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Old 22nd June 2012, 08:58 AM   #461
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Quote:
Originally Posted by Bonsai View Post
You can never escape the fact that as frequency goes up, phase shift accumulates with the 'conventional' topologies we talk about in audio. The thing that stops us easily getting ultra low distortion without having to do circuit gymnastics is the output stage bandwidth
I think phase shift must ultimately accumulate with ANY circuit.
By the time you add typical gate stopper resistors to MOSFETs I am not sure if the speed is so much better as to be worth the effort.
What I currently study is how fast I can make a conventional BJT OPS.
The NJL4*** transistors are nominally 35 Mhz Ft. Is the typical 1-2 Mhz unity crossover frequency really the best one can do?
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Maybe we have to re-think this whole story...
THD sub 100ppb at 20KHz at any power level up to full power into 8 Ohms
Now the e-amp is finished you can start work on this

Best wishes
David
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Old 22nd June 2012, 09:31 AM   #462
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Hello Dave,

Check out the 2SA2040/2SC5707 (available from Mouser) - max current 8 amps, linear HFE vs. Ic up to about 3 amps, minimum HFE of 200, fT > 200 MHz for 100 mA < Ic < 2 A, Cob @ 10 V Vce 50/28 pF respectively. These are small transistors so have very small SOA - you would have to use several in parallel* in a rail-tracking amplifier (supply their power from a low-voltage floating supply whose midpoint is driven by a second amplifier; frequency response of two amplifiers must match very closely in the audio band).

* note five in parallel still gives under half the Cob of one MJL4302.
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Old 22nd June 2012, 10:14 AM   #463
Bonsai is offline Bonsai  Taiwan
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Dave/Harry

Yes, I agree bipolars are also an option.

For the mosfets, I think there has to be a lot more focus on layout to enable one to exploit more of the potential bandwidth. I am currently very involved in mostfet power switching and external layout is absolutely critical for clean switching (we are talking 2-3ns rise times and 10ns fall times). So, you have to treat the PCB layout of the output stage and the mosfets as a system.

C'mon Edmond, put your thinking cap on - but remember, 12 trannies max!
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Old 22nd June 2012, 10:17 AM   #464
Bonsai is offline Bonsai  Taiwan
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"Now the e-amp is finished you can start work on this"

I am taking a break. For a few weeks ;-)
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Old 22nd June 2012, 11:18 AM   #465
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Originally Posted by HarryDymond View Post
Hello Dave,
Check out the 2SA2040/2SC5707
Thanks for the recommendation but breakdown Vce too low for me. I want to build a few amplifiers and a research project on rail trackers is just too much!

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Originally Posted by Bonsai View Post
For the mosfets, I think there has to be a lot more focus on layout to enable one to exploit more of the potential bandwidth. I am currently very involved in mostfet power switching and external layout is absolutely critical for clean switching (we are talking 2-3ns rise times and 10ns fall times). So, you have to treat the PCB layout of the output stage and the mosfets as a system.
That is the reason I have chosen BJT. I have no experience in MOSFET and I don't need a research project there either! If you have the expertise then they probably have more potential.

And 12 trannies max? I plan to use that many just in each OPS!
JBL pro drivers

Best wishes
David
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Old 22nd June 2012, 11:38 AM   #466
PHEONIX is offline PHEONIX  Australia
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Hello Dave

By 12 Trannies in the OPS do you mean 6 PNP & 6 NPN . Do you plan to use these in an error corrected output stage for lower THD.

Regards
Arthur
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Old 22nd June 2012, 12:02 PM   #467
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Originally Posted by PHEONIX View Post
By 12 Trannies in the OPS do you mean 6 PNP & 6 NPN . Do you plan to use these in an error corrected output stage for lower THD.
Hi Arthur

Yes 6 PNP & 6 NPN outputs + pre drivers and drivers so 16 in total. 3 kilowatts of On semi's finest transistors. No HEC, don't need another research project there either! I think Andrew's (Bonsai) and Dadod's amps (for instance) show you can obtain distortion that is below any reasonably audible threshold with conventional OPS and just advanced compensation. The advanced compensation is as much research as I need. Any one want to start a study team on Lurie's "Classical Feedback Control"?

Best wishes
David

BTW. Where are you in Oz?

Last edited by Dave Zan; 22nd June 2012 at 12:17 PM. Reason: Added BTW
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Old 22nd June 2012, 12:04 PM   #468
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One conventional method I see used by accuphase to remedy the bandwith situation is the use of cascoded drivers. Its also not just conventional cascode but they use a hawksford type cascode on their drivers. I have experimented with it but so far I am unable to get such a setup stable, I still dont know how they manage it but theyve been at it since the 90s so there must be a way. Maybe someone has a schematic of later year model, I would be interested in seeing it. Any comment on how much this could improve matters ??
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Old 22nd June 2012, 12:10 PM   #469
PHEONIX is offline PHEONIX  Australia
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Hello Homemoder,

How do you know they actually cascode the predrivers in the Accupase.

Regards
Arthur
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Old 22nd June 2012, 12:19 PM   #470
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From the literature they have on their amps. They show simplified schematic and describe the circuit on nearly all their amps but not the fine details. It works to a certain extent, I have tried on a blameless the problem arises with oscilation and to a lesser extent the temp compensation which gets affected as well. No matter what I have tried Ive failed to get it stable.
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