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Old 31st March 2013, 06:17 PM   #271
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Some of the resistors in the schematic, like R47/46 (volume), R36/37 (bsc), or R49/53 (LF level) are actually potentiometers. The resistors around them are there so I can use lower value pots, limit the pots' range and hopefully increase precision and temperature stability.

I'm also sorry for the awkward part numbers.

I'll add more gate stoppers, I was hoping that with keeping paths as short as possible I could without them.
And maybe I'll admit defeat and add a 10u cap at the LF output...

Oh, and I attached what I hope is a less fuzzy version of the schematic, same awkward part numbers though.
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Old 31st March 2013, 06:28 PM   #272
Zen Mod is offline Zen Mod  Serbia
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Pa speaket

(moi didn't looked at these details ....... moi dumb )

edit - Dave posted while I was editing his schm ...... but I think it will be useful
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Old 1st April 2013, 02:00 PM   #273
Kreisky is offline Kreisky  Italy
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... it would be so nice if one day someone make a PCB of these lovely XO ,
for newbies like me

Bye bye,

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Old 1st April 2013, 07:05 PM   #274
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Thanks Papa Pass! The gate stoppers did the trick!

The circuit is up and running, and the output offsets are now all close to 1mV (-0.6mV/1mV for the HP, -1.6mV/0.9mV for the LP, left/right channel). I have it playing with the input volume control at 12 o'clock right now, so there is enough gain in my system it seems, no additional gain stages required

The final circuit is like the one I posted earlier, but with 221R gate stoppers at each and every gate. And one 2200uF 50V Panasonic FC capacitor per rail for decoupling and peace of mind.
To avoid potential ground loops, the XO's ground is only connected to the DCB1 at one point, namely through the cables coming from the power supply. Since the circuit is biased in class A I see no potential harm in doing so.

It may not really look like it, but this may be the quietest circuit I have ever built. Even with getting the ear right up to the speaker, one can barely tell if the B1XO is on or off.

And the sound/performance is everything I had hoped for. This circuit would definitely be worthy of a real proper pcb! And a lot less work too...

So, thanks guys for putting this together, and also for helping me with troubleshooting!
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Old 1st April 2013, 11:01 PM   #275
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Well done!
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Old 2nd April 2013, 07:39 AM   #276
AndrewT is offline AndrewT  Scotland
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Quote:
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.........The low pass multiplies all the offsets from each stage, resulting in a net output offset.
Quote:
Originally Posted by Rodeodave View Post
Thanks Papa Pass! The gate stoppers did the trick! ........... and the output offsets are now all close to 1mV (-0.6mV/1mV for the HP, -1.6mV/0.9mV for the LP, left/right channel).................

The final circuit is like the one I posted earlier, but with 221R gate stoppers at each and every gate. And one 2200uF 50V Panasonic FC capacitor per rail for decoupling and peace of mind...........
So nothing to do with accumulation of offsets.
It was jFETs lacking gate stoppers.
Thanks for the warning.
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Old 2nd April 2013, 09:00 AM   #277
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Quote:
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So nothing to do with accumulation of offsets.
It was jFETs lacking gate stoppers.
Thanks for the warning.
Some additional detail on that:

Like I said, the problem was mainly present in the low pass (LP) section. It didn't matter if I had the circuit powered from a somewhat wonky regulated benchtop PSU or the DCB1's regs, the output offset of the LP was somewhere between +100mv to +150mV. There also was a 5mVpp high frequency fuzz going on, but I didn't pay too much attention to it. the high pass section was doing fairly well. At this point the circuit was exactly like in post #271, with gate stoppers only at points I thought were crucial. The decision to do so was partly motivated by your post #214, AndrewT

What was suspicious though was the fact that the output offset changed with the orientation of wiring and touching the cases of the caps. It also changed with waving the hand over the circuit, definite red flag. In hindsight this makes sense I guess, since, without gate stoppers, the JFETs in the LP pretty much only see a capacitance to ground, and this high impedance will cause problems no matter how short the wiring is.

With gate stoppers for each and every gate the circuit is stable and very, very quiet.

So, the lesson, dear reader: Make liberal use of gate stoppers.


What is being achieved by that? As can be seen in the 2sk170's datasheet, at V_DS=10V the input capacitance C_iss is 30pF. Together with the 221R gate stopper resistor this is effectively forming a high pass with -3dB at f=1/(2pi*R_gate*C_iss)=24MHz. That's unexpectedly high.
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Old 2nd April 2013, 09:16 AM   #278
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And my post214 conveniently does not even mention gate stoppers.
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Old 2nd April 2013, 09:52 AM   #279
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...

A filter feeding into B1 should not have anything between the filter output and the very high input impedance of the B1. Do not add any of the Rin setting components and/or band limiting filters that are used in a standard pre-amp/buffer.
I guess I read that as "just use a pair of naked JFETs". Clearly didn't work.

Use your gate stoppers, folks.
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Old 12th April 2013, 10:43 AM   #280
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I don't have enough 2SK170's to build the crossover, would there any disadvantages in building the buffers with BF862 other than fiddeling with SMD.
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