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Old 10th May 2007, 11:43 PM   #11
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Hi CS:

First of all, I'm rookie in electronics.
Second, my inglish is not my mother language, and please accept my apologies.
And now, to the point, we can use a regulated PS +/- 15/24V to fit opa445 and larger voltage to the power stage.
Third: can replace the TIP 29/30 with power darlingtons to drive in cascode a pair of mosfets?

PS: electronics is a hobby to me, I'm a dentist.

Cheers
Braco
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Old 10th May 2007, 11:45 PM   #12
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Sorry, I forgot the picture.

Cheers
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Old 11th May 2007, 01:08 AM   #13
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Refering to: http://focus.ti.com/lit/ds/symlink/opa552.pdf (page 9)

IMOP: suggest adding, as per TI recommendations, double bypass caps on the op-amp power rails ... one electrolytic cap (10 uF or more) and one plastic cap (0.1 uF, polystyrene or polypropylene or MKT) per each rail ... this snubbs down the op-amp for the better distortion & slew rate specs. (Put the plastic caps as close as you can get to the op-amp power pins = works best.)

TI ran their numbers with +/0/- 28 VDC rails to get those better specs (examine the graphs, etc.). If you can swing that, using a common supply rail configuration for both op-amp and outputs, then you won't need exotic or complicated voltage offset control or any DC blocking caps in the audio path (other than C1 in the original schemo, at least).

Noting that the feedback & input resistors indicate ~ unity gain through the op-amp (good), so if you can keep to the original MOSFET outputs, you would have a very decent little amp. Of interest: mounting the op-amp away from the MOSFET heatsinks would allow the internal op-amp circuitry to "run free" and do it's own short circuit & thermal limiting and if you stick with the MOSFET drivers in the original schematic, since the MOSFETs are voltage followers instead of current followers like transistors, there would be virtually no heat problems (also good IMOP).

As for the final tweak, probably all that's needed other than clean power ... the variable resistor (bias) setting is done with a simple sign wave ~~ 800 to 1200 Htz, centering any crossover distortion (@ the +/- 0 VAC point) ... run 'er up to just below clipping and measure across the outputs with a cheap 'scope & dummy 8 Ohm load.

Your original schematic is vaguely similar to the aussieamplifiers.com NX series = which use discrete FETs instead of an op-amp. Good stuff, dead cold in operation, great distortion specs, plenty of guts, clear highs and solid bass ...
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Old 11th May 2007, 07:36 AM   #14
cs is offline cs  United Kingdom
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Quote:
Originally posted by bracoraab
Hi CS:

First of all, I'm rookie in electronics.
Second, my inglish is not my mother language, and please accept my apologies.
And now, to the point, we can use a regulated PS +/- 15/24V to fit opa445 and larger voltage to the power stage.
Third: can replace the TIP 29/30 with power darlingtons to drive in cascode a pair of mosfets?

PS: electronics is a hobby to me, I'm a dentist.

Cheers
Braco
Hi Braco,

Yes I think your circuit using bipolars should work, but I would use some bias between the bases (as you did with the mosfets), to avoid crossover distortion.

You could also use higher power darlingtons such as the SAP16N/P, which would be good for the 100W version (+/-45V rails).

Also, Fasteddy's suggestions on supply rail decoupling are good ones.

CS.
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Old 11th May 2007, 08:08 AM   #15
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Quote:
And now, to the point, we can use a regulated PS +/- 15/24V to fit opa445 and larger voltage to the power stage.
Hi,

Then the OPA's output can only swing say +/-10V, and that's not enough to drive output tets running say +/-30V.
So the OPA needs equal or higher PSU voltage than output devices.

Regards,
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Old 11th May 2007, 10:08 AM   #16
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better 2-3V higher than equal....but this is problem if you do not have aditional windins on your transformer....but the idea is nice....using filtering cap's at the output and regulated power supply for the front end that has regulated voltage a little bit higher than the output...
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Old 11th May 2007, 12:08 PM   #17
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OK,
thanks to all of you guys, I'm going to design a teoretical circuit and put iton the the forum to discuss the details.
Cheers
Braco
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Old 11th May 2007, 09:52 PM   #18
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Hi, guys

Here, the last modifications.
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Old 11th May 2007, 10:36 PM   #19
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You could put a 100-250 ohm trimpot between R5 and R6 and balance out the even harmonic distortion fairly easily. Did it 40 years ago, it should still work.
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Old 11th May 2007, 10:47 PM   #20
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Hi, John

You mean like this?

Cheers
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