FET Mixer/buffer help

[Buffalo Tom]

Hi. I wanna build a simple mixer thats summing two inputs to one output. It should also have HighZ input and LowZ output. This schematic is put together by looking at the cornish buffer, amz fet buffer and different schematics I found on google looking for ”FET Audio Mixer”. So its combination of different ones. Can you guys say anything by looking to it. Does it look all right for what I wanna do? Am I missing any part or do I have to many parts?   Will this circuit get me 1MΩ input impedance and 150Ω output impedance?

Thanks for your time!

[PRR]

> do I have to many parts?

Yes.



> have HighZ input

The inputs are not hi-Z "to" each other. "Only" 200K apart. (True for your plan also.) This is almost always ample. But if you NEED >1Meg as for a piezo, or a hot-wind pickup, this will load-down. The only real fix is two hi-Z buffers, mix resistors, and make-up gain and buffering after.

[Transmogrifox]

The only real fix is two hi-Z buffers, mix resistors, and make-up gain and buffering after.

The way you are doing gives you a guaranteed gain cut of 6 dB which may be acceptable if you're going into an amp that you never max out (just turn the volume knob up higher).  PRR has provided a good simplified solution for that approach and it's a good circuit if you don't need really high input impedance or output impedance <500 ohms.

The method below is one way to combine the buffers and make-up gain into one stage and get near to 150 ohms output impedance.  The below circuit is designed for approximately unity gain on any input.  The 1Meg to ground could be changed to 1Meg pots and then you have mix controls if wanted.  It does give you a phase inversion which usually isn't a problem, but some people have a belief that keeping the output in phase with the input is important for some reason.

The reason for the BJT output is because you can't easily get output impedance from a FET buffer down to less than 300 ohms easily.  The BJT can give about 20 to 30 ohms, so combined with a 120 or 130 ohm resistor you get pretty close to 150 ohms.  I don't know how important 150 ohms output is for what you're doing -- if you're driving line-in on a PA system then you could just take the output directly off the FET drains at the 2.7k resistor.  If you are driving long capacitive lines or something then 150 ohms is probably a good target, but using a FET instead of a BJT for output at total impedance of ~500 ohms would probably be ok in most cases.  I'm not sure the FET adds any special effect on the output stage of my circuit -- just mojo if you really like FETs.

[Buffalo Tom]

> do I have to many parts?

Yes.



> have HighZ input

The inputs are not hi-Z "to" each other. "Only" 200K apart. (True for your plan also.) This is almost always ample. But if you NEED >1Meg as for a piezo, or a hot-wind pickup, this will load-down. The only real fix is two hi-Z buffers, mix resistors, and make-up gain and buffering after.

Thanks for your reply and the "new" schematic. Looks better now!
But how about gain cut of 6 dB? Will this mixer give me a volume drop of 6 dB.   

[Buffalo Tom]

The only real fix is two hi-Z buffers, mix resistors, and make-up gain and buffering after.

The way you are doing gives you a guaranteed gain cut of 6 dB which may be acceptable if you're going into an amp that you never max out (just turn the volume knob up higher).  PRR has provided a good simplified solution for that approach and it's a good circuit if you don't need really high input impedance or output impedance <500 ohms.

The method below is one way to combine the buffers and make-up gain into one stage and get near to 150 ohms output impedance.  The below circuit is designed for approximately unity gain on any input.  The 1Meg to ground could be changed to 1Meg pots and then you have mix controls if wanted.  It does give you a phase inversion which usually isn't a problem, but some people have a belief that keeping the output in phase with the input is important for some reason.

The reason for the BJT output is because you can't easily get output impedance from a FET buffer down to less than 300 ohms easily.  The BJT can give about 20 to 30 ohms, so combined with a 120 or 130 ohm resistor you get pretty close to 150 ohms.  I don't know how important 150 ohms output is for what you're doing -- if you're driving line-in on a PA system then you could just take the output directly off the FET drains at the 2.7k resistor.  If you are driving long capacitive lines or something then 150 ohms is probably a good target, but using a FET instead of a BJT for output at total impedance of ~500 ohms would probably be ok in most cases.  I'm not sure the FET adds any special effect on the output stage of my circuit -- just mojo if you really like FETs.

THANKS! Very interesting.. How about power? Just +9V ? Thanks again

[Transmogrifox]

Yeah, it's designed for 9V in.  Should be good for low battery (7V) up to 12V.  If feeding it with line input levels you probably want a solid 9 to 12 to avoid clipping.  It could be adapted to 20V supply by adding a non-zero bias to the FET gates.  After about 12V you cease to gain much benefit from increased supply input unless you adjust the bias to re-center it.

At 9V supply it's good for 2 Vpp on both input channels without much distortion.  Clipping is distinctly visible at about 4 Vpp on one channel, an driving 2 simultaneously somewhere around 3.25 Vpp.  It has enough headroom at 9V for most non-amplified guitar at typical max of 1.5 Vpp, and probably good for most pedal outputs since it can easily do 3 Vpp on both channel inputs without clipping (6Vpp output).