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Denon PMA-1560 unobtanium MOSFETs

PE9ZZ

PE9ZZ

Event Horizon
Subscriber
I am working on two of these beasts. Both have one channel blown to smithereens due to the driver MOSFETs failing. These devices, 2SJ78 and 2SK215, are low power versions of the well-known lateral audio MOSFETs like 2SJ161/K1057 and have a TO220 case with drain and source terminals swapped. I decided to toss them altogether and replace them with a MPSA42/92 and MJ15032/3 combo. One channel worked like a charm but the other channel was impossible to tame.

To make a long story even longer, I decided to put the whole amp into LTSpice. All of it. Turns out that it is very sensitive for the power supply voltage of the diff amp. If it is lower than say 10 V all hell breaks loose. Incredibly, the simulation has the same properties as the live amp. That doesn't happen often!

Right now I'm still gathering courage to continue working on the thing. Attached the state it's in now-
denon_pma-1560_ku-9210_final.png

What can be seen here is the behavior when the diff amp supply is rising. If that were to happen with the amp in operation the results will be catastrophic. It may well be the reason these amps fail.
 
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You have a lot going on there. I see three gain stages, cfp driver, lots of taming for sure. I agree, post your sim.
 
i cant see the schematic too well .i don't see any zeners . whether they will do any good i do not know .
 
Ylli said:
Wanna post the .asc?
Click to expand...
As zip file. As well as the screenshots and the models (.sub). Also noticed I uploaded the wrong screenshots. The two uploaded now (big ZIP) show the effect of VR501 on V(OUT) and I(bias). Ramp on the diffamp supplies. An amp should not become unstable if the bias is cut off but this Denon does. Highly annoying.
 

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I don't get it. I can reliably blow up this amp in the sim (attached). Gone in 10 µs. In real life it selfdestructs. There is a structural issue with the Sziklai pairs TR519/523 and TR521/525. If it goes into runaway mode it oscillates violently at 1 MHz. What it triggers I have NO idea. Sprinkled capacitors all over the place, nothing helped.

If I replace the Sziklai pairs with just an emitter follower the amp is stable. It becomes a bit like the Rauch amp above. Even the +/- 16 V supply ramp is stable. But surely this amp can be made to behave? These Denons fail left and right and it looks like this is the root cause. Very frustrating I cannot find what's going on here!
 

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Wow. Just wow. What a rollercoaster ride. Whatever I did made no difference. Either it started to oscillate without bias or it started to oscillate with bias. This amp is one temperamental beast. But I got it tamed and have a fix for those unobtanium MOSFETs. See the attached amplifier module schematic. Finally it is stable:
_IGP5438_crop.jpg

I removed five capacitors, two transistors, a resistor and two diodes. I replaced several resistors and one capacitor. It's clear the Japanese engineers ran into the same issues I did only they didn't have the tools I have available. Mainly LTSpice. The latest version also attached (ZIP).

Fun fact: while it took me weeks to find the solution it only took me minutes to fix the second amp...
 

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And of course this gives me the opportunity to do a shootout between my bipolar solution and the original MOSFET driver. Once I fixed the tone control issue with the second, silver amp (this one is black) I will do it. Same setup as the VFET vs. MOSFET shootout I did with the Sony TA-4650. With which I couldn't hear any difference whatsoever.
 
Shootout time!

denon_pma-1560_shootout_closeup.jpg


At first, the black Denon sounded a bit... darker than the champagne one. More pronounced bass, if you will. However, when I tried to replicate the semi-blind A/B test the difference had disappeared. The results became random. Tried for an hour to hear a difference but couldn't. Even though the silver Denon still has all its additional components on its boards and the black one is stripped as in the schematic above. Weird huh...
 
Thanks to share sims from PE9ZZ, here is what I'm going to do.
Remove FETs and BJTs and make new BJTs compound stage with C2690/A1220 and C5171/A1930 with floating output.
Sim is looking good and I hope it will work and sound as with laterals.
 

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Last edited:
Prolineaudio said:
Thanks to share sims from PE9ZZ, here is what I'm going to do.
Remove FETs and BJTs and make new BJTs compound stage with C2690/A1220 and C5171/A1930 with floating output.
Sim is looking good and I hope it will work and sound as with laterals.
Click to expand...
Did you make this work?
PE9ZZ said:
Wow. Just wow. What a rollercoaster ride. Whatever I did made no difference. Either it started to oscillate without bias or it started to oscillate with bias. This amp is one temperamental beast. But I got it tamed and have a fix for those unobtanium MOSFETs. See the attached amplifier module schematic. Finally it is stable:
View attachment 1877006

I removed five capacitors, two transistors, a resistor and two diodes. I replaced several resistors and one capacitor. It's clear the Japanese engineers ran into the same issues I did only they didn't have the tools I have available. Mainly LTSpice. The latest version also attached (ZIP).

Fun fact: while it took me weeks to find the solution it only took me minutes to fix the second amp...
Click to expand...
Did you use mje 15032 / 33 combo , or did you use mje 15034 / 35 combo?
Currently working on two of theese , where both had defective mosfets along with many smaller resistors.
Between both amps there was enough parts to get one amp working in original configuration.
 
Maybe this will help...
I know someone repaired a PMA-1560 and modified the pre-amp stage (I think that'a where your problem is) using the schematics of the PMA-1520.
Hope it helps...
Good luck!
 
Just to thank to PE9ZZ, for sharing the "mod" on PMA-1560. I was able to revive a POA2800, following your directions.After 2 weeks tweaking with fake 2sk215 and 2sj78, no way to adjust this beast, blowing finals, fuses,etc... Just one question, transistor base of TR514 must be to collector of TR518 (-1.3v) or to ground (0v) from the input connector?
I´ve tried both and seems to work fine...
BTW I use for finals NJW1302 and NJW3281
Thanks again!!!
 

Attachments

  • capture pma1560.JPG
    capture pma1560.JPG
    142.6 KB · Views: 72
Peterk6 said:
transistor base of TR514 must be to collector of TR518 (-1.3v) or to ground (0v)
Click to expand...
It goes to ground. I should have erased the voltage value at that point. TR514 just acts as a level shifter for TR510, that is the reason both (seem to) work. These schematics don't use ground symbols and that makes them confusing to read. Like the ground lead for C530. WTF is it doing there...

Your output transistors are fine.
 
Me too, I want to tell thanks you, very thanks you since a long time (one or two years...) to PE9ZZ. So, I've a PMA-1560 since a very long time, but it was out of order.
After a lot of FET burned, I've found your forum. I dreamed of it, and PE9ZZ did it. Thanks a lot. And excuse me for my bad english.
The next days, if you are agree, I will post photography of my modifications (I've made new circuit board).
Kind Regards. ;)
 
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