[net.audio] Tubes/bipolar/FETs

seifert@ihuxl.UUCP (D.A. Seifert) (06/28/84)

Since we seem to be discussing tubes vs solid state, how about
tubes vs bipolar vs FETs?

power-FETs are supposed to have some of the 'good' qualities
of tubes. One of these is 'soft-clipping'.  My theory is get
a big enough amp or more efficient speakers and don't worry
about clipping.

Another spec that came out about the time power-FETs did is
TIM (and its siblings).  FETs tend to be fast and have good
bandwidth/low TIM.

Are there any other theories of why FETs sound better than
bipolars?  Anyone think bipolars sound better than FETs?

For FET amps, I'm thinking of the Sony 5650 and the Hafler.
(power amp, not pre-amp!  Everyone I know who had the Hafler
preamp has replaced it. Something about the bass not being flat.)

-- 
	_____
       /_____\		Hey, Woodstock, have you seen my sunscreen?
      /_______\	
	|___|			    Snoopy
    ____|___|_____	       ihnp4!ihuxl!seifert

fudd@fluke.UUCP (Mark Freeman) (06/29/84)

---

This is a reply to some comments by D.A. Seifert:

	power-FETs are supposed to have some of the 'good' qualities
	of tubes. One of these is 'soft-clipping'.  

	Another spec that came out about the time power-FETs did is
	TIM (and its siblings).  FETs tend to be fast and have good
	bandwidth/low TIM.

	Are there any other theories of why FETs sound better than
	bipolars?  Anyone think bipolars sound better than FETs?

The advantages of FET's over bipolars that I am aware of are:

1) They are positive temperature coefficient devices.  This is mainly
an aid to designers.

2) They are faster devices.  This may help increase slew rate.

3) They are closer to linear devices.  This is probably a more
important factor in reducing TIM.  When slew-rate limiting was
"discovered" by the audio industry, high loop gain was identified as
the culprit.  FET's are square-law devices (and V-FET's are are almost
linear devices), and so allow less feedback for the same level of
distortion.  Some designers have gone to the extreme of "no feedback"
(well, each stage has its own feedback), to prevent the evil TIM from
appearing.

When monolithic op-amps with FET inputs appeared, they were noted for 
their fast slew rate.  But not for the reasons you might expect.  
They were fast because the FET's had lower gain than the bipolars they
replaced.  This allowed the miller capacitor in the voltage gain
stage to be reduced, and delayed the onset of saturation of the first
stage (the cause of slew rate limiting).

An excellent article about this subject, from someone at National, I
believe, appeared around 1975.  I can dig it up, if you really want to
get into this subject.

dmmartindale@watcgl.UUCP (Dave Martindale) (06/29/84)

Another theory I've heard claiming that FETs were better than bipolar
transistors is that FETs do not have the tendency to "thermal runaway"
that bipolar transistors do, and thus no circuitry to compensate for
it is necessary, and thus there is one less thing to introduce nonlinearities.

I hadn't heard bad comments about the Hafler preamp before - which one
is it (Hafler 100 or 110) that "everyone has replaced"?

pmr@drutx.UUCP (Rastocny) (06/30/84)

My reference to teh CJ/Hafler comparison was the "equivalent priced
Hafler," that being the DH-110.  The Hafler sounds OK out of the box,
much better than a lot of products in the same price range, but not as
good as others.

While we're on the subject, can anyone hear differences between
bipolar preamps (let's just confine our discussion to this simple
voltage amplifier first) and FETs?  Theory says we shouldn't.  What
does reality say?  Has anyone recently "upgraded" their preamp and
noticed these differences I reference?

		Yours for higher fidelity,
		Phil Rastocny
		AT&T-ISL
		..!drutx!pmr

seifert@ihuxl.UUCP (D.A. Seifert) (07/04/84)

> I hadn't heard bad comments about the Hafler preamp before - which one
> is it (Hafler 100 or 110) that "everyone has replaced"?

Asked Joe about it yesterday, and he said they were the 101s.
(ok, is it 100 or 101?  -sigh- ) He said the 110 was supposed to be
much better.

The problem was that the frequency response in the bass region was
not flat.  (and perhaps they both had money burning a hole in their
pockets?  :-)   )

Once again, this only applies to the 100/101, not to Hafler products
in general.  I am quite pleased with my DH 200 power amp.  Joe bought
two of them and strapped them for mono. (We WILL keep our speaker
wires short!)

	Florescent light hum is cruel and inhuman punishment!
-- 
	_____
       /_____\		Hey, Woodstock, have you seen the demagnetizer?
      /_______\	
	|___|			    Snoopy
    ____|___|_____	       ihnp4!ihuxl!seifert

rs@hou3c.UUCP (rs) (07/09/84)

Just a peripheral question... What is the "slew rate" of an amplifier.
I have seen this spec for amplifiers but have not found a source
that defines it.

Any pointers to information on sound system engineering would also
be appreciated.

Thanks,
Bob Switzer
AT&T Bell Labs
...!houxf!hou3c!rs

fish@ihu1g.UUCP (Bob Fishell) (07/09/84)

(oo)
The "slew rate" of an amplifier is a measure of its ability to respond to
an instantaneous change in the voltage at it input from one DC value
to another, for example, from -3 volts to +3 volts via a hard switch.
This will cause the amplifier's output to "slew" from one voltage to
another at some rate, given in volts per microsecond, which is dependent
on the speed of the internal circuitry.  I believe that bipolar transistor
amplifiers have the highest slew rate in the pack.
-- 

                               Bob Fishell
                               ihnp4!ihu1g!fish

ron@brl-tgr.ARPA (Ron Natalie <ron>) (07/11/84)

Slew rate is how fast it can change from one audio signal to another.  Very
important for square waves which require infinite slew rate.

-Ron

ron@brl-tgr.UUCP (07/11/84)

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Slew rate is how fast it can change from one audio signal to another.  Very
important for square waves which require infinite slew rate.

-Ron