Universal Buffer - Single Input w/ Volume Control

I am very interested in purchasing the universal buffer, I have a project in mind that reuses the chassis (an Audio-GD Fun) and currently only have in the plans a single source, a RaspberryPi running Volumio with a HifiBerry DAC. The chassis has an ALPS 20KAx2 (20k ohms) POT that would be convenient to reuse.

Being that I currently only plan on a single input, I do not know how the input selector implements volume control - is it just simply running the signal through the POT? I.e., to use the universal buffer with a single input, simply running the signal through the POT meets the same signal quality of using the input selector with volume control?

Thank you!
 
I would have the same question! I was wondering if I could just wire the volume pot in front of the universal buffer.

This would mean, power supply, universal buffer and volume control for a balanced "preamp".
 
If you only need a single input then don't bother with the Input Selector. Personally I would just solder microphone cable directly to the volume pot.

First identify the three terminals on the pot: Turn the volume control to minimum and measure the resistance from the centre pin to each of the outside pins on the pot. The pin that shows a resistance close to 0 Ω is the ground pin. Then the other outside pin is the input signal and the centre pin (the wiper) is the output signal.

Then just put the volume control in front of the Universal Buffer and power the Buffer with a Preamp Power Supply and you're set.

If you don't like soldering directly to the volume pot, I suggest getting one of the many Alps RK271 PCBs from Bezos' Bookstore, ePay, etc. You're welcome to use the one I offer as well, but it doesn't have screw terminals so it can be a bit harder to work with when you're connecting bare wire.

Tom
 
Thank you Tom for the quick response, I am ok with soldering and that work but wanted to make sure I was not missing something based on product descriptions.

The chassis will be able to be expanded, that will be on a future adaptation.
 
Is there some agreement on which POT to choose ? TKD 4CP-2511 or 4CP-601, I see also recently EIZZ pots have some good reviews or should I stick with an ALPS RK27 and be happy.
 
I'd go with a 5-10 kΩ pot with logarithmic/audio taper. I hear good things about the TKD pots. I've never heard of EIZZ, but that doesn't necessarily say anything. I don't have a strong opinion either way.

Tom
 
Everything has been delivered for my universal buffer pre-amp! I'm a bit confused how to wire the pot.
I had planned to use some shielded mogami cable from the XLR input to the pot, but looking at this PCB for the pot, it requires 4 ground inputs- outputs. I only have 1 ground/shield per mogami cable.

Do I then use mogami cable for the signal and leave the shield in that cable unconnected and then just add some extra ground cable to the 4 GND connections to the chasis?
Another option is to use the mogami microphone cable and then add 2 additional grounds...


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I would wire one of those input ground pins to Pin 1 of the XLR input. Wire Pin 2 to IN A and Pin 3 to IN B.

Then connect the output ground to Pin 1 of the input of the Universal Buffer. OUT A goes to Pin 2 of the Buffer input and OUT B goes to Pin 3.

Repeat for the second channel.

Tom
 
Ah ok, just skip some of the GND input/output, seems to make sense as you are right the Universal buffer only needs a single GND input.
Thx!
 
I think I would rotate the power supply 90 degrees clockwise and keep the AC input shorter and along side of chassis, in line with a straight shot to the IEC. Maybe push the Buffer a bit more to the right.
 
I would rotate the power supply 180 º and scoot it all the way up to the AC input to keep the wiring short.

Scoot the Buffer down to the pot so the connections to its input become as short as possible. They're higher impedance than the output so easier to inject interference into. I'd run the input connections along the right chassis wall to the pot.

The outputs of the Buffer are not very sensitive to interference, so they can be longer. I'd still run them along the right side of the chassis, though.

Don't forget to connect the IEC protective earth terminal to the chassis.

Tom
 
Hmm, this could be interesting as the hole in the faceplate is 6mm and I need it 7mm for the potthreads :) so this requires some... drilling.
How is the pot mounted - Diy mount of some description?
 
Pieces have arrived but need to go on vacation!
Two layout options...
I pluggged a dac straight into the Mod-86 and it sounds awesome so lets see whay my meddling/solderong does to the sound..hopefully nothing.
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Scoot the Buffer down to the pot so the connections to its input become as short as possible. They're higher impedance than the output so easier to inject interference into. I'd run the input connections along the right chassis wall to the pot.

The outputs of the Buffer are not very sensitive to interference, so they can be longer. I'd still run them along the right side of the chassis, though.

Hello Tom and All,

Tell us about higher impedance connections being more sensitive to interference, how / why?

For application of knowledge.

Thanks Bob
 
With inductive coupling the EMI source will introduce an unwanted current into a circuit. If the circuit has high impedance, the induced voltage, V = I*Z becomes higher.

You can make a similar argument around capacitive coupling.

Tom
 
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