There was a very similar mosfet amplifier design published in ETI in 1980. To test the resilience of the mosfets they actually shorted the speaker outputs with a screwdriver when the amplifier was delivering high audio output power. The mosfets survived. However they said that doing this test resulted in a damaged screwdriver and an urgent need for a clean pair of underpants.
Shift the bridge rectifier to the front of the case so it's wiring is well away from the signal input cables and star earth return cables. I built a similar amplifier many years ago which produced no noise or buzz at all. Sticking my ear right against the speakers, it was impossible to tell if the amplifier was switched on.
I improved the sound quality of the Maplin mosfet amplifier considerably which gives much deeper bass and much clearer mid and treble. Just strap out C1 and replace C3 with 220uF. The input capacitor C1 is a dc blocker that isn't required as long as you have dc offset loudspeaker protection. Saying that, any signal source you use to drive the amplifier shouldn't ever have any dc on it's output anyway.
Thanks for all your comments, if I was building it now I would not have put the bridge rectifier on the heatsink as it doesn't really get that hot & then ac wires would be away from the circuit boards. But that was 30 years ago when I had less knowledge. I suppose I could still move it but the owner is quite happy with it so that won't be happening! Yes the mosfets are virtually indisputable. I've never known one to fail. Of course I remember eti magazine, one of several I used to get every month. Best wishes nick
I achieved a much improved sound (similarly), but by using two completely separate power supplies (two transformers, two bridge rectifiers) and using two huge smoothing capacitors on each rail of each separate supply! ... In effect it's two 'mono-blocks' but used in stereo. The unit is still going strong today, and puts to shame some commercial units that claim they are 500 watts output (peak power times two). 😆
I went through all the problems you had and mostly came to same conclusions. So redesigned the pcb with extra decoupling and that fixed all the problems. The problem with zero volts line is keeping them all at the same potential which means very short leads especially for power wires. Any wire has inductance and resistance and to minimise that means quality low impedance cable and short runs.
Yes redesigning the pcb is probably the answer, but in this case the hum is down in the nano watt region now & the owner is happy with it, its barely audible on my celestion speakers with my ear right next to them, best wishes nick
@@nigelwright7557 yes on mine some was on the power supply 100 hz & some was getting in on the input as was 50 hz, I've got an identical amplifier I built myself & looking on a spectrum analyzer you can see the various frequencies including harmonics of 50 & 100 hz. Yes they can be unstable I've burnt out several zobell network resistors! I found putting a capacitor to earth after the output inductor helped stabilise it
pig of a problem to sort out. you could try low value caps across each leg of the bridge to try and remove the switching noise. other than that i dont know.
Great Video Nick Why Did You Put a Cable Tie On The Screened Input Going To The Amplifer Module Dont Like The Way Its Dressed Would Haved Moved It Away From Those Other Wires Going Round That Massive Transformer To Me That Transformer Needs Futher Away From The Amp Boards Mosfet Amps Seem To Be a Bit Touchey On Cable Dressing And Can Become Unstable Thanks For Uploading Regards mike P.S. You Can Hardly Hear The Hum Through Your Massive Ditten 44s !!
Hi Mike, it actually made no difference having that signal cable in the wiring loom, i did try it in various places, watch out for the next episode coming soon where I finally get to the source of the problem
The left ch is too close to the toroid... carbon steel plate between the toroid and board could help. Thicker is better, they could be several thinner.
If I knew what I know now when I built this 35 years ago I wouldn't put the transformer so near to the boards. I've got the noise level down significantly and the owner seems happy with it. Thanks for the comment
No the problem was not related to the transformer, it turned out to be a faulty decoupling capacitor on the noisy board, see the second part where i identify the problem, now its completely noise free on both channels
There was a very similar mosfet amplifier design published in ETI in 1980. To test the resilience of the mosfets they actually shorted the speaker outputs with a screwdriver when the amplifier was delivering high audio output power. The mosfets survived. However they said that doing this test resulted in a damaged screwdriver and an urgent need for a clean pair of underpants.
Shift the bridge rectifier to the front of the case so it's wiring is well away from the signal input
cables and star earth return cables. I built a similar amplifier many years ago which produced no noise or buzz at all. Sticking my ear right against the speakers, it was impossible to tell if the amplifier was switched on.
I improved the sound quality of the Maplin mosfet amplifier considerably which gives much deeper bass and much clearer mid and treble. Just strap out C1 and replace C3 with 220uF. The input capacitor C1 is a dc blocker that isn't required as long as you have dc offset loudspeaker protection. Saying that, any signal source you use to drive the amplifier shouldn't ever have any dc on it's output anyway.
Thanks for all your comments, if I was building it now I would not have put the bridge rectifier on the heatsink as it doesn't really get that hot & then ac wires would be away from the circuit boards. But that was 30 years ago when I had less knowledge. I suppose I could still move it but the owner is quite happy with it so that won't be happening! Yes the mosfets are virtually indisputable. I've never known one to fail. Of course I remember eti magazine, one of several I used to get every month. Best wishes nick
I achieved a much improved sound (similarly), but by using two completely separate power supplies (two transformers, two bridge rectifiers) and using two huge smoothing capacitors on each rail of each separate supply! ... In effect it's two 'mono-blocks' but used in stereo.
The unit is still going strong today, and puts to shame some commercial units that claim they are 500 watts output (peak power times two). 😆
I went through all the problems you had and mostly came to same conclusions. So redesigned the pcb with extra decoupling and that fixed all the problems.
The problem with zero volts line is keeping them all at the same potential which means very short leads especially for power wires.
Any wire has inductance and resistance and to minimise that means quality low impedance cable and short runs.
Yes redesigning the pcb is probably the answer, but in this case the hum is down in the nano watt region now & the owner is happy with it, its barely audible on my celestion speakers with my ear right next to them, best wishes nick
@@ptronix
I found on mine the hum was from the power supplies. The modules can be a bit unstable so I increased the 27pf caps a bit.
@@nigelwright7557 yes on mine some was on the power supply 100 hz & some was getting in on the input as was 50 hz, I've got an identical amplifier I built myself & looking on a spectrum analyzer you can see the various frequencies including harmonics of 50 & 100 hz. Yes they can be unstable I've burnt out several zobell network resistors! I found putting a capacitor to earth after the output inductor helped stabilise it
pig of a problem to sort out. you could try low value caps across each leg of the bridge to try and remove the switching noise. other than that i dont know.
Just tried that Rob made no difference!
Great Video Nick Why Did You Put a Cable Tie On The Screened Input Going To The Amplifer Module Dont Like The Way Its Dressed Would Haved Moved It Away From Those Other Wires Going Round That Massive Transformer To Me That Transformer Needs Futher Away From The Amp Boards Mosfet Amps Seem To Be a Bit Touchey On Cable Dressing And Can Become Unstable Thanks For Uploading Regards mike P.S. You Can Hardly Hear The Hum Through Your Massive Ditten 44s !!
Hi Mike, it actually made no difference having that signal cable in the wiring loom, i did try it in various places, watch out for the next episode coming soon where I finally get to the source of the problem
The left ch is too close to the toroid... carbon steel plate between the toroid and board could help. Thicker is better, they could be several thinner.
If I knew what I know now when I built this 35 years ago I wouldn't put the transformer so near to the boards. I've got the noise level down significantly and the owner seems happy with it. Thanks for the comment
Is the noise simply because the problem board is nearer to the transformer? Maybe try turning the transformer a little or shield it if possible?
No the problem was not related to the transformer, it turned out to be a faulty decoupling capacitor on the noisy board, see the second part where i identify the problem, now its completely noise free on both channels
@@ptronix Thanks, yes I've watched it now. Must have been leaky, electrically speaking.
👍
Not an earth loop is it?
No Simon, been there done that, got rid of all the earth loops, there were a few. Watch out for the next episode coming soon!