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?

Here's a link to the schematic....

[mjg]

I would think, with the FV-1 being digital, that you won't find any audio to probe on any pin other than 1 and 28.  I'd concentrate on the components after pin 28, maybe something is off there?

Seems odd that you still get the hiss when you have the effect bypassed.  If it's true bypass on your switching, then the effect shouldn't have any way to add hiss when things are bypassed.  Maybe have a close look at your stomp switch to make sure there is nothing shorted there? 

[bal704]

The pedal is setup so that it is hard by-passed with 3PDT.  No hiss.  When you engage the pedal, you can turn on reverb/chorus/boost in any combination, or no effect at all.  When the signal is going thru the FV-1 I get hiss no matter which effects are activated or not activated.

Just so I understand, if I'm getting hiss on pin 28, why would any components after that make a difference?

[mjg]

Ahh, ok, that makes it clearer.  I thought initially you were saying you got the hiss when completely bypassed.  That's not the case, so i guess the problem is most likely in the FV 1 chip or the code that it's been programmed with?

Did the build come with a pre programmed FV 1, or is it one you had to buy separate?  Did you need to program it yourself?

Anyway, I don't think you will find anything useful with an audio probe on any of the other pins, it's all just inputs to a digital system, which spits out the audio on pin 28. 

I guess you've checked the usual things like soldering, to make sure the chip is getting a good stable power supply and the crystal is connected properly?

[bal704]

I bought the kit of parts from the vendor.  He soldered the FV-1 chip on for me, and provided me the firmware chip (along with the other parts).

I've already reflowed the solder joints, but I'm going to check again.  I'm used to working on tube amps, and these connections are tiny....

[mjg]

I guess another thing to try would be to ask the vendor if it's meant to sound like that?  (I'm assuming it shouldn't, but hey, can't hurt to ask them!)

[bal704]

Yeah, I talked to the vendor.  It's not supposed to sound like that.  He suggested an audio probe, which led me to find out I have a clean input and a hiss on the output of the FV-1 chip.  Which led me to ask the question in this thread about tracking the audio through the FV-1 chip.

[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.

[madbean]

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.

[bal704]

I pulled the firmware chip, and grounded pin 13.  The hiss was still there.  The output had reverb on it btw.