Mixer Design

[yano]

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:



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

[gez]

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).

[gez]

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

[gez]

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...

[yano]

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.

[smoguzbenjamin]

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

[smoguzbenjamin]

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.

[gez]

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.

[Peter Snowberg]

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

[yano]

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...

[yano]

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:

Excess Voltage Indicator:


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

[gez]

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?

[Brian Marshall]

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

[Boofhead]

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

[R.G.]

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.

[yano]

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?

[gez]

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.

[gez]

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).

[gez]

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

[yano]

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!