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Old 30th May 2004, 08:56 AM   #1
Magnum9 is offline Magnum9  Australia
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Join Date: May 2004
Location: Perth WA
Default Help troubleshooting Jaycar sub amp hum

Hey all,

I'm hoping that someone else out there has also built one of the 320W RMS Playmaster Jaycar Subwoofer amps from Electronics Australia Magazine 1995. If not, maybe somebody can help diagnose my fault anyway.

Mine is slightly different to the standard design in that I installed an additional pre-amp module before the main input because I felt my Pioneer receiver wasn't producing enough signal and output was poor even with the sub amp turned up to max. It may in fact be this module producing the problem. Unfortunately I do not have the circuit diagrams for the amp any more, but if someone is familiar with the design, and can advise which resistors to change on the standard pre-amp/crossover board to give higher gain, I could ditch the extra pre-amp altogether.

Anyway, on to the fault.

The amp worked faultlessly for about a year, when it first started to hum. As stated in the errata at the time, I replaced the BC556 transistors with BC640's due to voltage breakdown. Since then I have had to replace them once again just recently due to very slight hum.

The strange thing though is that after I turn the amp off, the "clip" LED flashes a few times and when it does so, sharp bursts of sound are emitted from the speakers. It does this 4 or 5 times before going silent.

Another fault has only occurred since I started satellite logging from my PC. When the sub amp is on and the PC is logging, changing channels on the Humax gives a really annoying hum, but only through the sub amp, the main receiver and speakers don't do it. If I turn off the PC, no hum. Also if the receiver is switched to something other than the Humax, no problem. So it appears that the DVB-s card is interacting with the Humax, but only passing it to the sub amp for some reason. Bear in mind the sub amp gets is signal from the main receiver and the other speakers aren't running any crossovers that would eliminate any particular frequency.

Anyway, any ideas? There are two 160V capacitors (polarised elctrolytics) on the amp module board, (the same circuit that had the faulty transistors) a 100uf and a 47uf which could be the culprits. I would just replace these anyway but none of the garden variety electronics stores in Perth has 160V caps. There are also some big **** caps in the power supply but I don't think these would contribute to the problem.
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Old 31st July 2004, 04:20 PM   #2
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Join Date: Jul 2004
Location: Hobart, Tasmania
Hi Magnum9,

I don't have the Playmaster sub amp, but I do have 2 Playmaster Pro Series 3 amps, which are essentailly the same circuit, but with only 2 mosfets per rail instead of 3. I built a Jaycar kit recently, which had the BC640 transistors as per the EA errata for the sub amp (they have the same 75 volt supply rails). But, the original Pro Series 3 article in EA used BC556's, and my other amp, which I bought ready built by another tech, had the same problem as your amp in both channels, and the originally specified BC556 transistors. The clipping LEDs flashing, and clicks in the speakers, is the "motorboating" that EA mentioned in the errata, so when I replaced the BC556's for BC640's the problem, including clicking, LED flashing and low level hum, disappeared. Interestingly, the problem only occurs when something is plugged into the inputs, as that completes the circuit to earth.

So, to get to the point, I would try replacing those BC640's with a new set, or at least look at that part of the circuit around Q3 and Q4. If you need the circuit diagram and description, they are available in Electronics Australia of April and May 1995 - they're available in most public libraries around Oz if you don't have them. The circuit includes plenty of voltage readings to compare your amp with if you have problems. If some of the other parts such as resistors and capacitors around Q3/4 have also been stressed by the failing transistors, then replacing them might allow Q3 and Q4 to work without failing. Try taking voltage readings around there to check for any other faults. If you don't have a transistor tester, a good test is to check for 0.6V across the B-E junction of working transistors while they are in circuit. Hope that helps. And, if you are searching for components in Perth, Altronics should have most things. If they don't, you should be able to get anything that's in your amp using mail order from Jaycar or Farnell ( http://www.jaycar.com.au and http://www.farnellinone.com.au ).

And lastly, I notice that there has been a bit of discussion about how the Pro Series 3 sounds in relation to the old Tilbrook ETI Series 5000, which was an earlier mosfet power amp. I have a working Series 5000, which I built in the early 80s (it's been very reliable), upgraded in the power supply as per later Tilbrook/Playmaster Pro Series amps (i.e. no fuses and heavy duty wiring between 8000uF capacitors and power supply rails). The Playmaster Pro Series 3 is a much better sounding amp than the ETI Series 5000! (And most other early kit amps.) Is it better than recent Silicon Chip amps, such as the SC480 and 350W amp? Probably not - they are very good. But, it's without doubt better than ANY valve amp (unless you like distortion - it doesn't have that fuzzy, 'warm' distorted sound )!
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Old 2nd August 2004, 01:09 AM   #3
Magnum9 is offline Magnum9  Australia
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Join Date: May 2004
Location: Perth WA
Thanks for that.

I found part of my problem. When I had installed the pre-amp inline between the inputs and the existing crossover module, one of the wires got reversed, which was resulting in a bad ground loop hum. In addition to this, I was testing it at work on a source that is not earthed, which even after fixing the above problem, still resulted in ground loop hum. After taking it home to double check it my system, it seems okay.

There is still a very slight hum which is hardly noticeable, but I will replace the caps and see if it gets rid of it.
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