In article 4a7d2929.352510515@localhost, Don Pearce
wrote:


The reason why Naim amplifiers don't is that they failed to consider
exactly this problem in their design. If they had just used Figure 1
(which you deride) they might have made an acceptable product.

I can't comment on any current designs by Naim as I've not measured or
studied them. I should also say that it is quite possible to design an amp
which is unconditonally stable without it having an explicit output series
inductor. However, that said, I did many years ago do bench measurements on
a Naim power amp. And, yes, it gave bursts of oscillations on audio
waveforms when used with a cable that had low series inductance.

So this problem is not simply a theoretical one.

In my years of design I discovered something important. It doesn't
matter what is the maximum frequency you intend to put through
something. Your design must encompass the maximum frequency at which the
active devices can produce gain (something like Ft). It is all too easy
to end up with an audio amplifier which is so marginally stable at 30MHz
that it can oscillate into some loads. When that happens, yes, there
will be sonic consequences.

Yes. This is my experience as well. Perhaps enhanced for having designed
systems for up to over 300GHz as well as for the relatively low audio band.
Quite interesting to find harmonics or out-of-band oscillations for these.
:-)

I also recall using 'RS' UHF modules that all oscillated at about 1.5GHz. I
guess the makers only used scopes and analysers that went up to about
1GHz...

It is all too easy to make an amplifier that looks OK on a test bench
connected directly to a test load - then find it bursts into oscillation,
or its other properties alter - when given some other load. I've also seen
this happen when someone was using an oscilloscope that didn't reach the
oscillation frequency. So the audio waveform became distorted, but with no
visible sign of the RF bursts until they tried a faster scope.

Must admit I am surprised that Eeyore seems to have missed this point.
Although I can appreciate that audio engineers may not know the techniques
used by RF and microwave engineers to measure something like cable
properties.

Slainte,

Jim

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