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gjo 16th March 2004 07:24 AM

Volume pot question
I want to upgrade the volume control on my preamp, which is a 50K pot, using a Noble stepped unit. I can only find 100K units. What is the downside to useing a100K unit?



mig-ru 16th March 2004 07:50 AM

Less distortion, if you consider that a downside.

gjo 16th March 2004 08:13 AM

Thanks for the input. So it actually may be better to use this? I'm not worried about the sensitivity-the exisiting pot is a bit too sensitive for me!


Positron 17th March 2004 03:08 AM

Hi Gjo,

With a 100k pot, the high frequency response at any point of rotation (not at zero rotation or max rotation though) will be lower compared to the 50k resistance pot (assuming the pots have equal internal capacitance to ground).

If the input capacity of the tube is low, such as by using a CF, the 100k pot won't do as much damage to the HF response, assuming the wire from the wiper arm to the tube has low capacitance to ground: as if the wire has high capacitance to ground and the tube were run common cathode.

In any case, the frequency response will be its worse at the mid- resistance point of rotation using either pot. This is not the same as the physical mid rotation of the pot.

Of course if you have a source with vacuum tube output, and capactive coupling, it may help the low frequency response. But if the source has low output Z, and is DC coupled, the 50k pot would be prefered over the 100k pot.

As far as distortion is concerned, if the source has a low output Z, the distortion will be very close, if not exactly the same with the 50k control compared to the 100k resistance control. If the output Z of a tube output source is high, the 100k might create a lower distortion scenario. But then the high frequency response will be more critical than ever and harder to preserve because the IC capacitance will lower the HF response. So use low capacitance ICs in this scenario.

Hope this helps.

SHiFTY 17th March 2004 04:28 AM

Make sure it is a logarithmic device!

gjo 17th March 2004 08:46 AM


Thanks for the input.


gjo 17th March 2004 10:20 AM

Interestingly Bill Fitzpatrick has some 50K Noble pots currently available I just saw the thread a little while ago. I will pick up one of those and avoid all the problems, I think.


Mike C 18th March 2004 10:38 AM

Shunted Pot
May I suggest a shunted pot volume control (using say the 50k Noble
pots you mention).
It's very simple; all you need is a couple of high quality resistors.
See my Nina preamp on this site for the cct.

It gives most of the benefits of a stepped attenuator for very little
cost; but do use high quality resistors.

With say a 68k series resistor, the resistance in circuit will typically
be 70k to 80k ohms most of the time, which should not be a problem.

It will be marginally less sensitive, but from what you say this will
not be a problem.

If you have a balance control, I'd suggest you remove it or bypass it.
If you need a balance facility, consider two mono pots; it sounds a
lot better than a balance pot.

If you go for two mono pots, consider this. Get 50k stereo pots,
wire the two sides in parallel (earth to earth, wiper to wiper) to
give you two 25k mono pots. Use these with say 50k series resistor
and you should get very good sound and less sensitivity; the latter
will allow easy adjustment of volume and balance.

Or just buy two high quality 25k mono log pots, if you can find them ...
I've been looking for quite a while without success!

gjo 18th March 2004 12:09 PM


I will relook at my options after I lay my hands on the Noble 50K pot. Thanks for the input.

Positron 18th March 2004 02:07 PM

Problem is...
The problem with the shunt idea are two fold.

1) The input inpedance changes with the volume setting if I understand your concept correctly. Not horribly bad unless a source has tube and/or capacitive output.

2) One still has the problem of worse high frequency response because of the 68k resistor increases the total resistance of the circuit, while the internal capacitance of the input circuit remains the same.

A single 25k section of a pot is best, as paralleling increases the capacitance in the circuit, hence lowering the HF response, but you has little choice since 50k dual is the only one available.

Dual monopots is a great idea, get rid of that balance pot if at all possible.

Hope this helps.

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