"Eeyore" wrote in message
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Trevor Wilson wrote:

"Eeyore" wrote
Trevor Wilson wrote:
"Eeyore" wrote
Trevor Wilson wrote:
"Eeyore" wrote
Trevor Wilson wrote:
"Eeyore" wrote
Trevor Wilson wrote:

* Use an amplifier which can deal with 2 Ohm (or better) load
impedances,
without Voltage drop or current limiting (no MOSFETs or
valves,
of course).

MY mosfet amps will drive 2 ohms no trouble !

**That's what you think.

Do you know anything about them ?

**About the ones you use? Nothing.


Tell me about your objection to mosfets in this regard.

**I've never heard ANY MOSFET amp drive Quads adequately. Yours

may well be different.

Is your objection to them based on any specific scientific principle
is
what I was trying to get at.

**Indeed. Several. MOSFETs:

* Exhibit much higher levels of distortion, at low bias currents, than
(modern) BJTs.

Ok, I don't use them at low bias currents.

**Smart move. Of course, using BJTs eliminates that little inconvenience.
BJTs are MUCH better at low bias currents and slightly better at high
bias
currents. Win - win.

BJTs also have far worse cross-over characteristics.

**Nope. That would be a myth. Decently matched devices, coupled with modest
bias currents will eliminate all cross over distortion.



* Exhibit a negative tempco of gm. BJTs do not.

Is that a real problem ?

**Sure. If you're using the MOSFETs within a feedback loop. Or outside
one.

Why ?

**Think about it.



BJTs allow for external VI limiting, which, in turn, allows for ensuring
the
limiting can be kept outside any feedback loops.

I fail to see the distinction here between the two. In any case mosfets
require
far less VI limiting, being more durable, which is a 'good thing'.

**Not really. As a designer, you have no control over the point where
current limiting operates, nor what happens when it does. However, MOSFETs
do certainly have clear and unequivocal advantages. They are certainly more
durable and lack storage effects, thus providing an excellent HF response (I
built a MOSFET amp and measured a flat response out to 800kHz at 500 Watts!
Very impressive). None of which allows them to equal decent BJTs though.
BJTs are STILL lower in distortion. Particularly at low bias currents.



* Exhibit somewhat higher levels of distortion, at high bias currents,
than (modern) BJTs.

Gm related. Indeed. I use multiple devices in parallel with an
attentant
increase in output stage gm.

**Good stuff. Of course, BJTs are still better. And cheaper.

Since when was cheaper about good sound ?

**I did say they were better. Cheaper is just a bonus.


--
Trevor Wilson
www.rageaudio.com.au



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