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DIY Stompboxes => Building your own stompbox => Topic started by: tatems on February 27, 2007, 05:00:31 AM

Title: GGG Mini Mixer - Sucking Tone
Post by: tatems on February 27, 2007, 05:00:31 AM
Hey all,

I just built the GGG Mini Mixer on a pcb board i etched (using the GGG design). One problem i face is that it sucks the high end of my tone. I was wondering if this is normal or something is wrong with my mixer.

Cheers

Tatems
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on February 27, 2007, 07:43:40 AM
Is that when your guitar is plugged straight into a channel?  If so its the same loading from that channels vol pot as when you turn your volume knob down on your guitar, and would be a weakness of the simplicity of the design (like passive vol pedals).  The easiest solution is to put a buffered pedal between the guitar and the mixer, or build a buffer inside the mixer for one of the channels, to make that a high impedance input.  I like the GGG JFET buffer for things like that.  I'd only do that for one channel, if you're doing parallel effects or electronic instruments in the other channels.  If you're using it to mix multiple guitars (or other instruments with passive electronics, like mics, Wurlitzers, etc), buffer each of those channels before the volume pots.

To buffer all 4 channels you could save some parts and use a quad opamp like a TL074
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: tatems on February 27, 2007, 08:11:15 AM
Great, thanks for that.

I'm actually building it for a friend so he can play with other guitarists or bassists at home. I think it'll be fine without the buffers. So just to clarify when i plug the guitar into one of the channels i turn the guitar volume and the mixer channel volume fully up. I then play and the high end is not a prominet as when i play without the mixer. So as you said i would need a buffer to match my impendence then.

Cheers

Tatems
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on February 27, 2007, 08:32:53 AM
Without some sort of high impedance buffer, he'll get the tone suck with the existing mixer design.  The impedance of the mixer is only 10K, the value of the pots.  For guitar you'd want an impedance at least 10 times that.  You could try a stiffer pot, 100K would be a good start.  You'd then want to make the summing resistors (now 100K) 1M then to make it work right and not have the channels interact together.

The buffers followed by the stock mixer would sound better, in my opinion.  Depends how much time you feel like spending.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Mark Hammer on February 27, 2007, 09:48:31 AM
First, the mini-mixer uses an inverting op-amp, which I gather has a lower input impedance than a non-inverting one.  For guitar, that shouldn't, in theory, pose too big a problem.  My sense, however, is that the brunt of such simple mixer circuits anticipate low-impedance sources, such as line outputs or microphones.  If you feed the mixer with an effect pedal, usually the output impedance of the pedal will be low-enough that it falls in the desired range.  If the mixer is fed directly from the guitar, though, especially if it's through a long-ish cable, you're asking for trouble.

Certainly the use of higher value pots (e.g., 100k or greater) is an excellent suggestion.  Maybe the best insurance, though, would be to leave the pots as is, and use a quad op-amp, like a TL074, and use the two additional op-amps as high input-impedance non-inverting unity-gain buffers, just ahead of the mixer pot. 

What does such a stage look like?

This is essentially the circuit you see on the right side of page 15 here: http://www.ortodoxism.ro/datasheets/texasinstruments/tl074.pdf or at the top of the screen here: http://europa.spaceports.com/~fishbake/mixer/mixr.jpg
but with the necessary additional components to make it work right using a single-ended power supply.  The .1uf cap prevents any stray DC feeding the buffer, the 470k to Vref provides a 4.5Vdc bias, and the 1uf on the output removes the DC before feeding the mixer pot.  The 1M resistor on the input prevents the input cap from popping when you plug in.

What does this get you?  There will be no direct impact on levels from each of the inputs, however, with less signal loss from high-impedance sources, the two buffered inputs will appear to be a bit louder and brighter.  Not enough that substantial level-pot setting changes are required.  The two unbuffered inputs will be perfectly fine for lower-impedance sources, like CD/tape outputs, buffered effects pedals, etc.  The two bufefred inputs will now be able to directly accept things like guitar or bass without any tone sucking.  Of course the downside is that the existing PCB layout won't accommodate these changes.  On the other hand, this is not a particularly complicated circuit, so perfing or vero-boarding it should not be too painful or time-consuming.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: bluesdevil on February 27, 2007, 07:24:48 PM
Wow, thanks a lot for that Mark! I'm just now getting into learning about applying buffer circuits and still trying to wrap my head around the impedance thing, but it's starting to make sense to me after reading threads like this one.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on February 27, 2007, 08:12:31 PM
Excellent, Mark, thats what I'd do if it were my project.  You could even skip coupling caps between the buffers and the channel volume if the volume pot is tied to Vref rather than ground.


Here's something I put together to foot the bill, because the tone sucking problem with the GGG mixer interested me:  here's an opamp buffered 4 channel mixer. This uses a tl074 and tl072, low parts count being the goal, no bells nor whistles.
(http://i4.photobucket.com/albums/y114/processaurus/4channelbufferedmochamixer.gif)

I didn't use the inverting summing stage, but I doubt the channels will interact anyway and if they do, it will only be a little bit.  Anyway, this ain't no Neve.  Its set for unity gain, that could be changed by adding two resistors and a cap to making the final stage a non inverting amplifier, with a set amount of gain.  Higher power supply voltage would give it more headroom, as would rail to rail opamps, if it sounds like it clips on transients.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: fixr1984 on February 03, 2008, 10:34:33 PM
Its been almost a year but what the heck if someone can help.

I'm most likely missing something here but on your schem pin 11 is 9V and pin 4 is ground but
if you look at the Data Sheet (http://pdf1.alldatasheet.com/datasheet-pdf/view/25382/STMICROELECTRONICS/TL074.html) they are reversed.
Misprint? Supposed to be that way?

Thanks
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: R.G. on February 04, 2008, 12:20:35 AM
Trust the datasheet.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: fixr1984 on February 04, 2008, 08:01:06 AM
thank you
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on February 04, 2008, 08:21:57 AM
Good eye! I think the quad opamp in my schematic program is backwards, someone else pointed out the same error in another thing I did.  If you already made a circuit for it, the nice thing about quad opamps is you can just flip the chip 180 degrees and everything is the same except the + and - supply pins are swapped.

I fixed it in case anyone wants to make it.  Thinking about it now, the last opamp stage would be better to have a fixed gain of about 10, rather than a unity gain buffer, because then you can get quiet signals up to a normal level, and amount of noise this would add is negligible, and headroom is preserved because you control the signal volume before the gainstage.  With 10x gain the channel volume knobs in the 12 o'clock position would be approximately unity.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: fixr1984 on February 16, 2008, 04:32:05 PM
I just noticed that pin 1 and 2 in the first stage should be connected.
I was going nuts trying to figure out why input 2-3-and 4 worked great but input 1
was dead.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on February 17, 2008, 05:03:16 AM
Wha? it's always been connected :icon_wink:  I drew in the option of kicking the gain up as well, with R12, R13, and C7.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: fixr1984 on February 17, 2008, 09:38:35 AM
 ;D 
If anyone is interested i have a verified pcb layout for this one.
Just doing some finishing touches in paint shop.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: rasco22862 on February 17, 2008, 11:08:42 AM
;D 
If anyone is interested i have a verified pcb layout for this one.
Just doing some finishing touches in paint shop.
Im very interested :)
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: fixr1984 on February 17, 2008, 03:39:02 PM
Here you go. Not the smallest layout but it does work. I printed mine at 3.5'' x 1.75''
For the pot connections they are labeled as p (pot) and L (lug)



(http://i91.photobucket.com/albums/k288/fixr1984/Schematics%20and%20layouts/mixer3.gif)

(http://i91.photobucket.com/albums/k288/fixr1984/Schematics%20and%20layouts/mixer2.gif)
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: fixr1984 on February 18, 2008, 11:19:25 PM
Heres the updated version. unverified but should still work. Per the schematic this version will give a gain of 10x.
The first version will be unity.


(http://i91.photobucket.com/albums/k288/fixr1984/Schematics%20and%20layouts/mixer4.gif)


(http://i91.photobucket.com/albums/k288/fixr1984/Schematics%20and%20layouts/mixer21.gif)
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on February 19, 2008, 02:24:05 AM
cool!

a couple corrections:  R12 should go to the inverting input of IC2B rather than the output.

Also the Vref supply coming off the output of IC2A is not connected to anything, should be going to the bottom of all the pots and one side of R1 through R4 to bias the inputs to 1/2 the supply.

I think a TL074 would work much better than LM324 because it is low noise and also has JFET inputs.  Actually, thinking about it, a modern rail to rail opamp would work best for the dual opamp for the most headroom on the output. 

If 9v isn't enough headroom, the power supply voltage could be as high as the opamps can take.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: fixr1984 on February 19, 2008, 08:08:00 AM
Vref is connected on the pnp, i did that in paint shop cause i had forgotten to do it on the layout.
Do you mead r12 on the layout or the schematic? I have 12 and 13 numbered backwards to the schem.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on February 19, 2008, 06:20:11 PM
Ah, I just looked at the layout.  The 1K, that is connected to C7, its other end should go to pin 6 of IC2, the inverting input, rather than pin 7, the output.  The way it's on there now, it'll work, but will be unity gain.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: fixr1984 on February 19, 2008, 06:27:28 PM
I see what you mean now, I will have to correct that.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: fixr1984 on February 19, 2008, 07:30:06 PM
cool!

a couple corrections:  R12 should go to the inverting input of IC2B rather than the output.

Also the Vref supply coming off the output of IC2A is not connected to anything, should be going to the bottom of all the pots and one side of R1 through R4 to bias the inputs to 1/2 the supply.

I think a TL074 would work much better than LM324 because it is low noise and also has JFET inputs.  Actually, thinking about it, a modern rail to rail opamp would work best for the dual opamp for the most headroom on the output. 

If 9v isn't enough headroom, the power supply voltage could be as high as the opamps can take.


I updated the layout and transfer. Had to have R13 stand on end to fit.
I put an LM324 as a choice simply cause RS has them and if you need one in a pinch it will still work.
I tried both and didnt really notice much of a difference. I would still say the the TLO74 is first choice.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Wounded Paw on February 19, 2008, 10:03:20 PM
So would this be a good solution to mix guitars with keyboards?
I built a ggg mini-mixer with an LPB on the guitar channel to boost it a little and buffer it but ended up with distortion.  So I lowered the amount of boost and padded the lpb output but now it sucks high end pretty badly.

I'm gonna try this design with buffers on all channels but is there any easy way to bring the keyboard levels down a bit so that they're closer in level to the guitar signal?  I mean so that with all pots at midway the guitar level would be close to the keyboards levels.  with lowest part count and pcb size?
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: fixr1984 on February 20, 2008, 12:07:58 AM
So would this be a good solution to mix guitars with keyboards?
I built a ggg mini-mixer with an LPB on the guitar channel to boost it a little and buffer it but ended up with distortion.  So I lowered the amount of boost and padded the lpb output but now it sucks high end pretty badly.

I'm gonna try this design with buffers on all channels but is there any easy way to bring the keyboard levels down a bit so that they're closer in level to the guitar signal?  I mean so that with all pots at midway the guitar level would be close to the keyboards levels.  with lowest part count and pcb size?

Having a buffered input is supposed to solve the tone suck problems associated with the GGG mixer.  I have yet to make
Version 2 but I would guess with increassing gain by 10x you should have some pretty good flexibility in pot adjustment.
I built my version 1 into a 4" x 6" RS box.

(http://i91.photobucket.com/albums/k288/fixr1984/Pedals/DSCF1007.jpg?t=1203483938)

(http://i91.photobucket.com/albums/k288/fixr1984/Pedals/DSCF1005.jpg?t=1203483985)
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Ben N on February 20, 2008, 01:25:56 AM
With an impedance matching transformer and xlr input, and jacks for post-buffer pre-mixer line-outs, that could make a nice Tunstallish mixer for a looper input.

http://www.musicplayer.com/article/kt-tunstall/Jun-06/21172 (http://www.musicplayer.com/article/kt-tunstall/Jun-06/21172)
http://www.youtube.com/watch?v=Z-RM3IAZ9_s (http://www.youtube.com/watch?v=Z-RM3IAZ9_s)
(http://www.bloggingmuses.com/archives/weebastard.jpg)
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: rasco22862 on February 20, 2008, 10:06:11 AM
What about 4 VU Meters ? :icon_biggrin:
Like these:
http://www.circuit-projects.com/10-led-vu-meter-project-by-lm3915-and-lm324.html
http://sound.westhost.com/project60.htm
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: rasco22862 on February 21, 2008, 07:30:54 PM
Heres the updated version. unverified but should still work. Per the schematic this version will give a gain of 10x.
The first version will be unity.


(http://i91.photobucket.com/albums/k288/fixr1984/Schematics%20and%20layouts/mixer4.gif)


(http://i91.photobucket.com/albums/k288/fixr1984/Schematics%20and%20layouts/mixer21.gif)
So , is this layout verified?
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: fixr1984 on February 21, 2008, 09:28:14 PM
The only change I made was adding R12, R13 and C7. That will give you gain of 10x.
For some reason if there is a problem with those and I dont believe there is just omit those parts
and jumper R13 and it will be unity gain.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Wounded Paw on February 22, 2008, 11:56:52 AM
(http://i4.photobucket.com/albums/y114/processaurus/4channelbufferedmochamixer.gif)

So what difference would it make to replace one of the volume pots with say a 100kA or even 250kA instead of 10kA?
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on February 24, 2008, 06:09:05 AM


So what difference would it make to replace one of the volume pots with say a 100kA or even 250kA instead of 10kA?

It might not make a big deal, you could use higher pots and scale the summing resistors (R5 to R8) up accordingly.  It is nice to keep them lower because lower impedance = lower noise, but its also nice not to have to buy parts, if you have some 100K's laying around.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Wounded Paw on March 04, 2008, 02:17:47 PM
Okay so  I built the 4 Channel Buffered Mixer for a band that was mixing keyboards and guitars together after the GGG mini-mixer didn't work out for them.  I originally built it without the boost on the summing op-amp ie pins 6 and 7 jumpered together.  It worked fine, no distortion, no tone sucking but the output was significantly lower.  So I added in the R12, R13 and C7 section to boost the output but now the guitar signal is noticeably distorted.  I've rechecked my work and don't see anything wrong.  What might be going on here?
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on March 04, 2008, 06:38:36 PM
hmmm, does it get less distorted when you turn its volume down?  That would tell if the distortion is happening in the last stage or not.  If it is, you could make R13 larger, 2K would lower the gain to 5x.

Might be good to double check the opamp's appropriate supply pins are indeed getting 9v, are grounded, all the non inverting inputs are getting the 4.5v bias they deserve.
no tone sucking but the output was significantly lower.

I see a problem with the design without gain compensation, summing the channels to the non-inverting input means the loudest one signal can get with all the other channels turned down is about 1/4 volume!  Imagine that the 100K summing resistors are actually a voltage divider, if one is tied to the signal, and the other 3 are tied to Vref (which to an AC signal will behave the same as ground), that means the three 100Ks in parallel from the non inverting input to Vref look like a 33K resistor. 

Hadn't seen it that way.  The traditional inverting summing stage would be more appropriate.  Or the gain compensation will work.  Mr. Wounded Paw, if you can post your voltages I'd have a better idea what's up with your mixer, I can't see anything in the design that would clip.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: kriista on December 06, 2009, 06:05:24 PM
Sorry to bump such an old thread, but I built a mini mixer and wasn't happy with the results.

Is this layout/schematic here verified as working as per the corrections mentioned in the thread?

I'd like to build a 4-1 buffered mixer (using instrument level inputs) and I'm not really tech savy enough to mod/troubleshoot something that isn't working right from jump.

(http://i91.photobucket.com/albums/k288/fixr1984/Schematics%20and%20layouts/mixer4.gif)


(http://i91.photobucket.com/albums/k288/fixr1984/Schematics%20and%20layouts/mixer21.gif)
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on December 06, 2009, 07:31:33 PM
Hi, I was looking this over, trying to see what the problem was, the schematic will work but there is an error in the V2 layout, the buffered 4.5v supply (Vref) that comes out of pin 1 of IC2 didn't get connected to anything it was supposed to.  It should go to all points on the schematic marked Vref.

Also a FET input opamp (TL074 the most common) should be used for both IC1, an lm 324 would be inferior (though it might sound OK).  It's the chevy nova of opamps.  And a schottkey diode is better than a 1n914 for D1, but that's a little thing.

With R12, R13, and C7 in, the gain is only about 2.5x, not 10x, so I would up R13 to 47K, that will give gain up to ~12x.

It is a simple circuit, you could build it on perfboard, and if you had problems we could help solve them.

Title: Re: GGG Mini Mixer - Sucking Tone
Post by: kriista on December 06, 2009, 07:49:10 PM
Does 2.5x gain mean 2.5 times louder than unity gain?
Is having 12x better sound wise? I'm only going to use this as a summing device, not to boost anything beyond it's regular output levels).

Not sure I follow on the Vref thing. So there's basically a trace missing from pin1 of IC2 going a bunch of places?

Lastly, is there some automated way to turn a schematic (specifically that one) into a perfboard layout?
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on December 06, 2009, 09:28:23 PM
Quote
Does 2.5x gain mean 2.5 times louder than unity gain?

Yes.

Quote
Is having 12x better sound wise? I'm only going to use this as a summing device, not to boost anything beyond it's regular output levels).

Soundwise they would sound the same, the gain is just nice insurance, in case something is real quiet, like some piezo equipped instruments or mic or something.  You could make R13 4.7K and have just a little more than unity gain if you don't want extra gain available.

Quote
Not sure I follow on the Vref thing. So there's basically a trace missing from pin1 of IC2 going a bunch of places?

the 4.5v Vref biases all the opamps to half the supply voltage, so the negative cycle on the audio doesn't get cut off by the amplifiers not being able to produce a negative (lower than 0v) voltage.  Yes, it is supposed to go a bunch of places, everywhere on the schematic that has VREF in a diamond shaped border tied to it.
Quote
Lastly, is there some automated way to turn a schematic (specifically that one) into a perfboard layout?

No, but luckily elbow grease is the original automation.

I don't bother with a layout, just start by making the board the size it needs to be, drilling holes in it so it can be mounted later, then stick the ICs on  in a roomy location with plenty of pads around them for the discrete components, then start from the inputs and work through the circuit, one connection at a time.  You're making a little copper road system, it doesn't matter the shape of the road, as long as it ends up in the right place and makes the right stops.  With a roomy board, there's a million ways to lay it out right.  Some are more clever than others, but the only thing that matters is if it works.

One could keep track of connections made on the schematic with a highlighter, if inclined.  I just kind of triple check everything as I go and continually review the little area I'm doing, checking the connections are right, polarity of components is right, no unintended shorts, that kind of thing.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on December 06, 2009, 09:39:04 PM
It can help, with perfboarding, to run a bare wire for a ground bus around some edges of the board, to always have a handy, nearby place to ground stuff.  Same goes for power and vref, if there's a bunch of connections on the schematic, otherwise just fanning them out with wires from the source is fine.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on December 07, 2009, 12:20:07 AM
Oh, I just read back a little, the Vref is connected on the P'n'P, just not on the layout, so fixr1984's layout should work fine.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: kriista on December 07, 2009, 07:56:18 AM
Is the PnP the thing under the layout? So V2 of that thing should work fine?

That makes sense with the 12x gain thing. I would probably run some piezo stuff every now and again, so I'll probably do that.


Is it correct to assume that PnP is 'press and peel' etching?
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on December 08, 2009, 08:12:02 AM
Yes, that funny blue stuff for etching pcbs.  I've actually never etched a pcb before, but have some kicking around...  Sure, go ahead and try it out, even if there's a little error with the pcb it'd be easy to kludge it right.

Title: Re: GGG Mini Mixer - Sucking Tone
Post by: welcomb on December 08, 2009, 11:16:23 AM
I see a problem with the design without gain compensation, summing the channels to the non-inverting input means the loudest one signal can get with all the other channels turned down is about 1/4 volume!  Imagine that the 100K summing resistors are actually a voltage divider, if one is tied to the signal, and the other 3 are tied to Vref (which to an AC signal will behave the same as ground), that means the three 100Ks in parallel from the non inverting input to Vref look like a 33K resistor. 

Hadn't seen it that way.  The traditional inverting summing stage would be more appropriate.  Or the gain compensation will work.  Mr. Wounded Paw, if you can post your voltages I'd have a better idea what's up with your mixer, I can't see anything in the design that would clip.

Would it not then be better to use an inverting summing stage and the other half of the op-amp to invert it back to phase? In which case we don't use an opamp to hold Vref
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: kriista on December 08, 2009, 08:02:16 PM
Hmm. Sounds over my head, but very sensible.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on December 08, 2009, 11:35:34 PM
Sure, you could definitely do that.  And it would be the textbook way to make a simple mixer.  Lots of valid options out on how to design a simple utility summing amplifier, and this would be a good place to share them.

This will work just like it's supposed to, though, to make a simple mix with a minimum of parts.  The amount of self noise introduced from the extra 4x gain this circuit uses vs the more regular inverting mixer will be negligible next to the noise of the instruments...

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.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: kriista on December 09, 2009, 06:51:46 PM
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?
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: welcomb on December 10, 2009, 10:48:52 AM
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.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on December 10, 2009, 08:34:53 PM
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 (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:

(http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=9426&g2_serialNumber=7)

EDIT: Photo link was weird
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: welcomb on December 11, 2009, 12:06:52 PM
Just a question with using a non-inverting summing opamp, does lowering one channel then affect the other channel? For example with all channels turned to max, and turning down one channel affect the other three?
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on December 11, 2009, 11:27:23 PM
In theory, yes, because there is no virtual ground at the summing point (the main benefit of the inverting summing stage), and the signals would go back through the other channels summing resistors and interact with where the pot is positioned, as far as sinking current into either the low impedance Vref source or the low impedance channel buffer opamp output, or having some added resistance from the pot in the way because the wiper is somewhere in the middle.

In practice, no, short answer is because the summing resistors are big and the other channels pots moving around are a small change in load.

Long answer is because the summing resistors are much larger than the pots, worst case, a pot from Channel B is right in the middle of its resistance, and the Channel A signal sees it through the 100K resistor from its channel to the summing point, then another 100K resistor back to Channel B's pot, as a 5K resistor to the opamp output (Lo-Z, with ground-like current sinkability) and a 5K resistor to Vref (Lo-z, ground-like), so those combined, behave at most like a 2.5K load in the middle of the pots resistance and less as you move to either side, which with the 100K summing resistor from  channel A to the summing point, then through the 100K summing resistor to Channel B's pot wiper, is a small variation in attenuating load on Channel A's signal at the summing point, 102.5K vs 100K. Doing the voltage divider math, at most 1.395% variation is possible in the volume of one channel if you wiggle the all the other channel's volume pots around.  Since to the human ear something has to be twice as loud to be perceived as a noticeable difference, these tiny variations are not an issue.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: kriista on December 12, 2009, 09:07:48 AM
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.

I was referring to the mention that the other channels would change when adjusting one channel, making it near useless live (the reason it's being built). That and a couple of mentions previous to that that it isn't working at all.

So the last schematic you posted and the last PnP layout are working/the same?
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on December 12, 2009, 06:56:35 PM

So the last schematic you posted and the last PnP layout are working/the same?


Yours working will confirm that the layout and schematic work.  Go for it!  The offer to help you debug it still stands, if there's a problem.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: kriista on December 12, 2009, 09:33:20 PM
As in, are those two without mistakes and would presumably work?


So the last schematic you posted and the last PnP layout are working/the same?


Yours working will confirm that the layout and schematic work.  Go for it!  The offer to help you debug it still stands, if there's a problem.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: fixr1984 on December 14, 2009, 10:08:34 PM
PNP has been built/verified twice. I have had no problems, I might go back and change
out R13 to get a little more gain but other wise its fine.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: kriista on December 16, 2009, 01:03:41 PM
Perfect.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: VPIF on December 20, 2009, 03:29:37 PM
I have tried to research this a bit lately, but my electronics-knowledge is still to basic to figure it out :-\

I have a Fender Hot Rod DeLuxe:

Pre-amp output impedance:  1,5Kohm max
Power-amp input sensitivity:  200mV for clipping
Power-amp input impedance:  54Kohm

Can I buffer the pre-amp output signal with a simple buffer/line driver (suggestions are welcome), split the buffered signal in four, run it (one line per effect) through a BOSS chorus, Line6 RotoMachine and TC electronic D-two, + direct signal and use the unity gain version of this mixer to blend the signals, thus using this for a parallel effects loop?

Thanks!
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on December 21, 2009, 01:11:27 AM
I have tried to research this a bit lately, but my electronics-knowledge is still to basic to figure it out :-\

I have a Fender Hot Rod DeLuxe:

Pre-amp output impedance:  1,5Kohm max
Power-amp input sensitivity:  200mV for clipping
Power-amp input impedance:  54Kohm

Can I buffer the pre-amp output signal with a simple buffer/line driver (suggestions are welcome), split the buffered signal in four, run it (one line per effect) through a BOSS chorus, Line6 RotoMachine and TC electronic D-two, + direct signal and use the unity gain version of this mixer to blend the signals, thus using this for a parallel effects loop?

Thanks!

Your preamp already has a low output impedance, so buffering it initially before you split it isn't necessary.  You could  probably just make a passive 4 way splitter too.

It will work how you want it to.  The simpler GGG mixer will work too, unless your guitar is getting connected directly to the mixer.

A phase switch is often desirable on a summing amplifier used for parallel processing, you could add one after the buffer opamps with a standard opamp unity gain inverting amplifier:

(http://www.swarthmore.edu/NatSci/echeeve1/Ref/SingleSupply/Invert.gif) with R1 and Rf being equal (10K would be fine resistor values), then you'd put a phase switch in, using a SPDT switch with the common wired to the top of the channel volume pot, one lug to the buffer opamp's output, and the other lug to the new inverting amplifiers output, to choose between inverted or non inverted copies of the signal.

The green ground on the diagram would be connected to the 4.5v Vref supply.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: VPIF on December 21, 2009, 11:24:29 AM
OK. Thanks for the great explanation Processaurus.

Maybe this will be a suitable small and simple circuit for me to try to make my first layout...

Suggestions as to type of inverting amplifier? I have searched the forum, but didn't find very much information (parts numbers etc.) I am not really familiar with the concept of inverting/non-inverting op-amps, but I will try to dig into it when I got some time on my hands!
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: kriista on December 30, 2009, 08:16:38 PM
Ok, finally got the board etched and am going to start building this badboy.

A couple of questions.

First, with the PnP layout, the components, do they go in on the etched side. As in, how they look on the layout version. So I would put the ICs right on top of the cover that's showing and not on the blank side of the board.

Second, would using 'hi-fi' components matter in this kind of circuit? Like using Burr Brown opamps, and nice caps for C1-4 give a better signal/noise/sound?
This little mixer is going to be an 'always on' part of my setup, and the better it sounds, the better it will be.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on December 30, 2009, 09:18:46 PM
You put components on the blank side of the board.  If you etched the board mirrored from how it's supposed to be, in this design, that's ok, because you'd just rotate the opamps 180 degrees to have the power pins be right, all the other pins are identical if you rotate it, pretty handy.

I can't guess whether you etched it mirrored or not, but you want +9v going to pin 8 of the tl072 dual opamp and pin 4 of the tl074 quad opamp, and ground to pin 4 of the dual and pin 11 of the quad, as is on the schematic.

If a board is done, but backwards, I've seen people solder things on the copper side of the board, a little awkward, but it works.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: kriista on December 30, 2009, 11:07:41 PM
It's etched exactly as seen in the image. So I take it that's 'backwards' ?
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on December 31, 2009, 07:22:32 AM
The layout with the parts is a top, component side view, so the bottom traces as shown are seen from an xray view, through the top side of the board.  So you want to print the PNP as shown on screen, so that transferring it mirrors it.  Kind of like if you were making a text iron on tshirt that you wanted to be legible from the inside of the shirt, holding it up to the sun, in that case you wouldn't mirror the print.

So if you don't want to waste the board, you can turn the opamps 180 degrees, and keep everything straight with a mirrored component layout, or solder them from the top, using insulated wire for the jumpers, and maybe soldering offboard wires onto the traces rather than through holes.  Kludging is an important part of electronics.

Ha, that would be strange to make iron ons on the inside of shirts, that are legible from the outside; a perfect venue for subconscious messages.  From SATAN!
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: kriista on December 31, 2009, 08:13:08 AM
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?
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on December 31, 2009, 10:07:28 PM
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).
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: kriista on June 10, 2011, 07:35:40 PM
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_files/shapeimage_3.png)
http://rodrigoconstanzo.com/The_Party_Bus.html
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: Processaurus on June 15, 2011, 06:56:17 AM
Cool project!

Quote
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.

Quote
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.
Quote
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.
Quote
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.

Quote
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.
Quote
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.
Quote
100uF at power input

That is the best suggestion your other forum made.  Put it downstream of the diode.
Quote
.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.
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: garfo on January 26, 2014, 05:46:56 PM
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_files/shapeimage_3.png)
http://rodrigoconstanzo.com/The_Party_Bus.html
Title: Re: GGG Mini Mixer - Sucking Tone
Post by: garfo on April 04, 2014, 04:35:02 PM
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 (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:

(http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=9426&g2_serialNumber=7)

EDIT: Photo link was weird