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Old 29th May 2011, 10:54 AM   #21
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And i suggest using 20kHz for THD testing as this is where an amplifier shows its true colors.

IF its above 0.08% at 20kHz, its not worth building because it will not sound good at all.
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Old 30th May 2011, 06:22 AM   #22
glennb is offline glennb  Australia
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Default R14

What is the purpose of R14 (27 Ohms, between base and collector of Q12) ?

Is the function of Q12 supposed to be like Q7 ?
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Old 30th May 2011, 08:45 AM   #23
Junm is offline Junm  Philippines
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Can anybody of the master board designers here kind enough to do the PCB layout and design...single sided will be okay...
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Old 30th May 2011, 03:52 PM   #24
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I dont mind designing a board.
It would be double sided with the output + driver xistors to one side for heatsinking. All components will be TH. Gerber files will be posted.
Elvee: I will need wattage values of Rs that are higher than 1/4W and #s of other components that should be on the heatsink.
Q: Is it worthwhile to epoxy Q3+4 and Q5+6 to each other to improve temp. tracking?
Cheers, E
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Old 30th May 2011, 09:15 PM   #25
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by mickeymoose View Post
I dont mind designing a board.
It would be double sided with the output + driver xistors to one side for heatsinking. All components will be TH. Gerber files will be posted.
Elvee: I will need wattage values of Rs that are higher than 1/4W and #s of other components that should be on the heatsink.
Q: Is it worthwhile to epoxy Q3+4 and Q5+6 to each other to improve temp. tracking?
Cheers, E
Thanks for your involvement.

-All resistors can be 1/4W
-The output transistors, Q8 and Q10 obviously need a heatsink, as do the drivers Q9 and Q11. All four can be a common heatsink, or be cooled individually, or grouped in any convenient way.
-Q5 and Q6 do not normally require a heatsink unless operation at supply voltages above 45V is desired.
-No direct thermal contact is required between transistors, but the following pairs should preferably be mounted side by side: Q3/Q4, Q2/Q7, Q5/Q6, Q12/Q13.
-For D7, any small TH schottky will do, BAT81 or BAT85 f.e.
-D4 and D5 can be any 3A or 5A schottky, 1N5820 or something larger in TO220.
-Splitting D8/D9 is not required, but gives more flexibility.

-I didn't include an inductive Zobel network, because it would only be necessary in very extreme cases, but provision for it could be made on the PCB (and strapped in most instances).


Quote:
What is the purpose of R14 (27 Ohms, between base and collector of Q12) ?
It provides a first order cancellation of the dynamic resistance of Q12, making the quiescent current largely independent of the supply voltages.

Quote:
And i suggest using 20kHz for THD testing as this is where an amplifier shows its true colors.

IF its above 0.08% at 20kHz, its not worth building because it will not sound good at all.
Is 0.08% a magic number?
Here is the sim at 20KHz, the THD is doubled compared to 1KHz, but as I said earlier, the sim is not reliable at higher frequencies, and I'll need to make an actual measurement to confirm the value.
Attached Images
File Type: gif THD20K.gif (35.6 KB, 1899 views)
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Old 30th May 2011, 09:23 PM   #26
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Well i may just be rambling here, but in my world, a amp that does 0.1% THD at 20kHz is a bad amp.

Though i realize i contradict myself here since the stereo npn amp simulates 0.1% or worse at 20kHz and kinda wonky phase, yet its one of my best sounding amplifiers.
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Old 30th May 2011, 10:18 PM   #27
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Very nice pictures, but what about dynamic characteristics, like small signal distortions right after huge long impulses?
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Old 30th May 2011, 10:37 PM   #28
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Elvee: It is your design. Show me where you would implement a Zobel so I can include it on the pcb.

Wavebourne: Good point, but wouldn't your ears/brain take more time to recover than the electronics? The 16V squarewave output into the 3 Ohm load shows that it does the job (power suply permitting).

Last edited by mickeymoose; 30th May 2011 at 10:43 PM.
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Old 30th May 2011, 10:51 PM   #29
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Quote:
Originally Posted by mickeymoose View Post
Wavebourne: Good point, but wouldn't your ears/brain take more time to recover than the electronics?
No.This is why coupling capacitors between driver and output tube "sound", and why interstage transformers in tube amps are used.
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Old 30th May 2011, 10:52 PM   #30
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A zobel link goes between the speaker output and ground usually.
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