Amplifier distortion help?

[Gandy]

For the last year I've had two pairs of fairly cheap passive loudspeakers set up that I use to monitor. One produces some mid range and all of the high end, where the other larger pair produces most mid range and most low end. Each pair has an identical amplifier that I salvaged from two identical pairs of old computer speakers. It is two separate speaker pairs with two separate, identical amplifiers, each with its own separate, identical transformer and mains cable.

As there is a 'bass' control on each of the amplifiers (seems about 200Hz), I use the pair as a pseudo crossover, with the bass rolled down on the smaller pair (that go mad and distort when any bass is present), and have it rolled full up for the larger pair, where the larger cones don't produce much above 14k-ish.

These amplifiers have always been taped to the side of the speakers, with the transformers mounted on the back. Since I am renovating my room I am finally getting around to boxing the amplifiers in. It is simple - mount transformers on a chassis, mount amplifiers on a chassis, connect a 1/4" unbalanced stereo socket to the input of both amplifiers (what I was already doing) and solder the outs to a quadruplet of RCAs to plug the speakers in.

Since I had just enough space for the PCBs, I decided to solder the inputs and outputs directly to the PCB at the rear, connecting it to the tabs that the 3.5mm jacks are soldered to. There is a mono out for right speaker as the circuit is hard wired to the left, and a stereo input jack.

I soldered it up and tested, and nothing worked. I went back and took all the leads off and eventually found where they all had to go. The 3.5mm sockets are the box type with no clue as to what was the tip or sleeve, so it was trial and error.

One amplifier is okay. Plugging the speaker in to the 3.5mm out works properly (but isn't yet wired to its RCA), but the left out is hard wired to one of the RCAs. Plugging a speaker into the wired RCA creates sound, but at a level nowhere near that of the other speaker's direct out. I am using the same speaker for all testing.

The other amplifier, with the same thing done to it has somehow gone askew. It does not produce any sound from the RCA, wired directly to the left speaker output, and from the 3.5mm right speaker output it can only produce distortion. It sounds like bog standard control-to-11 distortion - it is easy to pick out rhythm and some pitch, and the more low end the more distorted it becomes, but it's not mild distortion, it's a full on gritty noise, almost devoid of musicality. It produces this distortion at the same level as the other amplifier produces normal sound - the volumes are near enough equal when played.

I've rechecked my soldering, and it's all good. I've made sure nothing is touching anything else and I've checked for broken track or terminals and I have had a look and there seem to be no fried resistors or capacitors. It is soldered identically to the fully working one, and had the same things done to it; they were done as a pair. The transistor's radiator on the distorted amplifier is getting desperately hot as well - hot enough to melt wires and burn my worktop, whereas the other working amplifier's radiator gets hot, but just touchable.

The problems with the left one arose after the power supply developed some problem. It takes a 12V AC, with the transformer currently mounted on the chassis, and soldered and mounted the same as the other amplifier. At some point it stopped working. I thought it was the fuse, but tried it in the other amplifier and it was not. For some reason it's just stopped doing anything. as the two systems are identical, I can interchange transformer to PCB plug, and neither amplifier runs from that particular mains line. Both will run fine from the other, non-troubled mains line. The non working transformer does not heat up - it remains stone cold. I suspect it is the soldering from the mains line, and is not as important as the amplifiers at the moment.

Any help on this is highly appreciated. I was battling all night with these things, and have battled for a lot of today with them. I will post some pictures and sound clips if they are necessary. I have no idea what the problem is, since one of them's gone tits up and the other is fine, yet there is no difference between the two or what's been done to them. This system has worked lovely for a long while, and the speakers sound good, given that the whole thing was about £60. There is no way I can afford a power amplifier unless I can get one for £50, and I can't keep wasting my money on cases and chassis and connectors and things just to have to to and buy another set of computer speakers to butcher.

Thank you all in advance.

[Gandy]

Here are some photos.

The first is the transformers.


This is how the right outputs are wired.


Top of the non working amplifier. The unlabelled arrow is the headphones socket.


Underside of the non working amplifier.


Top of the working amplifier.


And underside of the working amplifier.



I should point out that both PCBs look the same. On both sides they appear identical - same colour, same artifacts, same soldering, same everything. There are no dark or brown marks or burns on either.

[henryf]

I think your best option would be to buy a working amplifier. Some cheap new ones are sold by Parts Express and Amazon.

[LeeYoo]

What I understand from your story is that the transformer is dead.
These type of transformers have an inbuild, non replaceable primary fuse inside the windings.
Easy way to check is with a DMM, set to ohms.
The two pins of the power plug should have continuity of a few hundred ohms.
Compare with the other transformer.
You may find that the not working transformer is open circuit.
Leo..