Hiss on FV-1 circuit

[bal704]

I'm building a Jed's Peds More!More!Moore! pedal.  This is a reverb/chorus/boost pedal using the FV-1 chip.

https://www.jedspeds.co.uk/product-page/more-more-moore

I've got it built and working, and it sounds great.  However, there is a fairly loud hiss that acts as a noise floor that really makes it hard to use.  The hiss is there regardless of whether the fx are engaged or not, and the hiss doesn't change when the guitar volume is at 10 or zero.  Kicking on the boost makes it a bit louder, but not much.

I've been building pedals and tube amps for years, so I know how to use an audio probe.  However, I've never built a pedal using the FV-1 chip.  I'm used to following the audio thru the circuit in a linear fashion, but it's not clear to me how the audio flows inside the FV-1 chip.

Using the audio probe, I have no hiss on pin 1 (input), and I have hiss on pin 28 (output).  It's hard to put the probe on the leads of the FV-1, but so far I haven't been able to find any audio on the pins to check for hiss.  Any tips on which pins (and components) to look at for the source of the hiss?

[niektb]

I wished the designer had tucked in some 100nFs on every power pin...
But how are you prototyping? Do you have it on a breadboard? A breadboard acts as an antenna so noise is failry common on a breadboard

[garcho]

Double check R8, C11 and C13 values.

[bal704]

This is a PCB/parts kit I bought from the vendor, so there's no prototyping.  He soldered the FV-1 for me, since I have no experience with fine pitch surface mount IC's.  He also provided the firmware chip.  I checked every value of every part before I installed it, and they were all correct.

I don't have the equipment to check the Y1 crystal however.

[ElectricDruid]

You could check C6 and C9 are soldered ok, correct way around, etc. Those two caps help keep internal voltages in the FV-1 quiet, so they're important.

Aside from that, there isn't much going on beyond firmware. Is 15pF the right cap for the crystal you're using?

I agree with Niektb. A few 100n's wouldn't have done any harm.

Try and get a good signal level going into the pedal so that the noise isn't so significant at the output, and so you don't need to boost it so much subsequently.´(which boosts the noise too, of course). It's possible that the problem is in the coding. It's pretty easy to introduce unnecessary hiss in an FV-1 program by reducing levels and then boosting them again.

[bal704]

C6 and C9 are oriented correctly.  I don't know much about the crystal I'm using.  There was no part number provided on the BOM, which is the main reason I bought the kit rather than source the parts myself.

What should the correct value be?  I have a 47pf I can jumper in parallel to increase the value  if needed.

I'll try a boost later to see if that helps....

[ElectricDruid]

If the cap you used is the one they supplied for the crystal they also supplied then we assume it's the right value for that crystal. If the chip runs ok then it sounds like it's working.

There isn't a single "right value" - it depends on the type of crystal. Values from few-pFs to 47p are typical.

[bal704]

I used my Barber Gain Changer as a clean boost to see if that helps.  The hiss was still there, no improvement.  When this pedal is bypassed, the same amp is dead quiet, no hiss.

I posted this question on another site, and somebody mentioned checking the circuit after pin 28.  I'm having trouble figuring out how that is possible, but maybe there's something I don't understand?

[garcho]

I posted this question on another site, and somebody mentioned checking the circuit after pin 28.  I'm having trouble figuring out how that is possible, but maybe there's something I don't understand?

That's what I was saying, check the low-pass filtering after the output of the FV-1.

[Digital Larry]

IIRC some suppliers have "bad batches" of chips, date code might illuminate that if Frank is following this.  Otherwise post something over at the Spin forum.

[niektb]

What kind of capacitors are C9 and C11?
If these are MLCCs then throw them out and put something in that doesn't suffer from the DC Bias Effect 

[bal704]

C9 is an electrolytic and C11 is a box type cap.

Looks like a date code of '2035' on the FV-1.

Regarding the low pass filter, I'll check that as well.  However, if I'm getting hiss on pin 28, what does the LP filter have to do with that?  is the LP filter there to get rid of the hiss?  Why does the hiss exist in the first place?

I've already reflowed the solder joints, but I'm going to re-reflow them to see if that helps.

[bal704]

So I touched up the solder joints, again, and looked it over with my magnified phone app.  I didn't see anything that stood out as a problem.  Hiss is still there.

[niektb]

The low-pass filter doesn't change the hiss if you measure directly at pin 28, what counts is if the hiss is reduced áfter the low-pass filter

[bal704]

I received a tip on another forum.  I did this, and the hiss was still there.

You may have just bad luck with that particular FV-1. There's one test you could try which is to eliminate the actual programmed patches as the noise source (IOW, if the algorithm itself had too much output or something else). Bit of a long shot but it will eliminate one possibility.

To do it, remove the EEPROM (U3 on your schematic) and temporarily ground pin13 of the FV-1 (this is attached to the 10k). This will enable the internal FV-1 programs. You can check those to see if you get the same hiss out of pin28 output.

[ElectricDruid]

You said initially that it was "fairly loud hiss". Is there any way we can quantify that?

[bal704]

Not sure how to quantify it.  I suppose any noise is subject to personal opinion about whether it's acceptable or not.  When I powered it up for the first time, I immediately knew something was wrong, as the hiss was loud enough to notice it.  If it had been a tube amp build, I would be looking at different tubes, caps etc.

[garcho]

take some pictures of your build, record some audio examples, did you ask Jeds Peds?

[bal704]

I talked to Jeds Peds, and his advice was to look in the middle of the circuit, which is what led me here.  I wasn't sure how to look into the audio path through the FV-1 chip.

Not sure how to post audio files.  Is there a hosting site, or just attach an MP3 file?