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Old 25th October 2012, 08:41 PM   #471
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Originally Posted by janneman View Post
@Loudthud: what was the circuit in the '170 drain lead used for your distortion measurements? Did you measure against earth?
Yes. The 100K input impedance of my Audio Precision System 1 is capacitor coupled. I just left the ground connected and moved the hot lead to different points. To double check, I connected the System 1 differentially across the 1K Drain resistor and there was very little difference in the distortion reading. In the scope photo the output is DC coupled before the output capicitor, the other two traces are AC coupled.

I'm using a linear bench power supply. My shop is about 2 miles from a 50KW AM radio station. At times it gets into things, the analyzer seems to need a ground wire clipped to the circuit under test. There is a 5 Megawatt TV transmitter about 300 yards from the shop, but I never noticed any interference from it despite using a 1 GHz scope.
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Old 25th October 2012, 09:20 PM   #472
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Originally Posted by Antoinel View Post
Loudthud:It seems to me that loop feedback is contaminating the intrinsic performance of Q4 [in janneman's schematic] which maybe pristine without it. What are the distortion properties at the source and drain of Q4 without loop feedback by comparison? How great is the performance of Q4?
Yes, inside the loop distortion can increase quite a bit. The non-linearity of Q1 and it's input capacitance create distortion that the feedback tries to correct. Feeding a square wave to the input you see a couple of hunderd precent overshoot at the Source of Q4 and a great deal more at the Drain. The output looks like an ideal bandwidth limited square wave.

I'll try to plot some meaningfull openloop distortion graphs.
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Old 25th October 2012, 09:47 PM   #473
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Originally Posted by Loudthud View Post
Yes, inside the loop distortion can increase quite a bit. The non-linearity of Q1 and it's input capacitance create distortion that the feedback tries to correct. Feeding a square wave to the input you see a couple of hunderd precent overshoot at the Source of Q4 and a great deal more at the Drain. The output looks like an ideal bandwidth limited square wave.

I'll try to plot some meaningfull openloop distortion graphs.
Thank you Loudthud.
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Old 25th October 2012, 09:58 PM   #474
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Originally Posted by janneman View Post
A few comments: if you carefully balance the bridged output for equal DC (half supply) there is no longer a need for output caps. If you want to play it safe, a single output cap on one side will do.

The bias current should be increased in the bridged mode of course. You don't get anything for free: quadruple the output power will also mean quadruple the (class A) dissipation and that results from a) double the bias current and b) two amplifiers.

@Loudthud: what was the circuit in the '170 drain lead used for your distortion measurements? Did you measure against earth?

jan
Thank you jan for your comments. I am glad that we agree on the options which can manage the mating of the outputs of 2 ACAs in bridge configuration.

This question posed by DIYers still hangs: Can I bridge 2 ACAs? I am afraid the answer is no for the following reasons:
  • The power supply is undersized.
  • The heat sink is undersized.
  • Doubling the idle bias needs specific modifications from Mr. Pass and/or his appointee.
Maybe others have a different verdict!
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Old 25th October 2012, 10:01 PM   #475
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Quote:
Originally Posted by Loudthud View Post
Yes, inside the loop distortion can increase quite a bit. The non-linearity of Q1 and it's input capacitance create distortion that the feedback tries to correct. Feeding a square wave to the input you see a couple of hunderd precent overshoot at the Source of Q4 and a great deal more at the Drain. The output looks like an ideal bandwidth limited square wave.

I'll try to plot some meaningfull openloop distortion graphs.
Well the overshoot is not about distortion, it is about gain-bandwidth, a different thing. The real test is to band-limit the square wave to say a 1st order 20kHz, then look at eventual overshoot.
You can always cause overshoot in any feedback amp if you make the rise/fall time high enough, but that's not really saying anything about the performance with music.

The reason for my question above is that I noticed that the distortion on the drain is almost double that on the source, which is curious.

jan
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Old 25th October 2012, 10:04 PM   #476
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Originally Posted by Antoinel View Post
Thank you jan for your comments. I am glad that we agree on the options which can manage the mating of the outputs of 2 ACAs in bridge configuration.

This question posed by DIYers still hangs: Can I bridge 2 ACAs? I am afraid the answer is no for the following reasons:
  • The power supply is undersized.
  • The heat sink is undersized.
  • Doubling the idle bias needs specific modifications from Mr. Pass and/or his appointee.
Maybe others have a different verdict!
The power supply is OK, needs no change, if it can supply enough current, or you use one with more current.
Increase heat sink, yes.
The bias current can be increased easily by lowering the 4 resistors that make up the bias sensing from say 0.68 to 0.39 ohms or so.
But I agree, it really becomes a different amp.
Just an idea I had...

jan
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Old 25th October 2012, 10:25 PM   #477
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Originally Posted by janneman View Post
The power supply is OK, needs no change, if it can supply enough current, or you use one with more current.
Increase heat sink, yes.
The bias current can be increased easily by lowering the 4 resistors that make up the bias sensing from say 0.68 to 0.39 ohms or so.
But I agree, it really becomes a different amp.
Just an idea I had...

jan
I agree jan. The DIYer will need a specific set of guidelines and a procedure to transform the current ACA to a bridgeable amp. Maybe Mr. Pass will create an ACA Turbo which can be bridged with no further DIYer modification of its circuit.
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Old 25th October 2012, 11:19 PM   #478
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Originally Posted by janneman View Post
Well the overshoot is not about distortion, it is about gain-bandwidth, a different thing. The real test is to band-limit the square wave to say a 1st order 20kHz, then look at eventual overshoot.
You can always cause overshoot in any feedback amp if you make the rise/fall time high enough, but that's not really saying anything about the performance with music.

The reason for my question above is that I noticed that the distortion on the drain is almost double that on the source, which is curious.

jan
Voltage at the drain in the split load phase inverter (an analog of the drain current) depends on the total impedance at the source which includes the gate of Q1 and it's gate stopper. The point of the overshoot being that Q1 is a substantial load. I tried the ACA using IRFP460's which have a Ciss over 4000pF. The results were terrible even with 3 Q4's in parallel and R9 at 330 ohms. Obvious distortion on high amplitude outputs even at 1kHz. The non-linearity of the capacitance was evident when Q1's drain voltage dips below it's gate voltage.
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Old 25th October 2012, 11:30 PM   #479
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Getting the analyer to plot distortion vs generator amplitude was not straight forward, it likes to plot against the analyzer amplitude. I think I got it right, but the graphs have some surprises. Q4's drain voltage is plotted single ended to ground and differentially across the 1K resistor. I didn't install the decoupling network that Jan used, but there is a 100uF cap bypassing the 19V right near Q4.
Attached Images
File Type: gif ACA1_closed loop.GIF (26.5 KB, 596 views)
File Type: gif ACA1_open loop.GIF (26.3 KB, 588 views)
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Old 25th October 2012, 11:49 PM   #480
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