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Old 26th April 2013, 03:20 PM   #1
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Default Denon PMA 737 schematic needed

Could somebody help in finding schematic for Denon PMA737 integrated amplifier? Or at least comment on topology? Owners manual claims that it is zero feedback amplifier!
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Old 15th May 2014, 07:49 PM   #2
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Question Denon PMA-737

I hope you have found it, ivanlukic! Otherwise, let me know!

Taking advantage of this open thread, I have a question regarding this model.

In the schematics we can see only one opamp, the M5218. Here below, highlighted.

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But in the printed board, we see two! Unless we have one opamp to each channel...

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If so, are they in charge of tone control??
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Old 15th May 2014, 08:01 PM   #3
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Looks like they are just using one opamp in the package. No idea why they didn't use a single opamp... cost probably if they had these available. Or they were deemed most suitable but the board layout wouldn't accommodate using just one.
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Old 15th May 2014, 08:27 PM   #4
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Looking at the component numbering it looks like one per channel.
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Old 15th May 2014, 08:36 PM   #5
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Quote:
Originally Posted by Mooly View Post
Looks like they are just using one opamp in the package. No idea why they didn't use a single opamp... cost probably if they had these available. Or they were deemed most suitable but the board layout wouldn't accommodate using just one.
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Originally Posted by KatieandDad View Post
Looking at the component numbering it looks like one per channel.
Looking still in the first picture, the lower highlighted part could refer to pins 5, 6 and 7 of that same opamp, but it still has "IC2 M5218P" marked and it's inside a blank box-diagram... Strange...

In the PMA-757 model there are 4 of these Mitsubishi "dual low noise" opamps. A "pair" (IC301, IC303) shown here. The other "pair" is IC302, IC304 and is not shown in the schematics...

Click the image to open in full size.
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Old 15th May 2014, 10:19 PM   #6
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IC303 is using both op-amps in the single package. IC301 is using only one.
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Old 15th May 2014, 10:25 PM   #7
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Lightbulb Denon PMA-737

Thanks, KatieandDad and Mooly!
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Old 19th May 2014, 01:14 AM   #8
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Default Bias: temperature and stability

I'm sure this have already been discussed many times here, but in this specific case...

Photos below show the right pair of 2SA1011/2SC2344 drivers in this model. I know they were designed like this (without heat sinks for dissipation)...

Click the image to open in full size.

Click the image to open in full size.

But due to extreme sensibility even to air flow variations, setting the bias has been a funny thing: voltage goes from the 16mV down to 10mV, or up to 22mV within some 30 minutes. And the drivers are pretty "hot" even with no signal applied.

General question: adding a heat sink to these transistors would improve on stability?

Last edited by Karl vd Berg; 19th May 2014 at 01:17 AM.
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Old 19th May 2014, 07:16 AM   #9
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Hard to say really. The real problem could be that the vbe multiplier doesn't track well or that it is influenced by heat from the other channel.
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Old 19th May 2014, 10:10 PM   #10
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Originally Posted by Mooly View Post
Hard to say really. The real problem could be that the vbe multiplier doesn't track well or that it is influenced by heat from the other channel.
The amp's heat sink has plenty of space to accommodate the 4 transistors/drivers. I'll put it aside for now. And see if I can do something later...

... I just recapped the PMA-757 with some 25 Epcos film caps instead of most 1uF electrolytics spread in this amp.

Click the image to open in full size.

IC301 and IC302 (one opamp per package) I left as they are.

Click the image to open in full size.

IC303 and IC304 use both opamps in the package. I had several pairs of opamps to test in this amp. This time none of them was a problem. Easy bias setting and pretty stable after some 30 mins.

Click the image to open in full size.

I liked here the OPA2134. It seems to have a huge "3D" sound.
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