Replacing Trimmer Pot in Phono Amp?

[Spur07]

Hi all

I can adjust the upper frequencies of my phono amp via these basic trimmer pots, but I'm thinking of replacing them with either a couple of decent potentiometers, or a set resistor. I've been told its a relatively simple fix. Do I need to remove them from the circuit to measure the range of resistance? And where can I buy some suitable potentiometers? Any assistance much appreciated.

http://i601.photobucket.com/albums/tt94/spur07/MJS/_MG_0162.jpg


Cheers


Paul

[Stratmangler]

Are the potentiometers broken?

[Spur07]

Are the potentiometers broken?

Hi Chris

No, they're not broken, but I was advised they're fairly crude. Is this incorrect? Would not a decent pot be preferable? At least something with knobs on I can twiddle with :)

[Ali Tait]

I'd be tempted to measure resistance of the pots out of circuit once you decide on a value that you like the sound of. Once you have the value, replace with a decent resistor, say Takman carbon or Audionote tantalum, both of which can be bought here-

http://www.hificollective.co.uk/componentshome.html

If you'd prefer pots they are also sold there.

[Stratmangler]

Hi Chris

No, they're not broken, but I was advised they're fairly crude. Is this incorrect? Would not a decent pot be preferable? At least something with knobs on I can twiddle with :)

If it works fine and sounds good then I'd be inclined to pop everything back into the box and put the lid on.
As it's a phono stage you need to have some degree of assurance that the EQ you're using is correct (ie RIAA) - if you mess about with the pots then you're chucking the EQ out of the window.

[Spur07]

I'd be tempted to measure resistance of the pots out of circuit once you decide on a value that you like the sound of. Once you have the value, replace with a decent resistor, say Takman carbon or Audionote tantalum, both of which can be bought here-

http://www.hificollective.co.uk/componentshome.html

If you'd prefer pots they are also sold there.

Ali,

how do i go about choosing a pot - do I just decide on a range of resistance after measuring the trimmers?

[Spur07]

If it works fine and sounds good then I'd be inclined to pop everything back into the box and put the lid on.
As it's a phono stage you need to have some degree of assurance that the EQ you're using is correct (ie RIAA) - if you mess about with the pots then you're chucking the EQ out of the window.

yeah, i understand chris. the amp was too 'hot' for my system when i got it off fleabay, so i have adjusted the trimmers back and forth in the past. I was just told it was fairly crude as pots go.

[Stratmangler]

yeah, i understand chris. the amp was too 'hot' for my system when i got it off fleabay, so i have adjusted the trimmers back and forth in the past. I was just told it was fairly crude as pots go.

In which case it's way off spec anyway....
HiFi Collective's a good place to look.

[Spur07]

In which case it's way off spec anyway....
HiFi Collective's a good place to look.

how do you mean chris? the amp was pulled form a studio or broadcast station - i just assumed it was set up for a system that was some way different to mine. sorry for the questions, i'm not really au fait with vinyl, etc.

[Mr Kipling]

Hi Paul,

I don't think they are actually variable resistors! I could be wrong, but I think they are compression type variable capacitors. What is the wording in the centre? Is there any markings on the sides?

Kind Regards,
Stephen

[walpurgis]

Ali Tait's suggestion of setting the pots to give the sound you want and then measuring across the pot and substituting decent resistors to that value is the way I'd do it. Just keep the pots in case you ever need to reset the trim and refit and remeasure accordingly to get alternative resistor values.

[lurcher]

Hi Paul,

I don't think they are actually variable resistors! I could be wrong, but I think they are compression type variable capacitors. What is the wording in the centre? Is there any markings on the sides?

Kind Regards,
Stephen

Yes, I agree, they look far more like variable caps than resistors.

[walpurgis]

Same will apply in that case, just measure for capacitance instead and substitute decent caps.

[Spur07]

Yes, I agree, they look far more like variable caps than resistors.

yes, spot on nick and stephen - i found out earlier on another forum that they are in fact variable capacitors used to fine-tune the RIAA equalisation. I've got the schematic.

I've had this advice:

If you can, try to see if they have a marking (e.g 220 pF) and buy a few capacitors at higher and lower values eg from Maplin (they cost pennies) and experiment to see which you like best. Or if you will be using more than one cartridge, maybe it is best to keep the capacitors variable?

and this:

For normal RIAA equalisation the cap should be 1.34nF. This is so together with the 56k resistor it gives a time constant of 75us to get the correct treble rolloff. You could use 1nf and 330pF in parallel to give 1.33nF - close enough. Check that there is not already a fixed cap in parallel with the trimmer.

This is a peculiar design. There is no way someone could adjust to the correct value by ear, so it is in effect just a preset tone control.

Not sure what to do :scratch: it's nice to twiddle, etc but I can't help thinking it would be better to have it set permanently at normal equalisation

Paul

[Mr Kipling]

Hi Paul,

There is a third option. Wire it as option two for normal R.I.A.A. and you can add say three capacitors of different values which you can add in parallel with say D.I.L. switches. Or you could fit a set of phono sockets and put the capacitors in some phono plugs, if you want to experiment a bit.

Kind Regards,
Stephen