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Old 7th June 2010, 09:28 PM   #1
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Default Adding Additional Outputs to An Existing Denon PMA-520 Integrated

The Denon PMA-520 and PMA-720, early 1990's integrateds, share the same operating and service manuals, as well as the main pcb board. The power supply is a bit more beefed up on the PMA-720, with 13,600uF per channel (vs 11,200uF for the PMA-520) and a different, presumably larger, toroidal transormer. The bias is their "Class A optically controlled bias system" you can see components of it in the schematic. Interestingly, Mr. Pass holds some patents for this technology, hopefully he got a few bucks for each one of these sold. Apparently, there were many made.

The PMA-720 is rated at 80Wx2 and the PMA-520 at 70Wx2 with distortion .007% and .008%, respectively. I can't get the schematic to be readable with the imposed file limit, so I've attached a link.

If you compare the schematics at the amplifier level, they are identical, save for the diode string (two of them zeners) and additional pair of output transistors in the PMA-720, both of which I have highlighted in the schematic. I have the PMA-520 and am considering dropping in the additional devices, as the board appears to be ready to take them without modification. I am also considering increasing the capacitance to about 15,000uF per side. Is this going to present a problem?

It appears the operating voltages of the two designs are identical, but the different model numbers for the transformers and the additional weight (1 lb 13 oz more for the PMA-720) point to a higher capacity transformer in the PMA-720.

Last edited by DreadPirate; 7th June 2010 at 09:34 PM.
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Old 7th June 2010, 10:05 PM   #2
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Isn't this a lot of work for such a small power increase? You can barely hear the difference between 70 and 80 Watts, and then only when listening carefully. Only a bit over 1 dB.

If you add power devices you must adjust the bias accordingly, as well as its temperature compensation. And with more power you certainly should have a bigger transformer and even a bit more voltage.
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Old 7th June 2010, 11:09 PM   #3
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It's only about $5US in extra parts and a few minutes soldering, so its not a biggy. I guess the question is, given a design with a single output pair, is there great benefit in adding a second pair of outputs, all things the same? Does the second pair of outputs make a magical difference, turning a somewhat good amp into one that rivals the big boys? Will the unit sound different or better given a change like this?

The PMA-720 does have a slightly different bias and I would implement that difference with the added outputs.

I'm probably going to throw in these extra components into my next mouser order anyway, would like the gurus here to comment on what the extra pair of outputs accomplishes, as well as the diodes. Maybe I'll learn something...

Last edited by DreadPirate; 7th June 2010 at 11:12 PM.
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Old 7th June 2010, 11:50 PM   #4
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The answer to your question is a resounding NO, the amplifier will not be materially improved. If the job is done correctly and you have some good luck, it may be a bit more reliable but I really doubt it.

The main reason for having two amplifier models with nearly the same output power is marketing. It enables careful price engineering to meet whatever they consider the competition. Another reason for not doing this is the risk of a mistake. One little error and you will have a pile of burned out parts on your hands.
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