Help in choosing a potentiometer as a "Passive preamplifier"

When the vol pot impedance is much higher than the Receiver's input impedance then you find that the attenuation of the vol pot is less affected by the Receiver impedance and any variations in Receiver impedance.
As the Receiver impedance comes down towards and even below the Vol Pot impedance you will find that variations in Receiver input impedance have a much larger effect on the output.

A 50k Vol pot with a 50ohm source has a maximum output impedance of {50000+50}/4 = 12512ohms. The minimum impedance is zero ohms. The variation in source impedance is >12k
It is usual to specify a Receiver input impedance of 10times this, i.e. 125120ohms as the minimum receiver impedance. Selecting even higher reduces the effect of incompatible impedances.
Where possible I try to get >20:1 ratios from Source to Vol Pot and from Vol Pot to Receiver.

I suggest you use a 10k stepper Vol Pot to feed the 47k input impedance of the receiver.
This lower value of Vol Pot will also tolerate the capacitance of the cables far better than using a 50k vol pot.

Thanks Andrew
I have no problems with the current setup in terms of impedance (unless you can see some glaring issue?). The 47kΩ figure is from the amp's spec sheet - so I presume it includes the stock volume pot, is that what you assumed? That 50kΩ figure is for the stock pot already in there.

The reason I considered a stepper is because the L/R channel balance of the stock pot isn't good (measured ~5-10% difference depending on setting.

What are you improving by reducing the 50k to 10k, and is there a downside to this?

Also - do you have any recommendations on regular pots that are a bit better matched - or are you a fan of the stepped ones too? I'd appreciate any part suggestions - ebay or other (happy to build if needed). Goldpoint well outside of budget unfortunately.
 
I agree with Ray. Those Chinese stepped attenuators are easily a "best buy" when it comes to reasonably priced volume pots.
Carbon pots(ALPS included) don't even come close when it comes to channel balance.

My first choice would be the Goldpoints, but they're over 10 times the price of the Chinese DACT Types.
 
Thanks Andrew
I have no problems with the current setup in terms of impedance (unless you can see some glaring issue?). The 47kΩ figure is from the amp's spec sheet - so I presume it includes the stock volume pot, is that what you assumed? That 50kΩ figure is for the stock pot already in there.

The reason I considered a stepper is because the L/R channel balance of the stock pot isn't good (measured ~5-10% difference depending on setting.

What are you improving by reducing the 50k to 10k, and is there a downside to this?

Also - do you have any recommendations on regular pots that are a bit better matched - or are you a fan of the stepped ones too? I'd appreciate any part suggestions - ebay or other (happy to build if needed). Goldpoint well outside of budget unfortunately.
your ratio of source impedance to receiver impedance is not good.

It is usual to look for at least 1:5 source:receiver and preferably >1:10, I prefer to aim for 1:20
Your 50k vol pot has an output impedance of {50k+Rs}/4 i.e. ~12k7
Your receiver impedance is 47k. That gives your impedance ratio ~ 1:3.7
I consider that far too low.
A 10k vol pot will give you ~ 1:18 and the 10k is more capable of driving the interconnect.
 
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This has been very good and quiet, with perfect balance. Very reasonable price,
and built in 10k to 250k values. Check out the internal photos.
DACT Type 21 Stepped Attenuator / Potentiometer 10K * D Shape Shaft * HiFi grade | eBay

Nice! At first glance I assumed it was a normal pot for that price. I didn't find those when I looked, glad I asked now. Thankyou.

What's the friction like to turn it compared to a non-stepped knob, do you think a motor drive from a regular volume control (eg. std motorised blue Alps) would struggle to turn it or be okay? I could always gear down my drive system anyway, just curious - motor thing is a future project.

your ratio of source impedance to receiver impedance is not good.

I feel like you're not understanding correctly, the 47kΩ value is the rating for the whole amp (from the spec sheet, i.e. at the RCA input) and the volume control already exists as part of it (so I'm not adding an external 50kΩ volume pot to a 47kΩ input impedance amp). I am unsure of the input impedance of the power amp stage itself (which is probably what you're thinking of).

My understanding was that the ratio of source impedance to amplifier impedance in this case was just the 50Ω (DAC) to 47kΩ?

Please correct me if I'm wrong though. I'm happy with the setup as-is aside from the balance mismatch so would prefer not to risk new problems.
 
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What's the friction like to turn it compared to a non-stepped knob, do you think a
motor drive from a regular volume control (eg. std motorised blue Alps) would
struggle to turn it or be okay?

The torque is very similar to a regular pot, and you don't notice any clicks.
There are versions with flatted 1/4" shaft, and a knurled shaft. Bushing size is metric.
 
The torque is very similar to a regular pot, and you don't notice any clicks.
There are versions with flatted 1/4" shaft, and a knurled shaft. Bushing size is metric.

Sounds perfect, thanks again.
I'm pretty tempted to buy one right away (in the same impedance as the standard built-in pot, which I'm replacing here), but Andrew has me confused so I'll wait for a response before deciding on impedance value.
 
.........I feel like you're not understanding correctly, the 47kΩ value is the rating for the whole amp (from the spec sheet, i.e. at the RCA input) and the volume control already exists as part of it (so I'm not adding an external 50kΩ volume pot to a 47kΩ input impedance amp). I am unsure of the input impedance of the power amp stage itself (which is probably what you're thinking of).

My understanding was that the ratio of source impedance to amplifier impedance in this case was just the 50Ω (DAC) to 47kΩ?

Please correct me if I'm wrong though. I'm happy with the setup as-is aside from the balance mismatch so would prefer not to risk new problems.
your initial description led me completely down the wrong path.

Just carry on.
 
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I'll wait for a response before deciding on impedance value.

In your system, there should be no problem at all with the usual 10k pot value.
Unless there's a problem elsewhere (acoustics, etc.), the channel balance will be
rock-stable centered throughout the entire range of the volume control.
Remember that the body of the control is intended to be grounded via the
mounting on the panel.
 
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I don't understand how you guys find 21 steps (or 24) sufficient ? I have a Noble pot which isn't a true DACT-style but has indents so that when you rotate it you get discrete steps even though it's 'just a pot'. I found that I wanted to set a volume in-between the indents and although I could persuade it to kind of balance in this no-mands-land it was annoying so I replaced it with a blue Alps. I concluded that I need a smoothly varying pot rather than one with indents, or rather than a DACT style. Somtimes a source signal is stronger or weaker and the volume control has to be controllable at 3 o'clock and other times controllable at 7 o'clock. I do wish the chinese guys would figure out how to make an affordable 50 step option.

I use 10k for a passive volume control, seems to me a good compromise between not loading the source, not being too high to 'drive' the amplifier and also a common value that's easy to buy.
 
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Somtimes a source signal is stronger or weaker and the volume control has to be controllable
at 3 o'clock and other times controllable at 7 o'clock. I do wish the chinese guys would figure
out how to make an affordable 50 step option.

You can add a series resistor before the pot for more attenuation, switching it out when you
want more gain. I think your problem may be with the taper.
Then there's this type: http://www.vaneijndhoven.net/jos/relaixedpassive/index.html
 
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