Cheap but non-degrading solution to add volume control between dac and amp.

[Old Listener]

Thank you for all the suggestions.

Anything that would take balanced inputs and do balanced out?

Would some thing like these do the trick and not affect the sound?

https://www.amazon.com/DYNASTY-PROAUDIO-Controller-Extremely-Adjustments/dp/B08HPPXZLH/ref=mp_s_a_1_9?keywords=xlr+volume+control&qid=1636162787&sr=8-9

Behringer Monitor1 Passive Stereo Monitor and Volume Controller

Passive Monitor and Volume Controller with Mono and Mute Functions

www.sweetwater.com

I have no experience with the Behringer device. I have used this TC Electronic volume control for some time and find it to be very satisfactory.

TC Electronic Level Pilot X Desktop Speaker Volume Controller with XLR

 

[solderdude]

Is there a way you could measure impedances at, say, -20dB ?

No need to measure. It's simple math.

For a 10k potmeter set at -20dB the input resistance is between 9.9k and 9.99k (depending on the load) and the output resistance = 900ohm.
For a 50k potmeter set at -20dB the input resistance is between 48k and 49k (depending on the load) and the output resistance = 4.5k.
This is somewhere around 30% of the volume control range.

Actually the 'worst' effect is achieved at -6dB setting (around 70% of the volume control range)

For a 10k potmeter set at -20dB the input resistance is between 9.9k and 9.99k (depending on the load) and the output resistance = 2.5k
For a 50k potmeter set at -20dB the input resistance is between 48k and 49k (depending on the load) and the output resistance = 12k.

A screened cable is about 100pF/m some audiophile ones may be worse. Lets assume 300pF/m and a 2m cable (so 600pF load).

For a 10k potmeter set at -20dB the -3dB point = 300kHz (-0.5dB at 100kHz) could be lower when the amp itself has a substantial input capacitance, usually adds about 100pF.
For a 10k potmeter set at -20dB the -3dB point = 60kHz (-0.5dB at 20kHz) could be lower when the amp itself has a substantial input capacitance, usually adds about 100pF.

Above shows the reason why one should choose a 10k over a 50k pot when looking for a passive volume control and why a higher capacitive load will cause a (gentle) roll-off when using long or 'exotic' cables.

Maybe with the exception of a few very rare tube sources no normal source will ever have problems driving 10k loads, in fact some power amps may even be lower than that.

 

[Rja4000]

No need to measure. It's simple math.

Well, if you know the pot impedance for passives, yes.

Do we know which pot they use for the Monitor 1 ?
I didn't find it in their "doc".

 

[solderdude]

Ahh you wanted to know the value of the pot used in the monitor 1.
No one seems to have measured the resistance. All of these professional passive attenuators do not seem to specify the resistance.
At 0% setting all we know is the output resistance will be close to 0 and at max setting the output resistance will be close the the source resistance..

 

[Rja4000]

All of these professional passive attenuators do not seem to specify the resistance.

Well, Radial does for SAT-2:
8kΩ for input and 1.8kΩ for output

They also provide AP measurements, by the way...
(see the "Graphs" tab in their specification page)

To be fair, I rather measure the input impedance between 25k and 38k.
Which seems to make sense when I have a look inside the box.

 

[Waxx]

I bought a Tishbury Mini Passive Preamp years ago and it's perfectly neutral as far as i can hear without measurements. It's a bit more expensive but offers 3 in 2 out switching, and the second out can be set independent of volume (for active subwoofers or so). I have it for +5 years now without issues. No balanced I/O altough.

Tisbury Audio - Mini Passive Preamplifier

A small and simple passive preamp by Tisbury Audio.

www.tisburyaudio.co.uk

 

[solderdude]

Well, Radial does for SAT-2:
8kΩ for input and 1.8kΩ for output

They also provide AP measurements, by the way...
(see the "Graphs" tab in their specification page)

To be fair, I rather measure the input impedance between 25k and 38k.
Which seems to make sense when I have a look inside the box.

Looks like a 10k potmeter. With that potmeter full open and the DIM potmeter being 50k one can expect the lowest input resistance to be around 8k
The output R is most likely the highest possible value so in -6dB position which would make the output R 2.5k but there is also the DIM potmeter in parallel (50k) so below 2.5k.
It could also be that in series with the output a 1.8k resistor is used so that the minimum output R is 1.8k (and max 4.3k)

Whether or not all other passive volume control manufacturers would use 10k pots is not certain.
It is a logical choice. 5k or 2k pots may be a too high load. 20k/25k, 50k or heaven forbid 100k is too high.

I also saw one specifying output R as 600ohm. This would mean a 2.5k pot (when the 600ohm is max output R) or it has a 590 or 604 ohm extra output R and they just specify the minimum value.

So it is likely most of them are 10k but would not bet my life on this.

 

[Rja4000]

Looks like a 10k potmeter.

It's written "25k" on it.

 

[solderdude]

Then it cannot have 8k input resistance unless it has a 12k resistor in parallel to it.
That would be a silly thing to do though as the output resistance would be around 6k at its worst position. When it also has an 1.8k at the output almost 8k.

 

[Rja4000]

Then it cannot have 8k

That's what I wrote: it does not.
(Quick) measurement gives me between 25k and 38k

Here is another photo.
The Dim pot also seems to be 25k.

 

[solderdude]

Both pots are linear. It is highly likely they turned the volume control into semi-log by adding resistors between wiper and ground.
Linear pots, in general, have better L-R matching between the 2 'gangs'.

There are also lots of other resistors in there as well as bipolar coupling capacitors.
This could well be the reason you measure different values. The actual input resistance (for AC/audio) can be much lower than any measured DC resistance.
This definitely isn't a simple volpot in a box.
One could wonder how many extra parts are in similar boxes (with mono/mute switches)

 

[nagster]

Thanks for this !
Exactly what I was looking for.

The D90SE is actually very impressive by itself.
I'm surprised there is so little difference between the pre90 and the D90SE direct.

Is there a way you could measure impedances at, say, -20dB ?

Impedance of MONITOR1 at -20dB position.

https://mediadl.musictribe.com/media/PLM/data/docs/P0C9G/MONITOR1_QSG_WW.pdf