Mods for RG Keen Mini Mixer?

[Jasonmatthew911]

Hi guys, it's been awhile since I've posted here...I'm about to build the simple RG Keen Mini Mixer, but I'm wondering if there are any modifications to improve this design?...Also, can I use 100K Audio pots instead of the 10K and have about the same results?...I assume it shouldn't really matter...

I basically want to use this to connect my normal guitar, an Octave guitar, and a mini Yamaha Reface DX synth to my guitar pedals and then my amp...Are there any recommendations to make this mini mixer better/cleaner for this use, or is it pretty good just like it was designed?...I'm just worried if this mixer will affect my guitar tone much...

Lastly, will running this mixer on 18V improve the clean headroom or won't really make a difference?...Hopefully someone here has tinkered with this and can give me some good Mod tips...I'd really appreciate it, thanks in advance for any help with this...

[R.G.]

To start with, it's not good for an unbuffered guitar input. The 10K pot is too low, and will load the treble out of the guitar signal.

This mixer is a very - very - low-feature, low budget mixer. There are a dramatic number of improvements that could be made, but it's a long list.

First, for guitar use, buffer the guitar input with some kind of buffer that offers at least 1M input impedance.

Simply changing the 10K pots for 100K at the input won't do it. 100K audio pots won't hurt it much.

"Better" is fraught with subjectivism. "Cleaner" is easier to do. As long as your input signals are under about 3V peak, this is pretty darn clean. But your synth is likely to put out several volts of signal, so you may gets lots of distortion or find you're always running it at nearly 0 for that input. Padding down the synth by 10:1 would help.

If you're going to the trouble to run from 18V, make it +/-9V instead and bias to 0V instead of half of the power supply. This lets you simplify many things about it.

There's a world of other stuff.

But no, within its signal and power supply limits, there will be no apparent change to your guitar tone IF you buffer the guitar so it's not loaded by the 10K input pots.

[Jasonmatthew911]

Thanks for the quick reply RG...So would the simple buffer below work for my guitar inputs?...Also, let me know the simplest way I can Pad down the Synth input, with minimum parts required (Can I do this with a resistor or 2, where in the circuit?)...I'm not looking for audiophile quality, just for all instruments to sound decent and pretty clean through my guitar amp with minimum parts required...I like to build small simple circuits to save on time and money, that's why I chose this Mini mixer design...What kind of instruments was this mini mixer originally designed for?...Since I have to add a buffer for a guitar and Pad down for a synth, it doesn't seem all that practical anymore...

Also, would 500K or 1M Pots make any difference for the Guitar Volume inputs?...About the 18V, for me it seems easier to use parts rated above 25V and run the mini mixer on an 18V power supply...I don't have a very good understanding to do +/-9V, that's more complicated for me...Can I still add all parts rated above 18V and run on 18V supply even with the extra buffer and still have more headroom?

[Jasonmatthew911]

I basically want to make a mini mixer like the Red Panda Bit Mixer below...I see it has 1M input impedance so it's mostly for guitar use, but it says it can be used for line level instruments as well and runs on 9V - 18V...This is exactly what I'd like to make...So maybe let me know if the simple buffer I posted earlier could work for all my inputs and then into the Mini Mixer?...Also, with Buffer would it help to higher the Pot values as well, or won't make a difference?...Would it not be good for my synth to go through the buffer or doesn't matter?....Will I really need to Pad the synth input with or without the buffer?...Sorry for all the questions, like always I'm just trying to keep it simple and not too expensive, thanks.

[R.G.]

Thanks for the quick reply RG...So would the simple buffer below work for my guitar inputs?

It would, although I would make a separate bias string of 10k+10K between +V and ground and run a 1M from the 10K junction to the + input, for noise reasons. Bypass the bias source with a 22uF or larger cap to ground. Audio would go into the + input through a 0.1uF cap or larger.

Also, let me know the simplest way I can Pad down the Synth input, with minimum parts required (Can I do this with a resistor or 2, where in the circuit?)

I would use a 10K + 1K resistor divider before one of the input pots. Synths usually have low output impedance, and so this should work with a minimum of fuss. You might get by with a 100K in series before one of the 10K pots, using the pot itself as part of a divider.

I'm not looking for audiophile quality, just for all instruments to sound decent and pretty clean through my guitar amp with minimum parts required.

You'll get that.

What kind of instruments was this mini mixer originally designed for?

The outputs of effects pedals.

Since I have to add a buffer for a guitar and Pad down for a synth, it doesn't seem all that practical anymore.

?? Why? Adding one opamp and one or two resistors for a pad doesn't seem all that tough.

Also, would 500K or 1M Pots make any difference for the Guitar Volume inputs?

At some point raising the value of the input pots gives you other issues of noise and hum pickup. You also have the issue that the series resistance into the inverting input of the opamp is 100K for each input, and even with a higher input pot, you'd start loading the guitar with higher pot levels. Don't get all twitchy at the thought of a buffer. Frankly, pedal designs would be a lot easier and guitars in general less noisy if there was a buffer inside all guitars.

About the 18V, for me it seems easier to use parts rated above 25V and run the mini mixer on an 18V power supply...I don't have a very good understanding to do +/-9V, that's more complicated for me...Can I still add all parts rated above 18V and run on 18V supply even with the extra buffer and still have more headroom?

That's fine. Go with what you know.

I basically want to make a mini mixer like the Red Panda Bit Mixer below...I see it has 1M input impedance so it's mostly for guitar use, but it says it can be used for line level instruments as well and runs on 9V - 18V...This is exactly what I'd like to make...So maybe let me know if the simple buffer I posted earlier could work for all my inputs and then into the Mini Mixer?.

A buffer is only needed on the one input that the guitar drives directly in the setup you described.

Also, with Buffer would it help to higher the Pot values as well, or won't make a difference?

With a buffer, the pot values don't need changed; that's what the buffer is for, to enable your guitar to drive the pots without any tone loss. Only the guitar input needs a buffer.

Would it not be good for my synth to go through the buffer or doesn't matter?

I am not familiar with your synth, but synths I have worked with in the past in general have larger output signals than guitars, and lower source impedance, which means they can drive lower input resistances. The synth input probably does not need a buffer, probably/possibly needs a resistor or two to lower its level, and will not be helped by going through a buffer. Don't use a  buffer on the synth.

Will I really need to Pad the synth input with or without the buffer?

It's somewhere between likely and possible. Since padding only needs one or two resistors, build it without the pad, and if you find you need the synth input pot turned down to barely on all the time, go stick a resistor pad in. You do not need a buffer for the synth, only for the input that has the guitar directly into it.

I know you're trying to keep it simple and inexpensive. Let me bring up a touchy subject: the cost of the enclosure, knobs, jacks, switches, power, pots and so on. If the electronics circuits were FREE, it would hardly change the cost of building a widget like this. The majority of the money will be spent on stuff that's not the electronics circuits themselves. Keep that in mind. It's an ugly surprise to many people.

[Jasonmatthew911]

It would, although I would make a separate bias string of 10k+10K between +V and ground and run a 1M from the 10K junction to the + input, for noise reasons. Bypass the bias source with a 22uF or larger cap to ground. Audio would go into the + input through a 0.1uF cap or larger.

Thanks a lot RG, is there any chance you can draw these Mods out on the Buffer schematic I sent...I have an idea, but I'm just not a 100% on where I need to put all these parts in the schematic...It would be great if you could illustrate these mods for me...I'd really appreciate it.




It's somewhere between likely and possible. Since padding only needs one or two resistors, build it without the pad, and if you find you need the synth input pot turned down to barely on all the time, go stick a resistor pad in. You do not need a buffer for the synth, only for the input that has the guitar directly into it.

I know you're trying to keep it simple and inexpensive. Let me bring up a touchy subject: the cost of the enclosure, knobs, jacks, switches, power, pots and so on. If the electronics circuits were FREE, it would hardly change the cost of building a widget like this. The majority of the money will be spent on stuff that's not the electronics circuits themselves. Keep that in mind. It's an ugly surprise to many people.

"You're right, but the thing is that I already have the enclosure, jacks, pots, knobs, and most the electronic parts...Actually I have a bunch of 100K Audio Pots and higher, that's why I was asking if I could just use the 100K Pots instead of the 10K...So, I'm gonna make the Buffer, hopefully you can illustrate your mods for me...Also, just want to make sure the 100K Volume Pots will be fine with the Buffer right?"

[Jasonmatthew911]

Actually don't worry about the buffer...I think if I'm gonna do a buffer I'd rather use the Klon buffer below...Klon Buffer shouldn't have any noise issue, right?