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Old 4th June 2012, 01:16 PM   #11
Bonsai is offline Bonsai  Taiwan
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Yes, you are correct dadod, providing local feedack is also a technique that can be used to reduce loop gain, but I chose to use simple loading in this design.
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Old 4th June 2012, 01:27 PM   #12
wahab is offline wahab  Algeria
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Pictures are great , the beast is very well built , among others
the boards are awsome looking..

Weird thing is Acrobat gives me an error message
for the PDF file....
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Old 4th June 2012, 01:29 PM   #13
djQUAN is offline djQUAN  Philippines
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Quote:
Originally Posted by wahab View Post
Weird thing is Acrobat gives me an error message
for the PDF file....
same thing here.
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Old 4th June 2012, 01:38 PM   #14
Bonsai is offline Bonsai  Taiwan
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Is the file opening on your side?
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Old 4th June 2012, 01:46 PM   #15
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Having reread the relevant pages 6 & 39, a few times, I think I've misread "tight" for "light". Apologies, but I'm still curious about the final thermal coupling scheme described for the output transistor.

Re: PDF files. I was increasingly finding this problem on some some sites a few months ago but Adobe Reader (also a free download) seemed to fix that. Worth a try?
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Last edited by Ian Finch; 4th June 2012 at 01:56 PM.
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Old 4th June 2012, 02:04 PM   #16
dadod is offline dadod  Croatia
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Could you tell what NFB setting you prefer best? You described your design with very good argument, and I was on similar line when designed my TT amp. It will be very intereting if you show your PCB layout.
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Old 4th June 2012, 02:07 PM   #17
Bonsai is offline Bonsai  Taiwan
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dadod, unfortunately I have not listened enough to say at this stage which type of feedback I prefer. I will need to live with the amp for a few months, let my ear tune in and then change to see which one I like. I can only comment after that. Maybe there is no difference ;-)
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Old 4th June 2012, 02:19 PM   #18
dadod is offline dadod  Croatia
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Quote:
Originally Posted by Bonsai View Post
dadod, unfortunately I have not listened enough to say at this stage which type of feedback I prefer. I will need to live with the amp for a few months, let my ear tune in and then change to see which one I like. I can only comment after that. Maybe there is no difference ;-)
True enough. What about PCB layout, it would be very useful(for me at least) to see how you did it?
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Old 4th June 2012, 02:19 PM   #19
Bonsai is offline Bonsai  Taiwan
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Ian,
basically what I found is that EF3's are quite hard to thermally compensate. You can get them stable over a narrow temperature range, but once the heatsink heats up, you get errors - in my case, the Iq increased significantly. The only way to get a reasonable setting was to set the Iq very low at ambient (now you know why some amps specify very low Iq at ambient) and then rely on the amp warming up so that after some time Iq settled at about the optimum value. On my Ovation Amp, I used a conventional 2 transistor circuit, but ended up having to fiddle with the amount of thermal grease between the sensor and one of the output transistors. Under quiescent conditions it is stable, but if It runs at higher power levels (so 80-100W) it is not stable enough.

To get round this I used a NTC on the e-Amp which has only a moderate effect at ambient, but at higher temperatures the resistance drops very quickly. If you look at page 40 and 41 you can see a detailed description of how this approach was calibrated during the design phase. With this scheme, Iq is set at two points over the operating temperature (thats why I called it two point thermal compensation) and this is much more stable than the usual single point approach.

That said, EF2's which are much more common in bipolar amps, are a lot easier to thermally compensate, so you can usually get away with standard Vbe spreader circuits.
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Old 4th June 2012, 03:25 PM   #20
dadod is offline dadod  Croatia
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Quote:
Originally Posted by Bonsai View Post
Ian,
basically what I found is that EF3's are quite hard to thermally compensate. You can get them stable over a narrow temperature range, but once the heatsink heats up, you get errors - in my case, the Iq increased significantly. The only way to get a reasonable setting was to set the Iq very low at ambient (now you know why some amps specify very low Iq at ambient) and then rely on the amp warming up so that after some time Iq settled at about the optimum value. On my Ovation Amp, I used a conventional 2 transistor circuit, but ended up having to fiddle with the amount of thermal grease between the sensor and one of the output transistors. Under quiescent conditions it is stable, but if It runs at higher power levels (so 80-100W) it is not stable enough.

To get round this I used a NTC on the e-Amp which has only a moderate effect at ambient, but at higher temperatures the resistance drops very quickly. If you look at page 40 and 41 you can see a detailed description of how this approach was calibrated during the design phase. With this scheme, Iq is set at two points over the operating temperature (thats why I called it two point thermal compensation) and this is much more stable than the usual single point approach.

That said, EF2's which are much more common in bipolar amps, are a lot easier to thermally compensate, so you can usually get away with standard Vbe spreader circuits.
My experience is that you have to separate thermal compensation of the drivers from thermal comensation of the output transistors. Use two separate transistors thermaly coupled one to the driver separate heatsink and other one to the main heatsink, look attachment(simulated only). In mine TT amp ThermalTrak+TMC amp I used TT diodes to compensate output transistors only and Vbe multiplier transistor to compensate drivers only. A predrivers is not necessary to compensate in mine opinion.
dado
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