Boogie Monster squealing

[Billyhank]

Hi everyone!

I just finished the Boogie Monster but I’ve got a squeal when I turn the gain up. It starts before halfway and just gets worse if I go up.
I checked everything and can’t find anything wrong.
Has anyone else had this issue and figured out how to make it stop or does anyone know why it would be doing this?

 

[Billyhank]

 

[Betty Wont]

Did you bias the fets? To what voltage? You might want to lower the bias until the squeal goes away. It might only take lowering the first one a bit.

 

[Billyhank]

I did. I biased to 4.5v. I tried the pedal before biasing to make sure it worked, but then I biased. The squeal wasn’t as bad after biasing but it was still there.
Before biasing it was squealing if I turned the gain up past 9 o’clock. After I had to turn it up to about noon before it squealed.
I can try lowering them though. Appreciate it!!

 

[temol]

You can also try to replace wiring - use shielded wire.

 

[Mir9]

I had the same problem on my build.
I read about raising the 3.9k (R11) to 39k from a few places. I put a 50k trim and adjusted till the noise went away and thankfully it didn't change the tone. It's just as heavy now, but without the squeal.

I have a real Dual Recto and it sounds pretty close when connected to the power amp.

 

[Billyhank]

Which trimmer did you change? All or just the one over the 3k9?

 

[Mir9]

Which trimmer did you change? All or just the one over the 3k9?

I replaced the 3k9 with a 50k trimmer as I read of others modding it to 39k. I didn't replace any of the existing trimmers.

 

[temol]

I'm afraid that with Q3 source resistor changed from 3k9 to 39k you won't be able to bias Q3 @ 4.5V. It's going to be closer to 7V, even with a trimmer set to 100k. So.. this lowers Q3 gain, less noise, no squeal.. but that's not a proper solution in my opinion. Similar effect would be just to set Q3 drain voltage to 7V without changing R11 value (not exactly the same but close).

 

[Elijah-Baley]

I built the Dr. Boogie on stripboard, so it's another story, but during the test of the circuit out of the box it squeals awfully. I changed the 3.9k resistor with a 39k with the a 39k (with a 500k trimpot I had instead the stock 100k to get the 4.5v). This solved just at low gain setting the issue.
I used shielded wires in the input, the output and even between the treble pot and the Volume pot (of course with the stripboard the pots have wires). Nothing. The only solution was to use a buffer in front.
Until I tried to make cut track on the board in the input and output stripe to make it shorter, because for a certain length those were close. This definitely solved the problem, with gain and all the eq pots at max I got no squeal, though I still have shielded wires, that probably I'll try even to replace with normal wires, if I can. Else I'll use them again while I'll box it up.
So I guess to shield the input and output wires and take away from each other is very important in circuit like this.

 

[music6000]

I did. I biased to 4.5v. I tried the pedal before biasing to make sure it worked, but then I biased. The squeal wasn’t as bad after biasing but it was still there.
Before biasing it was squealing if I turned the gain up past 9 o’clock. After I had to turn it up to about noon before it squealed.
I can try lowering them though. Appreciate it!!

Did you check to see if they are 4.5v after you got to the last adjusted transistor?
Did you max the Gain pot & check the Bias @ 4.5v on each transistor?

 

[music6000]

After you have the Bias correct as above:
You can also get a 50K Potentiometer and attach 2 wires to Legs 1 & 2, remove R11 (3K9) & temporary solder the wires to the 2 pads & slowly adjust to where your not getting oscillation with Gain pot at Max.
Dont knock the Pot, Remove wires & measure Resistance between the 2 wires & replace with a resistor to the same value!

 

[Chuck D. Bones]

I suspect that there is stray coupling thru the power supply rail. This circuit is supposed to emulate a tube preamp. Like the tube preamp it's emulating, this circuit has poor power supply rejection. Noise on the power supply rail couples into each stage and each stage couples signal and noise onto the power supply rail. C100-C102 are supposed to keep Vcc & Vdd clean, but no capacitor is perfect. I'd try increasing R102 to 1K in order to get more isolation between Vcc & Vdd. Usually, the input stage is at the end of the power supply filter chain. This circuit has the input stages at the beginning of the filter chain. C11 makes things worse by providing a high-freq path between Q3's drain and Vdd. I don't know why the designer did that, connecting C11 to ground instead of Vdd would eliminate that coupling path without changing the overall freq response.

 

[temol]

There's many possible causes here. It's a hi-gain circuit, packed tightly into 125B enclosure. Bare IN/OUT wires (blue wire from the input jack goes very close to gain and bass potentiometers). IN/OUT wires too long.
I've build 4 or 5 Boogies. All of them in 1590bb enclsoures, with shielded wires. Zero problems, no oscillations.

 

[Elijah-Baley]

I suspect that there is stray coupling thru the power supply rail. [...]

If it was that, it's enough to try it with a battery, I think. If with the battery it's ok it's a power supply issue, and to solve it the circuit need a better power supply filter, or a different PSU.
When I had this problem with my Dr. Boogie stripboard battery or PSU was the same.

 

[Chuck D. Bones]

A battery will be a cleaner source, but it won't fix a coupling problem on the power rail, at least not in this circuit. I'm not saying power supply coupling IS the problem, I'm just turning over rocks. Until we know what's causing the problem, we have to consider all possible causes.

 

[fig]

Not likely related, but just FYI @Billyhank
That output tab is awfully close!

 

[fig]

Gotta bump this.

When I read this, I built the one I had [using my last @PedalPCB 3PDT board..heh].

@Billyhank, hoping you've made some headway?

I’ve got a squeal when I turn the gain up. It starts before halfway and just gets worse if I go up.

The only time I hear squealing is with the out jack disconnected, or if I dime a trimmer (specifically t1). There is a good deal of noise however using the hookup wire (high-gain circuit), so I concur with the others regarding some shielding to reduce it.

@temol - will you offer a source for the shielded wire you used? I have some pushback that would work fine, but I built this one with hookup wire to see if I had the same issue(s).

I didn't read @Chuck D. Bones suggestion until after populating, but I'm willing to switch that resistor. As for C11, there is a nearby ground pad. You want I should re-route a leg to it?

I do have a question regarding the trimmers. It appears Q4 & Q5 share T4. Q4 is connected via the drain, and Q5 connected via the source. That means adjusting the trimmer increases resistance to one and reduces it to the other, correct? This is the behavior I saw when adjusting. This does limit the bias to a single voltage. On my build, I set these first, and matched the others to that. I also tried several variations using the ear, voltmeter, and a magic 8-ball.

I'll swap the wire as well, and offer any groovy findings.

 

[music6000]

Gotta bump this.

When I read this, I built the one I had [using my last @PedalPCB 3PDT board..heh].

@Billyhank, hoping you've made some headway?



The only time I hear squealing is with the out jack disconnected, or if I dime a trimmer (specifically t1). There is a good deal of noise however using the hookup wire (high-gain circuit), so I concur with the others regarding some shielding to reduce it.

@temol - will you offer a source for the shielded wire you used? I have some pushback that would work fine, but I built this one with hookup wire to see if I had the same issue(s).

I didn't read @Chuck D. Bones suggestion until after populating, but I'm willing to switch that resistor. As for C11, there is a nearby ground pad. You want I should re-route a leg to it?

I do have a question regarding the trimmers. It appears Q4 & Q5 share T4. Q4 is connected via the drain, and Q5 connected via the source. That means adjusting the trimmer increases resistance to one and reduces it to the other, correct? This is the behavior I saw when adjusting. This does limit the bias to a single voltage. On my build, I set these first, and matched the others to that. I also tried several variations using the ear, voltmeter, and a magic 8-ball.

I'll swap the wire as well, and offer any groovy findings.

Chuck is going to test on his breadboard to confirm his mods will solve this issue!
if you are willing to try it, C11 can be still connected to top pad & bottom unsoldered & connected to Volume 1 pad which is going to Ground.
Try that First before doing the R102 - 1K resistor.

 

[temol]

will you offer a source for the shielded wire you used? I have some pushback that would work fine, but I built this one with hookup wire to see if I had the same issue(s).

I can't recommend any particular brand. I just use whatever wire I have. Sometimes I buy it in local store, sometimes I just take old RCA cables and split them. Thin microphone cable is also ok.
You can take anything between 2-3mm in diameter, with good, dense braid, not too stiff.

ps. RG174. It's a 50 ohm coaxial cable. Good quality wire but bit stiff though.. RG178 is also ok.