Azabache - white noise normal?

[maoriente]

I finally got around to building the Azabache, need some help.

I changed the tone control's 47k to 4k7 and added an additional diode at the gate of Q4 for asymmetrical clipping. Everything else stock.

Really like how the pedal sounds...until I stop playing as there is a huge amount of white noise. The audio probe picks up the noise at Q2's drain. It's probably at the gate too as I have swapped that J201 with a few others with no change.   

I can't imagine this is normal, however after swapping all components between Q1 and Q3, the noise is unchanged.

I found that increasing the 150p gate cap values of Q2, Q3 or Q4 reduces the noise. At the moment, I have the 150ps at Q2, Q3 and Q4 each increased to 470p. Much better, and the pedal still sounds great, but the background hiss is still unacceptable.   

To nearly eliminate the hiss, I have to increase any one of those caps to 1nf. Seems extreme, and also seems to effect the quality of the distortion. (Increasing the 150p at Q1 or the 470p at Q5 had no effect on the hiss)

http://www.runoffgroove.com/azabache.png

Questions:
1. For those of you who have built the stock Azabache, do you have a ton of background hiss? Noticeable when not playing.
2. It appears increasing the 150p at Q2, Q3 or Q4 helps equally reduce the hiss, would it be better to increase at only one location? If so, Q2 or Q4?
3. Or would it be better to leave the 150ps as is and add a low pass filter somewhere after Q4?

Hoping to save this build, the hiss is a game stopper for me.

Mike





   


 

[Kipper4]

And if you change the drain trimmers by ear?
The given values will surely be a ballpark voltage to make it work.
Tuning is down to the builder no.?

[anotherjim]

I haven't built it, but I imagine that with all those JFET's stages, you can't help but get some noise.

You might get a worthwhile reduction in hiss if the high value resistors that the signal must pass thru' are metal film type. Especially those 100k ones before the diode pairs.

The 150p caps on the gates might work better fitted between drain and gate (or fit an extra cap there) - that would hit the hiss via negative feedback. In that position, 150p value might not be optimal.

[blackieNYC]

Mine is a little hissy, but I do love it. Haven't attempted to reduce it. I could probably measure it in a way you could reproduce. Knobs maxed, AC mvolts.

What was the intention with the tone control change 4.7k?

[maoriente]

Thanks,

Kipper4: Haven't tried to bias this one by ear, didn't use trimmers. I have each of them biased at 6v and 5v as suggested. The highest drain resistor is 20k. Maybe that one is causing to much gain?

Anotherjim: I did use metal film resistors, in fact the audio probed 1st picked up the noise just after one of the 100k resistors (I think at gate of Q3), I thought maybe it was faulty. I swapped it out, no change. Haven't seen a cap connected between gate and drain before, any suggestions on values?

blackieNYC: lowering (or bypassing) the 47k on the tone control centers the frequency response. With the stock 47k a flat frequency response is between 2&3 on the control.  That doesn't leave much CCW rotation to dial back treble. So at noon (or 5), your getting about a 2.5db treble boost and 2.5db bass cut. To me, that's like having the control between 7 & 8. Bypassing the 47k (or using a smaller value like 4k7) centers the frequency response so noon (or 5) on the control is a flat response. Now you can cut the treble/increase bass a bit more. With the control maxed, you get the same amount of treble boost as before, but you do lose about 2db in bass cut in the lowest frequencies. To me, the change makes the control more useful.   

Regarding the hiss, I forgot to mention I also bypassed the tone control section by jumping from the preceding 220n to the Q2 gate, noise was still there.

I haven't tried to dial out the hiss by rebiasing the jfets, I probably should give that a shot considering the tone control has no effect on the hiss. Unfortunately it won't be easy as I designed the board without trimmers. But it will be interesting to see what that does.

Also, I saw another post where franknh recommended changing out the notch filter at the end with a low pass filter similar to the Professor Tweed. http://runoffgroove.com/professor.png Think I will give that a try 1st as that will be easier to do on my board. 

[anotherjim]

Haven't seen a cap connected between gate and drain before, any suggestions on values?

I usually end up fitting 33pF or 47pF, mostly to help reject any radio stuff that might get in. 100pF might be the most that doesn't cut wanted signal too much. The idea is to add another low-pass filter pole to the one already there using the 150pF. 2 poles is a steeper cut without lowering the overall pass band too much.

BTW, sometimes various sources of external interference can amount to "hiss" in a circuit. It can be much improved when boxed up in a metal enclosure.

Also, rule out external power supply noise -  try battery.

Just for research purposes, I wonder if you could also try Pink Jimi's trick of adding a germanium diode as a kind of compensating noise feedback. Try at Q2. The diode would be cathode to drain , anode to gate. At first sight, this shouldn't do anything since the diode won't have forward bias. In practice, the diode leakage might give enough negative feedback to calm things down.

[maoriente]

Sorry for the delay guys, I had a pretty bad bike accident on Monday, dislocated my leg and haven't been able to walk.

Will be a couple weeks before I can get back to this

[blackieNYC]

Ouch! That sucks. Good luck

[maoriente]

ok, finally able to get to my workbench.

1. No change when using a battery for power source.
2. Adding 100pf between Drain and Gate of Q2 or Q3 knocks off some of the hiss. At Q3 gave slightly better results.
3. Have not tried germanium diode yet.

A couple observations that aren't making sense to me:
1. The hiss remains and is unaffected when removing Q1
2. The hiss goes away when removing Q2 (no change when swapping out Q2)
3. Adding the 100pf to Q4 did not affect the hiss. (Why not, same configuration as Q2 and Q3?)

This seems to indicate the hiss is being created between the 5k trimmer and the drain of Q2. I don't know what to make of the Q4 thing.

Think I'm going to remove 1 component at a time between Q1 and Q2 and see where the hiss drops out. 

[blackieNYC]

I hadn't heard of that tone control mod - I changed 47k to 4.7k, and yes the tone control operates in a much better way.  12:00 is pretty close to flat. And I don't consider mine a hissy pedal.
Thanks for alerting me to this mod. My Aza is just perfect now. I at least owe you some voltage measurements.

[maoriente]

Nice, I'm just going around in circles with mine.

Think I'm just going to build a new one and see what happens.

[Frank_NH]

"This seems to indicate the hiss is being created between the 5k trimmer and the drain of Q2."

The trimmer is 20K for Q2 - I assume that's what you meant?

In any case, fire up your current build and measure your source, gate, and drain dc voltages for all JFETs and report them here.  That may be resolve the mystery.

I built this on a my breadboard and didn't notice it being all that hissy.

[maoriente]

Thanks Frank_NH, that's encouraging. I will definitely build another if I don't get this one sorted.

Sorry, I meant it seems the hiss enters between Q1's trimmer and the Drain of Q2, but now  I'm not so sure...

Supply voltage 9.05v at the trimmers.

Q1  2n5457   Rd 2k7  D-6.04 S-0.53 G-0.0
Q2   J201      Rd  16k  D-4.83 S-0.40 G-0.0
Q3   J201      Rd  20k  D-5.03 S-0.30 G-0.0   
Q4   J201      Rd  16k5 D-5.03 S-0.40 G-0.0

[Frank_NH]

Those voltages look good to me.  So the JFETs are biasing correctly.

If I built this again on my breadboard, I'd experiment with putting a simple JFET or BJT buffer in front of the tone control.  I'm wondering if the lower input impedance of the tone circuit could give rise to noise.  Perhaps worth a try.

[maoriente]

So I built another from a slightly different layout, all new components accept the Gain and Tone pots.

Same results   Above 7 on the volume knob and the hiss is very apparent. Not much of an issue on higher gain settings as the volume control is around noon, and does sound great, but on lower gain settings the volume needs to be turned up and the hiss seems to take over.

The audio probe detects the hiss at the drains of Q2,Q3 and Q4, increasing at every stage.  At Q4 it's crazy loud, unlike any other pedal I've  ever probed. The J201 (smds) drain resistors are 13k on this one, with drain voltages just about 5v. 

I converted the notch filter at the output to the Professor Tweed low pass filter, by removing the 10k to ground, the 22n, changed the 68n to 2n2 and added another 2n2 from Volume lug 3 to ground. I had to increase the final capacitor to about 5nf to reduce the hiss, however this darkened and somewhat deadened the pedal.

At this point, I'm going to try to find a balance between increasing the 150p Gate caps and the modded low pass filter cap(s) at the output to see if I can tame the hiss enough without sucking the life out of the pedal.




 

[maoriente]

FWIW, here is what I settled on (for now):

I left the original circuit intact except I did adjust the tone control and replaced the output notch filter with the Professor Tweed low pass filter as mentioned above. 

I used a mini 2P4T rotary for the bright and scoop switches and added the following "hiss reduction" DPDT on/off/on toggle.

The center off position of the toggle is the original circuit which I'll label "Rich". This sounds excellent as long as the volume control stays below 7. Above 7 and the hiss is very apparent.

Toggle down will add a 1nf cap from Q2's gate to ground, which I'll label "Warm".  This gets rid of much of the hiss at the higher volume settings, but also takes out some of the richness of the distortion.  I selected the 1nf value by setting the gain control full CCW and the Volume control near full CW. 1nf took out much of the hiss at these settings, however I may reduce the value of this added cap as it's unlikely I'll be using the pedal with these extreme settings. The smaller value needed to kill the hiss the better. 

Toggle up will add 2n7 caps in parallel to the low pass output 2n2 caps (modified output as noted above), which I'll label "Dark". This squelches most of the hiss, but darkens the pedal quite a bit. I may also reduce the value of these added caps as again the 2n7 may be extreme and unnecessary. Again, the smaller the value to kill the hiss, the better.

Once I figure out the lowest gain setting/highest volume setting combination I'll typically use, then I'll determine the smallest cap values needed to knock out any unwanted hiss.