JOB circuit topology

Hello everyone,

I have recently came across a nice little amplifier from Goldmund - model SRA. It's a stereo 2x50W audio amplifier with a very nice sound. One thing that struck me was it's physical size - about 30cm x 30cm x 6cm ! The manufacturer uses a design approach in which the bandwidth/slew rate of the amplifier is very high (for an audio amp) and he calls it a JOB circuit.

It's small, it looks cute, but it's no little cosmetic piece whatsoever. It's a very musical and pleasant amplifier that costs about $1500 (surprisingly higher end price for such a size :)).

I did a search and found out that the JOB was first made by http://www.jobsys.com and Goldmund used it under license apparently.

Also, on the http://www.goldmund.com site you can see internal pictures of this little amplifier. 2 output power transistors (mosfets ?) per channel and little more. The heatsink is very small, and when I tested it myself, I could touch it no problem, it wouldn't hurt my fingers.

I'm not an electronics wizard but I do understand a little of it. Each designer has it's own approach to solve a problem but this thing, coupled with the sound quality I heard and the reviews I've seen seems to go against what I've seen designed - big class A amplifiers dissipating generous amounts of heat, or fast amps like the Stochino and others which have more complex circuitry than this one seems to have at first sight, etc.

Anyone ever investigated these kind of amplifiers and came out with some ideas of what is being used ? I was kind of amazed that I searched and couldn't find any discussion on the topic on this forum :)

Relevant links :

http://www.jobsys.com/
http://www.jobsys.com/jobstereodoc.htm

http://www.goldmund.com/old/phlib.html#secr
(inside pictures of the SRA amplifier, scroll down)


Best regards to all,

K.
 

mlloyd1

Member
Paid Member
2001-02-25 7:10 pm
Northern Iliinois
re: yeah, it does arouse the curiousity ......

Sorry, I can't post anything more than what you've found about the JOB circuit. I agree with you that it certainly sounds intriguing. The more I think about it, the more I think it's just a very well optimized version of a fairly conventional circuit. I am reminded of material I've read by David Rich in "The Audio Critic". Dr. Rich thinks very highly of of Bryston's design and performance. They appear to take a good basic topology and engineer the heck out of it to squeeze out the best performance and highest reliability the design is capable of. For example, very low gain stage distortion across the entire audio band (and beyond) without using cascoded gain stages.

Maybe someone who has one (or access to one) will open and give us some reverse engineering notes :)

Please ...
 
Re: re: yeah, it does arouse the curiousity ......

mlloyd1 said:
Dr. Rich thinks very highly of of Bryston's design and performance. They appear to take a good basic topology and engineer the heck out of it to squeeze out the best performance and highest reliability the design is capable of.

Same with NAIM (at least the early ones). I have heard it said that the circuit is essentially right out of the RCA ap notes.

dave
 
Hi all

The following might probably apply:

http://www.goldmund.com/old/tecampst.html

There is not much info though, but at least a little hint. When you look at these job amps you see that there is a module covered with a small heatsink which might in fact be a power op-amp plus some additional control circuitry. There seems to be no means for adjustments in this amps so there must be some functionality in these modules to overcome this (they write about using a DC servo, so there is just the bias current control that remains a mystery).

Regards

Charles
 
mikek said:
I would urge caution with Bryston's amps., its been reported that these are prone to cross-conduction, as the output topology does not facilitate efficacious base-charge extraction from the output devices....

cross-conduction? Could you explain (when it comes to SS i'm still pretty much lost)?

I used Brystons extensively when i was doing proPA. Good sounding and VERY reliable. The newest generations are even better. Somewhere here i have a 3B/4B schematic if anyone is interested in seeing it.

dave
 

mikek

Disabled Account
2002-05-15 1:12 pm
Howdy folks...

Cross-conduction......In otherwords, driving the amp. with a suitably high-freq. stimulus, (eg a 20KHz cosinusoid), will cause high, and uncontrolled shoot-through currents as one supply rail is intermittently shorted to the opposite rail.

This is a recipe for the rapid overheating, and eventual destruction of the output stage.

This is caused by a driver stage that is inadequate with respect to the rapid extraction of base-charge.

Such a facility is necessary for controlling the turn-off of one output BJT, while the device connected to the opposite supply rail is turning on.

Michael in Chigwell, Essex, UK.

:cool:

P.S: Bryston schematics are available from their website...