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D.I.Y. amp

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Re: D.I.Y. amp

Postby James Perrett » Fri Jun 15, 2018 8:07 pm

Folderol wrote:I still haven't run it at full power.

I keep meaning to suggest that you bring it over here sometime and we can compare it with my Hypex powered 405 and a standard 405 through the Tannoys.
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Re: D.I.Y. amp

Postby Folderol » Fri Jun 15, 2018 8:32 pm

No, :(
The problem is I would have to completely dismantle the existing scruffy panel to make a template, rending the amp unusable. The spacings for the controls look pretty equal but are not quite the same. This was necessary to fit everything in.

Aha! I've just had an idea :bouncy:

I need to take photos absolutely square on. We know the dimensions of the panel itself so everything can be worked out to scale from those :lol:

This would mean only one removal and replace... Wooo!
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Re: D.I.Y. amp

Postby blinddrew » Fri Jun 15, 2018 9:26 pm

I would make you have a ruler against the front of the amp* as well, otherwise you could be led astray by distortion from the lens.

Or on the desk in front of it would work as well, basically as long as it distorts with the fascia.
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Re: D.I.Y. amp

Postby Wonks » Fri Jun 15, 2018 10:46 pm

Preferably taken with a large telephoto lens from a long way away to minimise the distortion.
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Re: D.I.Y. amp

Postby Folderol » Fri Jun 15, 2018 11:22 pm

Well I had a go at this.

I had my camera on maximum zoom, and about 6ft away from the panel with the camera directly in line and resting on a table. The image pretty much filled the view.

On pulling the image into an editor, the only thing I had to do was rotate it about 2 degrees. The grid is showing it pretty square, with just the top edge very slightly concave. X/Y proportions seem spot on.

I've cropped the image and loaded it into Inkscape so I can annotate it, and if Sahib is still around here I'll ask if he's interested.
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Re: D.I.Y. amp

Postby Jumpeyspyder » Fri Jun 15, 2018 11:36 pm

If you get stuck, give me a call, Not done anything like this before but..

I can call in a few favours at work and get access to;

a/ UV printer (can print on anything flat; glass aluminium, slate, concrete, wood..)
b/ lasercutter

That amp deserves a great panel!!
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Re: D.I.Y. amp

Postby blinddrew » Sat Jun 16, 2018 9:50 am

If it were me, i might try making a wooden panel.
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Re: D.I.Y. amp

Postby Folderol » Sat Jun 16, 2018 10:34 am

@jumpey
Thanks. I may call on you depending on 'stuff'.

@BD
That really would not work. Lots of controls quite tightly packed, and it would look totally out of place with everything else being either black or copper coloured metal.

I have actually come up with an even cunning-er plan.

Rescale the image down to exactly 1:2 then create an overlay in inkscape with accurate position info, that can be printed on A4 paper, but also keep the image for reference.
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Re: D.I.Y. amp

Postby Folderol » Wed Mar 06, 2019 8:42 pm

Hmmm. Amazing how much dust has accumulated in this room.
{brush}
{brush}
{brush}{brush}{brush}{brush}
{brush}
{COUGH}{brush}
{brush}

That's better.

Now, about this long running saga...
Although the amp seems to have been behaving perfectly as far as I can tell, the business of that asymmetry at the point of limiting has continued to bug me.

On my last day at work, my bench was clear so I quickly knocked up a rat's nest of a fully symmetrical possible replacement for the 'middle' stage. This is where I think the problem is coming from. The test looked extremely promising.

Now, as I'm expecting to continue to mess about with electronics stuff for fun (and friends) I decided to get accounts set up with both RS and Farnell. I then ordered a few transistors, caps and a stripboard - just to make sure the accounts were set up you understand :tongue:

Also, over the last couple of months I've been pootling about with my drawing package and now have a full scale copy of the exact positions of where all the controls are supposed to be. Originally I had thought of making an overlay of some sort matching the current positions, but now my thinking is that it would be simpler to just make a completely new panel.

The drilling and cutting I can do myself, but I would have the get it etched elsewhere. My idea here is to get a black anodised panel so the etching will reveal the bright aluminium underneath.

So, once more, watch this space :bouncy:
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Re: D.I.Y. amp

Postby Wonks » Wed Mar 06, 2019 8:54 pm

So the whole idea was to show us your etchings all along! :D
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Re: D.I.Y. amp

Postby blinddrew » Wed Mar 06, 2019 9:49 pm

I was wondering if you'd got the panel sorted on this yet!
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Re: D.I.Y. amp

Postby Folderol » Sat Mar 09, 2019 9:26 pm

I was wondering if you'd got the panel sorted on this yet!

Well I've thought about it quite a bit :tongue:

Anyway, moving on, there is progress!

I've built up the replacement for the dodgy part of the amp onto a proper piece of stripboard with nice new components, and sort-of faked it driving the output stage. Stability is excellent. No nasties when hit with sharp edged square waves, nor with slowly ramping triangles. From cold (where the offset has been zeroed) there's no more than 1mV drift after about half an hour. When a sine wave input is run massively into overload (to almost a square wave) the offset shift is less than 2mV.

I'll do a few more tests to see if I can make it hiccup, then if all is OK I'll unhook one of the existing sections of the amp and try this in its place . I'll probably use the sub amp as if it all goes tits-up that's the one I don't currently use anyway.

Once I'm more confident I'll post drawings and pix.
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Re: D.I.Y. amp

Postby Folderol » Tue Mar 12, 2019 12:56 pm

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:
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Re: D.I.Y. amp

Postby Wonks » Tue Mar 12, 2019 1:04 pm

Should have doubled it to save on the postage for the next lot. :D
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Re: D.I.Y. amp

Postby Folderol » Tue Mar 12, 2019 4:08 pm

What makes you think I didn't?

Found one silly mistake that could have caused quite serious overheating, but not the instant explosive destruction :(

I'd carefully wound the bias preset to the extreme end... the wrong one.

Checked the new module, and that is still fine {phew}.

When the transistors arrive, I'll hook them up directly to the module and feed from my test (current limited) PSU to see if there's something else I missed, although I can't think of anything.
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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!

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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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