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robbie010

Amplifier Schematic Review

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Super Wammer

Evening all,

Following my earlier posts regarding my project amplifier, I have now worked out that it is a DIY Nelson Pass A40 power amplifier. Having thoroughly reviewed the amp and compared it to the published schematic there are a number of deviations from the published info and I am not skilled enough to review the schematic and discern any obvious design issues.

The main deviation appears to be the use of two seperate centre tapped transformers and a triac transister in the power stage.

I have attached a schematic drawing of the amp that I produced using LTspice. Unfortunately, I am not skilled enough to actually run a simulation in spice but if anyone is capable of reviewing the schematic and providing any feedback it would be much appreciated.

Thanks.

A40 Schematic.pdf

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Using two transformers give you separate power supplies for each channel which is a good idea.

Don't think you have the mains input side of the transformers correct. Is there really a grounded centre tap on the primary? The triac is probably a soft start for the transformer but something is missing?

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Super Wammer
3 minutes ago, Alex 54 said:

Using two transformers give you separate power supplies for each channel which is a good idea.

Don't think you have the mains input side of the transformers correct. Is there really a grounded centre tap on the primary? The triac is probably a soft start for the transformer but something is missing?

The prinary side earth wire of both transformers is grounded to the chassis. For some reason LTspice does not have the symbols for this. 

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1 hour ago, robbie010 said:

The prinary side earth wire of both transformers is grounded to the chassis. For some reason LTspice does not have the symbols for this. 

Rather than an tap it's probably an inter-winding screen, a quick check with a meter will tell you if it's actually connected to the winding.

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Super Wammer
9 hours ago, Alex 54 said:

Rather than an tap it's probably an inter-winding screen, a quick check with a meter will tell you if it's actually connected to the winding.

When I powered the amp up from the mains and tested the transformers output voltage I got the following results:

Secondary 1 to Centre Tap - 28v

Secondary 2 to Centre Tap - 28v

Secondary 1 to Secondary 2 - 55v

I assume this means that it is connected to the winding?

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31 minutes ago, robbie010 said:

When I powered the amp up from the mains and tested the transformers output voltage I got the following results:

Secondary 1 to Centre Tap - 28v

Secondary 2 to Centre Tap - 28v

Secondary 1 to Secondary 2 - 55v

I assume this means that it is connected to the winding?

You were measuring the secondary, what I was referring to was what looks to be a centre tap on the primary in your circuit?

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Super Wammer
24 minutes ago, Alex 54 said:

You were measuring the secondary, what I was referring to was what looks to be a centre tap on the primary in your circuit?

Ah, I see.

The primary side has a Red live, Black negative, yellow/green Earth (which is what you are refering to) and its also has a spare for 120v supply which osn't shown on the schematic drawing.

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1 hour ago, robbie010 said:

Ah, I see.

The primary side has a Red live, Black negative, yellow/green Earth (which is what you are refering to) and its also has a spare for 120v supply which osn't shown on the schematic drawing.

OK, so it is an inter-winding screen then.

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