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Old 18th April 2013, 07:50 PM   #1
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Default Modified Alembic F2-B

I'm going to be building a single channel bass preamp based on the F2-B. I think I have most of parts ready to go, just need to order the FET's and some caps so I can do a quick prototype. I happen to have one of the Stancor power transformers used in the F2-B, so the power supply will be identical to the original.

The buffered output should cover the main weak point of the F2-B, which is it's compromised ability to drive long cable lengths and the low impedance inputs of modern pro amplifiers.
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Old 18th April 2013, 07:51 PM   #2
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Schematic attached.
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Old 18th April 2013, 10:02 PM   #3
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Cool build!
I'd up the value of the output capacitor though. If connected to e.g. a power amp with a 10k input impedance, which is quite commen, the -3dB point will be at about 160Hz(!). WAY too high for a bass amp. You want it to go down to at least around 30Hz, which is achieved with a 530nF cap, which doesn't exist, so you could use 1uF and be safe for even lower peripheral input impedances.
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Old 18th April 2013, 10:28 PM   #4
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Of course, you're right. Pretty bad copy/paste error there. I agree, 1uF would be good.
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Old 19th April 2013, 07:26 PM   #5
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Old 19th April 2013, 11:15 PM   #6
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Updated the schematic, and placed an order for a Par-Metal 10-19082B 1U case.

I'm worried about the transformer fitting; even though the original Alembic is in a 1U case, and I have the same Stancor PS-8415 power transformer, the Stancor I have is 1.64" thick, which is the exact internal dimensions of the Par-metal case (1.74" - 0.1" for the top bottom panels)... We'll see how that goes.

Now I'll be working on the turret board layout.
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Old 21st April 2013, 09:33 AM   #7
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Cool, keep us posted.
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Old 22nd April 2013, 06:42 PM   #8
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Of course the F2B classic is two channels, and some of the best sounds are using both channels in series.
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Old 22nd April 2013, 06:44 PM   #9
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Are you talking about that Q1 as your 'buffered output'??
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Old 22nd April 2013, 06:47 PM   #10
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BTW when I did mine, I used two sub-mini toggle switches for the bright switch, with a much different cap on each...gave three different knees (one, the other, both).
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