JOB circuit by GOLDMUND

[Kal]

Hello,

I recently read a review of a power amp by GOLDMUND that uses the JOB-circuit. This amp seems to be breathtakenly fast... and great sounding!!!
And it is only made with a very few parts...

Has anyone more information about the JOB-schematic/circuit except for what is available at www.goldmund.com or www.jobsys.com?

Thanx

[lgreen]

I asked the same thing awhile ago, see my earlier thread:

http://www.diyaudio.com/forums/showt...965#post189965

I'm with you, I'd like to make one of these if provided with the right information on what's in there.

[arthur]

difficult to copy
it contains a double sided pcb with lots of smd transistors
no markings on the transistors also

[subwo1]

So far, the fastest topology I have found is current mirrors connected to the collectors of dual input pairs. They provide active base pull-down for the upper and lower VAS transistors. I wonder if that topology is what they use. Simulations I have done indicate a slew rate up to 200uV/s when used with complementary source follower mosfets. In the simulation, output device idle current was set at 500ma. Maybe these ideas will give you some guidance.

[Elso Kwak]

Originally posted by subwo1
So far, the fastest topology I have found is current mirrors connected to the collectors of dual input pairs. They provide active base pull-down for the upper and lower VAS transistors. I wonder if that topology is what they use. Simulations I have done indicate a slew rate up to 200uV/s when used with complementary source follower mosfets. In the simulation, output device idle current was set at 500ma. Maybe these ideas will give you some guidance.

Hi, I would replace the VAS transistors by Darlington connected transistors. The current mirror likes to see a high impedance as load.

[subwo1]

Hi Elso, it would just happen that we end up with such a trade-off. The fast, active pull-down of the current mirror likes the high impedance of a Darlington pair with slow response. But then several resistors would be needed to make up for the forgone self-biasing nature of the current mirror. Maybe lets just make that waif of a current mirror sweat and drive that single transistor.

[Charles Hansen]

Subwo1, try simulating the circuit with the Hitachi lateral MOSFETs as output devices. These are what are used in the Goldmund. Their lower input capacitance may speed things up even more.

Also, you could try adding separate emitter followers between the current mirrors and the VAS. I think this is what Elso meant, and it should also speed things up.

[Elso Kwak]

Originally posted by Charles Hansen


Also, you could try adding separate emitter followers between the current mirrors and the VAS. I think this is what Elso meant, and it should also speed things up.

Hi Charles,
No I meant something like the VAS stage as here:
http://home.kimo.com.tw/skychutw/ampzilla/index.html
Unfortunately the AmpzillaII schematic is gone but here is a detail.

[Charles Hansen]

I think the Ampzilla circuit is a variation on what Borbely called the "Lender circuit", where the emitter of the VAS is driven by one phase of the diff pair, while the other phase drives the base of the VAS. I can't see where the collector of what looks like an emitter follower is connected.

At any rate, adding an emitter follower to drive the base of the VAS will eliminate the slowdown from the Miller capacitance of the VAS. This is what I think we were both suggesting to Subwo1. However, I don't think it is possible to combine the "Lender circuit" with the current mirror.

[Kal]

Hi Charles,

I really like our amps, too!!! One of the best amps I ever heard!
But unfortunately I can not afford them at the moment...
I really would like to have your AX-7 in a unbalanced version(only RCA inputs).
For some reason I prefer unbalanced interconnects...