be used for biasing the input stage, so you still need it where it is now when you put this circuit before the input.

Finally, as you can see the non-inverting input of the mixer opamp is tied directly to ground (you have a resistor in your schematic). This is what makes the inverting input a virtual ground and the reason why this setup works as a mixer without one signal feeding through to the other channel(s).

Cheers,

Cabirio
 

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microcontroller preamp and mixer

Heres the complete circuit, can anyone see any issues with it? I built it and got no outputs from any of the op amps, just a lot of noise. Although I tried to put this on a small protoboard and I am sure that I messed up wiring/soldering. Basically The 1st 2 op amp stages go into the microcontroller and provide a bias at half of vdd and also gain, and buffer. The other two op amps are a mixer/pan from cabirio.
 

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You need a 10k resistor to ground from the + input at least on the second op amp, if not 10 k to capacitor to ground. (10 uf or so). Op amps have gain of Rfeedback/Rinput, so with 0 input resistor you have gain of infinity. This means the 2nd op amp will whang the output to the rail and latch there til the power is turned off. I decided not to experiment with TL074 because they have this latching feature. TL084 does not, and 4558 is probably okay for guitar work. 4558 is slightly hissy at high gain though. opa2134 is quite expensive, and for ultamate cheap sound I found some 33078's at $.38 each. 33078 does require close power supply bypass disk caps though (.1 uf) plus the 33 pf feedback resistor shorting cap (at high frequency) because they are too fast to avoid oscillation without it. (100 pf is in the sample, which is enough).
Debugging opamps with a dvm: Look that all inputs & outputs of a section are at the same DC voltage somewhere in the middle of the rails. If different voltages, usually a wiring error or latchup.
you can look for ultrasonic oscillations with an analog meter with a 50 vac scale or better a 20. There is a GC analog meter with a 50 vac scale for about $24 at the hardware store. Put a .047 uf cap between the negative lead & analog ground with an alligator clip lead. . High AC voltage & no sound at an output, you've got an oscillation. DVM can't see this, and scopes are expensive & require $50 probes that get stepped on. DVM also produce random numbers on music so you need an analog meter to trace music through your circuit. I use a transistor radio tuned to rock music for AC tracing. You can see the beats as meter jerks, which means you're seeing music instead of oscillation. Better is a simpson 260 analog meter with 20 vac and 2 vac scales. I paid $185 for mine and at 30 years old it still works. 30 year old scopes are full of electrolytic caps that don't work anymore.
Have fun.
 
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Monte McGuire

Member
Paid Member
2013-07-28 2:58 am
You need a 10k resistor to ground from the + input at least on the second op amp, if not 10 k to capacitor to ground. (10 uf or so). Op amps have gain of Rfeedback/Rinput, so with 0 input resistor you have gain of infinity. This means the 2nd op amp will whang the output to the rail and latch there til the power is turned off.

That -Rf/Rin formula is only the gain of an inverting stage - this is not a general purpose op amp feedback rule. In this circuit, no such latchup will happen. TL074-2 is a follower, and since its + input is driven by the output of TL074-1, then it will be DC biased properly, and has no need for any resistance to anywhere. Gain is equal to +1.

Op amp TL074-4 is an inverter, and a resistor on the + input would only be useful to combat input bias current induced DC offset. The TL074 is a JFET input op amp, so this is not a problem.

Basically, the circuit as drawn should work. Every input terminal has a defined DC voltage, and all feedback paths exist.
 

cabirio

Member
2015-09-30 6:43 pm
Spain
I also did not filter my op amp supplies enough (only 2200 uF on the AC side) nor did I put any bypass caps on them. Any suggestions?
Yes, put bypass caps on them ;) The TL07x datasheet recommends the usual 0u1 close to the supply pins. Best is to solder them directly to the opamp pins on the solder side of the PCB. Some LF bypassing is also a good idea at the point of entry of the supply to the PCB, say 100u per rail.

Can you post the schematic and describe the layout of your supply? Note that each section of the TL074 draws 1.4mA, that's 5.6mA in total, and some regulators require more current draw to work properly (you can always use an LED to fix this). Whether fixed (L78/9xx) or adjustable (LM3x7) they may also require bypass caps to avoid oscillation, depending on the layout. As for the main caps, 2200u should be enough.

Cheers,

Cabirio
 
Hi,

Drive the guitar amplifier directly and put the effect
unit in the effects loop where it properly belongs.

What is driving the effects unit in your plan ?

rgds, sreten.

This amp does not have an effects loop. I could have designed this circuit to fit right before the power stage, but chose to use the smaller signal, so I could use low voltage signal relays, and so I could have +/- 12v for my op amp supplies, (this amp is about 15-16 v peak at the power input).

One of the opamps is driving the effects unit.. If your asking how the effects are implemented its an stm32f4 discovery board. Filters are written in Matlab which convert Matlab functions to optimized arm DSP functions.
 
Hi,

An effects loop right behind the power stage is exactly what you need.

Amplifier input stages are designed for distortion and then add effects.
The last thing you need is effects driven by clean and then put through
input distortion channels, its all wrong, as real experience will illustrate.

rgds, sreten.
 
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Hi,

Amplifier input stages are designed for distortion and then add effects.

rgds, sreten.


I agree, but in this partticular design, effects stage is implemented as if it were a pedal with a wet dry knob, i realized later that i should have considered putting it at the power stage, but i left it as is, theres no reason why it shouldnt work.
 
The layout is somewhat like this. There are jumper cables going to another board with the opamp circuit.
This shows the regulator ground reference going into the charging loop to the right of the smoothing capacitors. THIS IS WRONG !!!!!!!
The regulator ground reference MUST be taken from the OUTPUT node where power is sent to the next circuit..