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Active vs Passive pre-amp and optimal impedance matching. Or how to go about optimal volume control for my set up.

Joined
Apr 10, 2020
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#1
I'm current running a SMSL M200 DAC -> Nobsound NS-05P -> Buckeyeamp NC252MP -> Elac DBR62, all with balanced XLR interconnects. I added the nobsound because going from m200 -> nc252 didn't give me very useable volume control. The M200 only allows 40-0 volume control, it's a bit unclear if it actually represents dB or just it's max output voltage divided into 40 steps. I was only using the range 35-25 before adding the Nobsound, now with Nobsound around 2-3oclock, I use 35-10 on the M200. Overall, I'm happy with how it sounds, I just want to understand if this is ideal or not.

Apparently as long as I'm attenuating at least 20db with the Nobsound, which I probably am, and running short cables to the amp, which I am, it should perform well. At least according to these measurements. I suppose I'd need to actually measure if mine has a 10k, 20k, or 50k pot, since it seems like there is variability https://www.superbestaudiofriends.o...uator-comparison-technical-measurements.7324/

Does this make the effective source output impedance much higher? Hypex specs a 47k input impedance, so my source impedance should be <4.7k, but ideally much lower right? Hypex apparently recommends 50hms. The M200 DAC has an output impedance that I assume is >1k(based on M400 review) into a passive attenuator of 10-50k, will this end up being a poor match for the 252mp?

A few alternative solutions I could try if it might be worth it:
1) I have an E30, which allows 99-0 volume control, I'm not sure if going that deep into digital volume is okay or not. I'd also need RCA -> XLR to connect to amp. I'd miss my M200 LDAC bluetooth.
2) Better DAC for this purpose? I'd be happy to sell my M200 and find another DAC, if the 5.5V RMS out and 1k+ output impedance aren't a good match for my system.
3) Active pre-amps such as this one end up with a 300ohm output impedance, maybe there are other options as well? https://www.ebay.com/itm/Douk-Audio...392460&hash=item3f86ad2afa:g:ddYAAOSwpUtd5N-h
4)In-line XLR pads from M200 right into 252 and back to digital volume?

Seems to be many ways to go about getting the volume to where you want it. I just want to make sure I'm doing it in an optimal way.
 
Joined
May 31, 2019
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#2
I am going to plug into this thread because I plan almost the same setup and also was looking for optimal preamp.
 

dreite

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#3
I like option #4. Easiest, best, cheapest.

Your system just has too much voltage gain. :) But if you like the M200 (notwithstanding the volume control adjustment range) it makes sense to keep using it.

Dave.
 

sergeauckland

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#4
A resistive passive 'pre-amp' will have a maximum output impedance at half volume (-6dB) which will be quite near the top on a normal logarithmic volume control. These used to have 20dB attenuation (i.e. 10% of maximum) at 50% rotation, so 50% of maximum came somewhere quite close to the top. The maximum output impedance then would be 25% of the volume control's resistance, assuming a low output impedance source, as it would be half the volume control in paralllel with half the volume control, so 1/4 of the total.

Unless you have very long cables, or very high capacitance cables, 5-10m of low capacitance cable would still not give you more than 1dB of attenuation at 20khz so negligible. I used a passive preamp with my first Meridian M2 'speakers and with 7m of RG59 cable, the 3dB bandwidth was 50kHz, so -1dB at 25kHz.

For most applications, a 10kohm log law volume control works fine, as pretty much any source will drive 10K (actually a little less as the destination's input impedance appears in parallel) and for short cables (under 10m) the drop in hf is inaudible except possibly for bats. If all one wants is a variable attenuator for active 'speakers, then a so-called passive preamp is ideal. Impedance matching is irrelevant, and as long as the source will drive the passive, and the cables aren't too long, there's absolutely no problem.

S.
 
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