What is the effect of a 22K res in series with a 25K volume pot?

[andyr]

My preamp has a 22K res in series with a 25K volume pot, for each channel. Can anyone explain what having this res there, does?

My thinking is that when the pot is fully wound round (ie. maximum volume, so the pot is out-of-circuit), the signal is still going through a 22K resistor - so I must be getting some signal attenuation.

The reason for my question is that I would like to achieve a bit more gain from my preamp - and I'm thinking that reducing the value of these resistors ( or removing them) would do this?


Thanks,

Andy

 

[nigelwright7557]

Its reducing the signal by a half approx.

 

[andyr]

Its reducing the signal by a half approx.

Thanks, Nigel - that makes perfect sense.

Can you suggest any negative effect if I reduced this resistor to, say:
* 12K
* 10K
* 4K7?


Regards,

Andy

 

[nigelwright7557]

What is driving into the pre amp ?

 

[andyr]

What is driving into the pre amp ?

The preamp wiring is as follows:
* input RCAs --> selector switch
* selected source --> 22K --> 25K volume pot.

Source inputs to the preamp are:
* phono stage
* digital tuner/internet radio
* CDP.

Regards,

Andy

 

[nigelwright7557]

I think the phono stages need to see 47k hence the 22k and 25k pot.
Its all about impedance matching.

 

[andyr]

I think the phono stages need to see 47k hence the 22k and 25k pot.
Its all about impedance matching.

Yes, I understand about Zin but AIUI while the Zin of a phono stage is, typically, 47K ... its Zout has no such specific requirment but should be at least 10x (and preferably 20x or more) less than the Zin of the preamp?

Given that the Zout of the phono stage is 100ohms, the 25K of the pot itself should be more than adequate?

Even if the Zout of the tuner & the CDP are as much as 1K (which is unlikely, I suspect), the 25K of the volume pot should still be sufficient?


Thanks,

Andy

 

[nigelwright7557]

If the phono stage is buffered then the 25k is more than sufficient.

The extra resistor might be some sort of input protection in case someone applies a high voltage.

 

[andyr]

The extra resistor might be some sort of input protection in case someone applies a high voltage.

Aah, OK. I shall ponder on this before I change anything.

Thanks a lot,

Andy

 

[AndrewT]

The Zout of the pot is seen as the source impedance by the receiver.
The Zout of a pot reaches a maximum of [Rpot+Rs(before the pot)] / 4
When you add that 22k in series with the 25k pot you have increased the maximum Zout max to ~12k
Zout min = 0r0.
That will give a very variable low pass filter as seen by the receiver.

 

[DF96]

Is the 22k standing in for a removed balance pot? If not, it is simply adding attenuation to compensate for some (expected, but not present?) over-amplification elsewhere.

Removing the 22k will increase the signal, and is unlikely to have any bad effects.