Short story

I use the MP1584 to step down 5VDC to 1.5VDC to power an audio equalizer taken from a Panasonic walkman (RQ-P255) from the nineties. However this is working great, sound is great, it introduces a heavy whine and don't know how to avoid this. See also the schematic below of the device I have build. What can I do to remove the whine?

Long Story

I have build/want to build a Micro Portable Stereo Amplifier With Battery, Equalizer, BlueTooth And Line-In/Line-Out (in a small candy case) so you can connect any speaker you want (nice to use outside in the garden/party) and you can use it as a BlueTooth Walkman. I have added an equalizer taken from an old walkman of the nineties, the Panasonic RQ-P255. This because most bluetooth transmitters don't support an equalizer and, some bluetooth speakers supports only EQ with presets (hate it).

Figure out how the equalizer works and it was very straight forward but requires only 1.5VDC input (the whole device, all other parts, operates on 5VDC). Because of positive reviews of the MP1584, I use this little device to step down 5VDC to 1.5VDC. This is working great except the whine it produces, very annoying. The device has an equalizer on/off switch and when I turn it off, the whine is gone.

What can I do to remove the whine?

The whine i'm talking about

This is recorded with a microphone of a Tascam DR07 nearby speaker. The picture below is the waveform 1600% amplified.

Whine 1600% amplified in Creative Studio

Notice: I don't have an oscilloscope.

Schematic of the device

Notice: I'm not a hardware designer or schematic professional, so maybe the schematic is not as it should but I think it is very clear what goal I want to achieve. The headphones connection is not in the schematic. EDIT: 200K must be 200 Ohms

Schematic TinyHifi Walker MK I

Setup of the device

Here some drawings how the device must be set up TinyHifi Walker MK I Setup Guide

What I got so far

BlueTooth module and switch and headphone jack not implemented yet.

TinyHifi implementation so far

What it should look like when finished ;-)

TinyHifi Goal

Used parts/devices in current setup:

  • \$\begingroup\$ One thing I noticed while having a very quick look at the MP1584 datasheet is this phrase: High-Efficiency Pulse Skipping Mode for Light Load. If I understand this correctly, this will actually cause the switching frequency to drop unless there is enough load on the output. For 1.5 volt out, try connecting a resistor between 10 and 100 ohm(-ish, this does not matter much) and see if the noise go away. If it does, then you might have a problem, because the "fix" is to draw more current, meaning that your batteries will drain faster. \$\endgroup\$ – pipe Dec 12 '15 at 22:41
  • \$\begingroup\$ @Pipe, thanks. Maybe I have to switch to another DC/DC converter for lower loads. Any suggestions? \$\endgroup\$ – Codebeat Dec 12 '15 at 22:49
  • \$\begingroup\$ No, and you should not accept my comment at face value, because it's still just a guess, and I don't know anything about switch mode converters. :) Did you try it out? It would be interesting to hear if my hunch was correct, and it actually does remove the noise if you add extra load. \$\endgroup\$ – pipe Dec 12 '15 at 23:52
  • \$\begingroup\$ Didn't help, low resistance cause too heavy current and short protection in the first DC/DC converter will be triggered. Found another solution, I use a LDO (voltage regulator gs1117ax) instead of the MP1584 right now with a 100uf capacitor on the out line which does the trick very well, whine is almost gone, some white noise left (not heavy) but could be a problem of characteristics of the 'unknown' equalizer and the maybe the noise of the DC/DC converter in the USB charger. \$\endgroup\$ – Codebeat Dec 14 '15 at 2:37
  • 1
    \$\begingroup\$ I would assume an ancient portable EQ to be noisy. \$\endgroup\$ – pipe Dec 14 '15 at 9:45

Most DCDC converters run well above 20KHz at your power levels .You are getting Audible noise and I think that you should put the oscilliscope.You will probably see ripple artifacts on the 5V input and 1.5V output that are in the low KHZ range.When you scope the mosfet drain waveform you will probably see shimmering on the rising or falling edges depending on how you have got your scope triggering.Even when the DC input and load are constant the shimmering will persist.This low frequency crud cuts through the orthodox SMPS filter components like a knife through butter because those components are designed to attenuate the switching frequency.Experimentaly loading the converter may cure this because these low frequency instabilities tend to be worse at light load .If the load experiment nails this then you can increase the buck inducter and you will probably have silence or use a smaller buck converter which will in effect be more heavily loaded.If things get too bad consider linear post reg using a 1.5V LDO and setting the buck up at say 2.5V .

  • \$\begingroup\$ Ah, thanks for the answer! Appreciate it. Just edited the question with an example of the whine sound. I don't have a oscilloscope :-( I think you just want to say the converter is not intended for such low load? Just a few mA. What kind of DC/DC converter do you suggest (links or modelnumber)? Is there a board for the 1.5V LDO? \$\endgroup\$ – Codebeat Dec 12 '15 at 22:44
  • \$\begingroup\$ Use a LDO (voltage regulator gs1117ax FODE), found on a old PCB, instead of the MP1584 right now with a 100uf capacitor on the out line which does the trick very well, whine is almost gone, some white noise left (not heavy) but could be a problem of characteristics of the 'unknown' equalizer and the maybe the noise of the DC/DC converter in the USB charger. I think the equalizer board can handle 3.3V but sometimes the gain is too heavy, so want to bring it down to 1.5V. The documention explains to set a resistor on Vin and Vout but it's not clear to me what kind of resistor that should be. \$\endgroup\$ – Codebeat Dec 14 '15 at 2:55
  • \$\begingroup\$ Datasheet: secosgmbh.com/datasheet/products/LDO_Regulator/SOT-223/… \$\endgroup\$ – Codebeat Dec 14 '15 at 2:55

Your first task here is debugging/finding the root cause of your problem. It is most probably related to noise induced/coupled from one or both DC/DC converters.

First simple advice for you to try:

  • Disconnect both DC/DC converters.
  • Power your circuit with a decently cleaan Power Supply Unit. A simple low cost linear supply will do.
  • If the noise is gone, good news, now you can try to pinpoint which one of the converters is causing the problem.

After the debugging phase, you can move on to redesign part of your system in order to minimize or filter our the offending noise.

  • \$\begingroup\$ Thanks for the answer! Appreciate it. Like I said in the questiion, there is an on/off switch for the equalizer. When I turn it off, the whine is gone (still audio but not gained). Besides, the used amplifier is an class D amp that is very sensitive to EMI so I think the implementation before the second DC/Dc converter is okay. The problem is the second, the one I am talking about. I have already tried it with batteries directly connected to the equalizer board and there is no whine. \$\endgroup\$ – Codebeat Dec 12 '15 at 22:55

I would use just a linear LDO from the 5V or 3.7V (battery) like many suggested before, because the current consumption of the equalizer is too small to justify switching mode power supply. High sensitivity and high impedance preamps are generaly sensitive to perturbations. Event through the ground line noises may come and render the configuration useless... you have to always design the audio chain in such way that the ground line (and also +/- Vcc's) to be from the main supply to the speakers (if not BTL), than to the power amp, then to the potentiometer, then to the auxiliary supplies stabilisers and then to the preamps. From power to sensitive circuits; that is always the order.


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