Class D Faults/reliability

[dincz]

I'm considering a lightweight class D power amp and wondered if any of you have experience with faultfinding and repair. I've repaired class A/B (bipolar and mosfet) amps and have found repair to be fairly straightforward - especially with mosfet output stages as they tend to take out only the output devices.

I'd imagine that the low thermal dissipation of switching transistors in class D amps would make them more reliable. Does your experience confirm this? And when they do fail, what are the more common causes?

[dincz]

A bit quiet round here. Does this mean they don't break down?

[BassBod]

Well, I've used a EA 500 (toroidal transformer, but "digital" power amp) and more recently an EA Micro (all digital). No issues. Nothing.

The only thing I've heard on the interweb is they might not like unstable power - ie generators on outdoor gigs.

The issue I can see is in the future (ie when not in production) they will be unserviceable, due to the surface mounted design and construction. While they are being made you can get new circuit board, but in a few years time I doubt that will be possible. But isn't that true of a lot of things?

[stevie]

I believe (simply from reading on here) that, when they go wrong, they are not easy to repair, i.e. the usual fix is to swap out the main board, which can be expensive. No doubt someone with hands-on experience will confirm or deny this in due course.

[henry norton]

From my experience the money is in the design rather than the components so if you're handy with pulling and replacing semiconductors from multi layer circuit boards (and of course, how to find the faults in the first place), then you could effect surprisingly cheap repairs. But there's allot more to them compared to simple mosfet circuits. Ultimately, if you're looking primarily at repair-ability, high quality class D amps with switched power supplies aren't what you want.

You want VAAAAALLLLLLLLLVVVVVVEEEEEESSSSSSSSSSSS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Edited January 25, 2011 by henry norton

[Phil Starr]

I think we are coming to the end of an era. No-one builds a computer at home in the sense of soldering components onto a circuit board and computer repair is a matter of unplugging a card and plugging a new one in. I doubt that a digital amp will be any different from a computer or an iPod in this. The other problem is that with nearly all electronics manufacture moving overseas it is getting harder to locate components. On the plus side I think that eventually digital amps are going to be a lot more reliable than the analogue stuff we are used to and these amps are going to offer ever more facilities and power at lower prices. If the copper in the transformer is worth more than the cost of a new amp then no-one is going to opt to repair.

That won't stop me from trying to repair the first digital amp I break however. I must get up into the loft and start to repair those Betamax's.....

[geoffbyrne]

The only problem I've had with my GB Shuttle 6 was when half of the pre-amp valve stopped working.

It took a while to figure, as I kinda forgot there was a valve in there in the first place, but once it was swapped, all was good again!!!

G.

[bigjohn]

I've had one go on me.

Was due to a fault (dodgy transformer) on a pre-amp, but a fault that didn't blow either of my mosfet power amps.

Repairing them isn't easy, parts are expensive and tend to only be available from manufacturers. In fact, the manufacturer of mine wanted me to send the unit back for repair rather than sell the parts.

I'm resigned to carrying round an extra 10kg for the trouble I've found.

[dincz]

Thanks for the responses. All food for thought, although I guess component-level repair is, or at least is becoming, a thing of the past whether we're talking class D or linear, analogue or digital, so maybe it's time to bite the bullet.

[Mr. Foxen]

Probably worth noting that class D and switch mode power supplies aren't necessarily 'digital', and 'being made with pcbs and surface mount components' is just construction and not to do with amplifier class. You could handwire a solid state amp, but it is even more effort than having to pick up a box with transformers in it. My solid state amp is handwired and it is lovely, and I should be able to solder bits in as necessary. Class D amps just suffer from the age in which the came into use, rather than an actual issue with the technology.

[dincz]

[quote name='Mr. Foxen' post='1104666' date='Jan 27 2011, 02:51 AM']Probably worth noting that class D and switch mode power supplies aren't necessarily 'digital', and 'being made with pcbs and surface mount components' is just construction and not to do with amplifier class. You could handwire a solid state amp, but it is even more effort than having to pick up a box with transformers in it. My solid state amp is handwired and it is lovely, and I should be able to solder bits in as necessary. Class D amps just suffer from the age in which the came into use, rather than an actual issue with the technology.[/quote]

Yes. My point was that gear in general is going surface mount (for me that means unrepairable) so I may as well go for lightweight class D/SMPS.

[BassBod]

Its worth noting the history of Walter Woods amps - esoteric, supposedly the first "digital" amp on the market. Handmade, using fairly conventional components...but part numbers removed, so only he could repair them. Keeping it small and boutique/expensive has worked for him for 30 odd years, but now the electronics industry has caught up (well, EA and AI at least?).

So I guess its still possible to make a modern lightweight digi amp as a one off/in low numbers. But you would have to charge rather a lot of money for one, if you had to make a living.

Personally, I'd think about it...I just hate the thought of throwing my EA Micro away when it eventually dies, and replacement circuit cards are long gone. So, if someone could make a similar product the old fashioned way I'd consider it an investment, of sorts.

[el borracho]

I had a Yamaha BBT500 for a while. I actually bought it in a rush as my Ampeg failed (again) on the day of a gig. Used it as my main amp for a few months but mid-gig it developed a fault. It had a rotary dial on with about 10 presets and started steadily scrolling through them - a couple of seconds on each one. Was errr - interesting (at least it was still working!) especially when it got to distortion and fuzz!! Tried switching it off and switching it back on again but still the same.
Took it back to the shop as it was still under warranty but they said Yamaha wouldn't supply them with spares and it had to go back to Yamaha.
In the month it took to get it back I managed to get the Ampeg sorted and moved the Yamaha on soon after.
Current GAS for one of those TC Electronics heads with all the red lights

[rmorris]

[quote name='dincz' post='1105633' date='Jan 27 2011, 09:53 PM']Yes. My point was that gear in general is going surface mount (for me that means unrepairable) so I may as well go for lightweight class D/SMPS.[/quote]

hi. But I don't see why 'Surface Mount' needs to be seen as 'unrepairable' Yes - some slightly different techniques are required but in some ways it's easier than conventional 'thru hole' fabrication.
Although it does depend on the pcb quality to a degree.

[flyfisher]

While it's true to say surface mounted assemblies are not 'unrepairable', I'd say they require a significantly higher level of skill than 'conventional' boards, even if they have plated-through holes.

The smaller size of the components is a big factor and multi-lead components (ICs) are more difficult to remove unless you have the right tools.

In contrast, ICs are easy to remove from a PTH board by cutting the legs first and then removing them one at a time. That's not really possible with SMDs.

The other thing that's making electronics effectively unrepairable these days is the prevalence of programmable devices. This may not be such an issue with bass amps (I've not been inside a modern digital bass amp) but most digital equipment these days is based around programmable logic devices. So, while it might be easy to buy a replacement chip, you're actually dependent on the manufacturer to supply a programmed part and, since the firmware is part of their IP, they are likely to charge a lot for it.

Fortunately, digital devices are very reliable these days!