Maplin 100W mosfet amplifier - power supply problem

[HBrunt]

I have just acquired a pair of ten year old working Maplin 100W amplifier modules, assembled from their late 1990s LP56L kit, each with its own independent toroidal transformer, bridge rectifier and pair of 10,000uF smoothing capacitors. The quiescent current measures about 90mA on each postive supply rail, so it is within specification.

The Antrim transformers are both rated as 160VA, 39V-0-39V (I confirmed this from the original Maplin parts list and directly with Antrim) so they ought to produce a DC supply voltage of around +/- 55V, which is also what is stated in the Maplin construction notes.

However, I am only getting +/- 47.8V on one channel and +/- 48.6V on the other. Also, there is quite obvious hum which sounds like mains ripple. And the mosfets are getting quite warm (without any speakers connected), although this may be normal.

I suspect the capacitors are not holding their charge and may be past their best - 10 years is a long time in the life of an electolytic. The four caps are incidentally marked Semcon 63V, +85C, 10,000uF.

Has anyone experienced similar symptoms? Is there anything I can do to rescue the specified performance, other than fitting new capacitors?

I did not go through any special reforming precautions, such as using of a variac to increase the supply potential gradually, but the capacitors did not seem to get hot, bulge or emit noxious fumes or anything else to suggest they were in distress.

Also, recommendations for capacitors would be useful. I have found some 10,000uF, 63V, 85C "computer grade" caps ("Suntan" brand) on the internet for only GB pounds 1.50 each (plus VAT) - from UK supplier Rapid - but that seems just too good to be without some sort of catch. Equivalent specification Panasonic caps from RS are over GB pounds 6.00 each.

[jaycee]

Measure the supplies with the amp disconnected. What you're saying sounds about right - the capacitors have had it, simply due to the age.

I would say you are best replacing them with a bunch of smaller caps, eg 2x6800uF 63V per rail, rather than two big capacitors. High frequency performance is better this way. Look on eBay for capacitors - you will find them cheaper there.

You might also want to replace any electrolytic capacitors on the amplifier pcb's as well.

[HBrunt]

Sorry, I should have mentioned that the rail voltages were measured with the rail fuses removed. I do plan to instal a DC offset detection circuit, by the way (obviously essential if only one rail fuse blows).

[jaycee]

Yep if the rails are sagging that much without load, and the transformer is humming, i'd say that's definitely a sign the capacitors have dried out. You should check the transformers alone and confirm the AC voltage output is right.

The warm MOSFETs are most likely just due to the bias current. Are they mounted to a heatsink, or is this just with the L brackets ? You should probably renew the thermal paste between the MOSFETs, insulators, and L bracket at this stage.

A DC offset protection circuit is a very good idea If it's the amps I'm thinking of, then these amps are basically a refined version of the Hitachi appnote for the original MOSFETs. There should be loads of documentation and such about them.

[HBrunt]

Thanks for your suggestions. I'll need to get myself an new oscilloscope or a better mutlimeter to measure the AC voltage.

The MOSFETS are mounted with what look like silpads on an aluminium L-bracket attached to the rear of a rackmount case, with dual heatsinks on the outside (probably all original Maplin specifications). They aren't getting too hot , just quite warm.

[nigelwright7557]

[Clockworktent]

Also, there is quite obvious hum which sounds like mains ripple.

Yep, there will be mains hum. The reason is that the PCB is poorly designed; the 0v return from the output stage is routed right through the input section. I had this amp driving me nuts for ages with mains hum until someone found the problem. I built a whole series of new power supplies believing that to be the problem.

However, the good news is that there is an easy fix. What we did was hack the board a bit to separate the power connections to the input and output stages; linking and decoupling the input power rails to the output rails which is where we moved the power connections to. I have some notes and images I can post if it would help.

Once we made these changes the amp was unbelievably quiet and was much improved in every department.

[Sonical]

Originally Posted by Clockworktent

Also, there is quite obvious hum which sounds like mains ripple.

Yep, there will be mains hum. The reason is that the PCB is poorly designed; the 0v return from the output stage is routed right through the input section. I had this amp driving me nuts for ages with mains hum until someone found the problem. I built a whole series of new power supplies believing that to be the problem.

However, the good news is that there is an easy fix. What we did was hack the board a bit to separate the power connections to the input and output stages; linking and decoupling the input power rails to the output rails which is where we moved the power connections to. I have some notes and images I can post if it would help.

Once we made these changes the amp was unbelievably quiet and was much improved in every department.

I'd like the information about how to improve the mains hum on the Maplin 150W Mosfet Amp....Please.

[nigelwright7557]

Originally Posted by HBrunt

Equivalent specification Panasonic caps from RS are over GB pounds 6.00 each.

There is free p+p at RS tho.

£3-50 at Cricklewood electronics but £3-50 p+p.