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Old 9th September 2011, 04:31 AM   #1
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Default Phils Amp - Progress?

Hello, recently I decided to build an amplifier. Why?
For a couple of reasons actually. I purchased a yamaha surround system that I couldn’t fix, but it had a heap of good quality components I wanted to use in a stereo. I also had a couple of 40/0/40 200VA toroidal transformers that I wanted to use.

Like many people, I enjoy simplicity and so I looked for the simplest good design I could find. I spent months looking in every little nook and cranny on the internet but sadly the designs presented were generally incomplete, poorly documented or completely unexplained.

The design I chose to use was originally created by the Uni of Sheffield, but it needed to be adapted to suit the salvaged components. This mission to use old stuff created some challenges but with the help of a few of you, I have got most of the job done.

The soft start is finished. The preamp power supply is done. The 240VAC wiring has been designed and the main pair of 56-0-56 power supplies are built. DC Protection is built and tested ok, so this leaves me at the stage where I have to build the amplifier boards to keep the project moving.

I want to post what I think is a complete amp circuit but I know full – well that it will need a tweek or two. I want feedback from helpful tradesmen or engineers. Specifically I want you to look at the base-blocker resistors on the drivers and output transistors – are they a good/bad idea? What about the 0.22 emitter resistors on the MJE drivers – good/bad. Input filter values, right/wrong? The 100uF bipolar capacitor that I used on the feedback network turned out to be 90uF, will this matter?

I would appreciate any comments on the circuit or pcb layout, especially any mistakes because I reckon there may be a few. I want to get the boards cooked up as soon as possible.
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Old 9th September 2011, 04:47 AM   #2
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Default more photos

the pcb layout is a bit of a dodgey photo, so I have cut it into two halves in order to give you some better resolution.

Here it is now:
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File Type: jpg pcb bottom half.jpg (211.1 KB, 36 views)
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Old 9th September 2011, 05:27 AM   #3
benb is offline benb  United States
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I'm only looking at the schematic, and have a couple of comments.

Your 90uF should be fine, it only changes the LF rolloff from (just a SWAG) 1Hz to 1.1Hz over using a 100uF, something like that, not a significant difference.

The wiper of the 470 trimpot should be connected to the top end of its resistance where it joins the 820 ohm resistor. With this connection, if the wiper loses connection with the resistance element, the transistor turns more on, dropping less voltage and causing crossover distortion. This is preferable to an open wiper turning off the transistor, causing an overcurrent and heating in the driver and output transistors.

While I don't consider myself an expert in amp design (surely more knowledgable people will be along to comment on those emitter resistors and things), there's nothing in the schematic that jumps out at me as an error or something wrong.
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Old 9th September 2011, 05:50 AM   #4
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If i were you, i'd throw away that useless heatsink and find a real cast/extruded one. Those alu plates with the corrugated alu/steel sheet mounted to them are useless as heatsinks, they shouldent even be allowed to be called heatsinks.
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Old 9th September 2011, 07:37 AM   #5
djk is offline djk
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I would use a 470Ω base resistor on the B649, a silver-mica for the 120pF (not a ceramic as listed), and use a 0.1µF film bypass on the 90µF in the feedback loop.
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Old 9th September 2011, 10:22 AM   #6
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Welcome back, Farmer Jack! Good to see the project taking shape.
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Old 9th September 2011, 04:37 PM   #7
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"throw away that useless heatsink and find a real cast/extruded one".

Thanks grumpy!

How about this one;
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Old 9th September 2011, 05:18 PM   #8
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Potentiometer aroud Vbe multiplier should be connected as in a leach amp .... otherwise if the pot malfunction and give an open circuit you will blow the output stage.
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Old 9th September 2011, 05:31 PM   #9
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Default Bring it on!

Keep the ideas coming I have made note of;

Warm salutations from South Africa.

Silver Mica 120p.

104 film cap in paralell with my 90uF feedback capacitor.

Throw the heatsink in the rubbish!

NOW, how about those base blockers so I can fire this beast up? I can smell the snags on the barbie and I'm ready to eat.
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Old 10th September 2011, 02:17 AM   #10
djk is offline djk
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The base resistors look OK, you may want to increase the value on the MJE340/350, and don't forget to add one to the B649.
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