Hello. I am having a problem with one of my amplifiers, DAP Palladium 1200 Vintage. One of the channels has a problem with clipping at a very low power, specifically at 22 volts with either 4 or 8 ohm load. The channel then goes into protection. The other good channel has no problem pushing 56 volts into a 4ohm load (800W). The clipping occurs on both channels if you use amplifier in parallel mode (both channels using first input).
I have ordered an oscilloscope and should receive it in about 2 weeks, but just wondering if anyone here would have any idea what could be wrong with the channel. I tried switching left and right channels and the problem is definitely in the amplification board, not input/output gain pot or power supply.
Regards
Google drive link to the schematic pdf: dap_audio_palladium_p-1200_sch.pdf - Google Drive
I have ordered an oscilloscope and should receive it in about 2 weeks, but just wondering if anyone here would have any idea what could be wrong with the channel. I tried switching left and right channels and the problem is definitely in the amplification board, not input/output gain pot or power supply.
Regards
Google drive link to the schematic pdf: dap_audio_palladium_p-1200_sch.pdf - Google Drive
I checked both channels. The bad one has about 2.2V DC offset at clipping volume. It actually rises from 0 to 2.2V in difference depending on the volume.
When you say it clips at low volume, do you mean that it goes to max output at low volume setting? That the gain is very high?
How do you know it is clipping?
Jan
How do you know it is clipping?
Jan
Did you check the DC on the input? That would explain that both channels clip if they are driven by the same input.
What happens if you swap the leads from the source to the power amp, does the other channel now clip?
Jan
The rail voltages must be okay since the 2nd channel works and they share the same power supply.
It is most likely one of the power transistors is gone. To find the culprit, measure the voltage over the emitter resistors (the 0,22 Ohm/5W ones at the last stage), it's usually also easily to be spottet in another way, it will get quite hot (or stays completely cold). There are ofcourse other possible suspects but this is the most likely one.
BTW, could a MOD please move it to 'Solid State'? It's not a class-D amp.
It is most likely one of the power transistors is gone. To find the culprit, measure the voltage over the emitter resistors (the 0,22 Ohm/5W ones at the last stage), it's usually also easily to be spottet in another way, it will get quite hot (or stays completely cold). There are ofcourse other possible suspects but this is the most likely one.
BTW, could a MOD please move it to 'Solid State'? It's not a class-D amp.
The amp got an input decoupling capacitor, so that's not very likely but extremely little effort and should ofcourse be checked.
By clipping I mean that the amplifier channel runs normal when the output voltage is below around 18-20v, but after that there is some noticeable distortion/clipping heard from speaker and the clip light on amplifier lights up. It then goes into protection. The clipping is also not connected to any gain/volume knobs. Just happens at 22v of output voltage into any kind of load.
I have tried swapping amplification board sides (both are identical, took them out and swapped places). They share the power supply anyway so it shouldn't be the problem.
Also, sorry if this is posted in the wrong forum.
I have tried swapping amplification board sides (both are identical, took them out and swapped places). They share the power supply anyway so it shouldn't be the problem.
Also, sorry if this is posted in the wrong forum.
That depends, that's not necessarily the case. I already had exactly such a case.
Also, I want to note again that if the amplifier is set to PARALLEL mode (both channels from first input), both channel amplifiers will clip (act the same as first broken amplifier) at 22v. It is possible that there is a problem before the end stage.
That is quite possible, but you pointed out you don't have a full equipment, that's easiest to check (temperature or voltage over the resistors), so why not star there since other things have already been ruled out?
Defective transistors show 3 behaviours:
1. fully conductive (short)
2. non conductive (open circuit)
3. out of spec behaviour (thermal error, gain, noise etc)
With the test I explained you can check for #1 and #2 with your actual tools, no soldering and no scope needed for it.
Jan is right, 1. is not likely since it would in most cases cause a much higher offset but it could still be case #2. That would not cause a big DC offset but it would change the offset with the parallel mode. Does it change in parallel mode? If not, it's pretty much in the pre stage or a stabilizing part (one of the zener diodes maybe).
Defective transistors show 3 behaviours:
1. fully conductive (short)
2. non conductive (open circuit)
3. out of spec behaviour (thermal error, gain, noise etc)
With the test I explained you can check for #1 and #2 with your actual tools, no soldering and no scope needed for it.
Jan is right, 1. is not likely since it would in most cases cause a much higher offset but it could still be case #2. That would not cause a big DC offset but it would change the offset with the parallel mode. Does it change in parallel mode? If not, it's pretty much in the pre stage or a stabilizing part (one of the zener diodes maybe).
I have checked the end stage transistors and they all seem to be fine. Any other suggestions? I will be getting a scope soon so I can post waveforms of the output signal.
I was searching on the internet for other similar problems and stumbled upon some thread with DC voltage offset control, and it struck me that there are some trim pots on the amplifier board. The pots were all glued with silicone but one seemed to come loose. I turned the amp on, connected the load and the multimeter with dc mode on and fiddled with the pot. I managed to get it around 100mV of offset at around 56V of output voltage, which is pretty much the same as the other good channel.
This is a class H 2-tier (switched rail) amplifier. There could be a problem with the rail switches if bad channel output is close to half of good channel. Remember these are cheap amplifiers, where "BTL" and "PAR" configurations are often achieved feeding the output of 1st channel through an attenuator resistorto to the input of 2nd, inverted or not, as this does not require additional signal (op-amp) stages, and there is no drop in gain due to changes in input impedance between modes that way.
Once I repaired an old non-class-H "promaster" amplifier, looking identical to old DAP audio inside, and it was like that.
Once I repaired an old non-class-H "promaster" amplifier, looking identical to old DAP audio inside, and it was like that.
Maybe I've got an old version but I just checked again and according to the schematics I've got it's A/B. The 1600 and 2000 are class H, I don't know if they changed it with the production restart on the 1200, but the 1200 and all smaller models were class A/B.
That would explain the clipping but not the DC offset, that would not affect it as long as only the lower rail is used.
Okay, a friend just confirmed, the DAP Palladium 1200 Vintage is Class-H. The schematics I've got are likely from the non-'Vintage' ones and they itself are correct but most likely a different amp version, I got an older version which only applies to the smaller amps, very likely below the 1200 model (despite the labeling). I cannot verify to which models it actually applies to though and it's not as easy as one might think to get the appropriate schematics since it's been modified a copule of times and sold under different brand names and versions too.
That rises different questions though. The protection circuit on the 'vintage' amps was extended, it also features among other, a clip and a DC detection. The DC protection should trigger at a lower level than at a 2,2V offset, and the clip proteciton should trigger in this case too, so it seems the protection does not work as intended either. I cannot confirm what's at fault, my schematics don't apply to this amp version apparently unfortunately. I'm sorry but I won't buy the schematics just to help, even if I'd really like to.
My suggeston is to proceed with my former proposed test (despite the usual procedure to test the power supply and rail voltages (which was ruled out already), the input-, pre- and then the output stages) to check the output transformers first since that's the most simple test since a transistor failure would possibly also explain the DC offset, mostly because it can be performed with minimal equipment. If that does not come to any conclusion, I would test the rail voltage switching, which can be done with a multimeter too, which is also a thing I would suspect as Eva already did. That already might need a scope though if the test with the multimeter does not come to any conclusion. The zener diodes are (as already mentioned) a prime candidate for rail voltage switch problems, they have to be removed from the circuit for testing though. On the pre stage the OPs might also be at fault (probably only partly) but that's likely not that easily to find without a scope and/or removing the OPs and testing them separately - which is a bit more of an simple effort.
@MODs: That is still a Solid State amp, so move it please. 😉
That rises different questions though. The protection circuit on the 'vintage' amps was extended, it also features among other, a clip and a DC detection. The DC protection should trigger at a lower level than at a 2,2V offset, and the clip proteciton should trigger in this case too, so it seems the protection does not work as intended either. I cannot confirm what's at fault, my schematics don't apply to this amp version apparently unfortunately. I'm sorry but I won't buy the schematics just to help, even if I'd really like to.
My suggeston is to proceed with my former proposed test (despite the usual procedure to test the power supply and rail voltages (which was ruled out already), the input-, pre- and then the output stages) to check the output transformers first since that's the most simple test since a transistor failure would possibly also explain the DC offset, mostly because it can be performed with minimal equipment. If that does not come to any conclusion, I would test the rail voltage switching, which can be done with a multimeter too, which is also a thing I would suspect as Eva already did. That already might need a scope though if the test with the multimeter does not come to any conclusion. The zener diodes are (as already mentioned) a prime candidate for rail voltage switch problems, they have to be removed from the circuit for testing though. On the pre stage the OPs might also be at fault (probably only partly) but that's likely not that easily to find without a scope and/or removing the OPs and testing them separately - which is a bit more of an simple effort.
@MODs: That is still a Solid State amp, so move it please. 😉
I could only find this sketch/preview of schematics pdf:
https://elektrotanya.com/PREVIEWS/63463243/23432455/egyeb/dap_audio_palladium_p-1200_sch.pdf_1.png
And the pdf in pay per view sites:
dap_audio_palladium_p-1200_sch - Bing
https://elektrotanya.com/PREVIEWS/63463243/23432455/egyeb/dap_audio_palladium_p-1200_sch.pdf_1.png
And the pdf in pay per view sites:
dap_audio_palladium_p-1200_sch - Bing
My schematics are on paper (bought a long time ago), I can't post them here but these are not the same as mine anyway. You can get the schematics you've posted here, wait for a while, then the DL-Link appears, you can download it as a PDF. It shows definitely a class H amplifier with clearly marked VL and VH +/- which are switched by 2 FETs each. There are no voltages or current values and test points though, which makes the hunt for defects and later adjustment not much easier, that results in A/B comparisons with the working channel.
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