Convert a BHK250 to a BHK300

Hi fellow audiophiles,

I just ordered two BHK250’s for bi-amping my (huge) two way speakers. Since all documentation (and economics for that matter) emphasize on the equality of both the 250 and the 300 I have the following questions:

  1. Assuming one has the skills and the tools: Can one convert a 250 to a 300…?
  2. Are there block schematics of both the 250 and the 300…? Like the ones Accuphase always put in their brochures.
  3. What are the exact differences between a 250 and a 300…?

For instance: on pictures of both machines I see (I assume) 4 speaker wires screwed on both PCB’s of the 250 (2 red, 2 black per side) while only two on the 300 PCB’s (two blacks on one side, two reds on the other side).

How does one “combine” two fully balanced, low impedance amplifiers to one, without bridging obviously, since that is impossible says the manual of the 250…?

Hope to learn something here, can’t wait for an answer…!

Greetz from The Netherlands,
Ronald

Inside the BHK300s all components are doubled up so that both channels work as one mono amplifier… If you could do that, which I believe isn’t possible with a brace of BHK250s, you would effectively have BHK300 mono amplifiers, which would comprise 2 channels, not 4 channels… In which case you should have bought the BHK300s.

I use 2 x BHK250s and the BHK300 monos. The BHK 300s are considerably better amplifiers than 2 x BHK250s. Sorry, not the answer you were looking for, but it is the way I find it to be. Best to ask Paul if there is an alternative approach with 2 x BHK250s but I feel you should have opted for the BHK300 monos.

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Hi Dirk, thanks for your answer I was not looking for… LOL
I can’t imagine the 250 is that much worse than the 300, since Bascom and Arny both have the 250’s. At least, that’s what they say on several video’s. Anyway: the reason I have the 250’s is that I have a realy, realy sensitive 7’ ribbon and a s***tload of 12" woofers per channel that I just will not hookup up to a single amp, espessially not an amp with a damping factor of “just” 350. Besides, selling two stereo amps is much easier then one set of mono’s.
Thanks again,
Ronald

I am not saying the BHK250s are poor performing amplifiers by any stretch but, the BHK300s are BETTER !! Like I said, I own BHK300s and 2 x BHK250s and there is a sizeable improvement with the BHK300s in my system…

I was just putting over the case that to make the BHK250s into one amplifier ( which you asked about ), you then have BHK300s, i.e. 2 channels not 4 … I am sure you will be delighted with the BHK250s.

I sure hope so Dirk. It just puzzles me how they tie those two (or four so you will) together. I mean, they have to come together at the outputs, which is sheer impossible since they are going to short one and other, being low impedance as they are.

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P.S. I think a simple sketch would solve the question in a heartbeat.

Hi Ronald, you need to ask Paul. The magic is in the way they are configured with all components doubled up and working as one channel. ‘Doubles the current with half the impedance’. The damping factor is waaay better with BHK300s. You would need to hear them side by side… 2 x BHK250s and the BHK300s…!

I wait for the two 250’s first. What counts for me is that they are fully differential/balanced, have the “same sex” topology (which I find interesting as it is an old Philips patent) and that glowing piece of glas in the front line. In the mean time I hope that ask Paul picks this up and gives me a reply. It was good talking to you Dirk, thanks for your input in the matter.

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You are welcome - Ronald. I am sure Paul will enlighten you.

John Bedini always used same sex outputs on his amplifiers!

Assuming one has the skills: Can one convert a 250 to a 300…?

No. There’s different boards and settings that aren’t just converting one thing to another.

Are there block schematics of both the 250 and the 300…? Like the ones Accuphase always put in their brochures.

We don’t generally publish schematics

This explains part of the answer: bridging amps isn’t the same as paralleling amps: https://en.wikipedia.org/wiki/Bridged_and_paralleled_amplifiers

Thank you Ted. That piece you mentioned implies that both amps have “the black terminal” as common to ground, which is not the case with the 250 since it is fully balanced… I hope…!!! Am I right…? They are fully balanced, right…?

Bridging amps (or any other paralleling of amps) is not for the faint of heart (or light of pocket). Some amps are designed to be bridged but many mono amps are already bridged inside and can’t be bridged again outside. I don’t know the details about PS Audio amps but I suspect that you shouldn’t consider bridging (or paralleling) them at all.

I don’t know whether I belong to either one of those groups Ted.

I am not in the business of buying two stereo amps to make two mono’s out of them nor was I about to. I want to bi-amp my stereo two way speakers, so I need two stereo amps. Since the speakers I have made are highly sensitive I am not aiming for power. But I do want them to be fully balanced. The competition from Auburn,CA for instance has one called the XA30.8. Nevertheless I have chosen for the BHK250 since I firmly believe in the capabilities of Bascom, Paul and you too for that matter.

Now: The only question I have is how (in the world) one can “combine” two “DIFFERENTIALLY COUPLED BALANCED” output stages into one “FULLY PARALLELED INSIDE / DIFFERENTIALLY PARALLELED COUPLED BALANCED” mono stage, knowing that one amp will short the other one and vice versa.

By the way: If the answer to that question is confidential, which I would totally understand, than that would be a valid answer also.

Thanks so much for your input by the way Ted…!

P.S. The capitals I used are to emphasize the exact words on the PS Audio website.

Just by studying all the pics on Google… Am I close…?

The Stereo version has two heat sinks each containing a full bridge output stage. In the mono version, the output devices of each heat sink are paralleled and each becomes one phase of the final output. Suitable jumpers and component subtraction in the front end take care of making this all work.

BHK

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Hi Ronald. Yes you are close in the overall topology. What you have in the output stage blocks is far from what is that circuitry!

Regards,
BHK

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Thank you much BHK… I am truly honoured and satisfied with this answer.
Yours,
Ronald