REL Q150E mk2 very light hum

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got it faulty and repaired (one of the 4 mosfets was out along with 2 fuses). Now it works fine, though some very light buzzing/ hum is still there. It's not audible, only if in some 10-20cm from the sub and could be felt with hand.

1. It's there with no RCA or Neutrik cable connected.
2. If I short the RCA socket it's still there.
3. Connecting with no Earth or switching it off on the sub does not affect on the hum.
4. Same hum was there when I connect another speaker.
5. It's about 48vdc on the PS caps. Tried to connect the scope and it was 100mv p2p offset, is that fine?

Any ideas what's wrong here? I have a second sub (Quake) and it's dead quite.
Should I try to swap the main ps caps (2x 6800uf)?


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This was a note from Bose Panaray sound system 502 array bass box and controller (Service manual and bulletins):
402™, 502®, 802® II Controllers
Power Supply Noise
Units Manufactured before 4/96

Symptom: Noise or hum from outputs.

Reason: The transformer could be causing the problem.

Solution: Replace transformer with new type.

If a customer returns a unit for service because of a problem with noise or hum replace the transformer with part number 180128-1 (100V/ 115V) 180128-2 (220V/ 240V). This service
bulletin supersedes service bulletin 181472.
tried the following with no luck:

1. Swapped the toroid to 2x33VAC 1.2A, same hum.
2. Removed 2 x 6800uf caps and put temporary 2 Nichicons KG 470uf (the only 50v I had), same hum there...
3. Removed the amp board from the back panel to clarify if that is related to grounding or add any noise/ hum and still the hum was there. Magic :D

looks like the problem is somewhere else...
One thing I didn't mention is that IRFP9240 I've replaced is not matched with other 3 mosfets and shows different voltage across the power resistor, however I doubt this could affect on any buzzing noise?!
Anybody have a service manual for this one?

I have the same one with same symptoms on my table.

Turn on pop can maybe not be avoided with this simple relayfree design, but the slight crackle is certainly something extra.

Somebody tried to repair it before me and replaced a handful of caps.
He said they had leaked. Not sure if it is true. Caps don't leak easily in linear applications.

I am a bit surprised that there is no auto-on and the manufacturer states in the manual how safe it is to have it turned on all the time.
I unplug 90% of my things when I leave my home and I have good insurance :)

Hope we find a solution together!

as it was mentioned, it was some issue with voltage regulator there, so i've tried to find out something relevant in my Q150. It turned out that without any manual or circuit it's nearly impossible to determine smd chips. There are 3x ZVY415 that act like some pre regulation stage I suppose, but I couldn't find anything similar in the web at all. Same thing is with transistors that are most likely used for regulation - DH S48.

One thing I noticed is that if I touch the axle of frequency pot (Rolloff), it starts to hum much louder. Could the problem be here :confused:


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Are you sure those 14-pin chips are not LM324 Opamps?

The others I could not find either.

I was going to separate crossover and poweramp to see which one is actually fried. But without the schematic it was not obvious to me how to do it :(

The power FETs might get "hissy" and "poppy" if they have run hot, so I was thinking about replacing them too. But it is a slim chance and I am getting really tired of this cheap crappy amp module :)

Anyway my friend is willing to spend up to 200euros on the sub to get it working, so I ordered a D-class board with crossover from ebay.

I know it will mean buying a suitable transformer for more than 50euros too, but I think it will be below specified budget.

This should give about 150W rms, instead of the factory amp which gives max half of that :D
yes, should be LMs then as you've stated.

It looks like Q12 & Q13 are voltage regs here with no filtering on emitters at all, only on their bases (great approach REL!). So another thing I've tried is I added 470uf elkos here. Unfortunately it hasn't changed much, the hum is still there and is almost the same. Perhaps replacing these voltage regs with something more decent would reduce the hum, but I'm not sure about the substitute...
I doubt that replacing all mosfets will bring any improvements here.

Btw, for 200 eur you could find same working model without any issues (I assume that it's not factory problem of this model) and delivered to you friend!

Good point is that they put at least 4 mosfets here unlike Quake model with only a pair and a tiny 50VA toroid :D
As a lastlast resort I tried to disconnect a pair at a time of the power fets.
No change.

So I started building it with all new parts.
A 2x18V 200VA transformer from Noratel (made en Sweden!) 47euros
4x6800uF 80V Samwha caps 20euros
A funny D-class amp board from china 34USD + VAT = about 35euros
Pots, rectifier, wire et.c. for about 20euros
= 122euros + some work :)
Friend paid the parts and helped me pick up some furniture, so it was a good deal to help each other out.

The awesome amp had a solderbridge missing, so that only one channel worked.
When I figured it out I got both channels working and some bridged power out :)
Also the pots would not really fit the back panel of the REL so I wired the pots separately and also swapped the wires to the crossover pot, which was originally working in the wrong way.
Turning the pot clockwise decreased crossover frequency...only in china.
Seller was also worthless at helping when I complained.

It is late and my spelling is probably bad :)

Anyway I have tried to add a bad photo. (I have more)


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nice work!
how does it sound now?

As to mine poor buddy, I've adjusted the bias (now it's 40mA, 130mA was the adjustment set by REL?!), shielded all 5 pots with foil and connected to the earth. That made a noticeable improvement, however didn't eliminate all the hum, something is still there, but it's way better now than it was :)

I think I'll stop here as the next step would be to swap this poor amp module...
It sounds good and powerful, but crossover is really bad.
Have to keep it turned to the lowest frequency to get ok filtering.

I think that with descent placement of the sub, my friend will be able to enjoy both music and movies with it.

With the 2x18V trafo I measured 132W RMS to 4ohm resistive with low dist. (amp is meant for 8ohms, since it is bridged)
With severely clipped signal I measured 210W RMS. (I do not know why I even measured this)

8ohms gave about 100W

Should have found a 20 or 21V trafo.

2x24VAC is too much for the poor module, with TDA8920 chip.
I had a 2x22V trafo, but under no load it also had slightly too high voltage.
I believe +-30VDC is the limit for the chip.

I think chip has over current protection at 7.5A, but it never kicked in for me when testing :)
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