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Reduce SMPS ripple noise for PreAmp?
Reduce SMPS ripple noise for PreAmp?
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Old 18th January 2018, 07:52 AM   #21
forestsgump is offline forestsgump
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Quote:
Originally Posted by trobbins View Post
... The simplest test for identifying if your preamp power supply is contributing noise/hiss that you can hear is to temporarily use a battery power supply for your preamp...
Appreciate your suggestion, am supplying fixed 32v from meanwell to this China-made amp; I would have difficulty supply this voltage on batt, in order to make comparison definite conclusive.
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Old 18th January 2018, 07:53 AM   #22
forestsgump is offline forestsgump
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Quote:
Originally Posted by tomchr View Post
... The switching frequency of the Mean Well units tends to be around 70 kHz. You can attenuation that with a well-designed CLC filter. Make sure to limit peaking in the filter as that will boost the noise (due to the resulting higher supply impedance) rather than suppress it.

One way to determine whether your preamp or power amp is the dominant noise contributor is to use a shorting plug on the power amp. Short its input. Measure the output noise. Then connect the preamp with the volume turned all the way down (or a shorting plug on the selected input). Measure the output noise. If the noise increases significantly when you attach the preamp, the preamp is dominating (or at least contributes as much as the power amp to the noise level).

Tom
Appreciate sharing your much needed experience, wonderful suggestion!
Got some ques. for you:

- this China-made amp, has both pre-amp NE5532 & Class D TPA3250 on the same PCB; doesn't shorting inputs of TPA3250 equivalent to shorting outputs of pre-amp NE5532 & result in damaging to pre-amp?

- does meanwell ripple noise will have any other impact to DAC's SQ, besides also introducing noise?
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Old 18th January 2018, 12:55 PM   #23
sarathssca is offline sarathssca  India
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Here one example of noise results for smps used in preamp/headphone amp:
JFET-MOSFET headphone amplifier J-Mo 3

and info regarding smps usage in audio:
SMPS for Pass A3

Last edited by sarathssca; 18th January 2018 at 12:58 PM.
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Old 19th January 2018, 06:51 AM   #24
forestsgump is offline forestsgump
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Quote:
Originally Posted by sarathssca View Post
Here one example of noise results for smps used in preamp/headphone amp:
JFET-MOSFET headphone amplifier J-Mo 3

and info regarding smps usage in audio:
SMPS for Pass A3
Thks for the lead, any diagram, actual values of component & connection how it is done?
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Old 19th January 2018, 11:47 AM   #25
SemperFi is offline SemperFi  Wake Island
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So the switch frequency is 70kHz, we think. Do u know the Iq of the ckt it will supply?

The 70kHz means u wont be able to hear it, but 200mV ripple may perhaps create some unwanted effect...? Anyways u want less ripple.

The SMPS is likely switching with shorter than 50% duty cycle pulses and the rise/fall times of the switch current is likely short. Guessimate 10% of Tp (Tp is 14us (70kHz)) so 1.4us.

There is some Rout of the SMPS, and u probably have some wiring, so there is some more impedance. U want 10mV ripple? Lets pretend Iq is 50mA. C=I*dt/dv = 50m*1.4u/10m = 7uF.
Choose 10uF. Go crazy and add 10ohm resistor between two caps, CRC, and u should see even less ripple.
Electrolytic with close leads, say 5mm, will do (Big electros are too slow). Using ceramics u need to use >> 7uF b/c they drop in capacitance as voltage is applied.
Layout (where u place the caps) is important. Think smallest possible loop for supply and return of current in/out of cap.
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Old 19th January 2018, 01:49 PM   #26
forestsgump is offline forestsgump
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Appreciate your coming in, on the actuals!
Is my understanding correct?

R1=10ohm (0.25W?), C1=C2=20uf cermaics/10u electrolytic.

Just curious, how is 70kHz determined? I happen to use Meanwell's LRS150
Would hope go below 1uV ripple
Does Iq relates to operating or quiescence current of PreAmp/DAC?
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Last edited by forestsgump; 19th January 2018 at 01:55 PM.
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Old 19th January 2018, 08:42 PM   #27
martin clark is offline martin clark  Europe
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Adding to what I've already posted, and Tomchr's comments: you just don't have to reinvent the wheel here!

Look at detailed references like the excellent Analog Devices 'Op Amp Applications Handbook (Walt Jung, 2005): it's available free, online, as a PDF and will reward close & repeated reading at any level.

Anyway; here's one suggestion it contains, in a section you really ought to read: the voltage noted is not important - how the idea works, is - it will work exactly the same, at 30v in, or 300v come to that:
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Last edited by martin clark; 19th January 2018 at 08:52 PM.
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Old 22nd January 2018, 02:11 PM   #28
SemperFi is offline SemperFi  Wake Island
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Quote:
Originally Posted by forestsgump View Post
Appreciate your coming in, on the actuals!
Is my understanding correct?

R1=10ohm (0.25W?), C1=C2=20uf cermaics/10u electrolytic.

Just curious, how is 70kHz determined? I happen to use Meanwell's LRS150
Would hope go below 1uV ripple
Does Iq relates to operating or quiescence current of PreAmp/DAC?
Tomchr mentioned 70kHz... I dunno, but it is high frequency ripple. Here's a suggestion. U dont want a very large C near the output b/c u dont know how the feedback loop inside the SMPS is... There should be a datasheet stating max C. If you buffer the output with some resistance or better a choke, even a CM choke, u can probably go much bigger with the C since u probably isolate the large C from the feedback loop inside the SMPS that way.
Ripple currents into the small cap can be high and with fast edges, so keep that first loop as small as possible.
How much C u need near your circuit being powered depends on it's output (load) currents and if they are pulsed as in a class-B type or if they are averaged to a constant value as in class-A. Treat the circuit being powered as if being fed with a high impedance power line, (current source), and the local caps should provide all currents for the signals. U can figure the slew rate if u know the load and signal amplitudes. Then figure out minimum C using C=Ic*dV/dt.
If u want to go advanced build cap-multipliers and cascade filters etc, but the impression is that u are not that experienced yet...?
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Old 22nd January 2018, 09:06 PM   #29
SemperFi is offline SemperFi  Wake Island
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Oops. The formula is C=i*dt/dv... sorry
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Old 24th January 2018, 08:09 PM   #30
jackinnj is online now jackinnj  United States
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Reduce SMPS ripple noise for PreAmp?
I've used the small, PCB mount Murata and Meanwell DC-DC converters -- in addition to the filtering on the output (or use of an LDO post regulator) you might find it helpful/necessary to put 100uH on the 5V input to the DC-DC converter. These devices radiate in both directions.

If Jan had some "Simple Switchers" available they'd be an excellent solution --
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