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"vintage" quasi complementary power amp
"vintage" quasi complementary power amp
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Old 12th June 2018, 12:36 AM   #11
steveu is offline steveu  United States
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Lets try this again with collectors connected:
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Old 12th June 2018, 09:23 AM   #12
catd is offline catd  Europe
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Hello steveu,
Thank you for your posts.
Quote:
Originally Posted by steveu View Post
I assume the real amp has a build-out choke+resistor...
That's right, there is the mentioned choke in the real amp, contained in a speaker-on-delay extra-PCB. (could be seen in one of the pictures under the link in first post)
Quote:
The current limit protection should have worked without causing problems so that bothers me. Do you have some kind of passive crossover with a series resonance that is ~shorting the amp?
It's more than 35 years ago when this occurs and I remember only, that I omitted this protection and all was OK.
Speakers was/are 3-way with simple 12db passive crossover.

As I wrote, I want to stay with the quasi complementary approach, and see what is possible to make this a bit better.
But I will try your suggestions if possible so far.
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Old 13th June 2018, 01:13 PM   #13
steveu is offline steveu  United States
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BTW, do the math and you find the original current protection was set to limit at 7 Ohms, not 4, not even 6.
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Old 13th June 2018, 03:30 PM   #14
catd is offline catd  Europe
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That's right.
I have one original PCB/Circuit of the Amp as shown in the first pic/post; was built-in to a guitar amp at the time.
A couple of weeks ago I made a test with generator and oscilloscope whether it still works after all the years.
It does.
With 4Ohm output load it started clipping/limiting at about 20V amplitude.
( The amp was sold back then as 160W sin at 2-16 Ohm. )
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Old 18th June 2018, 05:26 PM   #15
catd is offline catd  Europe
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Hello...

This is the last version of the amplifier.
The outcome of the simulations is fine for me.
It was possible to put all parts on a PCB layout at the given PCB size.
So I'll build it up real in the near future and test it and post the result here.

"vintage" quasi complementary power amp-quasi_amp_014-jpg
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File Type: asc quasi_amp014_thd_test.asc (9.8 KB, 13 views)
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Old 19th June 2018, 11:06 AM   #16
Bix is offline Bix  Australia
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"vintage" quasi complementary power amp
Cool, I look forward to your build. Thanks for posting your schematic.
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Old 19th June 2018, 03:18 PM   #17
Kay Pirinha is offline Kay Pirinha  Germany
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Q11 supposedly emulates a Baxandall diode. What about replacing it by a 1N400x?


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Old 19th June 2018, 04:05 PM   #18
catd is offline catd  Europe
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Yes, Q11=Baxandall-Diode.
1N400x instead? Why not?
It will be possible to replace the transistor by a diode in the real PCB-built, and see/hear if it makes any difference.

I read somewhere: -a transdiode would fit better, if one chose the right transistor (same as in positive half), or so-, can't remember, where.
Therefore I made the layout with the possibility on the PCB.
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Old 19th June 2018, 05:41 PM   #19
john_ellis is offline john_ellis  United Kingdom
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R5 and R18 should be the same- can't read the diagram, are they?
C7 is usually needed in quasi amps to suppress lower half oscillation. Might still be needed even with a complementary OP stage. I too recommend changing to fully complementary. There is no advantage with current transistors for NPN only. There should be a resistor in series with the collectors of the lower half OP transistors which is also connected to the anode end of the diode-connected transistor. You won't need that if you swap to PNP's, saving a voltage drop.
Rather than increasing C5 you might reduce the OLG with emitter degeneration resistors.
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Old 19th June 2018, 07:08 PM   #20
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by john_ellis View Post
There should be a resistor in series with the collectors of the lower half OP transistors which is also connected to the anode end of the diode-connected transistor.

Why? I think there's a demand for a resistor, or resistors, between the lower power device(s) and the output bus only if there's a current limiter.


Quote:
You won't need that if you swap to PNP's, saving a voltage drop.

Yes, in this case the resistors remain in the emitter circuitries.


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