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Old 1st July 2005, 07:41 AM   #21
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Yes maf_au you're on track. generally when I design an amplifier I include drain down resistors for the power supply. They only need bleed 5 mA or so .. so that by the time the lid is unscrewed they're as good as discharged.

A8 - you designed this unit? So the pins 1,5 and 8 can be relieved of components to implant a OPA627?

Sorry I don't know the design so without a schema.. I'm working blind.

Cheers,
greg
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Old 1st July 2005, 07:58 AM   #22
maf_au is offline maf_au  Australia
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Default schematic

Ok, here it is.

Not sure what the limits are here, hopefully this is small enough...

Michael
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Old 1st July 2005, 08:08 AM   #23
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Quote:
Originally posted by amplifierguru
Yes maf_au you're on track. generally when I design an amplifier I include drain down resistors for the power supply. They only need bleed 5 mA or so .. so that by the time the lid is unscrewed they're as good as discharged.

A8 - you designed this unit? So the pins 1,5 and 8 can be relieved of components to implant a OPA627?

Sorry I don't know the design so without a schema.. I'm working blind.

Cheers,
greg
Thanks.

Looks like the replacement caps aren't going to make it here today, I might spend some time on the weekend identifying and recording the differences between my amp and the schema, just in case there is something that shouldn't be...

Michael
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Old 1st July 2005, 08:15 AM   #24
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You're doing well. If you can permanently fit 22k 1W resistors across all your PS C's that would ensure some safety. They won't degrade performance.


The components on pins 1,5,8 can safely be removed if you want to use a OPA627. The 3K3 at the input filter may be picking up excess hum and noise, depending on the layout. Replacing with 1K or a parallel equiv might be worthwhile.


I wouldn't use an OPA637 in this cct.

Good luck,
Greg
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Old 5th July 2005, 04:44 AM   #25
maf_au is offline maf_au  Australia
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Default Caps

Well, the caps have all arrived. Danged if I could get any of the well-regarded brands and types locally in Australia, but these should get me going, maybe I'll tweak it later when I work out how to get those brands here...

Anyway, in place of the 2 x 220uf/16v I have been supplied with 330uf/16v. I cannot place these values on the schematic, I think they are in place of the 100uf caps shown bridging to ground on the incoming power + and - rails.

Should the 330uf caps be ok, or should I try and get 220uf?

No idea about how tolerant the circuit is to changes like this, the supplier didn't even mention the switch, so it's either commonplace, or an error.

Michael
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Old 5th July 2005, 06:33 AM   #26
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Hi maf-au,

The 330uF are substituted for the 220uF in the input leg of the feedback loop. If they fit they'll be OK though you may have a slightly different thump at on/off than before.

One weakness of this type of driver stage design is that as the driver transistors get warmer for whatever reason, the design tends to deflect more current to the stage and turning on the output harder. It's only minor but a point. I have a similar design here I've been assessing. So instead of the bias having a neutral or -ve tempco, it's slightly +ve.

Good luck.

Greg
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Old 5th July 2005, 06:51 AM   #27
maf_au is offline maf_au  Australia
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Hi thanks for the follow up.

Ah! I see them now - connected to pin 3 of the opamp.

Thanks,

Michael
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Old 5th July 2005, 01:38 PM   #28
maf_au is offline maf_au  Australia
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Default damage

Here is the damage from underneath the PCB
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Old 5th July 2005, 01:39 PM   #29
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And here is what I reckon I will cut out, what do you reckon?
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Old 5th July 2005, 04:14 PM   #30
maf_au is offline maf_au  Australia
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And here is the new piece inserted.

It's not pretty, but it should work ok, and it's a lot better than what was there before...

New caps are installed, just have to drill some holes for the MOSFET, line it up, do the point to point soldering with the other components effected by the burn, repair the pcb track I cut cutting out the damage.
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