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Old 21st September 2005, 09:05 AM   #11
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Thank you, Michael, i shouldnt post silly numbers,reverse transfer is indeed that low.
Still, works miracles for teaching me though.

I do take it that separating Vas and driver stage by adding a driver stage between Vas and output stage will be a big improvement to distortion figures ?

How about upgrading the trimpot to a Vbe multiplier, the value of the gate resistors and the feedback ?

The advised heatsink was a 0.4 K/W model, 2 amp bias.
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Old 21st September 2005, 12:27 PM   #12
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Thanks Jacco for the very informative explanation. I hope I can learn something and have the gut to experiment At least, if the Hitachis get broken, I will not have to buy them again

Spec above audible spectrum, is it really audible?

Have you seen pavel Dudek's DPA330? He did almost everything in the circuit. If circuits like that 20W or the crescendo really need other stages, why are they not there? It's not difficult and not expensive to add 2 more stages correct?

Will the problem discussed here be solved even partially, by adding such stages as found in DPP330? (Without the errorr correction, servo FB and other fancy components)
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Old 21st September 2005, 12:32 PM   #13
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It wasn't DPA380?
http://sjostromaudio.com/dudek/dudek3/pics/dpa380.jpg

http://sjostromaudio.com/dudek/dudek...s/DPA380_1.pdf
http://sjostromaudio.com/dudek/dudek...s/DPA380_2.pdf
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Old 21st September 2005, 12:37 PM   #14
Jay is offline Jay  Indonesia
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No, DPA330 uses Hitachis. DPA330 is the grand grandpa
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Old 21st September 2005, 12:42 PM   #15
Jay is offline Jay  Indonesia
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Oh, but they share the same topology. 3 transistors in front of the error correction circuit, that's what I was referring to.
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Old 21st September 2005, 03:13 PM   #16
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DPA330 schematic
The 2SD760/2SB720 Pavel uses as drivers for the mosfets are great devices:100 MHz, 2 amp max., also by Hitachi.
You can still get them at Nikko elec., not cheap though, expect more than $1.50 the piece.

(i am also still an aficinado of opamp dc-servo's, bipolar dc blocking capacitors are so outdated)
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File Type: jpg dpa330.jpg (97.6 KB, 580 views)
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Old 21st September 2005, 03:24 PM   #17
LBHajdu is offline LBHajdu  United States
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Looking at the topology of the Klasse A-versterker van 20 Watt amp, it looks very similar to the pass labs A75 but with transistors as opposed to mosfets.
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Old 21st September 2005, 07:10 PM   #18
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The cascoded Vas stage, yes.

But other than that i see more differences than resemblances.
The A75 was the last amplifier i constructed in 1995, after finishing Mr Thagard's 100 watt 1990 mono design, after that 10 years nothing.
The output devices i used for the A75 were 630/9630s because i couldnt lay my hands on TO3's.(see datasheet of the irf230)
Performance of the 20 watter is in no way comparable with the A75.


The A75 pdf's should be good reading, Jay.
They should still be on the download section of PassDIY.
I still need to scan the xerox copies of the Norman Thagard 100 watt design and mail it to a diyaudio member. Anyone else interested in having it can send me a word.
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Old 23rd September 2005, 03:32 AM   #19
LBHajdu is offline LBHajdu  United States
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you can send me a copy lbhajdu[at]optionline[dot]net.
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Old 23rd September 2005, 12:26 PM   #20
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Yes, jacco I'm interested in the Thagard. Please send me also, thank you.

Jacco, I haven't yet checked the pdf but I built A40 once. if I'm not mistaken they differ 'only' on the number of output transistors. And as I can remember they are BIPOLAR! I'm not looking for high class amp here (I have built enough free designs ), I'm just looking for a high class amp for my expensive K135/J50 But you seem to like the A75. Hmmm... I think NP himself don't like bipolar??
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