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Old 1st July 2013, 02:04 PM   #401
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by masood View Post
dear sir,
why transistors become heated and destroy when input of an amplifier is not shorted?
That could be a sign of instability (wiring/layout etc). Its not an inherent issue to this design.
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Old 1st July 2013, 02:49 PM   #402
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Blown amp due to open input is most often due to RF oscillation.
I came across this thread only now. The circuit in post 1 appears to be a typical single ended input stage with CFB with buffered VAS and CFP output stage. Should probably come out nice if the CFP stage doesn't give problems.
Interesting circuit. 60's input stage slowly moving on to an 80's or 90's output stage with refinements like CCS VAS load and dc servo control.

Has this been built yet ? Would like to know how it sounds.

I've tried a similar input stage with Darlington VAS and standard bootstrapped resistive VAS load and quasi comp MOSFET output stage. Sounds nice. Your amp has several refinements and a completely different output stage. Sounds promising. From what we hear , lots of people like the sound of amps with a CFP output stage ( eg. P3A).
Cheers.
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Old 1st July 2013, 03:05 PM   #403
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dear ashok,
thanks for giving answer but i want to know the principle .what is rf oscillation?plz explain and pardon me to this type of my silly question.thanks
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Old 1st July 2013, 03:21 PM   #404
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A quick answer is that the amplifier is generating RF ( radio frequency ) signals on it's own without any input. Some parts cannot handle such signals and heat up very quickly and break down. Sometimes very dramatically !

An in depth explanation is not possible through a forum. You will need to read a good book on electronics. Your question is not silly but a proper explanation requires you to read a book on electronics. I think a short answer here will not do justice to the subject.

You can start here:
Parasitic oscillation - Wikipedia, the free encyclopedia

And from " High Performance Audio Amplifiers"
http://books.google.co.in/books?id=-...lifier&f=false
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Old 1st July 2013, 04:24 PM   #405
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Thanks ashok Yes its been built and been in daily use since around 2005.

Starting post #283.
Post your Solid State pics here.

How does it sound... its all in the thread... check out some of the links in the first post.
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Old 1st July 2013, 05:27 PM   #406
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Hi Mooly,
I used a current source on the input transistor thereby negating the current flow through the main NFB resistor and made it very low in value.
I don't remember exact details now but when we did rigorous calculation of gain with NFB ( while studying !), simplification was valid only if NFB parts were much smaller than circuit elements.

In an unrelated circuit I used a resistive load with bootstrap and found it sounded better than a CCS in the VAS stage. Simulation showed lower distortion with CCS but a listening test showed otherwise.

So I used a resistive load with bootstrap in my version. Maybe it will sound better with a CFP stage which I had looked at but discarded because of the difficulty in getting N/P channel FET's easily. However it does sound very nice but there are better amps !

Filtering the input stage supply has become a standard. Ever since I heard the difference in an amp I made long ago. It was like chalk and cheese !
Never skipped that ever since. Regulated supply for the stages up to VAS is best especially if you can have an extra winding on the trafo or you use a voltage multiplier and regulator!
Note that some amps can sound great even without all this !

Cheers.
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Old 1st July 2013, 06:42 PM   #407
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I agree there is no substitute for listening, because ultimately it is that "sense" that has to be satisfied. Using higher voltage and filtered rails for the front end and VAS is definitely one technique that would bring benefits. This type of amp has a somewhat "average" PSRR as it stands, although in practice its never an audible problem, hum and noise being inaudible.

Interesting to hear your observations on the CCS vs more traditional methods...

I'm currently working on a successor to this design, and its evolutionary rather than revolutionary, but it does promise to take things up a notch, addressing most of the compromises of the present design. That said, this is an amp that has stood the test of time.
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Old 12th August 2013, 09:45 PM   #408
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Smile Help required

Hi Mooly, this was indeed a great thread and the discussions i am sure will be very much beneficial for a great working build. The pcbs made by Alex are wonderful. I am very much keen towards building this amp. Just require few inputs.
1. Can we use 3 watts resistor in place of 5 watts...will it create any issue. With the existing supply.
2. The ground of the speaker to be taken from amps star earth...and the signal earth also Connected to same star earth ...am i correct?
3. Can 2 similar boards be bridged at the same input voltage , with the help of a external opamp ckt. Then Will any design considerations to be taken in to account. Or the design remains the same? The output will be floating...
4. What is the best transformer rating...I mean the otimum VA. keeping excellent performance and cost to min.

Waiting for your reply ....thanks in advance...regards Saraswat.
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Old 13th August 2013, 07:53 AM   #409
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Hi, great to hear your thinking of having a go

1) Yes, 3 watts is OK for 8 ohm loading and the existing supply.
2) Speaker ground should return back directly to the PSU. The input grounds of both channels return separately back to the star ground.
3) Not really suitable for bridged operation. I certainly wouldn't recommend it.
4) A 300va transformer is ideal for a stereo pair.
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Old 19th January 2014, 05:25 PM   #410
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Hi mooly, hw r u?...pls check my pcbs. And give your views.....about to complete the same
The FETs are yet to be populated...have ordered them and they r on the way...just cantcan't wait to hear them play..have read about the fake FET s. Just wondering the ones i have ordered are genuine or not.

If you have any methods to test them easily for genuine pls do post the same...waiting for your reply ...regards ...
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