I am planning to connect my Ares Ii DAC via differential (balanced) connection to my home-build power amplifier (which is based on a TPA-3251 board from 3e Audio).
As the differential output of the DAC is rather high @ 2.4KOhm, I was thinking to use a 100KOhm 4-gang potentiometer (Alps RK27). My reasoning: a 100K pot delivers worst case a minimum of 25% of it's rated resistance (at 12 o'clock setting a 100K pot has a resistance of 2x 50K in parallel = 25K. And 25K is a little over 10 times the output impedance DAC, so the DAC's output doesn't experience voltage drops due to excessive loading of its output. So far so good, I guess.
Because the input impedance of the power amplifier (balanced input) is 20KOhm, I thought it would be wise to place a buffer between the 100K pot and the 20K input of the power amplifier.
However, the buffers for balanced signals I came across had rather low input impedances themselves: the one with highest input impedance was 48K, which is about half of the 100K potentiometer that is on the buffer's input side.
Finally my question: is this a problem and does this defy the sole purpose of the buffer, or does this combination still make sense?
Should I take a 50K pot instead, or 20K even? What would be the best trade-off?
As the differential output of the DAC is rather high @ 2.4KOhm, I was thinking to use a 100KOhm 4-gang potentiometer (Alps RK27). My reasoning: a 100K pot delivers worst case a minimum of 25% of it's rated resistance (at 12 o'clock setting a 100K pot has a resistance of 2x 50K in parallel = 25K. And 25K is a little over 10 times the output impedance DAC, so the DAC's output doesn't experience voltage drops due to excessive loading of its output. So far so good, I guess.
Because the input impedance of the power amplifier (balanced input) is 20KOhm, I thought it would be wise to place a buffer between the 100K pot and the 20K input of the power amplifier.
However, the buffers for balanced signals I came across had rather low input impedances themselves: the one with highest input impedance was 48K, which is about half of the 100K potentiometer that is on the buffer's input side.
Finally my question: is this a problem and does this defy the sole purpose of the buffer, or does this combination still make sense?
Should I take a 50K pot instead, or 20K even? What would be the best trade-off?
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The signal flows from the planned potentiometer via the planned buffer directly into this board (no extra caps or resistors on the input): http://www.3e-audio.com/wp-content/uploads/2018/07/DS_EAUMT-0140-2-A_Rev1.0.pdf
Basically via an op-amp ic towards the TPA-3251.
Basically via an op-amp ic towards the TPA-3251.
Thanks Rayma!
Currently I use a 10K pot (relays actually, but also rated 10K) after the DAC and that sounds good. However, I am now using the single ended output on the DAC, which has an output impedance of 1.2K Ohm.
Currently I use a 10K pot (relays actually, but also rated 10K) after the DAC and that sounds good. However, I am now using the single ended output on the DAC, which has an output impedance of 1.2K Ohm.
Ok, then R source = 1.2k
Does the DAC have an output coupling capacitor, some have 10uF to 100uF caps.
Does the DAC have an output coupling capacitor, some have 10uF to 100uF caps.
I can't find any schematic of the DAC, so I cannot say if it has caps on the output.
There are some caps near the ladder resistors, but not sure if they are output coupling caps or not.
There are some caps near the ladder resistors, but not sure if they are output coupling caps or not.
From what you wrote, I would say you are exactly on-track.
Just go for the highest input impedance 'balancing buffer' you can, and use the Pot. for its intended purpose 🙂
Your self created problem comes from a poorly chosen pot resistance (and that´s an understatement).
Just use a 4 x 10k pot , no buffer needed.
Besides, what kind of "buffer" has 48k input impedance?
IF you needed one, it´s easy to get 1 M input impedance by using humble TL072 or similar.
Just use a 4 x 10k pot , no buffer needed.
Besides, what kind of "buffer" has 48k input impedance?
IF you needed one, it´s easy to get 1 M input impedance by using humble TL072 or similar.
Good to see that you seem to agree with my approach, but JMFahey and Rayma both say that a 2.4K Ohm source into a 10K pot won't (likely) cause issues and that I should try that first. So I am still a bit undecided 😊
@JMFahey, so you don't foresee issues feeding a 2.4K impedance source into a 10K pot?
I couldn't find fully balanced buffer circuits and I don't dare to adapt single ended designs out of fear of introducing noise or making matters worse in other respects.
I did however find the specs of Neurochrome's universal buffer board, designed by @tomchr, which has input impedance of 48K Ohm, see https://neurochrome.com/products/universal-buffer#
This is better than https://www.eltim.eu/data/mediablocks/ELTIM BinBout-128x.pdf which has an input impedance of 9K Ohm for the 0dB gain version (up to 12K Ohm for the -6dB version).
I couldn't find fully balanced buffer circuits and I don't dare to adapt single ended designs out of fear of introducing noise or making matters worse in other respects.
I did however find the specs of Neurochrome's universal buffer board, designed by @tomchr, which has input impedance of 48K Ohm, see https://neurochrome.com/products/universal-buffer#
This is better than https://www.eltim.eu/data/mediablocks/ELTIM BinBout-128x.pdf which has an input impedance of 9K Ohm for the 0dB gain version (up to 12K Ohm for the -6dB version).
The output impedance of an R-2R DAC is linear and resistive, so you can get away without buffering in this instance as the pot is also linear and resistive... Beware of high output impedances in general though - often overloading such a source can cause its distortion to rise above spec. Also any current noise or input current non-linearity in an amplifier can multiply with a high impedance source to create extra voltage noise and voltage distortion not in the amp's specs (which may assume something like a 100 ohm source impedance)...
Here I'd worry 100k is going to be too high and raise the noise floor somewhat - 10k should work fine. Always have RF suppression caps on the input connectors, note, you don't want RF entering the box and rattling around...
Here I'd worry 100k is going to be too high and raise the noise floor somewhat - 10k should work fine. Always have RF suppression caps on the input connectors, note, you don't want RF entering the box and rattling around...
I will soon be in the possession of a 2nd hand Musical Fidelity X10-D, which is a tube buffer.
I'll do some testing in my single ended setup (between DAC and volume control; between volume control and power amp) and see (hear) what it brings.
I'll do some testing in my single ended setup (between DAC and volume control; between volume control and power amp) and see (hear) what it brings.
Its because you added a tube stage and you didn't really need a buffer unless you need more signal going in which I doubt it because going into a load higher in resistance than the minimum stated in the DAC specs, which increases the signal voltage.