Playmaster 300W amp humming and thumping
Hi, I'm hoping someone can shed some light on a couple of problems I have with an Electronics Australia kitset power amp I have. Its used as a power-amp for a 500WRMS JBL subwoofer.
1) Ever since I built it 6 years ago, its had a weird quirk: about 1min after switching power off, the attached speaker starts thumping. It starts off slowly (around 2 thumps per second) and loudly (about 10mm speaker excursion), and over the course of 5min the thumping speeds up (maybe 6 thumps per second) and dies off. Somewhere in the middle, the thumping seems to switch to double-time.
The amp also has a "clipping" LED on the front panel, which is meant to show if you're driving the amp too hard. During its thumping routine, the LED flashes in time with the thumping.
Yes, I have checked for animals and small people trapped in the box ;-)
2) The other quirk may or may not be related. Loud humming. Not when a source device is plugged in, but only when you touch the signal terminal of the input RCA lead with a finger. I don't know what freq the hum is (no ossillyscope), but it sounds low, like a truck horn. It might be 50Hz, but I don't really know what 50Hz sounds like.
The humming is quieter if you touch the shielding of the RCA lead with the same finger, or if you touch the amplifier case with your other hand. Is this expected/normal? As I said, it only happens under these conditions... the amp operates noiselessly and as-expected when plugged into my pre-amp and playing music.
Whats interesting, is that if you turn the amp off, and wait for the thumping to start, the thumping speeds up considerably if you touch the signal RCA terminal with a finger, and returns to its normal downwards spiral when the finger is removed.
Does any electro-nerd know whats going on? I've checked all the wiring/connections, and everthing is normal. The first thing that springs to mind re the hum, is an earth loop, but the symptoms exist even when the amp is connected to the mains power and a sub only (no other devices). I've checked the earthing, and the case is earthed, which is also connected to the earth of the input signal socket.
I'll scan the circuit diagram tonight, and post it up here, if that will help anyone.
Thanks for any insight!
The humming thing is quite normal - your body is acting like an aerial picking up 50Hz radiation from mains wiring around you, and you are feeding this "signal" into the amp when you touch the input. When you touch the earth as well, the signal level is reduced. This is a useful quick-and-dirty way to see if an amp is working.
What is the EA project number on this amp? A lot of amps will make some strange noises as the power supply drops out, usually due to the rails dropping at different rates. Large filter capacitors will help prolong the die down time. I would have thought the Playmaster used a "dethump relay" to connect the speakers after a turn on delay, and disconnect them immediatey when shutting down, thus avoiding these effects.
Thanks Centauri. Good to know that the hum is normal.
The EA project details are as follows:
File Number: 1/MA/67a
Title: Playmaster 300W Subwoofer Amplifier - 1
Date: Apr 95
The original 8000uF power supply caps died some time ago, and I replaced them with some 2nd hand enormous ones (can't remember the value). It thumped with the old caps, but like you say, the larger caps prolonged the thumping.
I emailed Silicon Chip magazine, who have superceded Electronics Australia. They replied with the following:
It sounds as though you have two problems. The hum is probably due to instability in the Mosfet output stages, they're probably at 100MHz or more (you can check that with an FM radio). Check all the gate capacitors for the Mosfets.
The slow oscillation could be related to the above, ie, motorboating. Perhaps some of your bypass capacitors on the supply rails are open-circuit.
SILICON CHIP MAGAZINE
Also, heres a photo of the case layout. As you can see, the new caps are rather large. Bit of an overkill, but they can't be TOO big, can they?
The only query I have with the layout, is about the torodial trans being too close to the PCB. Even though torodials are not meant to make noise, I've read somewhere that shielding them or placing them away from any signal is advisable. What do you think?
And Centauri, no, this amp doesn't have a de-thump relay, unfortunately.
Circuit diagram coming soon.
Thanks for uploading the schematic - I don't have the April issue, but do have the May issue with part2 of the project.
Comparing your photo with the original, I note you are not using the crossover board, and the input is connected to an RCA connector on the back of the amp. This raises a couple of points: 1) is the earth terminal of the RCA connected to chassis? (ie. not insulated) - if so, then insulate the RCA from chassis. 2) the original crossover board had a transistor which mutes (shorts) the input of the amp during turn-on and off - this may have prevented the problem (maybe).
It is a little difficult to see properly the earthing in the photo, but it looks as though the filter cap commons are wired to a tagstrip in front of the PCB.
I would be tempted to connect the two filter cap common terminals directly together with heavy bare wire, and connect the speaker ground, transformer secondary centre tap, and PCB earth to the centre of this bare wire (this moves the common earth from just in front of the PCB, as in the original, to the mid point of the filter caps, and should give better stability).
Don't go with Leo's reply - HF instability won't show up as hum and will result in excessive heating of the heatsink, and faulty bypass capacitors can cause motorboating when the amp is RUNNING.
Have you seen this?
Thanks D.N. yes, I found those on Google too ;-) I was excited at first, but after checking found that the instructions I built the amp from already had the changes made. (I bought the kit from Dick Smith Electronics, who provided their own instructions, etc, but referenced the source being EA April 95)
Centauri, thanks for your reply. I haven't actually seen EA April and May, and as I said above, I got the kit from Dick Smith. So, it seems that my kit didn't come with a crossover. (I built a crossover seperately, in another case). Yes, the input is on the back panel. From memory, it came with a plastic insulator, but I'll check tonight that I haven't shorted it to chassis anywhere.
Can you please email me a scan (if possible) of the May issue? It will be really good to see the muting circuit on the cross-over board, and try to incorperate that into my input. My email is "chris at btg dot co dot nz".
Sorry about the photo. They only allow 100k uploads here, so couldn't do a clearer photo. Yes, the cap commons are wired to the earth tag strip on the PCB. OK, will try moving the earth centre. That makes more sense. Seeing I'm moving things, I'll move the transformer closer to the front of the box too.
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