ACA V1.5 Illustrated Build Guide

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Nelson Pass

Nelson Pass

The one and only
Joined 2001
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dawa said:
Found this transformer at the Lundahl site. Would it be a way to connect
an unbalanced preamp or source to the ACA in mono mode?

Lundahl Transformers • LL1584-PHF3MX
Click to expand...

Being that the channels are inverting in phase, another way is to simply
drive one channel and feed its output through an appropriately large
resistor to the input of the other channel, taking the output off both
minus outputs of the ACA.

The "minus" output of the channel driven by the source would become
the (-) speaker terminal, and the other channel's "minus" goes to the
(+) terminal. The resistor value in question would be about 68k for the
early version of the ACA (same as feedback resistor minus 10K) and the
later version would be about 39K.

The result is a bridged amplifier. It will deliver about 15 watts into
8 ohms at clipping, but will not enjoy the distortion cancellation that
comes with driving the inputs balanced. The result is still negative
phase 2nd harmonic at ordinary levels.
 
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SirDanBaker

Member
Joined 2018
First of all, a big thank you for the build guide 6L6, was easier to put together than a Legoset, very nice picturework as well����.

A question, in an earlier version of the ACA you could put a 2.21k resistor between Q3 and R15 for an extra watt of power, can that still be done, or has it been incorporated in the 1.1 version?

Thanks! Dan.
 
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SirDanBaker

Member
Joined 2018
thanks for the answers, I thought it was but I was not sure.

Maybe a couple of n00b questions (the ACA was my first ever DIY audio project):

1. what would happen if I alter the Bias voltage?
2. Would a bigger/better powersupply have any impact?
3. There were two extra resistors in the package but no note as to where they would go in the circuit, could someone tell me why and what they would do?
4. How long (in your experience) does it take for the little monster to run in properly?

Thx for thinking along :)
 
6L6

6L6

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1) You get more bias. This, in an by itself, does little.
2) Yes. Get a 24v 4-5A brick and bias to 12v
3) Use those resistors to make the gain match the original ACA
4) Break-in happens between your ears, not the electronics. That said, there will be slight sonic differences as the amp comes up to working temperature from cold.
 
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6L6

6L6

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chrisb said:
Jim - think you could organize a group buy on custom engraved faceplates with that?
Click to expand...

George the amplifier ?I like it, but I think only 4 people would get the joke... :cool:

533527
 
6L6

6L6

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Joined 2010
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Bridge balanced monoblock configuration - I built up the 2nd amp and am listening to the pair now. (XLR input)

Wow.

Wow.

WOW!

For such a modest circuit with lots of choices and tricks to making it simple, beginner-friendly, and bulletproof, these have no right to sound this good.

Papa is a genius.

Period.
 
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