DIYstompboxes.com

A while ago, I was asking questions about building a mixer. Well, I've made some progress, and some came up with some questions.
Here is my current design:

(http://yano.lardpirates.com/electronics/schematics/mixer_schematic.jpg)

It still needs a bit of work, especially on the Voltage Reference Generator.
Notice that the Output 1 jack is a stereo jack, so that the mixer is only being powered while something is plugged into Output 1.

Some things I need advice on follow:

-For the op-amp, I assume I can use any dual opamp. I've seen the TL072 recommended, but I assume I could use an NE5532 in place of it, correct me if I'm wrong.

-I added the output to the peak detector as though I were adding another output, I assume this is correct.

-The comparator used in the Peak detector is an LM339. The original design used 3 of the 4 comparators, however my alteration only requires one. Is there a comparator that is essentially the same as the LM339 but has less "channels"? I would prefer to use something in maybe an 8pin package rather than 14.

-The Voltage Reference Generator is the thing that I know nothing about, I've never dealt with a reference generator before. The design shown here is from the original peak detector schematic (Dave Johnson's Acceptable Voltage Indicator). This circuit was apparently designed for a 12V system.
My guess is that I can use a 9V controlled power supply to supply the reference voltage, and then just use straight 9V from the jack/battery to supply the 9V.

-I just noticed, the ground on output jack 2 is connected to some other circuitry. It eventually gets grounded, but should I use a more direct ground (i.e. tie it to output jack 1's ground)?

-Should I buffer the per channel outputs in the same fashion as the mixed outputs? (100 ohm resistor followed by 10uF cap)

Thanks again, and if you notice any values that look wrong, don't hesitate to tell me. I'm pretty new to designing circuits.

Design elements taken from
Mini Mixer
http://www.generalguitargadgets.com/v2/diagrams/mixer_sc.gif
Simple Mixer Schematics
http://www.all-electric.com/schematic/simp_mix.gif
Acceptable Voltage Indicator
http://www.imagineeringezine.com/PDF-FILES/voltst1.pdf(//)

I've only skimmed through your post, so appologies if I've got the wrong end of the stick.

You can use ‘any’ dual op-amp though some are quieter/have more headroom than others.  Either of the ones you mentioned would be fine for this purpose.

Not too sure why a buffer has been added (second op-amp).  Is it really that necessary to have a non-inverted output?  I appreciate that the first amp is doing six times the work but would it really be loaded without the buffer?

Personally I’d rectify the output before feeding it to the comparitor, but then again I don’t know how you intend to use the ‘peak detector’/what it’s for.  Any chance of elaborating?

The ‘voltage reference generator’ is only necessary if you don’t have a regulated 9V wall wart.  You can just use a standard 9V adaptor or battery (though you might need some RC filtering if your wall-wart is noisy).

PS  There are a number of op-amps which could be used for the comparitor.  CA3130E for example.

Hmmm.  The comparitor is set to trigger when the output goes higher than 7.5V.  You might have a problem triggering it if the op-amps you choose for the mixer aren't able to swing withing 1.5V of the positive rail.

You could either lower the threshold at which it triggers (peak is set for a 6V peak-to-peak signal at moment), or use a CMOS output op-amp for rail-to-rail swing.  Just a thought...

Thanks for the feedback gez,

From what I've read, the reason you need a second op amp is so the output is back in phase. If you don't do this, you could end up canceling out signals later on (not in the mixer, but perhaps with some other audio hardware).

I must have screwed up the calculations for the resistor values, I was aiming to trigger at 2V, not 7.5V
The peak detector is supposed to allow the user of the mixer to avoid clipping in the amplifier, so its suppossed to light the LED when the output voltage exceeds 2V, but like you said, its "tuned' wrong. The person who designed that part of the circuit said that a rectifier probably wasn't neccesary.

Maybe I'm a bit confused on what the voltage reference generator does...is the V+ value some voltage lower than 9V? That was my assumption.

Thanks again.

If you have two identical signals comin through, it makes no difference. But if you mixed a signal before and after your mixer they would cancel out, yes.

But some distortion boxes have inverted outputs too, so I wouldn't worry about it. Just leave it as-is it will work fine ;)

Oh, by the way:

V+ = 9v or in any case just the +voltage coming off your PS (in this case a battery). Vref is used to bias the opamps and is usually 1/2 of the supply voltage. 4.5v in this case.

The ‘voltage reference generator’ (not that I’d use this term) is just a 9V regulator that takes an input voltage between 12-25V and churns out a regulated 9V for you to use in your circuit.  If you have a 9V regulated supply in the first place (decent wall-wart) then there’s no need to include it.

Voltage ref usually (well, in stompbox circuits at any rate) refers to the bias point (usually set by a divider) for the op-amps, and is most commonly 4.5V as Ben has stated.

Unless you intend to run the input signals in a side chain and combine the output of the mixer with the output of the chain, then it’s a waste of time inverting the signal again to get the mixer output in-phase with its input signals

2V peak-to-peak?  Then use a 6k8 and 11k for the divider instead of 3k and 15k.

For another comparitor, check out the LM311. :)

Thanks for posting your schematic. Building a mixer is not that uncommon of a question. It will be nice to point to this thread.

Take care,
-Peter

I'll keep the board posted on my progress, the design you see here is not really finalized, people will have to do some changes to make it work...

Alright, I've finished my design, and also moved it over to PSpice, a wonderful tool.  File size for these JPEGs has increased, the main schematic is 138k...hope you don't mind load time if you're on a modem...

Mixer:
(http://yano.lardpirates.com/electronics/mixer/ps_mixer_schematic.jpg)
Excess Voltage Indicator:
(http://yano.lardpirates.com/electronics/mixer/ps_voltage_indicator.jpg)

The 12 jacks on the left probably deserve some explaination. This is only a 6 channel mixer, but I implemented "flow through" jacks. As can be seen plainly, the jacks are an output of each channel after the audio pot. The main reason for this is because I plan to use this mixer to do some recording...Therefore each instrument can use their own amp, and then I can record the unamplified outputs.
One thing I'm unsure of though, is should I buffer these "inline" outputs?

Thanks.

I'm also going to be posting some sort of webpage detailing my build here:
http://yano.lardpirates.com/electronics/mixer

I think I understand what you mean by 'flow through' inputs.  They're for balanced inputs that don't need attenuating right?  You don't need to buffer them, but you do need to connect them up differently.

As shown, there's a danger that you'll shunt the inputs to ground if the wipers of the pot's connected to the other jacks are set low.  You'd need to connect each of these inputs via a cap and 10k resistor to the -ve input of the amp.

This means the amp will do double the work so is going to be slightly noisier, but the amp you've chosen is a pretty quiet one.

edit: maybe I've got the wrong end of the stick about these 'flow through' inputs.  Can you give us a little more info?

not the way i would build it... i would definitely either put a buffer on each input.

The input impedance is pretty low, 5k with the input volumes on max.

Following up on Boof's comment, a guitar into that setup would lose a lot of highs because of the input loading. It might make sense to make a couple of channels (or all of them) have input buffers. JFET buffers, or JFET input opamps would work fine. Just hook them up as voltage followers, with 1M resistors to your mid-voltage bias supply, and drive the 10K level pots with the outputs.

Some comments:
- you can use linear pots and taper them to a log/audio taper (see "The Secret Life of Pots" at GEO)
- you can use the buffered outputs as a "flow-though" output to ensure less loading on the device driving the inputs.
- 9V is minimal power supply headroom for a generic mixer. If you only do guitar through this, fine. If you intend it for more general use, think about putting it on +/-12V or +/-15V, which will also simplify the biasing and remove the need for quite a few coupling capacitors.

gez:
Those "inputs" your talking about are actually meant to be outputs, in order to plug instruments into seperate amps, while recording the unamplified signal. To make them outputs, should I wire them more like the other outputs, with a 100 ohm resistor folloed by a 10uF cap?
Perhaps though, I will just leave them out.

R.G.:
How would I implement a JFET buffer for every channel? Could you perhaps sketch out a schematic? I don't know what you mean "as voltage followers", I guess I don't know all the vocabulary.

Also, would it be benificial to raise the values of the channel level pots, to perhaps, say 50k?

I thought you were going to use this mixer for balanced signals (edit: meant line level - slip of the brain!  :oops: ), but if you’re mixing guitar and other low level stuff then the input impedance is far too low, as has been pointed out.

If you didn’t want to go down the route of buffering all the inputs, then it would be advisable to increase the values of all the pots/resistors in the mixer.  470k would be a good compromise.  This would give you input impedance of 235k when each pot is fully up which, although not brilliant, is ‘respectable’.  You could use 1M for everything, but you might have problems with gate capacitance sucking some of the sparkle from your signal if you use FET input amps - try it and see.  If you're not using one, I'd suggest investing in a breadboard!

I understand the ‘flow through’ thing now.  I would definitely follow RG’s  advice and use buffers if you’re going to do this.

Here's a voltage follower:

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar2.html

The input is coupled through to the +ve input and a 1M resistor connects from this input to the center of the 100k divider (V ref).

The only thing you have to watch for is the input bias current of each op-amp.  If you have six buffers all drawing current from the same divider it might load it slightly.  You'd have to check the data sheet of the op-amp you're using and do some calculations.  If it is going to be a problem you can make the divider's resistor values smaller and increase the value of the decoupling cap (or even use another divider or two for the buffers).

Here's a link for a JFET buffer.  It's the first thing I came across when I did a search, and I haven't read through the blurb, but figure 2 shows a typical source follower.  Just run it off your 9V supply.  Perhaps others can give you some better links (bit pushed for time this morning).

http://zebu.uoregon.edu/~strom/ph610/lab4.pdf

From what I've gathered, I should put a buffer on every channel...Once I get some spare time and finish reading all those links, I'll be back with a revised schematic.

any recommendations on an opamp to use for the voltage follower? or which jfet would be good in this situation?

Thanks all!

Here is what I think the input stage is going to look like. It seems to me that using the op-amps will be easier to build than the JFET buffer. Correct me if I'm wrong.

(http://yano.lardpirates.com/electronics/schematics/input_stage_proposal.jpg)

Questions:
-Should I increase the value on the level pot? If so, what to?
-Is the channel listen jack wired properly?
-Are there any op-amps that lend themselves to this application, or can I simply use generic, like a TL072 or NE5532...? Does anyone know of an op-amp with 6 pairs of inputs?

Thanks.

Comments on the schematics:

- the schemos imply that you will be using bipolar (+ and -) power supplies centered around ground; if that's what you're doing, great
- you will need some kind of resistor to ground from the noninverting inputs of the opamps to bias them; 1M should work fine
- think about putting a non-polar capacitor in series with the inputs anyway; any DC offset in the circuit coming in will be carried through the rest of the circuit, causing bad juju all around.
- It might be good to put a 10K resistor in series with the input jacks and reverse biased diodes from the signal line to the +  and - power supplies so that if someone plugs 120Vac into the input, a fuse will blow somewhere, but it will not smoke your whole mixer.
- You used some polarized caps in your first schemo; do not use polar caps where there will not be DC volts across them - they die on a 0V diet.

Input pot at 10K is fine; Channel jack is OK; NE5532 is a *great* opamp for this.

Alright, more questions, more schematics...

(http://yano.lardpirates.com/electronics/schematics/mixer_020404-1247a.jpg)

Were the polarized capacitors you were talking about the ones in that box?

(http://yano.lardpirates.com/electronics/schematics/input_stage_020404-108a.jpg)

The questions I have for the input stage are in the image...

My power supply is either going to be a 9V battery or wall/small wart at 9V...but I'm considering 12V. I guess these are bipolar, I'm not really familar with the term. Thanks a lot, R.G. for your input.

A few ideas.

If you want to power this off a 9V plug, you'll need to make a few changes.  As RG said, with a bipolar supply (+ and -9V) you'll be right with what you've got.

For 9V supply, you'll need that powers upply box.  It gives you a stabilised 4.5V supply that you'll definately need.  (that 4.5V is often called Vbias).

Put that 10k resistor after the 1M resistor, and put a 0.1uF cap (in-line) before the 1M resistor. Now, connect the 1M resistor to the 4.5V supply - the junction of R8 and R9 - instead of earth.

Also, shift that polarised cap to *before* the 10k "level" pot.  Having done those things, the signal path from input cap to output cap will operate happily at 4.5V, allowing you to power this from a single-sided power supply of 9V.  (ie you can hook up pin 8 to 9V and pin 4 to 0V (instead of -9V))

If any of that is unclear, ask again.

So, what I'm understanding is that a bipolar power supply (in this case) has a -9V ground, rather than 0V?

And those mods that Brett listed, they are not neccesary if I'm using a bipolar power supply?

If I were to use a bipolar power supply, would that have any impact on the rest of the circuit (rather than just the input stages)?

How can I make a bipolar power supply, or where can I buy a +/- 9V wallwart?

Thanks in advance.

You wanted a peak detecter set for 2V peak-to-peak, suggesting that you're using low level signals.  You're only dealing with unity gain in each stage of the circuit too.  You can get away with using a 9V supply no problem.

If you're using a regulated 9V wall-wart you don't need the extra circuitry Bret mentioned, i.e. regulator fed from 12-25V (or whatever it was).edit: if that is what Brett was implying (apologies if it wasn't).

Quote from: yanoSo, what I'm understanding is that a bipolar power supply (in this case) has a -9V ground, rather than 0V?

And those mods that Brett listed, they are not neccesary if I'm using a bipolar power supply?

If I were to use a bipolar power supply, would that have any impact on the rest of the circuit (rather than just the input stages)?

How can I make a bipolar power supply, or where can I buy a +/- 9V wallwart?

Thanks in advance.

With a bipolar supply 0V is still earth and not -9V.  The positive inputs of each op-amp connect to 0V so a divider for bias is unnecessary.

Using a bipolar supply gives you more headroom, but if you’re only dealing with 2V peak-to-peak signals it’s not necessary, you can get away with just a 9V supply.

This mixer is intended for guitar level and line level inputs, straight from the instruments, essentially.

My plan calls for power to be supplied via a standard BOSS-type wallwart (for maximum flexibility), is this acceptably regulated, or should I use, say, a 12V wallwart going to some sort of regulation circuitry?

Also, I never intended to use a bipolar supply...mainly because I didn't even know what they were when I designed this ;)

Boss type wall-wart is fine...s'what I use! :)

I've got something of a site going for my mixer project, its located here:

http://yano.lardpirates.com/electronics/mixer

Some of you mentioned earlier some things about polarized capacitors in the main mixing circuit...(http://yano.lardpirates.com/electronics/mixer/mixer.php)...what were you getting at?

Also, what % tolerance capacitors should I use?

And what of polyester film? Should I use this for the .1uF capacitor?

I gather from your previous posts that you're running the whole circuit from 9V and not using a bipolar supply?  If that's the case you need DC blocking caps in the input stage and you need to reference the ground connections (except the jack grounds) to half the supply voltage (center of the 100k divider - though it would probably be wise to make the resistor values smaller and up the value of the decoupling cap).

The input stage as shown is for a bipolar supply.  If that was what you intended then it would be an idea to include input caps (non-polarised).

In the mixer section the 'master level' pot has DC voltage across it so will probably crackle when turned.  Either stick it at the output AFTER the output cap OR connect the lug currently connected to ground to Vref (centre of the 100k divider).

Take a look at RG's "Quick & clean bias voltage thread" - might be an idea to provide the bias for all those amps?

You might be able to get away with replacing the 10k feedback resistor in the second op-amp with a 10k pot set up to go from 0 ohms - 10k as you turn it.  This would act as the master level control and saves you a resistor.  With a FET input amp there wouldn't be much leakage current through it (hardly any voltage across it) so it probably wouldn't crackle.

I learned some interesting things today. I will require a voltage divider per channel. If I try and use the same 4.5V lead, the signals will mix prematurely and result in distortion of the actual frequencies. I think it will be more practical to use two 1M resistors rather than the LM386 way.

This is my rethought input stage:
(http://yano.lardpirates.com/electronics/mixer/stages/input.jpg)

C3 is included so that the channel level potentiometers will not interact, at least that's what I think they're supposed to do. Can someone tell me if they are neccesary or not?

Edit: Can I connect the channel listen jack between C3 and R3?

QuoteI learned some interesting things today. I will require a voltage divider per channel. If I try and use the same 4.5V lead, the signals will mix prematurely and result in distortion of the actual frequencies.

I'm curious - where did you "learn" that? It's not necessarily true.

If you make a bias voltage from resistors only, then there may be some basis in fact. However, every bias reference for more than one stage uses a capacitor or some other means to reduce the impedance of the bias voltage to the point where it shunts all the signal to ground, producing a quiet spot where the signals are swallowed by ground.

The commonest way to do this is with a big capacitor to ground from the joint resistor spot.

The LM386 bias trick is even better than the R-R-C bias trick, because it *actively* reduces the impedance of the bias voltage, with the amp doing everything it can to suppress any voltage movement of the point. The signals can't mix to any noticeable degree and then get back out to the signal path.

I've been expiramenting with pspice simulations, and the waveforms appeared to be quite distorted when the 4.5V was all connected to each other...

So you think the 386 "trick" will work?

I think at this point, my best bet will be to start breadboarding and expiramenting.

With your pspice simulations, did you put a big capacitor (e.g. 100uF) from the junction of the bias resistors to earth?  If you did, there should be no interference between channels, even if they use the same resistors for the bias voltage divider.

Also, don't forget that with your 9V supply you'll also need to use the bias voltage for the mixing stage (and the associated input and output caps to isolate the stage with respect to DC).

so you're saying I should bias the inputs of the amps in the mixing stage?

I assume there's no reason to unbias and then rebias the signal between op-amps.

I think what Brett's trying to say is you've used a divider (haven't got the schematic in front of me but I believe it was two 100k resistors) in the mixer section and the positive inputs of the mixer amps connect to the centre of this divider.  You should be able to bias the op-amps used in the input stage using the SAME divider - connect a 1M resistor from each + input to the centre of the two resistors - PROVIDED the junction of this divider is decoupled by a LARGE value cap.

It would be advisable to make the value of the divider resistors smaller, or use RG's 386 idea (yes, it will work!).

Ok, the divider resistors have  vanished since the I last looked!  :)

For the voltage ref (4.5V) either use a divider with a large value decoupling cap, or the 386 idea.

about the aoutput buffer why don`t you transform it to a gain stage you can use a gain control instead the volume.

gez:
Yes i'm planning on going with the 386...do you think it can provide enough current for all the channels as well as the summing opamps? My guess is that'd i'd be safe with an LM386-3.
What do you think about C2 (2.2uF)? Do I still need it with that voltage divider gone?

12afael:
I'm reworking parts of my design...the input buffer (now that it has opamps in it) i will be adding pots for gain in the feedback loop for the input, but the outputs will just be on a volume control. In fact though, I could put a pot in the feedback loop for the 2nd summing opamp for gain as well...I'm not sure what all i want to do yet.

About C2.  You need it because you don't want any DC voltage across the potentiometer.  Because that signal is at DC ground after the pot, you'll need to re-bias the signal in the mixing stage (ie isolate it with input and output caps and connect it to Vref/Vbias)

Quote from: yanogez:
Yes i'm planning on going with the 386...do you think it can provide enough current for all the channels as well as the summing opamps? My guess is that'd i'd be safe with an LM386-3.

Op-amp inputs are designed to draw very little current for their bias.  FET input amps don't require input bias current, but they do have leakage so you'll still see quoted figures for 'bias current' in data sheets.

Total it all together and it's pretty insignificant, but enough to cause loading of a divider if the resistor values are on the large side.  The 386 is an ideal solution and it'll provide a rock-solid bias to all the amps you use.

Quote from: brettBecause that signal is at DC ground after the pot, you'll need to re-bias the signal in the mixing stage (ie isolate it with input and output caps and connect it to Vref/Vbias)

One thing I've done in the past is direct coupling from op-amp input buffers to a mixer.  Using amps from the same chip doesn't seem to cause offsets in the mixer.  Whether it would here though I couldn't say as there are more than four inputs so different chips would have to be used.  The pots coming off the output of each input buffer are  connected to V Ref too, so it might be possible (well, it would with just two or three inputs and a quad chip).

Quote from: yanoWhat do you think about C2 (2.2uF)? Do I still need it with that voltage divider gone?

You've reference the 100k pot to half the supply voltage so theoretically you could directly couple everything.  But, as I've said above, using different chips might create an offset in the mixer.  I've only ever used simple mixers as part of larger circuits, so I couldn't say whether this would be a problem.

The ground connection for each input jack should connect to ground and not V ref.  Provided you leave the 100k pots connected to Vref you shouldn't need an input cap for the mixer.

The last schematic you linked to has C2 as 10u and not 2u2.  Are you referring to a different part of the circuit?  Could you link it? (I'm a bit confused/slow!)

I've been changing schematics a lot, renaming parts...i've got the jacks on ground now, i got a bit crazy thinking about virtual grounds.

Anyway, the latest schematics can be seen here:
http://yano.lardpirates.com/electronics/mixer/stages/input.php

I was referring to C2 in the mixing stage, which is a 2.2uF cap to ground, coming off of the power lead of my power input jack, i'm not sure its neccesary anymore

All the schematics can be seen from here:
http://yano.lardpirates.com/electronics/mixer/stages/

I probably won't be posting any schematic changes until i get this breadboarded out...but i'll only change them if i can't get them to work  :wink: .

The 2u2 is just connected up across the rails.  If it’s there for filtering then it’s a waste of space.  You have a connection for ‘voltage reference generator’ shown in the mixer section coming off +9V.  This would go to the + power connection of your 386 and Vref is taken from the 386’s output (4.5V).

The ‘channel with +6dB gain control’ input schematic is a little off.  You’ve got the 100k pot wired with one leg to ground so it’ll screw with the DC bias of the amp as it’s turned.

If this were me, I’d replace R2 (100k resistor in the first schematic) in the feedback loop of the buffer with a 250k pot set up so that it’s resistance increases from O ohms as it’s turned.  I’d then do away with R3, the 100k pot at the output, as it would now be superfluous.

By using the -ve input you’ve lost the high input impedance you could have had by using the +ve input, which means your ‘buffers’ are a bit of a waste of time.

Read back through this thread, look at some other schematics and hopefully it’ll click. :)

Man, I'm confused.

The two inputs of an op amp are not equally high resistance?

Hi Yano
Don't despair.  You've made a terrific journey just to get this far.

The +ve input of an op-amp provides a super-high input impedance.  So high that the effective impedance is usually set by the biasing resistor (between Vref and the signal line).  THis is often 1M or thereabouts.

The input impedance of the -ve input of an op-amp is set by the series resistor in the signal line.  To avoid resistor noise in the input and feedback resistors, the value of that resistor is usually kept low (often 10k or thereabouts).  

Does that make sense?  What it means in practice is that for guitar signals, etc, that need high impedance, you are better off using the +ve input of the op-amp for the input stage, and biasing the stage with a nice, big resistor (e.g. 1M or 2.2M).

What about the LM386 biasing approach, how does that compare to using 1M or 2M2 resistors?

The reason for using 100K resistors was in order to reduce filtering of frequencies below 100Hz (with the 100k resistor, the filtering starts below 15Hz or so).

QuoteWhat about the LM386 biasing approach, how does that compare to using 1M or 2M2 resistors?

Think of  it this way. With two-resistor biasing, you get an input impedance (that is, loading on the thing driving the input) of the parallel combination of the two resistors since the power supply is an AC short circuit. For 1M resistors, that's 500K, for 2m2 resistors, 1M1.

OK as far as it goes. However, there is a lot of voltage across those resistors and they are high valued, which is the ideal setup to make the resistor noise a prominent part of the input signal.

The LM386 bias source is a very low impedance source of bias. If you connect it directly to the inputs, it shorts out all of the input signal. So you put a high value resistor in series with it, maybe the same 1M or 2m2 values. Now the input signal at the input sees the high value resistor as a load, and you get low loading of the signal. The resistor has a low DC voltage across it, which minimizes its noise contribution.

In the schematic shown here (linked from http://europa.spaceports.com/~fishbake/mixer/linemix.htm )

(http://europa.spaceports.com/~fishbake/mixer/mixr.jpg)

What is the function of the .1 and 100uF caps on the power supply lines?

[Edit 03/01/04--I answered my own question]