Working on a Mixer...

[LyleCaldwell]

I'm building a dual mono 4x1 active mixer with unity gain, and I'd appreciate any feedback or troubleshooting.  As this has been cobbled together from existing designs out there (none of which did exactly what I want) I don't claim it to be mine, and if anyone wants to use it, feel free (but wait until I know it works).

Besides making sure the schematic is good, I was wondering if anyone could tell me how to use a single channel opamp per side instead of the dual channels shown.  I don't need the balanced outputs that this schematic has, but my previous design using a single channel opamp was inverting the phase, which I'd like to avoid.

Here's the schematic:  http://psionicmedia.net/lyle/music/NewMixer.jpg

Thanks in advance!

[ESPguitar]

Hi:P

I'm sure somebody here will help you:)

Didn't suhr help you a bit?

RB

[LyleCaldwell]

Yes, very much, but I don't want to keep asking him for free advice, particularly since this is how he earns his living.  I did order the book he recommended.

[ESPguitar]

I understand..

RB

[cd]

What you want is a non-inverting summing amplifier.   Re-wire the first opamp only (get rid of R6 and the rest of the 2nd opamp) so it looks like this:

http://content.edu.tw/vocation/electronic/tp_ss/content.files/opa/opa_noninvert_add.html

[LyleCaldwell]

Like this?



As I don't read Japanese, am I correct in reading that "RN" in my schematic should be 100K?  Every input on Bus A has a 100K resistor, and that's what the bias resistor is.

[Maneco]

two details:
the circuit posted later is an inverting mixer stage

but the new mixer schematic,the first one,looks nice,it should work...the only part unclear is in the rectifying of the ac suply...but there are many +- 15 v schematics ,you should take a look at rane's schematics...   www.rane.com

[LyleCaldwell]

Maneco,

Heh, that actually is taken from one of Rane's schematics, taking a 9VAC supply and turning it into +/-15VAC.

Are you saying the schematic CD linked would invert the polarity of the signals?  I thought it was positive in, positive out (which is what I want).

[cd]

Ummm... no, let's see: let's call the 100k resistor in the feedback loop Rf.  The Rn resistor should be the same as the "A" resistor.  However, if you want unity gain, the formula to calculate the amount of gain is (1+Rf/Rn).  So *technically* you can't get 1x gain, only 1+a tiny amount gain.  So you have to make Rf a really small value if Rn is going to be large.  Say Rn is 10k, then make Rf 100ohms - this will give you 1.001x gain; the tiny bit extra will not be audible.

[LyleCaldwell]

Hmm, other schematics have said the resistor in the feedback loop divided by the resistor in "A" gives the overall gain (so 100K/100K = 1, obviously), and increasing the value of the feedback loop resistor adds more gain if needed.  

But all those designs had "A" going into the negative input on the opamp and "A" was in parallel to the feedback loop.

You're saying that running "A" into the positive input would eliminate the polarity reversal but that there is a different formula at play (is it because the input is no longer in parallel with the feedback loop or because the positive input of an opamp has a different impedence than the negative input?).

I'm just trying to get all this straight.  If "A" is 100K and Rn is 100K, then Rf would need to be 1K, right?  Which would still give me a gain of, wait- my calculator says the gain would be 1.01, not 1.001.  Are you sure of your math there?  100 divided by 10000 is .01, not .001.

Did you mean to say 10ohms instead of 100?  In which case I should use an Rf of 100 instead of 1K, right?  Because that would give me a gain of 1.001.

Last (at least for now), is there any reason not to use one dual channel opamp for the dual mono mixer vs using two single channel opamps?

[Dragonfly]

Lyle Caldwell.....

from REC.AUDIO.PRO ?  i used to hang out there quite a bit

i see you've falllen to the "dark side"   :twisted:

andy carrell

[LyleCaldwell]

So here's what the proposed changes would look like, if I understand correctly:

[LyleCaldwell]

Lyle Caldwell.....

from REC.AUDIO.PRO ?  i used to hang out there quite a bit

i see you've falllen to the "dark side"   :twisted:

andy carrell

Hey, yeah, it's me.  I used to love R.A.P. but the right wing bullshit got to be too much.  I still lurk occasionally.

[Dragonfly]

Lyle Caldwell.....

from REC.AUDIO.PRO ?  i used to hang out there quite a bit

i see you've falllen to the "dark side"   :twisted:

andy carrell

Hey, yeah, it's me.  I used to love R.A.P. but the right wing bullshit got to be too much.  I still lurk occasionally.

yep....

i never went back though ...

this forum is VERY cool...probably the most civil, friendly, and helpful ne you'll find....

andy

[cd]

Yup, the gain equation for inverting vs. non-inverting is different.  For inverting, the gain is:

Rf/"A" - so if you have a 100k feedback resistor, and a 100k input resistor, you have 1x gain (unity)

Careful though, if you need a high input impedance, your input resistor needs to be really large, so Rf needs to be large, and you get more noise.

[LyleCaldwell]

This is a line mixer run in an amp's effects loop to allow parallel effects routing, so everything will be low impedence line level, though unbalanced at -10.  

Thanks for clarifying the inverting gain issue.  But again, do I need to aim for a factor of 1.001 or 1.01?

[cd]

Thanks for clarifying the inverting gain issue.  But again, do I need to aim for a factor of 1.001 or 1.01?

For the "illusion" of unity gain?  Pfft, doesn't really matter, just use whatever part you have lots of in your parts bin  You could use a jumper, but I forget why that's not desirable - something to do with closed loop gain and oscillation?  I don't remember, but I always use a low value resistor.

[LyleCaldwell]

Yeah, you're right.  Thanks.