Azabache White Noise Problem - Help?

[dancloud69]

Hi all,

I've been building pedals for a little less than a year now, and I am really happy with almost all of what I have built. I also have spent quite some time reading about the theory, or sorting JFETs, etc.

One of the pedals I really like is the Azabache from ROG. However, my build has a white noise problem similar to what nate77 describes in this thread:

http://www.diystompboxes.com/smfforum/index.php?topic=106485.0

I would not call the noise level crazy, but it is loud enough that it makes the pedal almost unusable for recording.

I've built the circuit on a perfboard using the original layout from ROG:

http://runoffgroove.com/azabache.html

I only had to spread the layout horizontally in a few places to make space for the wide FKP2 caps.

The components are 100% according to spec, only addition is a 100uF cap from collector of Q5 to ground, because I initially had some high frequency oscillation issue. The 100uF cap removed that oscillation (as described in the original Azabache thread in this forum).

The capcitors are all film (FKP2 and MKS2), the resistors are metal film. I already tried replacing the trim pots with fixed resistors, but the difference was neglectable.

The white noise is independent of the gain pot setting. During my debug sessions I noticed that it even stays about the same when I short pin2 of the gain pot to ground, which I guess tells me the noise does not come from the input buffer or the tone control section.

Replacing the J201 with different ones (I tried several in each position) didn't make a significant difference, and shielding the wires also didn't really change anything, the noise definitely comes from within the circuit.

The noise is significantly reduced when I short the gate of Q2 to ground, and even more when I short the gates of Q3 or Q4 to ground. So for me it looks like the noise ist simply generated in each of the gain stages and subsequently amplified in each stage.

I've spent many hours trying to debug the noise issue, but didn't find any issue with my layout and also the components seem to be OK.

Can anyone of you great people help me how to continue? Is the noise perhaps even expected, e.g. because I built on perfboard? Or would it make sense to continue debugging.

Thank you in advance for any hints. Apart frome the noise issue, it's such a cool pedal...

Regards,
Dan

BTW: Great forum! Great hobby!

[nate77]

When I read the title for this post I thought I was experiencing déjà vu!! I went through the same process of elimination as you, almost verbatim. The noise floor increased with each gain stage and was not a product of any specific part or section (as far as I could tell ). I also went through it component by component with an audio probe and only found what you found, the noise originated from the. It it and was amplified by each gain stage. I absolutely love this circuit and desperately want to use it but I cannot use it to record or even in a love situation as it just has too high of a noise floor.  I really, REALLY hope that we can sort this out. I built mine using a PCB from th customs, but it is 100% faithful to the schematic. I also did not substitute any components. Anyone out there with some insight for us??

[Kipper4]

Welcome aboard Dan.
I'm not familiar with the circuit, haven't built it so take this with a pinch.
Is it boxed up?
Have you tried biasing the transistors differently? A hair less voltage?
It took me ages after building to get the dr boogie to play nice without the squeel. Once I sorted the biasing it works nice.
I only compare the two because of the multiple transistors. I too biased the dr boo as suggested in a post then had to change it.
Like I said take it with a pinch

[dancloud69]

Kipper,

thanks for the welcome! 

yes, the circuit is boxed up, and I've played around with the biases quite a bit.

Actually, the circuit is behaving quite tame, apart from the ever present background noise.

In principle, each of the three gain stages has a pair of clipping diodes in front. Each has a gain of about 5 (as far as I can tell), but the first two are subsequently clipped again so that the overall gain is reduced. However, since the noise is far below the clipping level of the diodes, I guess it's amplitude is amplified far stronger than the guitar signal. At least that's my theory.

That said, what I will try to do is replace the J201 with lower gain 2N5457 or even 2N5952, while adapting the source resistors. If my theory is correct, the noise should be reduced while naturally the overall gain of the circuit is reduced. But since I almost exclusively use it for clean tone, that may be acceptable. I also don't necessarily need the pedal to sound too close to an actual Fender amp...

I'll keep you posted about the results of my experiments.

Dan

[duck_arse]

dan and nate - each of the fet gates in the azabache has a 150pF cap fitted. prehaps try removing these one at a time, see if the noise gets worse, or if one makes more worse than the others. you could then try increasing the value, 180pF, 220pF etc, see if it helps. it will of course be killing some the treble as you go higher value .....

[dancloud69]

Duck, thanks a lot!

I tried this, and it really helps. Actually, i went more extreme and added a 560p in parallel to the 150p at the gate of Q3. The noise loses a lot of treble content. The guitar didn't lose too much treble content to my ears (maybe it's an old age thing). I would guess that going less extreme would also have helped.

Before trying this, I had, as explained above, already replaced Q2 and Q3 with 2N5457s (while adapting their Rs to 470 Ohm), while keeping the J201 for Q4, which by itself had reduced the hiss enough that I would have been OK with using the pedal. The max. gain is reduced significantly, which is not a problem for me. However, I will at some point have to change the gain pot, as with that change it only is useful above the 12 o'clock position.

With both changes, the noise is now really low in relation to the guitar sound. For now, I am a happy person. Again, thank you very much for your advice!

Nate, you should really try Duck's suggestion to increase the miller caps. If reducing the treble of the hiss doesn't help enough, try replacing the J201 with 2N5457 (starting with Q2, while lowering its source resistor from 1.5k to 470R). If you do that, in order to keep the gain pot working across its full range, you will have to increase the 2.2k resistor from gain lug 1 to ground, though.

Best regards,
Dan

[nate77]

Thanks dan an duck, I'll give it a shot. I really dug this circuit and decided to give it a little rest after an exhausting debugging process. I appreciate the advice and feel a touch less insane since it's happened to at least one other person. Thanks guys!

[duck_arse]

Q3 you say? if it were me, I'd start with the noise Q1 might be producing, because each following stage has a chance at amplifing that noise, and adding their own. propagate and cascade = avalanche (which looks a lot like azabache .....)

dan, I'm just glad to have fluked one.

[dancloud69]

duck,

my thinking process to increase the cap at Q3 is as follows:

- Q1 didn't contribute significantly to the noise, as I wrote in my first post, because even when I grounded the output of the gain pot, the noise was about the same
- Q2 contributed most noise, most probably because its noise was amplified by Q3 and Q4
- Q3 still contributes noticibly to the noise. I thought, if I increase the cap before Q3, and thus lower the corner frequency of the hpf, it will influence the noise generated by Q1, Q2, and the components before Q3 at the same time
- The noise introduced at Q4 is not amplifed afterwards, it was almost neglectable in my experiments. I thought if I increase the gate cap for Q3 and not Q4, I will retain the overtones introduced by both stages which would alter the sound less.

Altogether, the outcome is not bad. If I find time, I will play around with it a bit more. But the last two days I've been more happy just playing guitar into the pedal :-)

Thanks and regards,
Dan