Yamaha BX-1 monoblocks: another pair pops up!

cdfac

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Yippee! Well, you could also think of it as the third working orphan on AK, at last count...

In perfectly acceptable shape (to me) and advertised as both being nonworking, one ended up being JUST FINE! :banana: The other had a loose transistor banging around and goes into protection. Got my fingers crossed that's the only problem. They were packed exquisitely in a microwave/TV-style shipping box, my only complaint being that they could've added some bubblewrap in place of some of the foam. These things are HUGE :jawdrop: Together they take up at least 80% of the volume of a PC5002M, I'd say. Avionic has agreed to take the repair job on, and he'll do a recap as well.

Here's some shots of the broken one. Both are in similar shape, which appears better than the ruthless flash shows. Larger-than-average coffee mug for scale:

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I was snooping on google to see what you had here, and low and behold there was a link for the service manual pdf.... thought you might like this-

http://www.audio-circuit.dk/Schematics/Yamaha-BX1-pwr-sm.pdf

Brian

I posted the same thing here..
http://www.audiokarma.org/forums/showpost.php?p=1725272&postcount=1
I was snooping on google to see what you had here
100 watts of "class A " power - reguardless of 4 ohm or 8 ohm load..Will warm up a small room in no time.:yes:
 
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And it is also one of Yamahas most expensive Hi-Fi amps with an MSRP of $2000 (yes $4.000 for a stereo setup), there are not that many of those around.

And as avionic mentions, you have to add the running electricity bill :)
 
100 watts of "class A " power - reguardless of 4 ohm or 8 ohm load..Will warm up a small room in no time.:yes:

100W per amp output, so around 400W per amp heat dissipation.

Good thing winter is coming soon....double duty as space heater and entertainment. :D
 
i hooked up the working one to an RS1b panel tonight. gets warm quickly, which is good for listening purposes. stand above it and you can feel it on your face! sounds excellent, though, lots of potential. it's a little hard to review a single mono amp in a stereo setup so i won't bother saying more until i've got both working.

i wonder what the dynamic power rating is. if you could get rid of the heat, i bet the amp would come close to doubling down into 4 Ohms RMS-wise, but as it is, seems like they need the 4-Ohm switch the lower the rail voltage so the amp doesn't melt..
 
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well, i probably listened an hour total tonight. it made me even more excited, so i figured i'd add just a wee bit to my previous observations, all fairly subjective of course and probably inaccurate...

it reminds me the most of the 5002M, but it's probably better (the BX-1), which is neither totally expected nor unexpected by me. very open and neutral but tending toward bright more than dark, otherwise difficult to characterize. also reminds me of the M-85 in "Auto A" but with no haze and more oomph/depth. the power supply seems to be very overbuilt and the slew/rise specs look phenomenal if they are even remotely accurate. my thoughts may change with both amps set up, but either way i don't think these guys will be going anywhere other than Avionic's Spa :no:
 
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Can a skilled schematic reader look at the diagrams on the last couple pages of the manual and tell me exactly what the differences are between the 4- and 8-Ohm operation modes? Of primary interest is whether the supply rail is variable. I had always assumed the supply dropped into 4 Ohms because of the transformer wiring in the block diagram and because of the identical power ratings into both impedances, but then I realized that perhaps it was just the bias that changes and that the 100W rating may merely be the maximum Class A output into 4 Ohms. Any thoughts?

http://akdatabase.com/AKview/albums/userpics/10004/Yamaha BX-1 Service.pdf
 
Chris on the schematic.Look at the secondary side of the power transformer
When selecting 4 or 8 ohm with the switch you are actually selecting which taps of the transformer your using RE 14.3 vac or BE 17.9 vac through relay RY751 AND RY752.Which raises or lowers the Bias power pcb's + / - rail voltage. by roughly 5 volts DC. The 4/8ohm switch also selects which bias selection is needed for 4 or 8 ohm.
 
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If you look at the block diagram on page 7 it show the 4-8 ohm switch swiching to a different winding on the transfomer. This means a different supply voltage.
 
ok, that's what i originally thought. how does the math work though? P = 28V^2/2/4R = ~100W for the lower rail. however, P = 36V^2/2/8R = ~80W, not the 100W you'd expect. am i missing something?
 
ok, that's what i originally thought. how does the math work though? P = 28V^2/2/4R = ~100W for the lower rail. however, P = 36V^2/2/8R = ~80W, not the 100W you'd expect. am i missing something?

Yep..20 watts :D Your volts should be RMS volts in the formula. Which will be some what lower than your rail voltages. 8 ohm rail is +/-30vdc --4 ohm rail is +/- 20vdc

http://www.the12volt.com/ohm/page2.asp
 
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haha! reason i'm asking Dave is that i'm wondering how safe it will be to run a 4-Ohm load on the 8-Ohm taps with the addition of the cooling fan. that extra 60 or more Watts would probably come in handy if i want to use these on the Infinities.
 
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