I find some of this hard to believe. I guess a little bit of knowledge
really is dangerous.

If anyone around here other than me knows how to design a transistor
amplifier stage, they know the cap is critical to some extent. You
cannot make the input impedance really high, like you can with a tube
(valve) amp.When dealing with bipolars, almost any stage needs an
emitter resistor and maybe even some bootstrap. Based on the gain of
the transistor you can raise the input impedance, perhaps even use a
common collector stage in front.

The problem is for a low distortion audio stage you need to drive the
base with a smoothly changing current, not a solid voltage.

I have experimented too, I have made amps distort in a certain way, so
I could see it on a scope as well as hear it, so I am not a babe in the
woods here.

Now I say this, you make the input impedance and gain of the stage what
you want, then select a coupling capacitor that will pass effectively
down to like 10Hz. But when driving a tube (valve) or FET it doesn't
matter as much. On high input impedance devices of course you need a
resistor, in the case of a tube, from grid to ground as well as a
cathode resistor to provide negative bias. In the case of FETs, well
some of them are enhancement mode and don't need a source (cathode)
resistor. Still if you intend to couple capacitively, there must be a
resistor across it.

When the input impedance is really high, it makes sense not to shunt it
out with a low value resistor. All you need is enough mhos to keep the
leakage and a few other things out of the picture. The higher the value
resistor, the lower value capacitance needed. Reactance + Ohm's law,
simple.

On that note I will add, keeping the input impedance high is good for
good sound. No capacitor is perfect. The thing is, what is imperfect
about capacitors ? ESR and ESL. For the uninformed these are effective
series resistance and inductance. (or just not up on all the three
letter acronyms)

The impedance of a capacitor is the vector sum of it's actual reactance
(admittance) and ESR and for higher frequencies the ESL must be brought
into the equation. ESL can usually be ignored for audio frequencies,
except maybe in high power crossover capacitors (in speakers, I mean
passive crossovers)

No matter what the quality of the coupling cap, the higher the input
impedance that it is couplinjg to, the better the quality. This is
because the ESR and ESL become very minimal when a capacitor is feeding
a high impedance.

Also, I totally disagree with switching to bipolar caps in any
equipment, unless you have speakers so cheap that....nevermind that. To
explain:

Capacitors have several variables. Size, capacity, voltage rating,
ESR/ESL, longevity and cost. You shouldn't go lower in capacity and you
can't go lower in voltage. If you get smaller caps they are likely to
not have as low ESR and ESL ratings as the originals. If you get
bipolars they will be bigger or have higher ESR and ESL at any given
rating. That is unless they are made for current and if that is so they
will either be bigger physically, or very expensive.

If the OP can supply me with a schematic of the unit, I might be able
to come up with some good modifications. Wouldn't be the first time.
Pioneer and Realistic both made mistakes in the FM stereo decoder, that
was one I couldn't handle. The problem was inside the chip. All I could
do is throw in a couple resistors to reduce the distortion but that
resulted in a subdued high frequency response. See that's the other
thing, in transistors, you never put a capacitor across an analog
output unless it is ballasted, that is has a resistor in series with
it.

Really the problem with these recievers was partly over modulation by
the FM stations, but this IM and THC was coming from the earlier
generations of PLL stereo demod chips. Although the signal wasn't
really over modulated, it was more than the engineers expected. The
result was that it sounded great on classical and some other types of
music, but on hard rock it was almost intolerable. Especially on
material that was REALLY in stereo, I mean had alot of seperation.

If anyone really wants to improve sound quality, let's face it, as long
as the amp has less than 1% THD and IM, get away from it. No speaker is
that good. Wonder why there is no THD or IM ratings on speakers ? You
would shreik.

My speakers are actually among the very very few rated for THD. The
rating is 0.7% at 1 watt 1Khz sine wave. And that is a fantastic rating
for a speaker. At 10 watts it is probably up there close to double
digit, but that is ALL speakers. They used to say that what comes out
of a speaker is only as good as what goes into it, and that is
absolutely true. But setting sights on the real problems is better than
wasting a ****load of money on things that will not solve the problem.

I paid $400 for these BAs, they were ten years old and I paid MSRP for
them, why ? Because I heard them. They rival Dahlquists and Cantons,
other really good speakers of the days of yore. Big names do not even
approach the sound quality of certain speakers. Pioneer, Bose, even
Technics alothough their electronics used to be pretty good. Macintosh.
Macintosh for _____ sakes !, not even close. Macintosh may have made
some of the finest electronics in the world for audio, but they did not
really excel in speakers IMO.

The only Mac I ever had was a Stereotech 1200, their first reciever.
Little known, but I have it still even though it fried out. I had some
Mac copies of their monoblock tube amps, but that is not the same.
Later they came out with the Mac 4100, their first "official" reciever.
Was nice too, cost alot too. But the brochure had the lid off, and you
could see ALOT of similarity to the Stereotech 1200. ALOT. I mean it
used the very same power amps, with darlington outputs in the SJ series
from Motorola. Nice smooth gain curve on those SJ series outputs, very
smooth for a darlington.

Anyway, I say this to all, if you want better sound, 90% of the time
you need to look at speakers and speaker placement. Amps are pretty
much how they are if designed properly. If they're any good at all you
won't hear a difference. Of course they can put better tone controls on
one and make it sound better in a certain environment. But the speaker
makes the most difference.

JURB