GGG Mini Mixer - Sucking Tone

[kriista]

Well it's already etched, and I don't think it'll be too hard soldering to the trace side (sockets for opamps being the hardest I would guess).

So any idea on using Burr Brown opamps as far as improved signal path?

[Processaurus]

I've never a/b'd them but I'd bet the increase in performance in this case would be slight...  Both are very good at linear amplification, and the gain here is modest.  If you're going shopping, and want to get fancy, maybe get a low noise rail to rail opamp for the dual (this would afford an extra volt or two of headroom), and a tl074 for the quad.  If you feel like getting the burr browns, just make sure it specifies the model has FET inputs for the quad (for high, guitar friendly input impedance).

[kriista]

Sorry to necro this thread, but I'm building this mixer again, for use in another project (rather, a v2 of the same project)

I'm going to try to go as quiet as possible this time, using metal film resistors and good opamps and such.

What are model numbers for good/quiet/headroom dual and quad opamps?
I went with WIMA caps for the input caps before. Anything better there?

Also, the following changes in values were suggested to me on another forum (along with including bypass caps at the power input and opamps).

R5-R8 = 22k
R9-R10 = 10k
D1 = 1n4004
100uF at power input
.1uF across opamp power

I want to basically go as 'hi-fi' as possible with it this time as I'm using it much more than I had originally thought.

Here is the project the first version of the mixer went into:

http://rodrigoconstanzo.com/The_Party_Bus.html

[Processaurus]

Cool project!

I'm going to try to go as quiet as possible this time, using metal film resistors and good opamps and such.

How noisy was your last mixer (like, if all the volumes are turned down is the hiss unacceptable?)?

Metal film is good.  Low noise opamps are good.

What are model numbers for good/quiet/headroom dual and quad opamps?

I usually just use the TL07* family for my pedals, cheap and good.  If those weren't quiet enough, I think the design would need to be changed to a more sophisticated one, rather than turning to audiophile components.

I went with WIMA caps for the input caps before. Anything better there?

Whatever film caps are around will be fine.  Not worth getting exotic.

Also, the following changes in values were suggested to me on another forum (along with including bypass caps at the power input and opamps).

R5-R8 = 22k

I don't like getting lower than 47K, but you can try it and see if the channel volumes get weird and interactive.  If you must get lower, than use stiffer pots, like 5K.

R9-R10 = 10k

OK.  Those can be whatever.  47K'd be fine.  470K is high, but fine.  Remember the 1 uF cap is bypassing the voltage the opamp sees, so resistor noise is no issue.

D1 = 1n4004

Unnecessary. 1n914's are rated to 100mA.  If you wanted to do something fancy there, use a Schottky diode (like the 1N5818).  They have a lower voltage drop.

100uF at power input

That is the best suggestion your other forum made.  Put it downstream of the diode.

.1uF across opamp power

That's a good suggestion too, for noise.  If you bypass the opamps individually with .1 caps the idea is to put it physically as close to the pins as you can.

If this design isn't cutting it for you performance wise, I could imagine a more hi fi version with more parts, with discrete FET buffers, variable gain with an inverting opamp stage on each channel (rather than attenuation followed by fixed gain here), and an inverting, unity gain summing stage.  Possibly a boosted power supply for more headroom.

[garfo]

I've seen this post and I'm very interested, I'll explain why.
I've built the GGG mini-mixer to use it with my bass.My idea was simple.Mix the sound that comes from the preamp of my bass into a splitter box.Then splits one channel to a buffer and to the minimixer and the other one goes to a Turbo Rat in series with a Boss GEB-7.
When it's time to mix both channels it is really hard to make it sound real good.First, for the clean channel to be able to be heard I had to lower the input resistor to 47 k so that gain could be increased and therefore louder.Second, the other channel with the rat, the rat will only sound good if I turn the channel volume way high towards max.At this stage when I mix the clean channel evrything is way too loud.If I don't max the pot the rat will sound buzzy and fuzzy.So, my queston is: should I quit the GGG minimixer and try this TL074mixer instead!?

Sorry to necro this thread, but I'm building this mixer again, for use in another project (rather, a v2 of the same project)

I'm going to try to go as quiet as possible this time, using metal film resistors and good opamps and such.

What are model numbers for good/quiet/headroom dual and quad opamps?
I went with WIMA caps for the input caps before. Anything better there?

Also, the following changes in values were suggested to me on another forum (along with including bypass caps at the power input and opamps).

R5-R8 = 22k
R9-R10 = 10k
D1 = 1n4004
100uF at power input
.1uF across opamp power

I want to basically go as 'hi-fi' as possible with it this time as I'm using it much more than I had originally thought.

Here is the project the first version of the mixer went into:

http://rodrigoconstanzo.com/The_Party_Bus.html

[garfo]

Why should r13 be 47k instead of 10k, anything to do with gain?How would I do to not increase gain too much?

The opamp buffered Vref is good because a resistive divider would need to suck a lot of current to hold itself steady, you want it to run 10x as much current as its load, and the 4 10K pots on the buffer opamps' outputs (a low impedance source) divide to a present Vref with a 2.5K load, so you'd want a resistor divider with two 250 ohm resistors and a big old cap, that's 18mA just for the Vref.  There was another thread recently about other rail splitting techniques that was interesting.

I don't quite get this part. Why not use bigger resistors for the divider? Didn't find much in the search.

Bigger resistors will make the reference voltage softer, and sag when there is a load on it (like when the buffer opamps swing higher or lower).  It stops being a dependable reference voltage when the rest of the circuit can push and pull it around (which here, can cause weird bleed problems or distortion, as the refence voltage would be pulled around at audio frequencies.

http://en.wikipedia.org/wiki/Voltage_divider

See "Loading Effect"

So are there any slightly higher part count schematic/layouts that a not super techy person like myself could just build without having to wrestle with too much?

Kriista, we were just discussing different ways of designing a circuit like this, not be repetitive but the schematic and layout as drawn will work, no design skills necessary.  I'm taking a fair amount of trouble to explain this project to you.  If assembling it is over your head, maybe come back to it after a few other pedal projects.

here's an updated schematic, fixed the resistor values in the last gain stage, and corrected the power pins on the quad opamp (note they were correct on the layout), and made the part numbers match Fixr's layout.  Note R13 should be 47K, not 10K as on that layout:



EDIT: Photo link was weird