VHS Hiss issue

[SimonS]

I've just finished building a VHS and it has a background hiss/white noise. This happens regardless of whether it is supplied from a 230vAC>9vDC wall adapter or a 9v battery.

The noise is present with any or the three effects and when the boost is selected the volume of the hiss varies with the hiss. Each of the effects does work normally but with this background hiss.

There is no hiss on Bypass.

Shown below are three traces. The first is at the input jack. The second at the input (pin 1) of the FV-1 and the last at the output (pin 28).

Is this likely to be a faulty FV-1 or one of it's associated components?

 

[Chuck D. Bones]

Can we see the FV-1 output with a 1ms/div timebase?

 

[SimonS]

Here you go. Pin 1 and pin 28 again.

 

[zgrav]

Do the readings at pin 28 change when you switch the chorus and reverb in or out of the circuit?

 

[SimonS]

No.

 

[SimonS]

To clarify, the hiss is not present when the effect is bypassed.
If the circuit is engaged with the FSW while the 3 effect switch are off the hiss is present.
If the Chorus and/or reverb are switched on there is no change in the hiss nor is there any change with movement of the chorus or reverb knobs.
If the Volume is turned on then the hiss is the same with the control at min and increases with clockwise movement.

 

[zgrav]

Here are some other things to try --- If you socketed the eeprom, try removing it and see if that changes the measurements you are getting at pin 28. You could also switch the FV-1 to its internal program and see if that changes the hiss you are getting (you can search the forums for how to do that since I do not recall right now). You can confirm you have connectivity between pin 1 and pin 2 for the FV01, and check to see if you get the same reading on pin 2 that you get on pin 1. Also check and see what you are getting on pin 27 for the FV-1, since that is a separate output.

People have different experiences with FV-1 builds, maybe some of the FV-1 chips are noisier than others -- check out this thread: https://www.madbeanpedals.com/forum/index.php?topic=28893.0

 

[SimonS]

OK, i'll have a read in the morning.

 

[Chuck D. Bones]

FYI, the FV-1 uses delta-sigma modulators as the A/D & D/A converters. It is normal for there to be a certain amount of noise coming out of the device, but it should be at a high freq that is filtered out by subsequent circuitry outside the FV-1. Can you check the clock freq on pin 9?

 

[SimonS]

Right in reverse order...

Pin 9 Freq 32.861khz withe a Vpp of 0.9v

No changed with the eprom removed.

Freq and Vpp on each pin

1 0/0
2 0/0
3 0/0
4 0/0
5 88/4
6 4.9/2
7 0/0
8 12.5/3
9 32.9/900
10 32.9/667
11 0/0
12 0/0
13 vrb/2
14 vrb/2
15 98.6/4
16 17ish/2
17 vrb/1
18 0/0
19 0/0
20 vrb/2
21 60ish/4
22 0/0
23 vrb/2
24 0/0
25 0/0
26 0/0
27 67.6/15
28 67.9/18

Notes:
All freq's in kHz and Vpp in mV
Measured with the three switches in the off position (towards the foot switch).
Those freq's listed as 'ish' or vrb were varying +/- 30k or more (basically, the higher the Vpp the more defined the freq is).

Edit to add that pin 5 of the TL072 is 67/12 and pin 7 gives 67/10 as does the output jack.

 

[Robert]

The VHS is noisier than all of the other FV-1 designs here. I've compared it against a JHS VCR and the VCR was just as noisy, if not more so.

This probably has a lot to do with the fact that there is no analog dry signal path, all audio passes through the FV-1 IC, even when in clean boost mode with no other effects active.

 

[zgrav]

SimonS -- thanks for the measurements from your board. I don't recall posts from other folks about the VHS that mentioned hiss.

So the VHS in all forms uses the 100% wet signal because there is not dry signal. By comparison, in most cases the dry signal would be as loud or louder than the audio from the FV-1, making the hiss sound relatively smaller at the output.

Is there still likely to be some variation in the amount of noise from one FV-1 chip to another? For comparison, I have had a lot of variation in the noise level I get from PT2399 chips.

 

[SimonS]

OK thanks for that.

I can say that this is the second VHS that I built and this one is considerably noisier than the first.

I did try this afternoon to swap the FV-1 from my Hydra. The one in the Hydra came out ok but the VHS pulled a pad so that board is now in the scrap bin. I'll wait for the restock and order another.

 

[Chuck D. Bones]

Here are two screenshots from my Arachnid. They show the same signals at two different timebases. Blue trace is measured at the Output jack, green trace is measured at pin 28 of the FV-1. MIX is set for 100% wet, VOLUME is unity, MODE is set to Dual Pitch Shift. This is what I would expect to see. The noise on pin 28 is quantization noise and is inherent to any analog / digital conversion. Delta-Sigma modulators generate their quantization noise at high frequency, away from the audio band. This makes it easy to filter out. The difference between the two traces is due to three low-pass filters. The VHS has two low-pass filters: R20-C11 and C13-VOLUME control.
My Arachnid pedal is very quiet to the ear.
The FV-1 is essentially 100% digital inside. The factory should be detecting and rejecting any chip that isn't 100% perfect. If one chip is noisier than another, that makes me think the problem is at source of the chips. Someone could be selling either fakes or rejects.

Any possibility that the VHS code deliberately injects noise for a more "vintage" sound?

 

[Etherealreaper]

So I was having a hiss issue really bad with a few fv1 pedals and had not realized that the 3.3v regulators i was using were bad, they were putting out 6.6v I changed it out and the hiss was gone. Just something to check.

 

[Chuck D. Bones]

You are lucky that didn't kill the FV-1. Where did you buy them?