Help in choosing a potentiometer as a "Passive preamplifier"

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
 
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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.
 
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.
 
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?
 
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Joined 2011
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.
 
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Joined 2010
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
 
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What is the difference in an expensive potentiometer and a cheaper one? Like an expensive ALPS pot vs a $2 Alpha?

From what I am reading it seems like the major difference is channel imbalance. Can't this be solved by adding balance controls with another potentiometer? Would an Alpha pot + channel balance sound better then an Alps pot alone?
 
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What is the difference in an expensive potentiometer and a cheaper one? Like an expensive ALPS pot vs a $2 Alpha?

It depends if you believe that the conducting /resistive element has an effect on the "sound".

I believe it does...however many think it all sounds the same.

So in theory its channel imbalance and you can correct this with a balance control.

The other issue is the wiper type and contact resistance with the conducting /resistive element.

So you have carbon/conductive plastic/ stepped/ shunt/ etc.

Using law faked pots you have a choice of resistor type..also with a stepped unit. However if you think its all the same then use what you like. :D
You wouldn't bother about it on industrial controls except for current and life expectancy.
(Some sound awful..:D)

Shock and horror..I like the magnetic tantalum resistors better than the non-magnetic. (I find the type of resistor on a law faked pot has an effect YMMV)

Years ago everyone used things like this:
https://www.google.co.uk/search?q=old+potentiometer&biw=1455&bih=691&source=lnms&tbm=isch&sa=X&ei=rRKEVf3jLoX9UPLHi8AG&ved=0CAYQ_AUoAQ#imgrc=3ePDDltb3uuaXM%253A%3BhP8_wbAIRSBPFM%3Bhttp%253A%252F%252Fantiqueradio.org%252Fart%252FCleaningCarbonPot.jpg%3Bhttp%253A%252F%252Fantiqueradio.org%252FFirstStepsInRestoration.htm%3B768%3B576

And inside you can see the track and the wiper element:

http://s1219.photobucket.com/user/Kale021/media/sanyo_jcx2250/pict1344_zps4eb8ef46.jpg.html

So you have a rotating contact on the resistive element and also a connection to the wiper.

How others see it..
http://www.head-fi.org/t/528261/channel-imbalance-in-your-volume-pot-some-thoughts

Regards
M. Gregg
 
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Disabled Account
Joined 2010
NB I'm not a fan of ALPS many are,

I have used TKD, TOCOS (Carbon), and audio note.

I also like law faked pots (conductive plastic) doesn't have to be expensive AB or Bourns can be quite good with (Select resistor type).

Its a shame you can't get the above as dual concentric pots...

Years ago you could also get pots with taps for loudness..:)

Its down to a personal preference..(and signal level/cost)

Regards
M. Gregg
 
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Some people prefer switched attentuators and they may in theory be best, but many have also been disappointed to find that they can't have them without also introducing extra coupling capacitors and ground resistors. As in all engineering, you have to choose your compromises carefully.

Hi DF96,
I know this is an old post but could you elaborate on "introducing coupling capacitors and ground resistors"? When would this be required when swapping a regular carbon-track pot for a stepped attentuator / how can I work out if needed or not?

Feel free to link if already covered elsewhere.

If it matters, the numbers I know for the current setup:
- 50Ω output impedance (DAC)
- 50kΩ volume pot
- 47kΩ input impedance (power amp, presumably including that stock pot)

If I replaced the pot with a 50kΩ stepped attentuator, would any impedance relationships be modified? I'm unsure where the capacitance is also.
 
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.
 
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why not put a buffer opamp infront of the volume pot, essentially providing you with a constant high input impedance for the previous stage to see, and and a nother buffer right after the pot, so you make sure the amplifier will be fed from a low impedance sure at all times?
well.. actually i do this in practice and would like feedback.. allso got a pot at the input of the first opamp so i can adjust input impedance, and a pot at the output of the 2nd opampbuffer to adjust output impedance if i would.. like to.. change it..