Advice on upgrading a Denon PMA-925R - diyAudio
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Old 19th July 2011, 06:46 AM   #1
E30NA is offline E30NA  Denmark
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Default Advice on upgrading a Denon PMA-925R

Hi, i am curently studying, so the funds for new HI-Fi parts are low. I have a Denon PMA-925R stereo amplifier that i am considering giving an upgrade, instead of buing a new amplifier.
So i would like to have your advice on how to, and if it is a good idea?
I am considering changing the two 12000uf Elna caps to 18000uf Elna caps- are there other parts that i can change to give better sound?
The thing is that my brother has recently bought a NAD 80w amplifier, and i am amazad on the difference in sound. The NAD sounds much clearer, dynamic and powerfull, and when played loud, it has much better control of my 12" bas speakers.
Is there a way to make the Denon more dynamic and powerfull in the bas and on dynamic disharges? Or should i just forget it, and live with the thiner sound of the Denon until i have the funds for a better amplifier?
I am well aware that i will newer reach the quality of a new modern amplifier, but is it possible to make the denon better with limited funds?

Regards
Jens
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Old 19th July 2011, 08:31 AM   #2
E30NA is offline E30NA  Denmark
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Heres a link to the manual and wirring diagrams:
Denon PMA-925R | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
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Old 19th July 2011, 08:56 AM   #3
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Moin (as we say here) Jens, as it looks from the inside, it seems worth a try ...

Click the image to open in full size.

At least the transformers look decent, and there could be worse than realy switched inputs, a Panasonic plastic film potentiometer and an active bias control. Adding just some more 6000F won't change much IMHO, you should at least double or better triple the capacitance to have an effect. There is enough space to do that, consider mounting the capacitors on veroboard and wire that arrangement to the original solder points. Use thick wire, keep it as short as possible- high currents flow there.
Change the rectifier bridge for some ultra-fast MUR860 diodes. Change the crappy BA4558 and NJM2068DDC opamps for OPA2134 (except the phono stage if you don't have a turntable) and decouple them as close as possible with 10F Panasonic FM (or something else in that league) and 10nF film foil in parallel.
I've stored there service manual here. (Scroll down and click the 'Slow Download' icon)

Have some reading about amplifier sound and power supplies: : Amplifier Sound - What Are The Influences?, Solid State Power Amplifier Supply Part 1

Last edited by Groundloops; 19th July 2011 at 09:11 AM. Reason: typo
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Old 19th July 2011, 09:37 AM   #4
E30NA is offline E30NA  Denmark
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Moin (We use that phrase here in DK to )

Thanks a lot, that was the kind of answer i was looking for. I am no electronics master, so my knowledge on how to fabricate new boards are wery limited. I asume that the new capacitors should be wired in paralell?
Can i drop the new rectifier bridges and opamps/capasitors on to the exesting board? Or do i have to fabrikate a new board to be soldered in where the stock diodes and opamps are soldered to the board?
Sorry if my questions are a bit newbee..

Regards
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Old 20th July 2011, 08:51 AM   #5
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Desolder the bridge rectifier D702 and place the MUR860 diodes as shown in the rectifier's footprint. Quite self-explaining ...
The capacitors must be wired in parallel. You don't have to make boards. Just get a veroboard and a drill to widen the holes. Do it like that:

Click the image to open in full size.

Use thick wire!

Change opamps IC201 and IC202 for OPA2134PA, you also could try LME49720NA. Change C213/C214 and C258/C259 for 10F Panasonic FM or Elna Silmic or whatever with low ESR. Desolder the opamps the the capacitors, then scrape off the solder mask coating where I placed the blue dots:

Click the image to open in full size.

There you solder the small film capactitors, no need to drill holes. Solder in the new opamps, then the small film capacitors, then the Pana FM or whatever you like.

About decoupling there is an interesting thread here. A good source for audio partss in small quantities is here.
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Old 20th July 2011, 04:07 PM   #6
E30NA is offline E30NA  Denmark
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Now i understand, thanks
It shouldnt be to difficult for me to do.

I have a question on what electrolytes to use. From the thread you have linked to, i can see that capasitors from different makes have an efect on the sound. Is there a brand that you can recomend me me to buy for the big electrolytes? I would like a bit warmer sound than the Denon have now- if it is possible?
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Old 21st July 2011, 07:31 AM   #7
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Fancy psu capacitors

Decent psu capacitors

For the sound I can't do even only a guess. Higher capacitance gives more punch, especially on higher volumes. Have a look into the schematic and follow the signal trace from input to the speaker posts. All what is in that trace affects the sound.

Making a psu beefier is a good starting point. Do it, listen to it, then think further.
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Old 21st July 2011, 07:31 AM   #8
E30NA is offline E30NA  Denmark
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I have ordered two Mundorf M-Lytic AG 22000uF Electrolytes, 4 Elna Silmic II 10uF and 4 Obligato 0,1uF for the upamps.
The rectifiers will be Motorola MUR860, and the upamps wil be Burr-Brown OPA2134PA.
If the 22000uF dont do the job, i will order two more M-Lytic electrolytes for a total of 2x44000uF.

I cant wait to hear the result I will of course give some feedback, when the amplifier is up and running.
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Old 21st July 2011, 07:37 AM   #9
E30NA is offline E30NA  Denmark
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We must have just crosed each other with the threads.

I wil try my new mods for some time, and then make i desission if i want to upgrade the capasitance more than the 22000uF.

And again, a realy apresiate the effort you have made to help me- thanks
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Old 21st July 2011, 08:54 AM   #10
E30NA is offline E30NA  Denmark
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I forgot to mention, that from the calculations on the website TNT Audio, i need 1000uF per peak A in 2Ohm to be on the safe side. That should mean:

Sq root of (420/2)*1,41= 20,43A and 1000*20,43= 20430uF

Then i should be in the balpark with the 22000uF Electrolytes.
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