Off Topic but need help on a Bryston 4B ST Amp

[zhangtony]

Hi, Sorry for the OT thread but I encountered a situation on my Bryston 4B ST amplifier.

The 4B ST has been placed without using for over 1 year. Then I have it set up yesterday, it plays, but I found something strange: The cone of woofer speaker moves foward and backward in a low frequency (say 1 Hz)!!!! And I am very sure this is not related to the music anyway.

I had made some test then:
1) if the amplifier disconnect from the pre amplifier, it is normal and the cone won't move

2) The cone moves in that strange way when music plays. especially when there is a strong output, like a drum strike, the speaker cone moves greatly.

So I guess there is something wrong with the 4B ST? It works not stable or what? Is this what people called "DC offset"? (I am new on electronics, sorry)

Then the following question is, can I try to repair the machine by myself? If this is just because some dust and just need some screw driver turn, I can do it.

I am living in China and have no idea where can I send it to repaired. I had send mail to Bryston asking for the closest repair center but I need to use the amp urgently.

Thanks for your opinion.

 

[SY]

1) if the amplifier disconnect from the pre amplifier, it is normal and the cone won't move

That suggests that it's your preamp, not your amp that's the problem.

 

[zhangtony]

But.....

Thanks for your advise!

I had tried 2 preamps as well as 2 CD players, and to make sure, used 2 set of cables, but the same.

I guess the 4B is not stable, it is ok when it is not playing, but have some DC output when put music into it.

 

[djk]

Envelope Distortion

It is a common, but seldom understood problem, and doesn't receive much discussion.

It can enter the chain in the recording process too.

http://www.audioasylum.com/audio/general/messages/219136.html

http://db.audioasylum.com/cgi/m.mpl?forum=hug&n=68792&highlight=envelope+distortion&r=&session=

Posted by djk ( M ) on April 18, 2004 at 20:06:39
In Reply to: Not the amp class or amp topology posted by Jon Risch on April 18, 2004 at 15:24:15:


"A simple cure is to use very large RC time constants, and stagger them by at least an octave apart."
I agree, and add:

the RC pole in the feedback loop must be a higher frequency than the RC pole formed by the load impedance and the power supply C, and the amplifier should have an input RC pole higher than the feedback pole.

"Most Class A DC coupled amps seem to be immune (Gee, I KNEW there was a reason that Class A sounded good!)"

Mainly because of the pseudo-regulation of the power supply that class A causes. Nelson Pass spoke to this once in a letter to Audio Amateur WRT the benefits of class A.

Operating at or near full power can cause the poles to 'beat' against each other (when not spaced properly) and cause the loudspeaker to bottom out from the self-generated infrasonic signals.

A JBL 2235 can handle 1KW clean program material, but bottom out with a 100W amplifier driven hard. While watching the woofer cone 'jerk', you could actually see the RC time constants in the supply and feedback loop EVEN THOUGH the input to the amplifier was high pass filtered at 20hz.

The RC constant of a Bryston 4B power supply for an 8 ohm load is about 1.3hz, the feedback loop constant is around 0.34hz.

I would raise the feedback constant to about 3.4hz by changing the 470µF cap in the feedback loop to 47µF.

I would also create an input pole at least an octave higher than this, 7hz~10hz. This could be done in the preamp or CD player too, just change the output coupling cap if it has one.