Type of capacitors to use on audio path with VS1003

Designing hardware that uses VLSI Solution's devices as slave codecs such as an external MP3 decoder chip for a host microcontroller.
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Berry
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Type of capacitors to use on audio path with VS1003

Post by Berry » Sun 2019-05-26 4:09

hello

I’m working on an embedded project where I’ll be using the VS1003 MP3 decoder along with a class D amplifier to play back audio using a mono on PCB speaker in differential mode (SM DIFF), as well as an option to play through external speakers. I'd like to inquiry on what type of capacitors folks usually use for audio signal path. Looking at some of the reference designs for the VS1003, I see regular ceramic capacitors are used. From my research, it looks like film capacitors are recommenced for audio, as ceramic ones have strong non-linearity with respect to temperature, frequency and bias voltage. It turns out there’s class 1 C0G (NP0) ceramic capacitors that are much more stable than regular ceramic caps, so that’s what I plan to use. However, they usually come only up to 0.1uF. Based on the reference designs and recommendations for this chip, I needing the following caps (see attached circuit):

3n3
4.7n
10n
47n
1uF (not shown in the circuit, this is for RCAP)
10uF

For all of the above, except for the 1uF and 10uF, I’ll be using C0G ceramic capacitors. Since COG don't exist for 0.1uF and 10uF, I’ll be using a film capacitor for the 0.1uF and a electrolytic capacitor for the 10uF. I have the following queries:

1. What’s VLSI Solutions’ recommendation on capacitor types for audio path? Does my plan sound reasonable?
2. The 10uF caps for blocking DC usually have large tolerance, at +/- 20%. Is that a problem? What if the two differential signals are quite a bit off due to this large tolerance?
3. For the RCAP recommend cap of 1uF, it says here it’s okay to use 0.1uF with minimal impact: viewtopic.php?f=10&t=127&p=743&hilit=rc ... citor#p743
a. I may use 0.1uF or 10uF, instead of the recommended 1uF, depending on other cap requirements, since I have no need for 1uF caps anywhere else. Would using 10uF (10x higher than recommended) be a problem? How about 0.1uF?
4. Can any of the caps in the attached circuit be removed, are they all necessary?

Thanks!
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audio_circuit-Our Circuit - take 2.jpg
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Panu
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Re: Type of capacitors to use on audio path with VS1003

Post by Panu » Sun 2019-05-26 16:36

Hi!

The capacitors for audio output are generally not so sensitive as the capacitors for audio input. Your plan to use linear caps seems good, low ESR is a nice feature. But I fail to see the reason for the filters R5+C24 and R4+C27. Why do you have them? It would seem to me that the filters fro L to A and from R to B should be sufficient. Am I missing something?

For the RCAP, please treat it with the greatest respect. It's the most important cap of all. Don't use vias for the RCAP, really don't. Put the RCAP to top layer (the same layer as the VS10xx) and place it right next to the VS10xx so you can route the pins directly from the cap to the VS10xx on the top layer.

-Panu
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Berry
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Re: Type of capacitors to use on audio path with VS1003

Post by Berry » Mon 2019-05-27 0:26

Good to hear from you Panu and thanks for the feedback. I did a couple of posts before and didn’t hear from you so thought you maybe no longer with the company 😊

Regarding the “R5+C24 and R4+C27”, I got those by looking at the USB example here: http://www.vlsi.fi/fileadmin/software/V ... der_03.pdf, they were placed just before the amplifier. I wasn’t sure if they were required or not for my case, as I was trying to combine the circuit from that example along with your guidelines in this post: viewtopic.php?f=9&t=69 , since I couldn’t find one reference design that shows the caps and resistors required for my case (using both a PCB mono speaker and a mono line out). Would love to remove those if they serve no purpose for my circuit. Can you please confirm?

Can you also comment on the requirement on the cap value for RCAP, if using either of 0.1uF or 10uF would you expect to see issues?

Lastly, the large DC blocking caps (10uF) have large tolerance, at +/- 20% from all manufacturers it seems. Is that a problem, since L and R channels may see different tolerance as a result?

Thanks!

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Panu
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Re: Type of capacitors to use on audio path with VS1003

Post by Panu » Mon 2019-05-27 10:04

Berry wrote:
Mon 2019-05-27 0:26
Regarding the “R5+C24 and R4+C27”, I got those by looking at the USB example here: http://www.vlsi.fi/fileadmin/software/V ... der_03.pdf, they were placed just before the amplifier. I wasn’t sure if they were required or not for my case ... Would love to remove those if they serve no purpose for my circuit. Can you please confirm?
Ok, got you. Maybe you didn't look quite closely enough, the signal is taken from BETWEEN the R and C to the D class amp in that pdf... so it's in fact the same filter as R38+C1 in your schematic. Thus, the R38+C1 is enough. Take the signal from between C1 and C24 to the amp.
Can you also comment on the requirement on the cap value for RCAP, if using either of 0.1uF or 10uF would you expect to see issues?
Compared to 1uF, 0.1uF would have more noise and 10uF a longer settling time.
Lastly, the large DC blocking caps (10uF) have large tolerance, at +/- 20% from all manufacturers it seems. Is that a problem, since L and R channels may see different tolerance as a result?
No, just a slight variance of the cutoff frequency. But in reality, if the caps are from the same production lot, they're probably quite similar.

-Panu
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Berry
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Re: Type of capacitors to use on audio path with VS1003

Post by Berry » Mon 2019-05-27 23:30

Ok, got you. Maybe you didn't look quite closely enough, the signal is taken from BETWEEN the R and C to the D class amp in that pdf... so it's in fact the same filter as R38+C1 in your schematic. Thus, the R38+C1 is enough. Take the signal from between C1 and C24 to the amp.

You're right Panu, I overlooked that. I noticed the values for R and C in the USB example are quite a bit different than the reference design here: viewtopic.php?f=9&t=69 (1k vs 0.47K Ohm, 3n3uF vs 4n7uF). Can you explain why that is? The reference circuit implements a low pass filter (labelled “sigma delta”, cut-off freq ~10k Hz ) followed by a highpass filter (labelled “DC block”, cut-off freq 0.2Hz ). So for lineout the frequency range passed based on those values are 0.2 Hz to ~10K Hz.

Following the USB example, only the lowpass filter (“sigma delta”) is required for the amplifier since it’s differential but the lowpass filter cutoff frequency there is ~33K Hz. The audiable frequency range is 20-20K Hz. Can you please elaborate why the filter in the reference design only goes up to ~10k Hz, instead of to ~20K Hz? While at the same time, the USB example goes all the way to 33K Hz , way beyond the audible range. Is there any advantage to that?

I’m thinking of changing my ciruit to an R38 and C1 of 1K Ohm and 8nF, respectively, to get a cutoff frequency of ~20K, since my speaker can go to 20K Hz. Am I approaching the problem correctly and do you see any issues with that?

Thanks!

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Panu
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Re: Type of capacitors to use on audio path with VS1003

Post by Panu » Tue 2019-05-28 7:49

Hi!

You're right, it's about the cut-off frequency. The sigma-delta modulation noise is pushed to frequencies over something like 100 kHz. These must be suppressed at the sampling frequency of the D class amplifier. If they're not, then the noise is aliased down to all frequencies and a hiss sound will occur. The lower the cutoff frequency, the safer it is regarding different types of D class amplifiers. Different designers have made different choices in regards of which amplifier they use, and what kind of bass/treble response they are aiming for.

I would play it safe, choose a lower cut-off frequency and increase treble via software, since the VS1003 has built-in bass/treble adjustment.

-Panu
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Panu
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Re: Type of capacitors to use on audio path with VS1003

Post by Panu » Tue 2019-05-28 11:07

Hi!

I checked out your amp IC's datasheet, please see page 5:
The 30-kHz low-pass filter is required even if the analyzer has an internal low-pass filter. An RC low-pass filter (1kΩ,
4700pF) is used on each output for the data sheet graphs.
I think that advice could be good for that IC. But if you experience noise in your final circuit, then increase the C or the R.

-Panu
Info: Line In and Line Out, VS1000 User interface, Overlay howto, Latest VSIDE, MCU Howto, Youtube
Panu-Kristian Poiksalo, VLSI Solution Oy

Berry
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Re: Type of capacitors to use on audio path with VS1003

Post by Berry » Wed 2019-05-29 7:43

Hi Panu

That makes sense, thank you for the explanation. Based on this feedback, I'll go ahead and change to the higher cutoff freq (1K Ohm & 4.7 nF) as shown in the updated circuit attached.

Thanks!
Attachments
audio_circuit-Our Circuit - take 2.jpg
audio_circuit-Our Circuit - take 2.jpg (81.66 KiB) Viewed 498 times

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