Help in choosing a potentiometer as a "Passive preamplifier"

[Wayne Parham]

Originally Posted by AudioLapDance

Ohh, the Analog Devices AD5206 RDAC does up to 6 ch! (AD5204 = 4ch) That would be great for anyone with a 5.1 setup (HT, surround, ... etc) and those with active crossovers (2 and 3 way!)

Are there any opamps onboard or is it just the resistor network?

Sorry, I didn't see your post until just now. I'm sure you probably found the datasheet by now, but if not:

• Analog Devices AD5204/AD5206 4/6-Channel Digital Potentiometers

[UMarcus]

What are the most sound quality solution for a potentiometer as 'Passive PreAmp' ?

Did anyone comapre some solutions ?

There are :

1. Potentiometer like ALPS (motorized)
2. Relay based Attenuator with resistor array
3. IC based volume control

It should be possible to remote control the volume, so i exclude the switch based solution which is near to relay based solution (?)

Thanks, regards
Marcus

[UMarcus]

I'm create a thread for this question

http://www.diyaudio.com/forums/analo...-solution.html

[rayma]

Originally Posted by ExtraCredit1

OK so I am using a 10k Alps and have the issue where there is no low end until the pot is turned up to almost max. I am using a Iphone as a source. Should I use a 5k? and if so does anyone know of a good quality 5k volume pot?

Thanks!!
Mike

That doesn't really make any sense. But, have you tried a buffer before or after the volume control? That might localize the problem.

[Chris Hornbeck]

Originally Posted by ExtraCredit1

OK so I am using a 10k Alps and have the issue where there is no low end until the pot is turned up to almost max. I am using a Iphone as a source.

Is the pot wired to progressively short the source's output? With a finite output coupling cap, that would give you those symptoms.

All good fortune,
Chris

[AndrewT]

No.
The source sees the vol pot resistance in parallel with a variable impedance.

When the vol pot is set to minimum output the parallel impedance is effectively infinite and the source sees only the vol pot resistance.

When the vol pot is set to maximum output the source sees the vol pot resistance in parallel to the input impedance of the receiver after the vol pot.

eg.
a 10k vol pot
a 100k receiver with and RF filter of 47pF and a cable of 300pF and etc...
At minimum output the source sees the 10k.
at maximum output the source sees 10k||100k||47pF||300pF
and the source must be capable of driving this impedance adequately.

[DF96]

Not if the pot is wired back to front, as is sometimes done by newbies or those who prefer 'innovation' to sound engineering.

[cspirou]

This is a good post.

What about when the potentiometer is integrated in a stereo amp? Is there such thing as having a wrong value or using too high of a potentiometer resistance? The problem of a low pass filter forming from cable capacitance seems completely avoided. So why wouldn't I want to use a 50k or 100k potentiometer for volume control?

[rayma]

Originally Posted by cspirou

This is a good post.

What about when the potentiometer is integrated in a stereo amp? Is there such thing as having a wrong value or using too high of a potentiometer resistance? The problem of a low pass filter forming from cable capacitance seems completely avoided. So why wouldn't I want to use a 50k or 100k potentiometer for volume control?

The following stage may have by design too low an input impedance to use a higher impedance pot.
A 50k pot has at most a 12.5k output resistance, so you'd want at least 5 or 10 times that for the line stage's input impedance.
Also, internal wiring, or the line amp's input capacitance, may require something more like a 10k pot to preserve bandwidth.

[M Gregg]

Its all depends on,

How much series resistance there is with different settings of the volume control..at full volume low series resistance / at low volume higher series resistance..thats not taking into account the drive of a cable if its remote or the lack of a cable if its intergrated..so from the point of an integrated amp the drive required after the control is different to at a remote distance.

miller capacitance and series resistance..is another issue.
Another is the current available for drive is divided at the pot.

So with a higher value pot you get more series resistance at low volumes, however at higher settings the dividing resistance is higher to ground.
(as usual its a trade off) So you are asking for a linear device with varying parameters of real life conditions.
(You are changing the circuit and expecting the same behaviour in all positions). ie even expecting a passive to be plugged into other amps and work the same..
how audible that will be is another issue.

Then there is the loading effect of the value of the pot on the driving stage (input device) and if there is a cable before the pot (connection from device to passive) or if its on the output stage of the source before the cable.

So you want maximum drive for the cable..unless its integrated. Trouble is you have an input cable and / or output cable (with a passive pre).
ie a load on the drive stage with a cable load and a divider driving into an output cable plus input load(amp).

Regards
M. Gregg