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Old 10th April 2010, 04:01 AM   #81
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Default Finally measure the little composite amp

So the little guy is run on ±30 volts with this lame bad ground set up in the picture. Slew rate is 16V/µs. Test signals for wave forms is at 10 volts peak to peak. I hope the rest is clear by the pictures. Load conditions are under each image. Blank is just the resistor. The last row of three are capacitors only. Matrix is THD. Bottom values are with an 80kHz low pass filter on the distortion detection circuit. For the square wave a 50 ohm resistor was placed across the input. With the 50 ohm resistor and no signal output is 400µV rms wide band... 2-250kHz Av= 20 Higher gain would improve stability of course also.

Yes this amp will oscillate driving these bad loads as soon as it gets hot which happens really fast...I mean really fast with the 1µF at 50kHz. But that is to be expected. The amp design was just thrown together and there are at least 5 optimizations I know of to make it significantly better and more stable including a better input IC than the LF412.

The amp is just sort of there on the bench and not really proper ground with the power supply yonder. My hand for the camera is not to steady but should be good enough. Looks like a pretty good amp to me but others may disagree which is just fine. Said I would post the measured data so here it is. Believe it or not as you like but it is what it is.

Enjoy... I hope.
Attached Images
File Type: jpg DSCF0979.JPG (108.9 KB, 913 views)
File Type: jpg SCOPE.jpg (92.9 KB, 897 views)
File Type: jpg DISTORT.jpg (43.2 KB, 865 views)
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Old 10th April 2010, 01:17 PM   #82
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Default Wildly low output impedance

Measured the output impedance at 10kHzand 12.5 Vrms... at least I tried hahaha. When I referenced to a different ground to the star and not the wire ground going to the load from the star and the feedback/ouput point I got no change in reading at all on a 5 3/4 digit meter. Yikes! Certainly less than .008 ohms at the output wire and load ground wire so damping of 1000 into 8 ohms and 10kHz. These points include traces and one length of 18ga wire 3 inches long. Messed with this quite a while with the proper ground and decided a 5 count variation is about what the oscillator/meter does short term so 100µohms, non-sense I say! High gain with feedback should have low output impedance anyway.

If it was 10 ohms open loop (reasonable?) and 100db of feedback (feedback is correct) that is 100µohms. That is just plain crazy low. Damping of 80,000? I do not believe it! Have been simply switching the load (4 ohms) on and off and taking readings all the while. Anyone got a better idea? Never tried to measure this anyway because I knew it had to be extremely low so who cares?

Listening experience says this chip amp does have fantastic bass and sounds better than most amps I and other listeners ever heard contrary to some other post....not just my ears?
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Old 11th April 2010, 04:34 AM   #83
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Quote:
Originally Posted by thetube0a3 View Post
I went from all 1950s and 60s Fisher and Scott amps to a Bryston 4B to home built LM3886 amps.

The only thing I can say to people who claim discrete is better is, good luck matching your transistors even remotely as close as those in a chipamp. And to people who claim they can be matched close enough, I guess you can argue any point so use what makes you happy. My fisher and scott amps sounded great, the Bryston 4B sounded great and gave me bragging rights. The LM3886 amps give me great sound and simplicity. And since bragging rights are useless when it comes to getting the job done, I'm sticking to my lm3886's

Build and use what makes YOU happy. Thats what its about.
Theres more to a good amplifier than matching transitors.
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Old 11th April 2010, 12:43 PM   #84
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Come to think of it , a Bryston 4B does sound like a chip amp , with power...
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Old 11th April 2010, 12:50 PM   #85
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Originally Posted by a.wayne View Post
Come to think of it , a Bryston 4B does sound like a chip amp , with power...

You know- I tend to agree. Keen observation.

Glad my "chip (pronounced cheap) amp" above does not sound as bad as the Bryston 4B! The others I have heard, yes.
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Old 10th June 2010, 01:06 AM   #86
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Quote:
Originally Posted by Mooly View Post
The ZD-50 looks quite impressive from a quick look through the articles and having a slook at the scope traces.
It has only one problem - not good clip behavior. To fix that it is possible to use non-linear feedback, but it is quiet complicated do to with LM3886 (because of it's Spike protection). On the other hand if do not listen the amp in constant clip, this behavior has nothing to do with the sound.
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Old 10th June 2010, 01:22 AM   #87
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Quote:
Originally Posted by AndrewT View Post
I think he meant open loop gain.
But at 10kHz that would imply about 100dB of feedback.
How does he make it stable with that much feedback?
Personally I do not see any problem to do it even greater than 100dB (using multi-pole compensation). However distortions could be not reduced as expected, cause there are other significant factors (such as input/output stage/power lines magnetic coupling).
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Old 10th June 2010, 01:30 AM   #88
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Quote:
Originally Posted by tomchr View Post
The LM3886 does show 2nd and 3rd harmonics above the noise floor at low power. It would be interesting to see if the composite amp could remove those.
I can say for ZD-50 - yes, it does. They are below the noise floor, as well as the noise floor (itself) is significantly reduced too. I could say it is almost perfect in all aspects except clipping. Clipping behavior is not good, but is not so bad (I posted the worst cases in the thread, however with an output filter it is much better).
PS. It looks like nobody got it here, but ZD-50 is not just a composite amplifier with a conventional feedback, it is rather an error correction. U2 works (almost) only with non-linear distortions, linear are compensated.

Last edited by Nickolay Shvydky; 10th June 2010 at 01:53 AM.
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Old 10th June 2010, 03:03 AM   #89
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Nickolay- Looked at the ZD-50 amp- pretty fancy stuff! Someone has gone to a lot of trouble to attempt solve problems. I used the "hit it with feedback" approach. I did not optimize this design and did not desire extremely low noise as parts in stock were used for values. I like the active LPF in front of the ZD-50 amp and when that is an option that is great! In post 60 of "MY ZD-50..." oscillation is visible in the clip recovery...hummm. Would like to see test data similar to my measurement just for comparison. Note I did not even trim DC offset but used a servo instead...easy but always increases distortion because of the differential pair imbalance. Many ways to improve my simple design.

I think the key point here is now there are clearly at least 2 designs which work very well and I know mine sounds very good according to over 30 listeners and hundreds of professional engineers who have used the system the amps are in.

Two great efforts!
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Last edited by sumaudioguy; 10th June 2010 at 03:10 AM.
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Old 10th June 2010, 03:53 PM   #90
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Quote:
Originally Posted by a.wayne View Post
Come to think of it , a Bryston 4B does sound like a chip amp , with power...
That is why I loved the Bryston amps, from the 4B till the 28B's so much, as well as the powerfull BPA chipamps (with nested feedback).

It is a matter of personal preference.

I like tight, clean and detailed. (don't confuse that with "harsh" or "aggressive") However even a single chip with good implemented nested feedback is tighter and more detailed in bass then any Bryston.

With kind regards,
Bas
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