fixing creative T40 speakers - one for the electronics geeks

[ik9000]

So my lovely T40 series 1 speakers just developed an interesting perma-fuzz. It's serious distortion in the mids, and turns (for example) the beautiful south's "need a little time" into a euro-synth fuzzfest of distortion. the longer it's left on the louder the distorted element gets. Seriously if I could sample this tone and put it onto my guitar rig I'd be chuffed, but for my speakers? No no no no no.

It affects both speakers. It does not affect the headphone through-pass. It does the same distortion when plugged into my hifi's headphone out (in a different room, on a different wiring loop). It is definitely the speakers, and it is not EMI (not from an external source anyway).

So it's possibly the amp element - though the volume and eq pots still all work fine with headphones plugged in. Which makes me think possibly the crossovers? This article suggests there is indeed a 2kHz issue, and that the original design uses a capacitor to limit it. If the controlling cap had gone, would that be enough to send it into this distortion heaven? (I'm presuming it's not the IC since the headphone output seems fine)

Any thoughts/tips on how to diagnose and resolve this? Has anyone had a similar issue, and how did you fix it? Particularly does anyone know where I can get a circuit diagram for these - my google hasn't yielded much fruit. I don't mind swapping out a cap or inductor etc - got nothing to lose at this stage, but be good to know a bit more where to target when I dive inside.

[g8ina]

The power supply often goes phut in these, maybe try another power source first ?

[peterb]

The amplifiers are often single monolithic chip - if you can find the part type number, you may be able to get a replacement. Unsolder the old one, solder in the new and jobs done. But try the obvious thins first - as g8ina says - PSU!

Edit:

Cct board layout here:



The main amp looks like a surface mounted design - which while replaceable can be a little more problematic than conventional components.

The data sheet for the TDA 8932B is here https://datasheetspdf.com/pdf/659811/NXP/TDA8932B/1

A quick search found it here https://picclick.co.uk/TDA8932BTW-TD...364162750.html

at a cost of £7.29 + £5 P&P - you might find a more local supplier though. The problem then is replacing it - or getting it replaced.

However, having established that it is a class D amplifier, it could be that one of the output filter capacitors is suffereing - although which one is hard to say - and again they look like surface mount devices.

[ik9000]

The amplifiers are often single monolithic chip - if you can find the part type number, you may be able to get a replacement. Unsolder the old one, solder in the new and jobs done. But try the obvious thins first - as g8ina says - PSU!

Edit:

Cct board layout here:



The main amp looks like a surface mounted design - which while replaceable can be a little more problematic than conventional components.

The data sheet for the TDA 8932B is here https://datasheetspdf.com/pdf/659811/NXP/TDA8932B/1

A quick search found it here https://picclick.co.uk/TDA8932BTW-TD...364162750.html

at a cost of £7.29 + £5 P&P - you might find a more local supplier though. The problem then is replacing it - or getting it replaced.

However, having established that it is a class D amplifier, it could be that one of the output filter capacitors is suffereing - although which one is hard to say - and again they look like surface mount devices.

So it's an interesting one. The socket is marked with two values, 20VAC or 27VDC - and I've no idea which of those the current supply is putting out (multimeter test to follow) but I have nothing that will supply that, not for the type of socket it needs to operate. And if it was the PSU wouldn't that still screw over any headphones coming off the speaker's headphone out socket - since that is powered by the same psu?

[g8ina]

I can prolly cobble a replacement PSU if needed.

[DanceswithUnix]

A laptop supply is probably close enough in voltage to work just fine. Amplifiers aren't like computers, feed them their full voltage for their full rated output but you probably don't use that whole range anyway.

If the PSU is ok then a fuzzy sound is most likely a burnt out speaker coil. That could be more problematic to get a replacement for. The amplifier chip would have all sorts of self protection circuitry in there, so that is least likely to be the problem.

[ik9000]

A laptop supply is probably close enough in voltage to work just fine. Amplifiers aren't like computers, feed them their full voltage for their full rated output but you probably don't use that whole range anyway.

If the PSU is ok then a fuzzy sound is most likely a burnt out speaker coil. That could be more problematic to get a replacement for. The amplifier chip would have all sorts of self protection circuitry in there, so that is least likely to be the problem.

well if it were a coil I would expect it to only affect one speaker - but both are doing it. When I get back I'll try the PSU against a multimeter, and then I'll try an 18VDC supply out of my pedalboard powersupply and see if that works (shouldn't do any harm to under-volt it right???) (assuming I can find a cable with a socket that fits in the cable draw of chaos)

[ik9000]

I can prolly cobble a replacement PSU if needed.

thanks + careful in case I end up taking you up on that

[DanceswithUnix]

well if it were a coil I would expect it to only affect one speaker - but both are doing it. When I get back I'll try the PSU against a multimeter, and then I'll try an 18VDC supply out of my pedalboard powersupply and see if that works (shouldn't do any harm to under-volt it right???) (assuming I can find a cable with a socket that fits in the cable draw of chaos)

As long as the PSU can supply enough current, just don't turn it up too high.

Speaker coils can usually take quite a lot of power as long as the amplifier feeding them is good, but if you drive the amplifier to the limit then the waveform clips. That distortion sounds bad, but the problem is the clipping isn't always symmetrical in cutting the top and bottom off the waveform. That lack of symmetry creates an overall DC voltage across the loudspeaker coil which creates damaging heat not sound.

So don't turn it right up. But that's a generally good bit of amplifier advice anyway