Passive preamp

[Flemming J P]

I have constructed a passive preamp (read a 100 kohm pot and nothing else)

Now I read somewhere on the net, that a preamp should have a high input resistance and a very low output resistance (which is not possible with a pot alone).

I know that if the input resistance gets too low, the will source act up, but what about high output resistance and power amp? What problems are there on that front?

Best regards
Flemming J P

 

[lohk]

You should keep impedances as low as possible.
A preamp output should be lower than the recieving input, otherwise even a short cable will act as a significant low pass filter, and will be very prone to RF influence (try your passive pre with a non-screened cable).

So 100k is IMHO far too much for a passive preamp, even 10k is critical. Try to get pot as low as your CD can drive. But as a matter of fact CD players usually can drive low impedances not very well, also because of their relativly high output impedance. And amps usually are much better driven by low impedances.
100k for me only seems suitable with an all-tube chain and short and very well screened cables.

Read more about the eventually at http://www.passlabs.com/ about the output-pot of the BalZen and the AlephP.
There is al lot of theory material about that in the net.

My personal experience with passive pres is mixed. Usually it is better to add a high-quality impedance converter at least. I tried a 100k pot once which sounded much worse than my usual preamp.

good luck
Klaus

 

[Flemming J P]

Hmmmm and I thought it was simple.

I had a minor problem with just using the 100 kohm pot, which was that the input signal was way too high, so a very steady hand was needed when turning the pot.

I have changed it a bit now. The input first runs through a 100 kohm resistor and then into a 10 kohm pot. This works like a charm. This should also be better! The amp now sees max 10 kohm and the input sees 110 kohm.

Both solutions sounds far more detalied than the original preamp. The old preamp that I've bypassed is the one found in the sansui AU-719.

 

[Dave]

You could put the pot in your power amp, to make an integrated amp. That would solve the interconnect problem.

 

[GAK]

The best idea is Dave's.If you can omit the preamp stage on your system ,is the best solution.
But if you want to have a passive preamp I think you must visit http://members.tripod.de/Promitheus
There's the best passive preamp I've ever seen.
Without the selector switch the results are going to be fine.
Regards

 

[AudioFreak]

Another point that has not been touched on is that if the the output impedance of the preamp is too high, the gain of the amp may be decreased (in some cases severely so).

 

[Flemming J P]

It seems that I've been lucky, because I don't have any major problems.

The only issue I've got is to figure out how much too dampen the signal before the pot.

How do you figure out if the output impedance is going to be a problem or not?

 

[AudioFreak]

to do it totally properly you need an oscilloscope, dB meter, frequency analyser and sweep generator.