Bottlehead Forum
Bottlehead Kits => BeePre => Topic started by: debk on March 02, 2013, 01:47:40 AM
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If I were to build the BeePre with two unbalanced outputs. Would I just replace the XLR jack with a RCA jack and eliminate the 10UF cap and two resistors that go to pin 3 of fthe XLR jack. Should I put a 1K resistor between the two outputs like there is in the Foreplay III. I intend to use one output to the Paramounts and the other to the sub amplifier.
Deb
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I was wondering the same thing today as I read through the manual.
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You can eliminate the two resistors and two caps per jack, then just parallel another set of RCA jacks.
The 1K resistors are optional, I don't generally recommend them.
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I have the same question with a twist (which I think I asked ages ago about the FPIII) - if my sub has a single RCA input, can I simply use a y-connector from the two additional (repurposed XLR) outputs, or is there a problem with doing that?
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Sorry, the Y adapter will short the right and left outputs together. That will make all outputs mono.
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That's kind of what I thought.
Seems like some kind of unity-gain summing amplifier would work to add the channels but keep them isolated, but it might be cheaper and easier to buy a sub with two inputs.
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That's kind of what I thought.
Seems like some kind of unity-gain summing amplifier would work to add the channels but keep them isolated, but it might be cheaper and easier to buy a sub with two inputs.
You can stick a Quickie between the BeePre and sub; summing its outputs won't bother the BeePre.
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That's certainly worth a try...the BeePre will be bumping the Quickie out of the signal path, as it happens.
A summing amp is easy to do with Op Amps - would that sound like crap, or would the bass tones not be as critical?
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The Opamp route is feasible, but you'll end up using lots of feedback to keep the gain under control, and actually more parts than the Quickie has.
Here's a page with a few solid state buffers:
http://www.beavisaudio.com/techpages/Buffers/ (http://www.beavisaudio.com/techpages/Buffers/)
I'd be most excited about trying the Jfet buffer, but use the 1N3819 since the part they reference is no longer in service.
In reality, I would be happier to use the Quickie than any of those circuits, but they are easy to build and good as experiments.
-PB
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Hi
From previous posts in this thread I was trying to figure out how you achieved balanced output on this pre-amp.
I might be a bit slow as thought you would need output transformers to do this. :-)
Cheers
Gn
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A balanced output by definition has equal impedances between hot and ground and cold and ground. We accomplished this by mirroring all the elements that make up this impedance and adding a resistor that is nearly identical to the output impedance.
Oddly enough, this is well covered by at least one transformer manufacturer.
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Hi Paul
Thanks for the explanation. Considering the amount of complexity that can be thrown at trying to effect balanced output, the use of one carefully chosen resistor is both simple and elegant.
I like it. :)
Regards
Gn
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If you want to pursue the solid state route, look into the BUF634, a unity gain chip designed specifically as a buffer and which is available as a TO-220 package.
I used this years ago as a preamp by putting a potentiometer and switching in front of the chip. I still think it sounds good and I think it would be transparent enough for use with a sub. It can certainly deliver enough current to drive even a greedy amplifier.
Mine still gets some use when I need a buffer / attenuator temporarily to test some wacko audio project.