Help required bridging Albarrys

[Dandy355]

I am a newbie here so think I'd better state my experience: I am ok with a soldering iron but ignorant of circuit design beyond the basics, so please bear this in mind in any response

I have Albarry M408 mono power amps that I would like to bridge. I was hoping to do this without surgery on the amps, by inverting the signal into one amp then taking the output “across” both (linking the –ve of one to the +ve of the other). I have a balanced output on the preamp, so can I simply use ground and hot into one amp and ground and cold into the other to achieve the inverted signal?

I understand that not all amps can be bridged, so I'm worried I might fry them. Has anyone any experience of bridging M408s? If they cannot be bridged like this, is there a low intrusion, safe alternative to bridging them in the way described above?

There is a schematic of the more powerful M1008 which has the same topology I believe here

Any help would be much appreciated.

[krmaudio]

ouch!

1st amp with standard input, use positive output terminal to speaker +

2nd amp with inverted input signal use positive output terminal to speaker -

please DONT do this: (linking the –ve of one to the +ve of the other). it will result in smoke and flames!

[Stuart Easson]

Nothing in the schematic of the amp would imply it cannot be bridged...but...

If you decide to go this route, your output will be from the +ve terminals of the amps, and I'd suggest you try linking the grounds, perhaps via chassis, perhaps via -ve speaker returns.

The schematic published for the more powerful m1008 does not suggest that Albarry amps have overbuilt output stages. With the same speaker load, bridging doubles the current and thereby power in each output stage.

With an ineffective output protection scheme (suggested by the discourse on the m1008) bridging, even done correctly, may still kill one or both amps, which may also wipe out the speaker.

Caveat emptor

Stuart

[sakis]

amplifie can be unstable since including 4 stupid transistors to my opinion ....so bridged will be twice unstable .....

ceep in mind if you do it that its very risky .... if you are looking for more power or generally for power that is not a solution

also a combination of a bridged amplifier cannot handle low loads ...especially this one wit tip 142-147 that have a reputation of NOT LIKING REACTIVE LOADS !!!!

regards sakis

[Dandy355]

Originally posted by krmaudio
ouch!

1st amp with standard input, use positive output terminal to speaker +

2nd amp with inverted input signal use positive output terminal to speaker -

please DONT do this: (linking the –ve of one to the +ve of the other). it will result in smoke and flames!

Oops.., yes... that's what I meant, thanks. Is splitting the balanced o/p from the pre ok for standard and inverted signals?

[Dandy355]

Originally posted by Stuart Easson
Nothing in the schematic of the amp would imply it cannot be bridged...but...

If you decide to go this route, your output will be from the +ve terminals of the amps, and I'd suggest you try linking the grounds, perhaps via chassis, perhaps via -ve speaker returns.

The schematic published for the more powerful m1008 does not suggest that Albarry amps have overbuilt output stages. With the same speaker load, bridging doubles the current and thereby power in each output stage.

With an ineffective output protection scheme (suggested by the discourse on the m1008) bridging, even done correctly, may still kill one or both amps, which may also wipe out the speaker.

Caveat emptor

Stuart

Thanks Stuart. I didn't realise the current would double in each stage. I'll leave this option I think as I wouldn't be confident enough to make the output protection mods suggested in the link.

Is there another way of wiring them (maybe with small mods) to maintain operating conditions?

Dave

[jez]

I would advise you not to modify these in any way......
They are the one model of amplifier that through bitter experience I now refuse to service
I have known these to blow up shortly after repair for no logical reason, even after replacing all the components! This has happened on more than one sample and yes, I know that this sounds stupid and that after ALL components have been replaced it should not happen. On all three occasions the repaired amps were soak tested,
tested for THD, frequency response etc and the bias stability was carefully monitored. In spite of all this they still blew up within a couple of days of being returned to the customer.
You have been warned.

[CBS240]

TIP142/147 are cheap Darlington BJT's that are made by everyone whose anyone (or not) in semi manufacturing (some of which end up either re-labled or don't even remotely come close to spec and thus are considered fakes....I may still have some of these fakes around here somewere.) I dropped these long ago when I became so frustrated with the amps suddenly occilating and melting outputs. These as with most Darlington pairs are not very linear which is one reason why they are not so popular in audio....unless you follow in the wake of those such as Dr. Hawksford and employ some form of error correction.

I would suggest leaving these as they are if they work now. Also I would not screw around with different impeadances at the output. I find these particular outputs to be fragile at best.

I wonder why they put the base of the Vbe multiplier to the wiper of the pot instead of between the pot and R8? It seems they are setting up for a new set of outputs if the pot opens or the wiper gets dirty.

[Stuart Easson]

...would be to eschew the use of encapsulated darlingtons and instead create a topologically similar arrangement using standard drivers and then (depending on power) one or more pairs of output transistors. Something like an mje1503x driving some mjl2119x outputs. Overall gain would be in the 1000's, but you can control the emitter resistor of the driver etc. For a 40-50 watt class AB amp you probably don't need more than a complementary pair of mj2119x transistors for all non-destructive loads.

You have most of the components (chassis, heatsinks, power supply etc) of a fine amp, there are couple of compromises Albarry made that you can fix and have a much improved amp.

We can discuss the fixes in this thread if you want to follow that route, or email me and I'll help where I can.

HTH

Stuart

[audiodesign]

See this
http://www.audiodesignguide.com/Albarry/index.html