The BA-3 as preamp build guide

I have to be doing something fundamentally wrong, or misunderstanding something. I'm operating under the impression that I have to adjust P1 to get 50mA at R10 ( since I have Toshiba 2SJ313/2SK2013 mosfets, and Toshiba jfets @ 9.6 mA idss ), while adjusting P2 to get 0 volts DC at the output. The moment I remove the meter from R10 offset - being measured here as DC volts between the signal and ground at the actual RCA output - jumps to nearly 2 volts then very slowly heads back towards 0. What am I missing? Am I measuring bias or offset at the wrong spots?
 
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Measured at the RCA out, after the cap

Measure off-set just before the output cap.

See pic.
 

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6L6

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I have to be doing something fundamentally wrong, or misunderstanding something. I'm operating under the impression that I have to adjust P1 to get 50mA at R10 , while adjusting P2 to get 0 volts DC at the output. The moment I remove the meter from R10 offset - being measured here as DC volts between the signal and ground at the actual RCA output - jumps to nearly 2 volts then very slowly heads back towards 0. What am I missing? Am I measuring bias or offset at the wrong spots?

Aha! Yep, you're not setting it correctly.

(And looking at my post on setting things, I think I see the confusing bit... Sorry...)

This circuit is a teeter-totter, and it's not as simple as P1= bias and P2= offset. They work together to get the job done, and are both responsible for bias and offset.

But, before that, let's state the goals of adjusting the pots -

1)You want to measure 1.0VDC across both R10 and R11

2)You want to set that in conjunction with 0VDC (or as close as you can get) measured from the "top" of the output capacitor to ground. (I.E., the input side of the cap, as the cap should always block the DC on the output.)

It's best to do this with 3 meters, one across R10, one across R1, one from the cap input to gnd.

Stating with the P1 and P2 to zero (And P3 set to it's middle and then ignored for this entire procedure)

Increase each pot an even number of turns until you see some voltage across the resistors.

Decrease or increase one of the pots (doesn't matter which) to zero the offset. -- this will take a little of the bias voltage away; that's normal.

Increase the pots again so you show more voltage (go in 0.2V steps or so) and then zero the offset again. It will feel like two steps forward, one step back, but again, that's normal.

Continue until you have 1.0V across R10 and R11 with zero offset.

Check your PM
 
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It's working!

Thank you for all your help, everyone - especially you Jim! You are all gentleman, and scholars. It's up and running and sounds sooooooo good. And I didn't even get electrocuted in the process ;). I'm certain my old CT-1000's have never sounded better. Amazing for 30 year old speakers. They show no signs of needing more wattage, even at irresponsible levels. I ended up having to back the bias off a smidgeon on the F4 after getting it situated in its final spot. Couldn't have had my hands on those sinks much past 5 seconds. Now all I have to do is tuck away all the wiring before the wife and kids get back in town.

Thank you much!
 

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OT - Kinda...

Not owning a scope or a distortion analyzer, I've been hoping to be able to see the P3 adjustments with the Visual Analyzer software. With some help from Terry McGee, the setup does respond to P3 adjustments on the spectrum screen.

My build has single turn trimmers in those two spots so the swing is large. Configuration is:

Laptop Spk out to BA-3 in.
BA-3 Spk trmnl to computer in.
1K sine wave generated by VA.
Notch filter applied.
Direct feed on Wave Option page.

Similar to Jim's video, maximum effect (~8:30) can be passed causing a reversal (~7:00). Passing back through the centered position all the way to 4:00 has only a minor effect.

It's certainly not pro lab quality in this test, but fun and encouraging as members dig into the new version of Visual Analyzer. Later this week I'll put in multi-turns at P3 and do the adjustments with speakers attached. ;)

VA/BA-3 Adjustment Video
 

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before ordering parts--
and without going into (important!) but often subjective theorizing about sound quality--

has anyone summarized the technical build differences
(power supply range (V or otherwise), output power range, parts cost, general finickyness),

of BA-3 FE + BA-2 output stage vs BA-3 FE + crippled F4?

even a simple "NO" would be appreciated as much as a link to a previous thread entry,
thanks,
 
many thanks for info/time,

I got caught up in difference between source resistors/schottky model nos,
knew I didn't know (useful, even if Rummy is an *ss)
what specs to look for on schottky data sheets, or
whether or not I was missing other important details in bias board.

ran across BA-2 OS months after poking around for days
on old "define crippled F4" posts (several threads)--

saved links, but didn't stop to make notes of cumulative details (my fault).

moving like a sloth (and looking at all options) until more money is nailed down,
but it's damn fun just to read above what I can understand,
good excuse to spend hours trying to grasp things like "cascode follower",
"long-tailed pair", "which schematic is this guy looking at", and generally
working the "down" side of "trickle-down" sport reading as best I can (and it's cheap).

Thanks again to both of you,
 
Questions about the transistors for this project -the answers may be somewhere in the threads, but I'm afraid I couldn't fine much info... :scratch2::

Anyone on the forum still selling the Toshiba Jfets or/and mosfets for this? Maybe matched?

Also, is there any consensus about the sound signature of the Linear Systems Jfets compared to the Toshibas?