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Voltage regulator bypass questions
Voltage regulator bypass questions
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Old 28th November 2019, 10:05 PM   #21
MarcelvdG is offline MarcelvdG  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.

Jan
Using an LM317 or 78XX, it can be a simple way to get rid of regulator noise though, if the decoupling capacitance is large enough.
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Old 29th November 2019, 07:48 PM   #22
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.

Jan
Okay, so do you think I could/should bypass R65 (especially with the LM317 regulator circuit replaced by the LT3045 board)?
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Old 30th November 2019, 03:53 PM   #23
Abtr is offline Abtr  Netherlands
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I did some blind listening tests with the crossover power section based on LT3045 versus LM317 and identified the correct chip 7 times out of 7 tests (source was an RME ADI-2 DAC). I really wonder if I can further improve the original power supply of the crossover boards:

Click the image to open in full size.

I already removed C15 and C65 and shorted pin 2 and 3 of the LM317 regulator for use with the LT3045 regulator board. Possibly R65 can be bypassed and the other caps may be removed or be replaced by a single cap. Each crossover board has three OPA1654 quad opamps to feed:

Click the image to open in full size.


Any insight will (again) be greatly appreciated!

Last edited by Abtr; 30th November 2019 at 03:58 PM.
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Old 30th November 2019, 04:14 PM   #24
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by MarcelvdG View Post
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).
I think what would happen if the '317 runs out of headroom is that the pass transistor will be driven fully open. So effectively you have a low impedance, 0.3V or so 'battery' in the line path. Should not do much harm, but no good either.

Jan
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Old 30th November 2019, 04:44 PM   #25
MarcelvdG is offline MarcelvdG  Netherlands
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At page 4 of https://www.st.com/resource/en/datasheet/lm317.pdf you can see the internal schematic of an LM317 and at page 8 a graph of the dropout voltage versus temperature with current as a parameter. It has a Darlington NPN output stage, so the voltage drop is around 1.5 V at low currents (20 mA) at room temperature. At 1 A it is around 2 V. In any case, we agree that it doesn't do any good when it is connected this way.
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Old 30th November 2019, 05:09 PM   #26
Abtr is offline Abtr  Netherlands
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LM317 set to 15V output and fed 15V from the LT3045 board degrades sound quality relative to when LM317 is removed from the circuit (pins 2 and 3 shorted).
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Old 1st December 2019, 11:22 AM   #27
Abtr is offline Abtr  Netherlands
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It's a petty Xkitz don't respond to my query about the series resistor and output caps configuration. I found many LM317 regulator diagrams with different (parallel) caps (see e.g. attached examples), but none have a series resistor.

I may just try bypassing R65 and remove all output caps (except perhaps C16) for use with the LT3045 board and then see what happens..
Attached Images
File Type: png LM317(0).png (83.3 KB, 75 views)
File Type: png LM317(1).png (5.6 KB, 74 views)
File Type: png LM317(2).png (9.4 KB, 75 views)
File Type: png LM317(3).png (7.9 KB, 29 views)
File Type: png LM317(4).png (31.5 KB, 30 views)
File Type: png LM317(5).png (6.6 KB, 22 views)
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Old 1st December 2019, 03:14 PM   #28
jan.didden is offline jan.didden  Europe
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Throw away R65. It's a misguided attempt to improve' something the wrong way.

Jan
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Old 1st December 2019, 03:45 PM   #29
Abtr is offline Abtr  Netherlands
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Quote:
Originally Posted by jan.didden View Post
Throw away R65. It's a misguided attempt to improve' something the wrong way.

Jan
OK, thanks. I'll try bypassing R65. Can you elaborate on what might be the idea behind r65 (noise reduction)? And what about the caps after R65? Should I remove them for the LT3045 regulator board?
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Old 1st December 2019, 04:00 PM   #30
jan.didden is offline jan.didden  Europe
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Originally Posted by Abtr View Post
OK, thanks. I'll try bypassing R65. Can you elaborate on what might be the idea behind r65 (noise reduction)? And what about the caps after R65? Should I remove them for the LT3045 regulator board?
Someone *probably* thought that he/she could lower the noise after the regulator further. But trying that it would completely swamp the low output impedance of the regulator. Like, you have a reg with less than 1 ohm Zout, which means the output voltage it is rock-steady with all the variations in load current that happen while amplifying a signal.

Now you place a Very Large resistor in series, wasn't it 4.7 ohms? So if the load current varies, so does the supply voltage, undoing what the regulator was for in the first place.

Then you place some capacitance after that 4.7 ohms. Tat helps for low output impedance but such a cap has easily a larger ESR than the regulator Zout, and especially at higher frequencies ruins it because of parasitic inductance.

Such a capacitor after a regulator (without the R) is not there to lower the regulator Zout but only for stability; the regulator is 'better' than that cap anyway.

So such things as R65 + cap are done by people with enough knowledge to be dangerous ;-)

Jan

Last edited by jan.didden; 1st December 2019 at 04:04 PM.
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