The Alpha Big Boy with Buttah (ABBB) 52w Class A Amp GB

[xrk971]

In case anyone wants to get the smaller MOSFET snubber boards that were designed for the ABBB amp, they are now available in my shop. A set of 4 (N channel and P channel MOSFET and NPN and PNP BJT) on a single panel for $15.

A member asked that these be made available as he needed a lower profile one than the bigger ones that were used on the Alpha Nirvana.

[pinkfloyd4ever]

Originally Posted by xrk971

In case anyone wants to get the smaller MOSFET snubber boards that were designed for the ABBB amp, they are now available in my shop. A set of 4 (N channel and P channel MOSFET and NPN and PNP BJT) on a single panel for $15.

A member asked that these be made available as he needed a lower profile one than the bigger ones that were used on the Alpha Nirvana.

Where/when would the smaller snubber boards be preferable vs the bigger ones? Could the Aleph J be built using these? I guess the AJ PCB doesn't have anywhere for the molex connector though...

I was toying with the idea of building an amp chassis with dual mono SLBs, RTR DC protection boards, and SFP, and ideally be able to swap between the Aleph J, Alpha Nirvana, and other amp boards. Not sure how realistic that is, but I know I can't afford to build a whole PS, chassis, etc for all the DIY Audio amp designs I want to try.

Edit: shoot. now I'm remembering that the Aleph J and Alpha Nirvana use significantly different PCB rail voltages, so maybe this isn't practical. I may have to settle for 1 chassis/PS for Pass / FirtWatt designs, and a second for the AN and others that use higher voltage rails.

[xrk971]

The member who asked me about the smaller snubbers wants to use them in the Aleph J.

[joshua43214]

Originally Posted by xrk971

The member who asked me about the smaller snubbers wants to use them in the Aleph J.

Finally, yes, I have an update.

I got busy with work again, and used my spare time to put together a chassis, I was finally able to get back to the boards.

I am still having some difficulty, though.

First, I powered it up with the mosfets disconnected, and everything ran fine.
I held my breath, and put power to it with the mosfets connected, and it ran with no smoke

The voltage drop over the output source resistors was right at .645 volts.

When I checked the voltage at the mosfet drains, the Pmos was about 39V, and the Nmos was about 42v.

I turned the screw on the trim pot near the PNP and the fuse blew. After fiddling with the trim pot and 3 blown fuses later, I ordered a variac.

I can start the amp with the variac set at 80VAC.
At ~80VAC, rail voltage is +/-22VDC with mosfets unplugged.

with the mosfets plugged in.
The voltage drop over the source resistors is about .6v
The Pmos is 16.2v at the drain.
The NMos is 18.2v at the drain.

Adjusting either trim pot has no effect. I unmounted the trim pot for the PNP, and it works fine.

Any thoughts?

Thanks,
-Josh

[xrk971]

0.6v/0.22ohm is about 3A - the trimpot really doesn't have an effect and we should have just left it out (like we do in later amps). The 3A bias current is correct. What is the DC offset at the output?

The fact that you are blowing fuses might be that it is in-rush current into the transformer and caps and you don't have a slow blow fuse? What are your fuses rated for?

Also, with variac, did you slowly bring it all the way to 100% output and do fuses still blow?

[joshua43214]

Originally Posted by xrk971

0.6v/0.22ohm is about 3A - the trimpot really doesn't have an effect and we should have just left it out (like we do in later amps). The 3A bias current is correct. What is the DC offset at the output?

The fact that you are blowing fuses might be that it is in-rush current into the transformer and caps and you don't have a slow blow fuse? What are your fuses rated for?

Also, with variac, did you slowly bring it all the way to 100% output and do fuses still blow?

lol, good to know about the trimpot...
I have to put in an order with Mouser, should I get a some resistors and replace the trim pots? If so, what resistance?


I am a using 5A slow-blow ceramic fuse.
The transformer is an Antek 6432 (600VA), my line voltage is typically around 125v.

I tried turning it on, and turning up the variac a bit at a time.
The fuse blew again.
Also, the C-60 on circuit board got very hot, and something smells burnt.

Feeling kinda frustrated with this project, I've put together alot of circuit boards over the years, and never had this many problems. I wish I understood the circuit better, I am certain I just did something obviously wrong.

Any help is really appreciated.
Thanks,
-Josh

EDIT:
After poking around, I noticed that R215 is blown. This is the 10k resistor between the base and ground for the NPN BJT.

I rechecked all the leads to the transistors and mosfets, nothing is shorted to ground to the heat sinks, and they are all connected to the correct sockets.

[xrk971]

It sounds like you have a short or solder bridge to ground plane somewhere - the CL60 should only get hot in the event of a dead short. It seems that this short may have also fried your R215 on the cap Mx portion of the board. If the CL60 get hot, there must be a shorted connection from the +ve or -ve rail to the ground. It probably is the +ve side as that is where the R215 is blown. Are you vertain X211 is wired correctly to the BJT?

The pot on the cap Mx does have ability to vary voltage by maybe 2v to 3v or so, but we have found that it is best to leave it maxed around 3v for best regulation.

Can you please show a closeup photo(s) of the board and also give us voltages at all active transistr/mosfet pins marked up in red on a printout of the schematic? This will help us to diagnose the problem.

Thanks,
X

[joshua43214]

I have some sad stuff in real life to deal with (step-father died), so my replies might be a bit slow.
Please don't think this is low priority, just more important things sometimes take precedence.

When I have a moment, I'll pull the board and get you some good high-res images.
I checked the mosfet drains to audio ground, no dead shorts.
Hard to give you real figures, one was about 0.9M, and the other closer to 3M.

I have seen spots of flux act as a path to ground when given enough current, I have a few suspect places to check closely. Will report back later.

I can try to get you some voltages, they will be with a reduced line voltage though.
I don't have any good 10k resistors (lost somewhere) to replace the blown one. All I can find are some Chinesium ones that I do not trust with any decent amperage.

Thanks again for your help,
-Josh

[Vunce]

I am so sorry to hear about your sad news Josh, my condolences to you and your family.
Troubleshooting an amplifier does not seem so important now, take your time.

[xrk971]

Yes, sorry for your loss Joshua. Take care.