Advice on using an active preamp with a power amp equipped with a passive vol pot

Query: Advice on using an active preamp with a power amp which has a passive vol pot

Excuse the lengthy thread title - any views anyone?

I have a 300B power amp (a Wavac) which has a passive volume pot. Over the past few months I have been using it with an active valve Wavac Preamp - much better takes the sound to another level. The usual advice with this kind of combination is to turn the passive vol pot up full and attenuate via the preamp. Which is what I have been doing. The power amp doesn't have an independent input for preamps BTW.

However. . . .

I remember a few reviews from Berning ZH270 owners who had used a preamp and claimed better results from NOT having the Berning power amp pot full on. So - I turned the passive vol pot on the power amp back about a quarter and I do beleive it sounds better. I've read a few reviewers who advocate experimenting rather than just turning the pot full on - but why?

Does this make any sense - has anybody tried this? I know that some amp oweners would disconnect the passive pot on the power amp altogether, but i dont want to do this.

One other thing, I found on some recordings I got very slight clipping with the power amp pot full on - my adjustments seem to have fixed that - again any explanations here?

Any opinions welcome and may be an interesting thread for other users with a similar pre/power combo

 

I've set my passive pot to the highest level I'll listen to with the lowest sounding source/CD.

 

It is true that when you have a separate pre amp it is usually best to set the pot on the power amp to max. This should couple the input signal more directly to the input circuits as the signal won't be passing through the pot track.

Real life can sometimes dictate that you must use the power amp pot - for example if the power amp has high sensitivity.

Be aware that passive pots act as filters and that the precise nature of that filter depends on the loading condition which can be complex to say the least where there is more than one pot in circuit.

Use the solution that sounds best.

 

Yes - I too have always understood max to be the best result on the power for reasons explained. But as you say - real life experience has produced different response, interesting to have picked up how many users of pre's with powers with passive pots experiment with the settings and don't turn the pot to maximum. Just wondered why this should be - then again why should a load of extra circuits and stuff in front of an amp sound better than a single wire passive.

I agree - good advice to use what sounds best, as always, but was curious to hear of other's experience. BTW posted this on Pink Fish too to see if I could get a variety of opinions.

 

Loading is usually the issue.

All systems are different but the worst culprits are sources and pre amps with high output impedance, valve units typically, which cannot properly drive the load. The load isn't just the amplifier load but includes the interconnect cable capacitance.
Effectively, the frequency response and distortion performance varies depending on the position of the volume control.

Passive can work extremely well, but usually only where the source impedance is low, cables are short and of low capacitance, and the amplifier input impedance high.
20k ohms is a good working minimum for solid state systems though even higher is preferable if driving with valves IME.

 

What Rob said.

Your active pre-amp is effectively overloading the power amp, and by attenuating that signal you are rectifying it.

In a similar situation previously;- bypassing the passive pot in the integrated, and using a resistor to drop the output of the active pre-amp provided a better result. YMMV.

 

Hi,

It's not this amp?



or this one



They both have a input impedance of 100k Ohms & a 1v senistivity. It's not the most senistive but has a fairly high input impedance. Usually passive work better with power amps of a much higher sensitive input. (lower value of voltage).

SCIDB

 

Hi - it's the first one listed MD300B -most definitely better though a preamp and with the passive pot turned back about a quarter.

Read a review of a SET Viva amp and the reviewer tried the amp with its own pot, pot disconnected with a pre and usig a pre through one of the normal inputs and significantly preferred the latter - but in this case used the Viva powerm pot turned full.

As Rob says it seems there's no right or wrong just try it out.

 

laurence, we've spent some time here with wavac 805 trying to improve its pretty disappointing performance. our findings were not honouring wavac much in terms of the parts selection. you maybe interested to hear that one of our first findings was the pots being really a weak point. this seems to be a common rule with the cheaper wavacs (cheaper - the 805s were 12k euros a pair) as we were rather disgraced to see what a pair of gain pots MD-572 has on its back (the ones on 6-7 years old piece with low use couldn't hold a stereo balance in central position).

things rather seriously changed once we've bypassed it and that would be my sincere recommendation. once you are using a preamp, there is no need for it anymore. keep things simple with as little parts as possible.

second - as mentioned before - due to a high imp impedance of 100k, you might be surprised to hear what a good passive preamp can do. this is a perfect scenario for it. in the similar setup i have at home, i've even found a music first copper TVC (which i generally nag about) doing job better than i've though it's capable of, given the fact that it's between a 2.5k source and a 100k amp. try to borrow one.

 

Thanks for that Gordan - I may bypass if it's very easy to do. Very surprised to hear what you say re quality of the Wavac passive - given the overall quality of components and price of their amps. However - the Wavac preamp is amazing and so far the best preamp i have tried, I am expecting it to stay. I did own a couple of Music First passives as you may know.

I'll still stick by my view of the MD300B - it sounds really excellent to my ears - however I have just tried a Viva 300B amp with the Wavac preamp and I think the Viva may have the edge!