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Nelson Pass A40 Help
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Old 11th February 2019, 12:02 PM   #21
Robbie010 is offline Robbie010
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Quote:
Originally Posted by jazbo8 View Post
Please don't start multiple threads for the same topic.
Sorry.....
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Old 11th February 2019, 06:17 PM   #22
Nelson Pass is offline Nelson Pass  United States
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Nelson Pass A40 Help
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Originally Posted by Robbie010 View Post
I have managed to draw up a schematic of my project amplifier and while it is most definately a Pass A40, it is not 100 true to the published information.
Your C4 compensation cap is huge electrolytic, the amp will work at DC only...

Maybe you would do better with 47 pf.

Where's the feedback resistor?

Input 2.2k resistor might want to be more like 1K.

That's all I saw in 30 seconds.

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Old 11th February 2019, 07:04 PM   #23
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by Robbie010 View Post
I have managed to draw up a schematic of my project amplifier and while it is most definately a Pass A40, it is not 100 true to the published information.

The amplifier uses two centre tapped transformers, one for each channel, it has a triac transistor in the power stage and a thermostatic switch, so I was hoping someone with the skill and knowledge would be willing to review the schematic and provide some feedback, maybe point out any potential design issues?

Thanks.
The type of transformer and the triac has nothing to do with the amplifier itself. Try to focus on one area, get that going, then look around to other victims ;-)
You have the original schematic from the article. That should be your reference.

BTW The speaker return should go to ground, not to the top of the R22.

Jan
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Last edited by jan.didden; 11th February 2019 at 07:08 PM.
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Old 11th February 2019, 07:08 PM   #24
Robbie010 is offline Robbie010
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Quote:
Originally Posted by Nelson Pass View Post
Your C4 compensation cap is huge electrolytic, the amp will work at DC only...

Maybe you would do better with 47 pf.

Where's the feedback resistor?

Input 2.2k resistor might want to be more like 1K.

That's all I saw in 30 seconds.

My bad...... the schematic was wrong! Sorry.

C4 is actually 39pf 160v, not 470pf 25v as per the drawing.

Is the feedback resistor R13?

The input resistor is as spec at 2.2k, should I change it out to 1k?

Updated and correct schematic attached.

Thanks again.
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File Type: pdf Pass A40 Schematic 11.02.19.pdf (76.5 KB, 25 views)
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Old 11th February 2019, 07:46 PM   #25
jan.didden is offline jan.didden  Europe
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Look at the original circuit for the feedback resistor from output to the base of Q2.
And C1 wants to be at the other side of R1.
Speaker return should go to ground.

Jan
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Last edited by jan.didden; 11th February 2019 at 08:15 PM.
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Old 11th February 2019, 07:47 PM   #26
wrenchone is offline wrenchone  United States
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The collector of Q6 should go to the collector of Q5 and Q7-8 base, not to thee +V rail. Just having a pot to set the bias current in the output stage is dangerous, as the amp will blow up if the wiper lifts on the pot.
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Old 11th February 2019, 08:19 PM   #27
mitchba is offline mitchba  Canada
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Don't know if this is helpful or not, but just in case...
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File Type: pdf a40PCBError.pdf (34.9 KB, 23 views)
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Old 11th February 2019, 08:44 PM   #28
wrenchone is offline wrenchone  United States
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Here's a little tutorial on Vbe multiplier schemes. I fixed a Sony receiver that used the first scheme and blew expensive output transistors when the cheap carbon bias adjuster pot had a wiper lift. I lost respect for Sony product after that experience. I also replaced the faulty pot with a more reliable cermet multiturn unit.
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File Type: gif VBE_mult.GIF (16.9 KB, 122 views)
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Old 11th February 2019, 09:08 PM   #29
Nelson Pass is offline Nelson Pass  United States
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Nelson Pass A40 Help
Quote:
Originally Posted by Robbie010 View Post

C4 is actually 39pf 160v, not 470pf 25v as per the drawing.

Is the feedback resistor R13?
It would be the first electrolytic 470 pF I will have seen.



R13 provides feedback about the output stage bias, but you are still
missing a resistor between the output and the Base of Q2. Something
like 2.2K @1 watt would do it.
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Old 12th February 2019, 02:35 PM   #30
Robbie010 is offline Robbie010
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Originally Posted by mitchba View Post
Don't know if this is helpful or not, but just in case...
Thanks for the heads-up on this, I have checked it against my circuit board and the correction has already been made. Or it was never done incorrectly in the first place.

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