How to increase the input impedance of a pre-amp ?

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DonOE

I did a search, but nothing answered this question to a satisfying degree.
I promised to add an inline volume control for these active speakers. http://www.esi-pro.com/viewProduct.php?pid=50

They are used in un-balanced mode, so apparently they have an input impedance of just 10 kOhms.

I know the speakers already feature a volume control, but 1) it's hard to adjust to the same volume on both speakers, 2) it's on the back of them.

I cannot find a small-ish dual-log pot rated at about 1k or thereabouts, so is it possible to increase the input impedance of the amp section of the speakers ? how ? Something tells me that it isn't as easy as just adding resistors on the output of the pot....

Any questions ? - just ask.

Thank you.

AndrewT

Hi,
the volume control on the active speaker is to set the sensitivity to suit the preamp output. I do not think the manufacturer intended it to control volume in the day to day sense.
Your preamp should be able to control volume. Similarly it should easily drive a 10k input load.
It should have an output impedance of less than 1k, most have.
If you only have a passive preamp then some maths will help.
If the passive is set to minimum volume then the output resistance is almost zero+10-0 with a parallel resistance of the source output +passive. When set to maximum the output resistance is the output resistance of the source with the passive in parallel. Worst case is when set to just above -6db (half voltage). The output resistance is a parallel combination of the lower half of the passive // the series combination of upper half of passive + source impedance.
The result is that the output impedance of a 1k passive is a maximum of just over 250R. If you want to use all of the 1k allowance ( and this is just a recommended not absolute) then a 4k7 will give Rout =1k18.
There is another solution; use a log faking passive. Buy a linear dual pot about 10 times final output impedance and add a log faking resistor of about 10% of the lin pot. Connect the faker between the wiper and the lower end of the passive pot.
A small cap added to the wiper can compensate for cable anomalies to correct frequency response at different volumes. I cannnot recall the details so you'll need to search. HIFi World did a mini project on it.

DonOE

It isn't as sophisticated as pre-amp. It's more like a cheap PC soundcard, unfortunately (not mine either). The only way to control volume is in windows volume settings, so it's a "digital volume control" that doesn't do anything about what is seen at the output regardless of volume.
I'm not sure of it's output impedance, but yes, likely less than 1kOhm.
Thanks for the indepths of different output resistances and impedances, it's very helpful, i think the best would be 1k rather than 4k7 in this case, the differences in load the soundcard would "see" would be very high with a 4k7 pot and low with a 1k. Am I right saying that it will be easier to control the volume using a 1k pot rather than a 4k7, i.e. less change in volume per degree of turning the knob ?

I found a 1K dual log pot: http://www.vejle-rc.dk/web-shop.asp?showItem=4285 I'm not sure how good it is though. 1K seems extremely rare.

AndrewT

Hi,
your sound card may have serious problems trying to drive the 1k pot. It may even struggle to drive the 4k7 pot.
The % rotation controls the % voltage out. So both pots will have the same sensitivity of volume to pot position.
The sound card will see the pot resistance // the 10k input resistance (but only when at maximum volume). So the variation of resistance seen by the sound card is effectively irrelevant.
I would recommend, without seeing all the specs, that the safer route is to buy a 4k7 dual log pot. and when things have settled down buy a secondhand preamp to control things properly and give a convenient choice of source.

DonOE

Hi again,
Ah ok.
If using, say 120 Ohms impedance headphones directly from the soundcard - that is about say 120 Ohms impedance regardless of frequency - it would work fine, but i'm not sure as to why using a 1kOhm would make any trouble, please enlighten me I thought that the 120 Ohms would be just a "load" like 1kOhm regardless of if it's impedance or resistance...

richwalters

How to increase the input impedance of a pre-amp ?

Bootstrap the input (technically better)......FET it ....or TUBE it.

richj

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