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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Help in choosing a potentiometer as a "Passive preamplifier"
Help in choosing a potentiometer as a "Passive preamplifier"
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Old 18th June 2015, 01:44 PM   #31
cspirou is offline cspirou  United States
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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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Old 19th June 2015, 12:47 PM   #32
M Gregg is offline M Gregg  England
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Quote:
Originally Posted by cspirou View Post
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.
You wouldn't bother about it on industrial controls except for current and life expectancy.
(Some sound awful..)

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=ol...tm%3B768%3B576

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

http://s1219.photobucket.com/user/Ka...8ef46.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/chan...-some-thoughts

Regards
M. Gregg

Last edited by M Gregg; 19th June 2015 at 01:12 PM.
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Old 19th June 2015, 04:00 PM   #33
M Gregg is offline M Gregg  England
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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

Last edited by M Gregg; 19th June 2015 at 04:12 PM.
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Old 10th January 2017, 12:57 PM   #34
cdsgames is offline cdsgames  Canada
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I have a 500ohm Z source...and Amp Z = 60k

From the above , whats the best pot to use ? 7k or 10k
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Old 10th January 2017, 01:25 PM   #35
AndrewT is offline AndrewT  Scotland
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Either will work.
Be careful to check the effects of the cables. Make them too long and the unbuffered outputs could end up with rolled off treble.
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Old 10th January 2017, 01:59 PM   #36
cdsgames is offline cdsgames  Canada
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I see (on the excel calculator) slightly better response with the 7k pot...enough to make a difference in sound ?
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Old 10th January 2017, 03:36 PM   #37
AndrewT is offline AndrewT  Scotland
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Quote:
better response
with respect to what?
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Old 1st May 2017, 06:43 PM   #38
zaib4tsu is offline zaib4tsu
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Quote:
Originally Posted by DF96 View Post
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.
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Old 2nd May 2017, 07:32 AM   #39
AndrewT is offline AndrewT  Scotland
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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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Last edited by AndrewT; 2nd May 2017 at 07:37 AM.
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Old 2nd May 2017, 11:56 PM   #40
Arty is offline Arty
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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..
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