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ccs-loaded sziklai
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Old 11th June 2008, 05:16 PM   #11
lumanauw is offline lumanauw  Indonesia
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You can also look at GregBall's (Amplifierguru) conceptual schematic. It make higher gain by bootstrapping the collector resistor of the driver.
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Old 11th June 2008, 05:48 PM   #12
ionomolo is offline ionomolo  United States
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I will give a look at the boostrap idea to check if it can go higher frequencies or perhaps compensate somehow the gate stopper + gate capacitance pole. I have no idea if this is possible so don't take it as a statement but only as a "thing i might check"

Thanks!
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Old 11th June 2008, 11:00 PM   #13
AKSA is offline AKSA  Australia
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Ion,

Use a 5mA CCS to power the circuit earlier described, and run 1.5A through the device with 0.47R source degeneration to reduce transconductance (and hence OLG) slightly. A 1.5A CCS below the CFP is required to sink the standing current to ground, of course. Measured Zout is around 80 milliohms and 3dB points are at 18Hz and 75KHz, with Zout halved if you use double the number of mosfets top and bottom.

You do not need to worry about bandwidth. Measured and subjective speed is astonishing. The challenge is how you will drive the bipolar driver. I use a tube with cap coupling. Speaker is cap coupled too, using around 2,200uF.

Hugh
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Old 12th June 2008, 04:30 AM   #14
jam is offline jam  United States
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Hi Hugh,

Have you ever tried a Sziklai with a mosfet driver and bipolar output?

Jam
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Old 12th June 2008, 05:10 AM   #15
AKSA is offline AKSA  Australia
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Hi Jam,

I was in your town in transit last year, hugely complicated airport....

No, haven't tried it because of rail voltage issues. But I understand Steve McCormack uses something like this in his very good amps?

Hugh
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Old 12th June 2008, 09:02 AM   #16
ionomolo is offline ionomolo  United States
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Quote:
Originally posted by AKSA
Ion,

Use a 5mA CCS to power the circuit earlier described, and run 1.5A through the device with 0.47R source degeneration to reduce transconductance (and hence OLG) slightly. A 1.5A CCS below the CFP is required to sink the standing current to ground, of course. Measured Zout is around 80 milliohms and 3dB points are at 18Hz and 75KHz, with Zout halved if you use double the number of mosfets top and bottom.

You do not need to worry about bandwidth. Measured and subjective speed is astonishing. The challenge is how you will drive the bipolar driver. I use a tube with cap coupling. Speaker is cap coupled too, using around 2,200uF.

Hugh
75 Khz (I assume closed loop) is more than enough for an amp without global nfb, but far too low if some feedback is applied.

Did you measure the slew rate? Was yours built with mosfets similar to the IRFP9240?

I have all the requiered parts on hand, so i will do an compare it with the opamp driven bjt sziklai. I have just realized that i did ask a couple of npn's for it instead of one pnp and one npn! I will have to look for a driver transistor locally.
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Old 12th June 2008, 10:16 AM   #17
AKSA is offline AKSA  Australia
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Ion,

With just 80 milliohm Zout you do NOT need gnfb - not required. And with the OLG of the bipolar driver CCS loaded the local fb factor is probably about 70dB anyway.....

Any voltage amp you use in front of this circuit block would need only local feedback, such as an opamp. A tube would NOT need fb, of course.

Slew rate is better than 20V/uS. More than enough for audio. Don't get hung up on the figures. Build the damn thing.

Hugh
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Old 12th June 2008, 03:15 PM   #18
ionomolo is offline ionomolo  United States
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I will build it with IRFP9240 + MJE15034 + 2 2N5087 for the upper CCS and a MJE15034 + MJL4281A for the big sink. I will bias it with 0R4 to get arround 1.6A.

Is that similar to your setup?

A lot of thanks for bringing this idea into reality.
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Old 12th June 2008, 03:30 PM   #19
jam is offline jam  United States
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Hi Hugh,

Next time you are passing thru please let me know, lunch is on me.

I was thinking of using a mosfet bipolar Sziklai output stage in an amp that did not use a global feedback loop.
I am not sure if Steve McCormack's amp used this topology or a standard follower.
I guess a schematic of the output stage would help.

Regards,
Jam
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Old 13th June 2008, 03:31 AM   #20
AKSA is offline AKSA  Australia
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Thanks Jam,

I will certainly remember that.....

I don't believe McCormack publishes his schematics, but we could put out an APB anyway.

Anyone seen schematics of the McCormack amps? Are they public domain? I understand he uses bipolar outputs and mosfet drivers, is this true?

Cheers,

Hugh
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