Thinking of building an amp out of my broken Audio Research D350B - advice please!

[w1jim]

OK, here's my bright idea.
Last year I purchased a broken Audio Research D350B power amplifier from a fellow AK'er - it was too cool to turn down.

http://www.audiokarma.org/forums/showthread.php?t=457511

After lots of research and tinkering I came to the conclusion that there was (at least) one unobtanium part that needed replacement. I actually found a source (a former AR employee) for one at the time but couldn't justify the $150 for the part just to find out that it wasn't the problem.
To add to that, I asked - "well what does AR do when these need repair", to which the reply was - "we tell them to buy something new, these weren't very good in the first place". They're big into tubes now - don't you know.

So, after ignoring this project for a year it's time to see what I can make of this.

I'm thinking - what would be involved in reengineering this into a modern SS power amp?

The old amp will contribute the power supply, enclosure, heat sinks, cooling fans, output and line voltage power meters to the new creation.

The power supply consists of 2 separate +50/-50 (with 120,000uf each) and a single +75/-75 (with 7,600uf) supplies.

What kind of advice can be offered as I bumble down this dark path?

 

[petehall347]

400w mosfet amp should be possible with the 75v lines

 

[hatrack71]

Man, I would have tried to restore it anyway no matter the part cost. Don't give up, it is not often you see that Audio Research. I'd love o have one of those. There isn't a part you can sub in?

 

[w1jim]

There isn't a part you can sub in?

Doubtful - it's one of the black modules on the amp board.
Even though they are obsolete AR wasn't willing to give me the schematic.

 

[w1jim]

400w mosfet amp should be possible with the 75v lines

I'd be using the 50/50 volt lines for the amp stage.

 

[petehall347]

did you compare voltages at the modules ? and did you take a look inside one of them ?
am thinking they will be full of discreet components that could be built onto a pcb board if needed .
maybe you could draw out the amp schematics yourself ?

 

[w1jim]

I tried to get into one of the modules - they are potted, chipping them open will only destroy them (further).

 

[bobrown14]

I tried to get into one of the modules - they are potted, chipping them open will only destroy them (further).

It says "analog" on the case cover.... maybe it's some sort of voltage/current regulator similar to what all automobiles have installed on the fuse block to monitor current/voltage to save gas.

Wow, I would keep trying to get the schematic, isn't the company still in business? Maybe negative pub would be enough to get them off the dark side.

Squeaky wheel and all... ask to speak with the... ahem... "manager"

You can't give up, that amp is so dang big, you gotta get it going. Look at is as a challenge (I'm sure you already did that) and think that if you can load it in your car and get it home you should be able to talk the schematic out of the Gate Keeper at AR. Heck offer to purchase and sign no disclosure clause. Pet the Gate Keepers ego.... then :butt1::butt1:

Cheers,
Bob

 

[w1jim]

I have the full schematic for the amp that they were kind enough to send me - just not the schematic for the modules.
It's unlikely they'll change their policies for my sake!

 

[petehall347]

it should be possible to figure out what the modules actually do from looking at the schematic .
there is some info floating around on the net . once its figured out what they do then you should be able to see if the suspect one is actually faulty or not .. maybe a swap with the other channel ? i take it one channel is working ?

 

[w1jim]

Here's the schematic - what do you think AM1 does?
http://www.audiokarma.org/forums/attachment.php?attachmentid=364547&d=1343168731

 

[w1jim]

I'm thinking of using 2 of these amp PCB kits - available in 300 and 500 watt models:
http://connexelectronic.com/product_info.php/products_id/78

Here's the manual:
http://www.connexelectronic.com/documents/LME49810_Audio_Amplifier_Module.pdf


Thoughts?

 

[petehall347]

Here's the schematic - what do you think AM1 does?
http://www.audiokarma.org/forums/attachment.php?attachmentid=364547&d=1343168731

looks like an input buffer .

 

[fpitas]

looks like an input buffer .

Something like that, maybe a discrete op-amp.

 

[whoaru99]

I tried to get into one of the modules - they are potted, chipping them open will only destroy them (further).

Sometimes you can soak them in a strong solvent to soften/dissolve the potting compound. May take a while but sounds like you may have a while.

 

[fpitas]

Sometimes you can soak them in a strong solvent to soften/dissolve the potting compound. May take a while but sounds like you may have a while.

Methylene chloride (sold under the Attack brand name) will slowly dissolve epoxy it's true; however, any epoxy encapsulated devices, or for that matter the PCB, may be dissolved too.

 

[fpitas]

I'm thinking of using 2 of these amp PCB kits - available in 300 and 500 watt models:
http://connexelectronic.com/product_info.php/products_id/78

Here's the manual:
http://www.connexelectronic.com/documents/LME49810_Audio_Amplifier_Module.pdf


Thoughts?

Those look fine, although with a bit more effort you could build one of the Pass amps:

http://www.diyaudio.com/forums/pass-labs/

 

[ConradH]

I repaired an ARC amp a while back and the circuit board looked exactly like yours. ARC was right, they aren't that good to begin with. The module is basically a high voltage op-amp with access to some internal pins. The ones on the board I did weren't bad, but I did consider how one might replace them. In that case I would have just picked up some HV opamps from Apex, and redesigned the servo circuit a bit so the extra pins weren't needed. It would make no financial sense to do for somebody else, but if you do your own work it's entirely possible. Note that the unit also had some bad ground connections and a rather poorly thought out grounding scheme.

To rebuild the unit as something modern, the first thing I'd do is get a copy of Doug Self's power amplifier book, followed by the one by Bob Cordell. Then you'll have some modern thinking and the fundamentals to choose a decent circuit.

 

[w1jim]

Those look fine, although with a bit more effort you could build one of the Pass amps:

http://www.diyaudio.com/forums/pass-labs/

.
I've looked at some of the Pass amp designs over at diyaudio but there are so many designs it would be hard to figure out which is most appropriate.
Also, are thoses mostly (relatively) lower power high current designs?

 

[chicks]

I repaired an ARC amp a while back and the circuit board looked exactly like yours. ARC was right, they aren't that good to begin with. ... Note that the unit also had some bad ground connections and a rather poorly thought out grounding scheme.

TAC was highly critical of ARC and Conrad Johnson for their poorly designed, overpriced, proprietary stuff. Lousy engineering at inflated prices. They were the darlings of the audiophool press, though.