B&K AV5000 II Bridging Help Needed

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I recently purchased a B&K AV5000 in need of repair. This amplifier is rated at 125 watts per channel into 8 ohms and has 5 channels.

What I'd like to do while I'm repairing it, is bridge pairs of channels to give more power into two channels for stereo use.

While I've repaired equipment as a hobby for nearly 25 years, I've never designed circuits so hence don't understand how I would go about bridging this unit. I'd rather not fry stuff experimenting, so I thought it might not hurt to ask if some of you folks with expertise would be willing to help.

I've attached the schematic for reference. I'm looking for the best solution that would also keep it stable.

For reference, per reviews, B&K did a very nice job on this amp. It's biased fairly high so it runs hot. I've read it tends to have the nice, smooth midrange that fets are so good at when the bias is turned up. The price was right and I figured it might make a nice two channel amp. I'd just prefer to have more bottom-end grunt if I can get it.

Any help would be much appreciated!

Thanks in advance,


B&K AV5000 Schematic.jpg
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Thank you zeonrider for the info.

I was hoping I could just tap part of the input circuit from one and feed to the other to make it work. Some amplifiers have a bridging switch to combine channels and I was hoping for a similar solution. I would rather not run the signal through an inverting stage if I don't have to.

Can I just run a balanced preamp and take + and - from the balanced output, run each to a separate channel input of the amplifer, then just take the speaker output from each of the positive speaker terminals?

Is it really that simple?

Would it be a problem if both input grounds were common?

Any help is much appreciated!


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I think then that your best solution is the one you've already suggested - take the + and - outputs from a balanced o/p of your pre. The speakers are connected, as you say, between the two positive terminals. Bear in mind that when operating in bridged mode, the speaker impedance the amp sees is halved - you said it already runs hot. It will surely run hotter when running in bridged. Lateral FETs aren't the best choice in such a configuration but I'm sure it will work.
I did notice that and it was why I asked about how to bridge in the first place. I should have been more clear in my question.

Where would that be connected to on the other channel? The input?

Since it's past the input stage, I thought it would be too hot to feed the adjacent channel input. Figured there would be a BR IN or something like that to connect it to and I didn't see that.

Any idea?

The amp is due to arrive on Tuesday so I'll have a better idea after I physically see it.


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