Why is "no preamp" best?

[NewBuyer]

To achieve voltage bridging, I've always heard that load impedance should be at least ten times the source impedance...

[Phil Leigh]

To achieve voltage bridging, I've always heard that load impedance should be at least ten times the source impedance...

100x it is and the higher the better really. Electric guitar pickups are a classic voltage transfer problem - a 1M impedance is ideal! (1000x?)

[cliveb]

I've been away for the Xmas break, and have just noticed this thread. Since it seems that it was started in response to a comment I made in another thread, I should try to clarify what I meant.

You can have a hifi set up wihtout a pre-amp? I currently use a Transporter directly into a NAIM power amp. However I am under the impression that I am using the Transporter's built-in pre-amp. Is this not true? If it is true why should the pre-amp be better than than the NAIM pre-amp I used to use?

Yes, of course there is a "preamp" in the Transporter: the analogue output circuitry that follows the DAC. When I said that "the best preamp is no preamp at all", I was referring to the addition of a separate preamp in the signal path. For example, if you feed your Naim preamp from the Transporter, you now have effectively two preamps in circuit: the Transporter's (which you can't avoid), plus the Naim. It is my contention that in most systems, the additional preamp is undesirable. In my experience, of all the electronic components in a HiFi system, preamps are the least transparent. (That's why really good ones cost so much).

There are exceptions. If your power amps have a low input impedance, they may benefit from the buffering stage of an active preamp. But the vast majority of power amps these days have sufficiently high input impedances that this is not really an issue.

NAIM dealers tend to say that using another pre-amp (rather than a NAIM) can introduce "instability" (yup, I know... not too technical, but then NAIM dealers tend towards the wooly rather than the technical). Patrick Dixon has said on these forums that the pre-amp should block very high freuencies (out-of-hearing range one presumes) to work properly with a NAIM amp, otherwise "instability" is the result.

Naim power amps rely on being fed a bandwidth limited signal to avoid being driven into instability. I don't know exactly why this is, but am told it is a result of the particular circuit topology used by Naim. Of course, Naim preamps ensure suitable bandwidth limiting. Most others don't (DNM is one other that springs to mind that does).

Therefore using a Naim power amp may be another possible case where a (suitable) preamp is desirable. That said, with good quality digital sources that have proper anti-imaging filters, I'd imagine that there is unlikely to be much high frequency energy in a signal anyway, so perhaps this particular issue with Naim power amps isn't so relevant any more.

I assume that if this is the case with NAIM it might well be the case with other power amp/pre-amp combos.

I think Naim are actually pretty unusual in this respect. Most other power amps don't have the same bandwidth limitation requirements.

[ModelCitizen]

Thanks a lot for your reply Clive. It is just what I was hoping for. Happy New year.
MC

[lafayette]

Never mind -- I figured out the answer to my question.