B&K AVR mods to make a multi channel power amp (2024)

Hi all,
I couldn't find an AK writeup for this mod so thought I would make a post. I have done this now to two B&K AVR's. One is an AVR-305 owned by a friend, and now an AVR-507 that I found. Both units had failures in the digital sections so I made them into multi-channel power amps. My friend and I both have Oppo bluray players with full 7.1 line level outputs so with this amp it's quite easy to make a surround system.

There are a couple of things that need to be done. I strip out all of the preamp and digital boards including the rear panel connector board. Then I reamed out enough of the holes to install RCA jacks above each of the speaker binding post pairs. There is a blue and green pair of wires for the analog input to each amp section. Blue is signal and green is common. The caveat here is that the green is not connected to amplifier ground. That connection originally came through the preamp board. This connection must be reestablished on the amp boards. Without this ground connection I had about -35 volts on the speaker jacks. I ran a wire from where R30 is located. This resistor is not installed by the factory so has an empty hole which is amp ground. A wire from here to the input connector pin for the green wire makes it all work. R30 locations are under the B+ power fuses on each amp section.

There is an enable circuit connected to each channel. This was originally connected to one of the control boards. You could add a small time delay relay but I just "hot wired" the channels. These are the orange wires from the power supply board to each channel. I connected a 1k resistor from a ground pin on one of the now unused connectors and the other end to an unused point of the orange enable wirs on the board. Boths end of the enable circuit, i.e., the orange wire group at each end of the power supply board are connected together on the board so it doesn't matter to which end the resistor is soldered.

The schematics are on Hifiengine.

Nice repurposing!
I have a Harman Kardon AVR 7300 (7x125w/ch) that has a VLSI chip that runs the preamp and similar functions. I shipped it Harman to have it replaced once, but when it died from the same problem a second time, in less than a year, I jus put it aside. I really hate to trash a monster multichannel amp like that, but I had no chops to refurb it like you did.

Funny enough, I replaced the HK with an Integra DTC 9.8 Pre/Processor,
and now that feeds a pair of B&K power amps - one stereo amp (B&K ST202 +) for the front mains,
and a 5 ch amp (B&K AV-5000 II) for the center, sides, and rears.
- I run the B-460 sub with an old HK Citation 22 in Mono mode (est 600w)
Love the sound of the B&K amps!

Congrats on the mods and integrating it all. Happy Holidays (and great listening/viewing) yourself!

P.S. How and from what source did you determine to set the channels' output transistor biases? Plus, what was unstable in Ch. 6 and 7? Volume, distortion, power handling, ...?

awillia6 said:

P.S. How and from what source did you determine to set the channels' output transistor biases? Plus, what was unstable in Ch. 6 and 7? Volume, distortion, power handling, ...?

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Neither the bias or the DC offset was stable in the two channels. Funny thing is that those two channels had broken binding posts which I replaced. They may have suffered some damage if the amp was on when it happened. I just assumed it was handling damage and not likely to have included any electrical failure. I got the bias numbers from a B&K document for some similar amps which have a similar power amp schematic. I just inferred that these should be about the same. It's a very low class AB bias so probably not that critical. I set the channels for 200mA per the attached document.

I found these documents at HiFi Engine: https://www.hifiengine.com/manual_library/bk-components.shtml

hjames said:

Nice repurposing!
I have a Harman Kardon AVR 7300 (7x125w/ch) that has a VLSI chip that runs the preamp and similar functions. I shipped it Harman to have it replaced once, but when it died from the same problem a second time, in less than a year, I jus put it aside. I really hate to trash a monster multichannel amp like that, but I had no chops to refurb it like you did.

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The service manual with schematics for the HK is on HiFiEngine as well. The power amp sections are all analog so it should be very possible to convert it to a 7 channel amp similar to my effort on the B&K. Maybe you have a friend that could tackle it. It's handy to have a 5 or 7 channel amp around for emergencies.
John

nerdorama said:

Neither the bias or the DC offset was stable in the two channels. Funny thing is that those two channels had broken binding posts which I replaced. They may have suffered some damage if the amp was on when it happened. I just assumed it was handling damage and not likely to have included any electrical failure. I got the bias numbers from a B&K document for some similar amps which have a similar power amp schematic. I just inferred that these should be about the same. It's a very low class AB bias so probably not that critical. I set the channels for 200mA per the attached document.

I found these documents at HiFi Engine: https://www.hifiengine.com/manual_library/bk-components.shtml

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Belated thanks for taking the time to post up the links and especially for the bias info. If only B&K had been clear and definitive in their technical documentation, this stuff would be a lot easier to deal with.

B&K AVRs seem to have lot of a failures in the digital sections. My guess is many were "bricked" along the way trying out firmware updates leaving them inoperable in a power on "waiting" state as opposed to a real parts failure.

Too bad somebody can't figure a recovery for that. Seems a shame to convert them over to just a power amp, they are really nice receivers if working.

Sonido24 said:

Hi nerdorama, Thank your for sharing this info! I'm trying to do the same with a B&K avr 307
do you mind if i ask you more information about this conversion?

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What would you like to know? I've been away from these for a while so I'll try to dredge up what I can offer.
John

nerdorama said:

What would you like to know? I've been away from these for a while so I'll try to dredge up what I can offer.
John

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Hi, thank you in advance! I'm new to B&K audio equipment 2019 and i own a B&K avr 507 my speakers transformed! The sound is unreal compared to other amps and receivers that i used to own.
I also own a B&K avr 307 that has problems with the Preamp section and i would like to do a conversion to a power amp. The picture with the brown wire connected R30 i get it,
i would like to ask you about the wire connected on the other side close to the top right fuse and a diode where is that cable connected to the fuse or the diode?
Also how do i check the bias or DC offset? I see that you posted that you set the channels to 200mA did you do that with the remaining channels as well? Thank you very much!

Best regards,

The other wire is connected the same. They terminate under the fuses to the unused R30 solder pad and the other end I soldered to the pin of the input connector. This has the result of tying the common side of the input circuit to signal ground of the amp section.

Bias is measured by removing one of the amp fuses and connecting an ammeter across the fuse holder to make a direct measurement of the idle current through the channel being adjusted. DC offset is measured across the dummy load resistor attached to the speaker binding posts as mentioned in the instructions which I attached in post no 7 of this thread.

Hope I've answered your questions clearly. Keep asking if you need more clarification.
John

nerdorama said:

The other wire is connected the same. They terminate under the fuses to the unused R30 solder pad and the other end I soldered to the pin of the input connector. This has the result of tying the common side of the input circuit to signal ground of the amp section.

Bias is measured by removing one of the amp fuses and connecting an ammeter across the fuse holder to make a direct measurement of the idle current through the channel being adjusted. DC offset is measured across the dummy load resistor attached to the speaker binding posts as mentioned in the instructions which I attached in post no 7 of this thread.

Hope I've answered your questions clearly. Keep asking if you need more clarification.
John

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Thank you sir! I will start this project sometime next week hopefully and i will upload pictures just to confirm that i'm not doing anything wrong, Thanks again.

Best,
Alex

nerdorama said:

The other wire is connected the same. They terminate under the fuses to the unused R30 solder pad and the other end I soldered to the pin of the input connector. This has the result of tying the common side of the input circuit to signal ground of the amp section.

Bias is measured by removing one of the amp fuses and connecting an ammeter across the fuse holder to make a direct measurement of the idle current through the channel being adjusted. DC offset is measured across the dummy load resistor attached to the speaker binding posts as mentioned in the instructions which I attached in post no 7 of this thread.

Hope I've answered your questions clearly. Keep asking if you need more clarification.
John

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Hello Nerdorama, I completed the project today but I would like you to take a look at what I did before i try it for the first time. I added a wire from R30 to the second pin For each channel, I also added 1k resistor on the power supply on both ends and finally
Connected the new rca jacks for each Channel, blue wire has the signal and green cable is the common.
Please take a look at the images, thank you!

Best regards, Alex