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D.I.Y. amp

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Re: D.I.Y. amp

Postby blinddrew » Tue Mar 12, 2019 5:10 pm

Bummer :(
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Re: D.I.Y. amp

Postby ef37a » Tue Mar 12, 2019 6:04 pm

Folderol wrote:Well today with the decks cleared, and after extensive testing of the new module on its pcb I carefully connected it up to the sub amp, checked all connections multiple times and switched on.

... I've just put an order in to RS for 28 quids worth of power transistors :madas:

Did you not manage to liberate a Variac when you retired Will? And do NOT let anyone buy you Duggie Self's amp book, it will just depress you further!

Dave.
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Re: D.I.Y. amp

Postby Folderol » Wed Mar 13, 2019 3:32 pm

The only variac they have is a massive lump that takes two people to lift - not exactly the sort of thing you can hide in your pocket. Besides, I wasn't expecting any kind of problem. The power stage has run for several years without issues and the new module tested out fine under quite extreme conditions. I still don't know what caused the problem, so am being mega cautious with further tests.

Anyway, wound-licking over and new transistors arrived, so ...

When I removed the remains of the two output transistors I discovered that the other three transistors in the power section escaped unscathed - so at least that part of the protection system I designed in had worked. Just to be sure I applied full voltage to them and controlled drive current {i.e. a resistor}. All three passed. The slight downside is that I spent more than I necessary.

Something that may be a problem is that although I bought extra output ones, I can't get anywhere near a matched pair. Frustratingly all the NPN ones have a gain close to 50, and the PNP ones around 110. That's most unusual. We'll see how that pans out on a test rig.
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Re: D.I.Y. amp

Postby Folderol » Sat Mar 16, 2019 6:03 pm

Well it's back to the drawing board I'm afraid :cry:

The good news is that the amp is now back as it was, and seemingly none the worse for the 'experience' apart from some scorch marks. I gave it a good blast for about an hour this afternoon - actually playing some music for a change :tongue:

However, tacking a spare pair of those transistors onto the new module and running under very controlled conditions immediately revealed the cause of the failure. Massive parasitic oscillations at a frequency that's so high a 10MHz scope can barely resolve them :o Under these conditions both transistors stay almost permanently 'on', effectively a short circuit across the supply.

I remember an old college professor once saying that the correct way to deal with these is not to cause them in the first place! With such a bad case I didn't even bother to try the usual dodges. Instead, I went over the board and design with a fine tooth comb to see if I could find any stupid errors - couldn't see any. My best guess is that the extra stage pushed the open-loop gain up too high - combined with the fact that modern transistors have an incredibly wide bandwidth. Something I forgot was that coupling the module to the output stage also increases to module gain as it no longer directly takes the feedback resistors.

I'll give this a break for a while, and maybe get on with the front panel instead.
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Re: D.I.Y. amp

Postby Folderol » Sun Aug 04, 2019 11:15 am

You were expecting me to report I had finally made the front panel... sorry :(

So no, not really any progress - be warned. Don't ever retire - you never seem to have time to do anything!

Anyway the other day (for some undefined value of 'other') there was an incident :o
I was happily playing some bit of improvisation when suddenly all sound cut out. The monitor was showing everything seemingly quite normal, but then I glanced at the amp display, to be greeted by both channels showing an offset of around -2V and wavering. The good news is that the monitoring system had done exactly what it was designed for and disconnected the speakers.

Puzzled, I switched on the Sub channel, which showed exactly the same behaviour, so this was clearly a common fault. Pulling out the input plugs proved that it was in no way an external issue, and the voltage started to quite rapidly go even more negative, and before i could reach the power switch suddenly slammed to full positive rail.

Today, withdrawal symptoms became so severe I dropped everything to investigate the amp. Cold checks revealed the internal fuse for the negative rail to the voltage regulators had blown. This is rated at 100mA, although only about 50mA should be drawn.

Here is the drawing for it. I was a bit relieved that this suggested it was unlikely to be anything to do with the (expensive) power stages, and initially thought it might be the -12V regulator or associated caps, but cold checks showed nothing suspicious.

My next move was to pull the other fuse, then power just this stage from my bench PSU, with current limiting set to 100mA. This immediately limited at 100mA for the negative supply and dropped the voltage to (curiously) around -12V. However, the output of that regulator was around - 9V, so it clearly wasn't shorting, and unlikely to be the problem.
These regulators feed the preamp stages, so I tried disconnecting them - no difference.

This suggested a problem with the -20V rail which is used by the drive stage of the amp itself, so I disconnected the feeds here and the other supplies all came up correctly, with both rails drawing about 53mA.

Cold checks seemed to show no problems with the PCB components, which includes everything up to the four power transistors for each output stage, and without the main supplies these just sit passively at 0V.

Finally I started to make live voltage checks. There really is only one common point, and that is the BC560 bias transistor (on the PSU drawing). This is mounted on the PCB in close proximity to the input transistors so that they are all at a similar temperature. I quickly discovered the collector was sitting at about -12V i.e. the reduced negative rail.

My first though was that the 100u cap might have gone S/C this cutting off the transistor, but that also had about 12V across it! Thinking that possibly the transistor had gone O/C (very unlikely) I switched off and did cold checks on it, getting perfectly correct readings.

Time for a cup of tea!

I must be getting {cough} old {cough} because it took me quite a while for the obvious to sink in. If the transistor was OK the voltage couldn't possibly be wrong... unless it was disconnected, and continuity testing showed that the base was, indeed, waving about in the breeze.

Confession time

All those components are physically on the same PCB track, so only a bad joint could be the cause. I shall now go and stand in the corner wearing a dunce's hat :oops:
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Re: D.I.Y. amp

Postby blinddrew » Sun Aug 04, 2019 12:27 pm

Just put it down to insufficient cups of tea...
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