Amp for sub...Recommendations

[ubergeeknz]

So I've acquired an old JBL Digital 12 sub which has a built in amp. The amp is dead. The driver is good. The amp uses a module S53AMI which I'm pretty sure is blown (it's been removed from the main board) - The rest of the board seems okay.

So what I have is a +-42vdc supply, all the necessary filters, and a 4 ohm driver. There's also +5v and +-15v on the board. The transformer seems to have a good heft to it. I would guess 150 or 200VA at least. What's a good amp to build, or module, to fit here? Or is it possible, and worthwhile, to repair these modules? Recommendations please!

 

[Fenalaar]

A Hypex UCD180 module would be usable here, as long as you have +-42V. It runs on voltages from 30 to 49 volts.

DIYclassd.com

You'll need a spot of free space of 65x25mm on the rear plate to attach the heatsink, though.

Johan-Kr

 

[ubergeeknz]

The hypex looks ideal! I would love an option at around half that price, but I'm probably dreaming [emoji16] we shall see

BTW. It's a ported box. So a bit of airflow/cooling shouldn't be an issue.

 

[JonSnell Electronic]

When R9 goes open circuit the amplifier output section stops working as there is no 6volt supply to the FM modulator.

 

[ubergeeknz]

When R9 goes open circuit the amplifier output section stops working as there is no 6volt supply to the FM modulator.

I'm guessing wrong thread?

 

[JonSnell Electronic]

The amp uses a module S53AMI that module uses a feed from the power to set up the FM oscillator.
This is a JBL Digi 12. See service manual.

 

[ubergeeknz]

I see! I could always check if that's the issue and not the module... I can see a cap blown across the output, so it got abused at some stage. Worth a try. Thanks.

 

[JonSnell Electronic]

My pleasure, let's hope it isn't terminal.

 

[ubergeeknz]

My pleasure, let's hope it isn't terminal.

Suffice to say, genius advice.

R9 was very high (and no wonder the heat which it puts out, I replaced it with 2 x 1k 2W in series and they still stunk for a while) and R46 low (5 ohm instead of 47) so I replaced both, put the module back in and low and behold... Life!

Unfortunately there is something oscillating in the input stage, putting a high frequency square wave to the input. There's also a square wave on the output, so the amp seems to work... Already checked all resistors around the opamps, and they seem okay. I've run out of time for tonight ... Any ideas on what might be worth a check, would be most welcome.

View attachment 783223

 

[tmuikku]

My old prosumer mixing console oscillated badly on the EQ section every once in a while. Some of the poti were losing contact turning the filter circuit into an oscillator. Not sure if this could be the case here. Anyway, extruding poti will take a hit sooner or later

 

[ubergeeknz]

I did get some more time on it... And I think after all the power amp module is bad.

The output is oscillating in a square wave almost rail to rail and that's what's causing oscillation to appear at the input (due to nfb). The frequency is around 90kHz.

The oscillation is not present (in significant quantity) at the opamp output.

Open to suggestions for repair or replacement...

 

[ubergeeknz]

Oh my. I just realised this is a class D amp so... Probably this is how it operates [emoji16] maybe it's ok after all. The signal is barely present at the speaker terminals after the filter.

 

[xrk971]

IRS2092 based modules work well with dual rail supplies and are about $20. For example:

200W IRS2092 Mono AMP Board Class D Amplifier Board | eBay

There is a thread on it here:
200W IRS2092 Amp for $20

 

[ubergeeknz]

Well, it looks like I don't need another amp [emoji16] stoked! Output top, input bottom, 100 Hz sine. Note, output has superimposed ~100kHz because D class.

 

[ubergeeknz]

Hooked up the driver and rattled the garage a bit [emoji16] all seems to be operating nicely. Thanks all for your suggestions and thank you especially @JonSnell Electronic for pointing me in the right direction on repairing the thing! And to think I was ready to just bin the module otherwise.

Now I just have to assemble it and sneak it into the lounge room. Hopefully such an inconspicuous black box achieves the required WAF. The grill cloth needs a clean and I might black out the bright orange JBL logo just to play it safe...