muting transistors bad?
Dave Rahardja
wrote:
>Howdy,
> I read an article on some guys home page and he said that some CD players
>use muting transistors at the output stage to short circuit the output
>during turn-on and turn-off. He recommends those transistors be removed
>because the parasitic capacitance across the emitter-collector will act like
>a voltage sensitive high frequency filter. The result is a lost of, quote,
>"fluency" in the music during normal operations. Is his claim vlaid?
The parasitic effect of transistors are quite small, probably in the pF
range (Crss for FETs). This, combined with an 8 ohm load, gives a cutoff
frequency in the GHz range. In fact, an unusually large parasitic capacitance
of 1uF is required for a cutoff of 20MHz. Even my cheap Radio Shack FET has a
Ciss of 7pF.
> He said that only CD players cost less than $900 use transistors for
>muting. All more expensive players use relays. If his claim is true, then
>how can manufacturers claim a +-0.3db response curve from 20Hz-20KHz even
>for CD players cost well less than $900 (like my Sony CDP997, it does use
>muting transistors. 3/ch in fact)?
I don't think the muting transistor's parasitic capacitance affects audio
output with any significance at all. But besides, I wouldn't trust those
specs anyway (grin)... especially those units made in Japan and SE Asia --
they have a way of exaggerating :)
Any comments, anyone?
/+_,--___ _|
++( P \_\ \ ---------------------------------
|++ U ) ) \_\ Dave Rahardja at ORU, Tulsa, OK
-++ ( P /__/\:: ---------------------------------
/++++__--~ |/
John Gilliver
: On 28 Nov 1995 06:14:21 GMT, uac...@mcs.drexel.edu (Andy "LaLa" Chang)
: wrote:
[]
: > He said that only CD players cost less than $900 use transistors for
: >muting. All more expensive players use relays. If his claim is true, then
: >how can manufacturers claim a +-0.3db response curve from 20Hz-20KHz even
: >for CD players cost well less than $900 (like my Sony CDP997, it does use
: >muting transistors. 3/ch in fact)?
: I don't think the muting transistor's parasitic capacitance affects audio
: output with any significance at all. But besides, I wouldn't trust those
[]
I'd agree - and I'd also be dubious about the use of relays for audio signals
- you never know quite what the mechanical conditions are inside the thing!
If it is a mercury wetted one or similar it _might_ be OK, but in general, I'd
be very dubious about mechanical contacts for audio signals. (Those shops
where you can switch between different systems often introduce far more
difference with the switch that there is between the systems!)
--
J. P. Gilliver | Tel: +44 1245 473331 x 2133 | Usually NOT
GEC-Marconi Research Centre | Fax: +44 1245 475244 or 478639|writing for GEC!
GEC-Marconi Ltd, GREAT | Home: G6...@soft255.demon.co.uk, G6JPG@GB7NNA
BADDOW, Essex, CM2 8HN, UK. | Email: john.g...@gecm.com (-:!
J. Szamosfalvi
: Dave Rahardja (dav...@pobox.com) wrote:
: : On 28 Nov 1995 06:14:21 GMT, uac...@mcs.drexel.edu (Andy "LaLa" Chang)
: : wrote:
: []
: : > He said that only CD players cost less than $900 use transistors for
: : >muting. All more expensive players use relays. If his claim is true, then
: : >how can manufacturers claim a +-0.3db response curve from 20Hz-20KHz even
: : >for CD players cost well less than $900 (like my Sony CDP997, it does use
: : >muting transistors. 3/ch in fact)?
: : I don't think the muting transistor's parasitic capacitance affects audio
: : output with any significance at all. But besides, I wouldn't trust those
: []
: I'd agree - and I'd also be dubious about the use of relays for audio signals
: - you never know quite what the mechanical conditions are inside the thing!
: If it is a mercury wetted one or similar it _might_ be OK, but in general, I'd
: be very dubious about mechanical contacts for audio signals. (Those shops
: where you can switch between different systems often introduce far more
: difference with the switch that there is between the systems!)
Actually with 'muting relays' the relay is *NOT* in the signal path
when it's not muting, so aside a pf or two stray capacitance it should
not effect the signal in any way.
As for muting transistors, I've heard from at least two people that they
introduce some distortion and their effect is audible.
Sam Goldwasser
This is what I wrote when this question came up originally:
> I read an article on some guys home page and he said that some CD players
> use muting transistors at the output stage to short circuit the output
> during turn-on and turn-off. He recommends those transistors be removed
> because the parasitic capacitance across the emitter-collector will act like
> a voltage sensitive high frequency filter. The result is a lost of, quote,
> "fluency" in the music during normal operations. Is his claim vlaid?
You don't need a license to operate a Web page.
> He said that only CD players cost less than $900 use transistors for
> muting. All more expensive players use relays. If his claim is true, then
> how can manufacturers claim a +-0.3db response curve from 20Hz-20KHz even
> for CD players cost well less than $900 (like my Sony CDP997, it does use
> muting transistors. 3/ch in fact)?
My 10 year old Technics uses relays and it sure cost a lot less than $900.
Let's do a little calculation:
Parasitic capacitance, say 100 pF (much much larger than likely).
Highest frequency of interest: 20 KHz.
|Z|=1/(2*pi*f*C)= 1/ (2*3.14159*2E+4*1E-10) = ~80K. Compared to the
output impedance of a typical final audio stage, say less than 1K. Yeh,
I will loose a lot of sleep over that. There are better things to worry
about than an immeasurable blip in your frequency response curve. Are
the transistors at the very output? Oh my gosh, you better start
investigating super ultra low capacitance audio cables costing at least
$1000 each with water protected oxygen free tapered oriented conductors.
But wait: you are connecting to an amplifier with non-infinite input
inpedance (perhaps, horrible as it may seem, non-uniform as well)?
Your setup must sound like crap! How can you even have it in the same
house with you?
The specs quoted by CD player manufacturers are so bogus in any case
that this is just a pimple on an elephant.
Ask him to provide a scientifically designed and implemented A-B comparison!
Or, get a life.
Sorry, I just could not resist. I hope this does not start one of those
semi-infinite, zero information content threads.
--- sam
John Gilliver
[]
: Actually with 'muting relays' the relay is *NOT* in the signal path
: not effect the signal in any way.
Please explain how they are not in the path. (I'm not being [intentionally!]
rude, I just can't see how.)
J. Szamosfalvi
: J. Szamosfalvi (sza...@saul6.u.washington.edu) wrote:
: []
: : Actually with 'muting relays' the relay is *NOT* in the signal path
: : when it's not muting, so aside a pf or two stray capacitance it should
: : not effect the signal in any way.
: []
: Please explain how they are not in the path. (I'm not being [intentionally!]
: rude, I just can't see how.)
They can short the signal to the ground when closed, that's why they're
not in the path in the opened (non-shorting, non-muting) position.