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Voltage regulator bypass questions
Voltage regulator bypass questions
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Old 23rd November 2019, 09:05 PM   #11
Abtr is offline Abtr  Netherlands
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Quote:
Originally Posted by sgrossklass View Post
You may want to hunt down the crossover's obvious PSRR problem then, in case you're not just fooling yourself that is. I see it seems to be a single supply job - how are they generating their Vcc/2 reference? That would be my first suspect. The circuitry at the input looks like maybe a cap multiplier and some passive RC, but I don't see where the rail splitting occurs. Maybe it involves C20? If the circuitry is inverting like I suspect it is, check where the noninverting inputs go.
Attached is the complete schematic (also see user manual)..
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File Type: png Xkitz Xover-2 v5.2.png (144.5 KB, 252 views)

Last edited by Abtr; 23rd November 2019 at 09:12 PM.
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Old 23rd November 2019, 09:14 PM   #12
Rick PA Stadel is offline Rick PA Stadel  United States
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Another possible reason to avoid generalized capacitor yanking -- at least two of those 1uF jobbies, C63 and C64, are local supply bypassing for a quad opamp.

Since this piece is single supply, I'm surprised any *interest in capacitors* doesn't begin with the coupling caps.

Cheers, Rick

Edit: oooh, yaay, complete schematic ..
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Old 24th November 2019, 12:18 PM   #13
Abtr is offline Abtr  Netherlands
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It seems something was upsetting the stability of the LM317 regulators. The much improved sound quality with LM317 bypassed is compared to the sound quality in the situation with the on-board power supply section intact and output voltage set to 15V (which is fixed and can't be adjusted in version 5.1 of the crossover boards which I use) while being fed by the 15V output of the LT3045-A board, which in turn was fed by a 17.5V LPS.

The idea was that without headroom for the LM317 to regulate the voltage (15V in and 15V out) it would be effectively out of the circuit. Apparently it doesn't work that way and shorting pins 2 and 3 results in a much better sounding crossover.

To be sure I ordered another pair of crossover boards. I'll set them to 15V and try them without LT3045-A and feed them directly with the 17.5V LPS. I used them this way in the past and although it was an improvement over the built-in crossovers of my subs I can't remember them sounding as good as they do now. But we'll see..
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Old 24th November 2019, 12:27 PM   #14
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by jackinnj View Post
I thought it might be instructive to put into a picture what PRR and others have related. C3 is "parameter selected" for 1pF or 100nF. All the other values are per the datasheet. (with a little parasitic inductance thrown in for good measure). The dark blue line shows the effect of the additional capacitor -- an additional impedance peak with Q~=2.9, so phase margin is only 18 degrees.
Very instructive Jack!

Jan
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Old 25th November 2019, 05:04 PM   #15
Abtr is offline Abtr  Netherlands
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I received the new crossover boards and fed them directly with the 17.5V LPS. The sonic difference is evident. The modified boards with LT3045 regulator and LM317 out of the way perform much better. The improvement is not subtle. The modified boards clearly sound more refined and less fatiguing.

I might make some ADC captures in Audacity and compute an FFT of the difference..

Last edited by Abtr; 25th November 2019 at 05:09 PM.
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Old 28th November 2019, 08:03 PM   #16
Abtr is offline Abtr  Netherlands
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So I asked Xkitz about the function of the parallel capacitors after R65. Still no reply. I guess C16 may be there for better transient response and the 0.1uF caps and C22 may lower overall ESR. I wonder if I can replace them by a single low ESR cap.

Given that different parallel output caps can oscillate and affect sound quality, could it be that the capacitor stack was designed for a specific audible effect (possibly related to the LM317 voltage regulator)?

Click the image to open in full size.
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Old 28th November 2019, 09:17 PM   #17
Mark Tillotson is offline Mark Tillotson
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Capacitors are paralleled for lower ESR and ESL, and often to allow physically smaller capacitors to be used (don't need mechanical support)
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Old 28th November 2019, 09:46 PM   #18
jan.didden is offline jan.didden  Europe
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R65 is an abomination. First you invest in a low Zout regulator and then you throw it away with R65.

Jan
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Old 28th November 2019, 09:55 PM   #19
Abtr is offline Abtr  Netherlands
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Quote:
Originally Posted by jan.didden View Post
R65 is an abomination. First you invest in a low Zout regulator and then you throw it away with R65.
Well, it's only 4.7 Ohm..
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Old 28th November 2019, 10:03 PM   #20
MarcelvdG is offline MarcelvdG  Netherlands
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Quote:
Originally Posted by Abtr View Post
The idea was that without headroom for the LM317 to regulate the voltage (15V in and 15V out) it would be effectively out of the circuit. Apparently it doesn't work that way and shorting pins 2 and 3 results in a much better sounding crossover.
That won't work well because an LM317 has a dropout voltage, so you still get a few volts of drop across the LM317 (precise value depending on current, temperature and what not). The reason Jan recommended it is that he was under the false impression that you would use the LM317 as a preregulator (unregulated supply - LM317 set to 16 V or 17 V or so - LT3045 set to 15 V rather than unregulated supply - LT3045 set to 15 V - LM317 set to 15 V).
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