Hi, Before everyone tells me these amps are rubbish, I already know. The thing I would like to know is why two amplifiers produce this strange buzz/hum on all channels with inputs shorted. I bought these two amps brand new, being rather naïve at the time, and I asked their tech guy (lol) who made these, and could he either contact them for me or get a schematic for these. I failed on both counts, and now of course they've gone.
Both amplifiers are slightly more quiet if I disconnect the cooling fan. It is fed from its own winding on the toroidal transformer. The shame of it is, these are not that badly made, in 55 years of electronics hobby history I have seen much - much worse construction. These use the 2SC5200/2SA1943 transistors, and have 5 pairs per side. This hum/buzz is only audible with no signal, but in a quiet room while searching for a track, it is obtrusive. Short of reverse engineering the schematic, I ask on here if anybody knows who made these for maplin, or if anyone has a schematic. The prosound 1600 is a completely different amp (class d ) I think, although typically I have seen schematics floating around on the web.
Thanks, john
Both amplifiers are slightly more quiet if I disconnect the cooling fan. It is fed from its own winding on the toroidal transformer. The shame of it is, these are not that badly made, in 55 years of electronics hobby history I have seen much - much worse construction. These use the 2SC5200/2SA1943 transistors, and have 5 pairs per side. This hum/buzz is only audible with no signal, but in a quiet room while searching for a track, it is obtrusive. Short of reverse engineering the schematic, I ask on here if anybody knows who made these for maplin, or if anyone has a schematic. The prosound 1600 is a completely different amp (class d ) I think, although typically I have seen schematics floating around on the web.
Thanks, john
Hi John, I don't have a schematic but the buzz/hum when inputs shorted could be a ground loop earthing problem. Check out Rod Elliot (ESP) for details of a ground loop breaker that should be easy to fit into your amps. Also check all your signal cables and make sure the phono connectors are clean. The other thing that may cause buzz/hum is ageing PSU capacitors with increased ripple, though that is less likely if it is happening on both amps. I am sure someone here will have more details or even a schematic lurking in their collection.
Hi, and thank you for your reply. These two amps have done this since day 1, but on the advice of an engineer friend I did replace the two main rail capacitors from 4700uF to 10000uF which made no difference. The only thing that gives me any idea is that this whopping great toroidal transformer is in the middle of the rack case. I wonder if magnetic interference is getting to the main amp pcb which is right next to it. The other thing I was going to try was to re-route the dc power cables that actually go underneath the amp pcb. Things will hopefully be more interesting when I can get at my fluke scope. It is a shame, because these amps fundamentally, are not really as bad as some people claim - quality wise that is. In the past, Maplin was one of the best component suppliers in England until they got too ambitious, and then became a "gimmick toy store" imho.
Thanks again
john
Thanks again
john
The toroid will not make electronic noise as you describe it. Toroids contain the magnetic field extremely well.
Check the centre point (between the main smoothing capacitors) is being used for the loudspeaker ground point and most importantly the input ground points. Use a 'star' configuration ensuring everything grounds to the one point.
If the input grounds connect to the chassis, make sure the chassis grounds only to the centre point and ensure your electrical earth point is connected to the chassis.
Check the centre point (between the main smoothing capacitors) is being used for the loudspeaker ground point and most importantly the input ground points. Use a 'star' configuration ensuring everything grounds to the one point.
If the input grounds connect to the chassis, make sure the chassis grounds only to the centre point and ensure your electrical earth point is connected to the chassis.
Hi John, that's important additional information the buzz/hum has been on both amplifiers since Day 1 as it points to a design or wiring error. Jonsnell's advice is spot-on. If that fails to kill the buzz/hum and you get your Fluke in action, look for tell-tale signs of mains ripple being injected into the signal path via rail-tied resistors/transistors such as current sources and biasing networks.
I remember going to Maplin head-office in Rayleigh, Essex many years ago for a job interview as a young engineer. I didn't make the grade, probably failed the psychometric tests, but neither did they in the end ;-)
I remember going to Maplin head-office in Rayleigh, Essex many years ago for a job interview as a young engineer. I didn't make the grade, probably failed the psychometric tests, but neither did they in the end ;-)
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I had a Pro-sound 200 and that was hum free.
Either ground loop or if they all do it a design fault.
I used to have a Maplin lateral mosfet amp that hummed badly no matter how it was wired up. It wasn't until many years later when I copied and improved the design the hum went away. Hum on the power rails was getting back into the input on the original. So I added RC decoupling on the front end and it was dead quiet.
It needs a bit of investigation with a scope. Without a schematic that isn't as easy.
Either ground loop or if they all do it a design fault.
I used to have a Maplin lateral mosfet amp that hummed badly no matter how it was wired up. It wasn't until many years later when I copied and improved the design the hum went away. Hum on the power rails was getting back into the input on the original. So I added RC decoupling on the front end and it was dead quiet.
It needs a bit of investigation with a scope. Without a schematic that isn't as easy.
Hi,
Apologies for delayed replies, in time honoured tradition our virgin super hub decided it was too hot. I can't disagree with it. Anyway I've still not got access to my proper scope, but it is possible to dis-connect the preamp section just by unplugging a 3 pin jst connector on the main amp board. This made absolutely no difference at all. I tend to think also that it could be a design issue, because the adverts at the time were "an all new design". Hopefully soon I can look at various waveforms on both power rails and the output.
Thank you to members that replied, I hope to find out more soon.
Regards john
Apologies for delayed replies, in time honoured tradition our virgin super hub decided it was too hot. I can't disagree with it. Anyway I've still not got access to my proper scope, but it is possible to dis-connect the preamp section just by unplugging a 3 pin jst connector on the main amp board. This made absolutely no difference at all. I tend to think also that it could be a design issue, because the adverts at the time were "an all new design". Hopefully soon I can look at various waveforms on both power rails and the output.
Thank you to members that replied, I hope to find out more soon.
Regards john
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