Hi Guys,
I'm looking for some help with my Maplin GA28F which I built from a kit about 38 years ago. It started life as an electric guitar amp, then an amp for my record player, and then as the amp for a homemade active sub-woofer. They said it’s virtually bomb proof.. OMG they weren't wrong! ... up until last week that is when I think it has finally packed in. So here's the issue....
On powering up, after about a second the speaker coil gets pushed out fully giving a loud POP/THUD noise. This repeats every few seconds.
Does anyone have any idea what could be causing this? Could it be one of the components has failed. The R15 4R7 3W,W/W looks a bit fried? I was thinking I could just replace all the components but this seems a bit extreme at this stage.
I'm looking for some help with my Maplin GA28F which I built from a kit about 38 years ago. It started life as an electric guitar amp, then an amp for my record player, and then as the amp for a homemade active sub-woofer. They said it’s virtually bomb proof.. OMG they weren't wrong! ... up until last week that is when I think it has finally packed in. So here's the issue....
On powering up, after about a second the speaker coil gets pushed out fully giving a loud POP/THUD noise. This repeats every few seconds.
Does anyone have any idea what could be causing this? Could it be one of the components has failed. The R15 4R7 3W,W/W looks a bit fried? I was thinking I could just replace all the components but this seems a bit extreme at this stage.
If R15 is fried then the cause of that is possibly high frequency instability, either that or the amp has been seriously 'abused' in a past life....
First things first. Do not have a speaker connected when testing. There is a fair chance the speaker may already be damaged from the DC voltage that is pushing the cone out.
Measure the 4.7 ohm. If it reads OK leave it be. Also check that both the plus and minus rails are present. There is always the possibility of an output transistor failing but these are lateral FET's and very rugged.
Check the rails first, then with the amp off make sure neither FET reads short circuit (or very low ohms) from drain to source.
First things first. Do not have a speaker connected when testing. There is a fair chance the speaker may already be damaged from the DC voltage that is pushing the cone out.
Measure the 4.7 ohm. If it reads OK leave it be. Also check that both the plus and minus rails are present. There is always the possibility of an output transistor failing but these are lateral FET's and very rugged.
Check the rails first, then with the amp off make sure neither FET reads short circuit (or very low ohms) from drain to source.
This happened to all 4 of my modules, I only noticed when the background hum became noticeable.
I traced it to oscillating output MOSFETs, visible on a scope at several megahertz.
After some experimentation, documented on another thread about this amplifier, I determined that the 100R gate resistors should be at least 200R, to stop the oscillations.
The oscillations become visible when the 4R7 Ohm resistor is open circuit.
Of course, you might have additional problems.
well normally these if made well are very good .. I recently got an amp that I made 20 years ago and still working well
Check things like poor build quality or dry joints as a small suggestion
Check things like poor build quality or dry joints as a small suggestion
But the circuit and PCB design is sound and reliable if made with care and attention with no counterfeit parts or poor substitutes
trust me I have been making these amps since the early 1980's ... THEY WORK
trust me I have been making these amps since the early 1980's ... THEY WORK
Yes, they work remarkably well considering the design flaw.
As an experiment, temporarily disconnect the 4R7 resistor in the Zobel network, and look at the output, with no signal at the input, on an oscilloscope.
If you don't have an oscilloscope, check whether the background hum level increases, then try to work out why the hum level increases without a Zobel network.
Just because all 4 of mine had this problem, doesn't mean that every one of them does, but I would think it's wise to check.
Thanks guys for all your comments.
I checked the rails and was only getting 30v on one of them and 0v on the other. I thought then this could be a problem with the power supply; either the rectifier or the fuses... embarrasingly it turns out one of the fuses had blown. No excuse, but I only checked the glass fuse with a visual inspection. Schoolboy error and lesson learnt.
So it turns out the GA28F is still going strong!
I'll take your advice and get the capacitors replaced. I have 30 year old NAIM Pre and Power amps and I recently replaced the Caps on those and I definately could tell the difference in sound quility. Before replacing them I had to let the amps 'warm up' for about 20mins to get the best out of them.
johnnyx - I'll also replace the 100R gate resistor to 200R whilst I'm at it.
I checked the rails and was only getting 30v on one of them and 0v on the other. I thought then this could be a problem with the power supply; either the rectifier or the fuses... embarrasingly it turns out one of the fuses had blown. No excuse, but I only checked the glass fuse with a visual inspection. Schoolboy error and lesson learnt.
So it turns out the GA28F is still going strong!
I'll take your advice and get the capacitors replaced. I have 30 year old NAIM Pre and Power amps and I recently replaced the Caps on those and I definately could tell the difference in sound quility. Before replacing them I had to let the amps 'warm up' for about 20mins to get the best out of them.
johnnyx - I'll also replace the 100R gate resistor to 200R whilst I'm at it.
After replacing all the electrolytic capacitors in the GA28F board I can honestly say what a difference it has made the the sound produced in my sub-woofer. I wasn't expecting there to be much difference, however, on powering up for the first time, the frequencies were much lower, less harsh but with a punch when needed. Now the low frequencies fill the room in a way that you realy can't tell where the sound is coming from, just the way a sub-woofer should be. I would recommend to anyone who is still using one of these boards, get the caps replaced for the sake of a few quid.
Note that the graphics for the MOSFETS are screw terminals, NOT transistor packages wrongly placed. Confusion avoided, I hope...
Hi Nigel, any chance of getting the gerbers for this design and the schematic you used. I assume you built and verified it?
Cheers,
Gary.
Hello fellow audio enthusiasts, i've been away from DIY stuff for too long, so recently I decided to make myself Maplin GA28F amp as I got my hands on several pairs of NOS 2SK405/2SJ115 Toshiba MOSFETS. As I understand they are not actual lateral ones, despite some of the sources stating otherwise. My question is if it is possible to implement these MOSFETS into Maplin without any changes in circuit topology (well at least major ones)?
Thank you in advance for your answers.
Thank you in advance for your answers.