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Old 2nd October 2002, 02:46 AM   #51
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Default 3-way CFP?

Hi Hugh.
Sorry I hadn't replied to your posts before. Yeah, I am interested in trying a CFP but as I want to keep an N-channel fet I might have to do a 3 stage. I'll just stick a lot of emitter degeneration in the second stage to try and keep the gain down to a reasonable level. Might even revisit a few ideas from the CDA amp. e.g. the error signal multiplication transformer cct, and/or the error feedforward cct using an opamp. They both worked more or less ok at the time but were not quite suitable for what I was doing then. BTW, who performs the "Post Modern Blues" track on that CD?

Hi traderbam
What I am also going to do is try a unity gain opamp buffer ahead of the nfb resistors and lower these to about 3k3 and 33k. I hope the much lower cct impedance will then make any fet capacitance variation unnoticeable. How much trouble wil I get into off you guys for using an opamp?

GP.
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Old 2nd October 2002, 09:00 AM   #52
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Aksa,
I'm trying to say that the CFP circuit as drawn doesn't really have an output impedance of its own...rather it has a high beta. Like a very high gain BJT. This beta is 500k (according to my educated guesses). So the output Z of the amp using the CFP depends on the Z that the input of the CFP sees. In Circlotron's circuit this is 100k-ohms. So the output Z is 100k/500k=200mohms. This is twice as high as the simple FET follower (according to my educated guesses).

In other words the driver stage output Z is critical to calculating the output stage output Z. In this case the output stage does not define the output Z on its own.

I recognize that the CFP has the benefit of reducing the influence of the FETs Cgs and Cgd and to linearize the gm.
I would be careful about omitting the Zobel network - the CFP is a feedback system in itself with a minimum 90deg OL phase lag because of the FET. If a pure capacitive load is driven this will add another 90deg. The Zobel will help to mitigate the phase shift. Especially beneficial when loop feedback is used.
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Old 2nd October 2002, 09:09 AM   #53
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Default evil thoughts

GP...considers using an op-amp

Why spoil this lovely discrete, metropolis-style design with a modern prefab? Like British sausages you never know for sure what they've put in them.

Why not use a simple emitter-follower BJT - something small with high gain.
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Old 2nd October 2002, 11:33 AM   #54
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Hi Bam,

OK, I can't argue with your theory; seems sound to me.

However, I have built it, worked it over, tweaked it, and listened to it - for about five years now, and it's very good, works brilliantly.

To answer your points:

>I'm trying to say that the CFP circuit as drawn doesn't really have an output impedance of its own...rather it has a high beta.

If you select a stage current, a drive circuit, and a load, then surely it must have a Zout. At 3A, two paralleled mosfets top and bottom, 50V supply, and 8R load, I measured better than 40 milliohms.

The circuit block as shown has a beta, or voltage gain, of one. The driver has high gain, of course, determined by the combination of it's collector load and the transconductance of the mosfet. While it makes sense to figure the Zout from the source impedance of the driver and the current magnification, I measured the Zout by reading off the output voltage unloaded versus loaded, and using the familiar (1 - loaded/unloaded) x 8R.

I seem to remember a 180K bias resistor on the base of the CFP driver, supplying roughly half Vcc, and a source impedance (from a tube, in fact) of around 15K.

>Like a very high gain BJT. This beta is 500k (according to my educated guesses). So the output Z of the amp using the CFP depends on the Z that the input of the CFP sees. In Circlotron's circuit this is 100k-ohms. So the output Z is 100k/500k=200mohms. This is twice as high as the simple FET follower (according to my educated guesses).


In my circuit, this same calculation is 15.2K/500K, which is 30.4 milliohms, close to the 38 milliohm measured. Pretty close.

>In other words the driver stage output Z is critical to calculating the output stage output Z. In this case the output stage does not define the output Z on its own.

Ah, I see now! Yes, of course, I was relating a portion of my amp design, not giving it all away!!! Moreover, I have not revealed how I interface the tube to the CFP; there is a further trick here which I cannot talk about.

>I recognize that the CFP has the benefit of reducing the influence of the FETs Cgs and Cgd and to linearize the gm.
I would be careful about omitting the Zobel network - the CFP is a feedback system in itself with a minimum 90deg OL phase lag because of the FET. If a pure capacitive load is driven this will add another 90deg. The Zobel will help to mitigate the phase shift. Especially beneficial when loop feedback is used.

Yes, Bam, perhaps, but I found that a Zobel slightly degraded the sound and yet the amp is quite stable into an ESL63, which it has been happily driving now for about three years!

Thank you for your continuing interest.

Cheers,

Hugh

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Old 2nd October 2002, 01:54 PM   #55
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Default Circlotron shows some backbone; laughs at peer pressure.

Well I put the opamp buffer in as I said I would and it fixed up the wobbly virtual earth node that was indeed getting pushed around by external things, probably by the varying fet capacitance. I replaced the 100k & 1M with a 3k3 and 33k as I said. Used half a Texas LTC2272 because I had a few. Will try and post a cct about 12 hours from now. With the source resistor bypassed and bootstrapping applied that little non-audio fet has a gain of almost exactly 3000 Close the loop from the speaker output and it drops to almost exactly 10. No oscillation problems but when I turn it off, as the rails are going down it makes a single blurt noise. Is it trying to tell me something?

Now that I'm satisfied that the cct likes to be driven with a low Z I've got no problem using an emitter follower buffer instead of an opamp. It has to swing +/- 2.7 volts into 3k3. Will anyone out there encourage my laziness by suggesting a cct?

GP.

PS. thanks heaps for the interest, guys / blokes / dudes.

PPS the opamp sounded just fine, but I was having imagination attacks because everyone tells me they are just no good.

GGP.
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Old 2nd October 2002, 02:12 PM   #56
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Default Re: Circlotron shows some backbone; laughs at peer pressure.

Quote:
Originally posted by Circlotron
PPS the opamp sounded just fine, but I was having imagination attacks because everyone tells me they are just no good.
Tell you what, close your eyes (very important) and just listen. What do you hear?

When I do it myself, I don't hear my overkill (according to some, the overkill). I hear purely the signals coming from my Ortofon FF15 + TT JVC direct drive, 70's, which are surprisingly good for being one the cheapest pickup available (25 USD). I'm talking about my phono amp.

It's important not to see the gear.
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Old 3rd October 2002, 02:26 AM   #57
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Default This is it now.

I really have to get some better speakers for testing. What I am using are a pair of Plessey C100 10" things in a single 100 litre sealed box, with a pair of ~40mm cone tweeters from some old Philips tv just hanging in mid air on their wires. The ones I have inside the house you just don't want to know about ^2.

GP.
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Old 3rd October 2002, 11:56 AM   #58
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Default Mother of all constant current circuits.

During the last week on and off I had been messing around with various constant current circuits, both mina and other peoples. The purpose is to use it as a drain / collector load instead of a resistor because it can act like a super high value resistor in that when the signal voltage across it changes the current through it hardly changes at all. Therefore, the transistor or fet attached to it gives heaps of gain and very good linearity. Anyway, here is something I threw together late this afternoon, and wow does it go! It has a slope resistance of greater than 400 megohms. Here's how I measured it:

I got a zero to 60vdc supply and attached it to the cct and put a 100R resistor in series, and put a decent meter across the resistor. Cranked up the voltage and it starts to work at a little over 10v. The voltage drop across the resistor (i.e. the current) stayed constant from 20v to 60v. What do I mean by constant?? I cant remember the exact figure but it was about 51 mA so say there was 5.12345 volts across the 100R resistor. At 20v the reading dithered from 5.12345 to 5.12346. At 60v the reading *still* dithered from 5.12345 to 5.12346 so you see it changed LESS THAN 10 microvolts across 100 ohms, which means less than 0.1 microamps even though the applied voltage changed by 40 volts. R=V/I = 40/0.0000001= 400,000,000 ohms mimimum! I'm satisfied with that.

Looking at the cct a little further, if you ran a resistor from the fet drain to supply rail, and replaced the LM317 "output" resistor with a collector-emitter connected across it you would have an outrageous cascode stage would you not? The output would be pure current and soooo linear. What's more, if you had a current source instead of the top load resistor, the stage gain would be nothing short of ridiculous raised to the power of a googolplex. If nfb was applied to bring the gain down to a sane level, the distortion should be improved by a similar amount. Overkill to the max.

GP.
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Old 3rd October 2002, 01:20 PM   #59
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Default CFP with N channel MOSFETS??

Ok, I have a MOSFET follower amp.. with a +12 -12 split supply. lower mosfet configured as a CCS. upper mosfet gain of 1.

I have always had trouble with gate capacitances and driving them adequately.

how does one use the CFP stage with N channel mosfets?

Thanks!!

Richard
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Old 3rd October 2002, 01:41 PM   #60
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Default Not my speciality

Hi Richard.
You would do well to ask Aksa about this. He is way better at that than I am. Sorry. Over to you Hugh?

GP.
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