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?

 

[Chuck D. Bones]

When it comes to terminating shielded wire, there are as many exceptions as there are rules. One end is probably good enough and the jack end is the easiest.

 

[fig]

Yep, soldered to jack. Mark it Donnie.

 

[Elijah-Baley]

I like the tone of my Dr. Boogie, I'm using a 9v PSU because I have just it. The only thing I don't like too much is the dynamic lacking, even if could be the nature of the effect. I have really little difference in distortion hitting the string soft or hard, and just setting very low the guitar volume pot I can decrease a bit the distortion.

I'm not sure I got what happen if we increase the voltage. If I use a 18v (or more) charge pump (re-setting the trimmer, of course), I understand I'll have more volume. But what about the amout of distortion? I think I'll get a bit less gain, but even more dynamic?

 

[Chuck D. Bones]

I initially thought increasing the power supply voltage would increase the headroom and dynamic range. My experiments and analysis indicated that raising the voltage increases the gain more than it increases the headroom. As a consequence, there is the same or less dynamic range because the higher gain keeps the last 2 stages saturated most of the time. If you want more gain, increase the voltage. I thought there was more than enough gain at 9V and it sounded better. YMMV.

 

[music6000]

Yep, soldered to jack. Mark it Donnie.

So, what is your final assesment of the pedal with the shielded wire now in place.

 

[Elijah-Baley]

I initially thought increasing the power supply voltage would increase the headroom and dynamic range. My experiments and analysis indicated that raising the voltage increases the gain more than it increases the headroom. As a consequence, there is the same or less dynamic range because the higher gain keeps the last 2 stages saturated most of the time. If you want more gain, increase the voltage. I thought there was more than enough gain at 9V and it sounded better. YMMV.

I thought the same thing you thought. But, honestly, if you think that the first thing you can notice when the voltage is higher (at least it was for me) is that it became louder, it's easy imagine that all the stage push more.
Another consideration is, if at higher voltage the stage push more, maybe we can to use the original 39k (in the dual rectifier schematic) instead the 3.9k. I know, but I could wrong, it was changed because a voltage question. That third stage maybe is used to push not so much before the last two.
If you still have the circuit on the breadboard you could try to supply it at 18v (or more) and change the 3.9k with a 39k or so to see what happens.

Yes, I think it has enough gain, even if it's weird to me say this, I was agree to sacrifice a bit of gain in favor of a major dynamic. But, after that thing you said... I'm not sure I want to add a charge pump, now. Actually I made one test at 18v, but the PSU I have is too noise (waste of money). Noise aside, I thought I had less gain but maybe I was wrong.

If the last two stages at higher voltage saturate more, do you think we could add a resistor to keep those less gainy?

I hears someone tell that this kind of jfet circuits to sounds good have to work at higher voltage. This guy in some forum I don't remember said that its Dr. Boogie works at 40v.

 

[Chuck D. Bones]

I have removed the Boogie Monster from my breadboard. I had tried increasing R11 to 6.8K, 10K and 39K. Not much sonic difference when the other three stages are driven to saturation.

If you want the same gain you had at 9V, just put the trimmers back to the same resistance you had at 9V. Once you've done that, try different power supply voltages and see if you like the tone.

I don't know if "this guy in some forum" explained how the circuit works, so I will. JFETs act like current sources. In this circuit the gate bias voltage is 0V for every JFET except Q5*. The DC drain current of Q1-Q4 does not depend on power supply voltage. It only depends on Vp, Idss and the source resistor. Those three quantites are constant, therefore the DC drain current is constant. Transconductance is a function of drain current. Since drain current doesn't change with power supply voltage, transconductance doesn't change either. The voltage gain of each stage is equal to the transconductance times the load impedance. The trimmers have the strongest influence on load impedance. If we change the power supply voltage but don't change the trimmers, then the gain doesn't change. If we increase the trimmers when we raise the power supply voltage, then the gain goes up.

*Q5 is a source follower. Its voltage gain is always unity. Since Q5's gate voltage is the same as Q4's drain voltage, changing the power supply voltage or Q4's trimmer will change Q5's drain current. Still, Q5's gain will always be unity.

FYI, J201s are rated for 40V max, therefore I would not recommend running them higher than 40V. I do not know how the filter caps are rated, so that's a concern.

 

[temol]

If you want the same gain you had at 9V, just put the trimmers back to the same resistance you had at 9V. Once you've done that, try different power supply voltages and see if you like the tone.

The gain would not be exactly the same.
See Boogie frequency response graphs below.
Red - 9V, transistors biased around 4.5V
Green - 24v, no bias adjustment
Blue - 24V, transistors biased around 12V.




See the output signal envelopes also

 

[fig]

So, what is your final assesment of the pedal with the shielded wire now in place.

It's a high-gain pedal with a LOT of 201s. Unwanted noise is a byproduct of this design.

Of all the changes or adjustments made, these two seemed to yield the most "bang for the buck";

1. Shielded cable - it mitigates EM amplification, which is needed with a circuit of this nature. I use 9v power supply cable. The insulator is delicate so you have to pay attention when soldering or it will melt well back into the length and you'll be dealing with a ground-short.

2. Biasing - Two schools here. a) in accordance with the design criteria and given formulae, or b) turn those cute little thingys until it sounds good. I set mine to taste by setting all pots fully CW, trimmers at max, and backing them down one at a time. I started with T4 and worked back. There is a third biasing option; but it's difficult to source the correct chicken bones, and improper use could be disastrous!

The TMB is pretty weak in my opinion with little more nuance than a tone knob. For me, this was a bigger factor than the noise. If you can't effectively shape the "preamped" signal, what's the point?

I initially jumped in to offer another build to experiment, in light of @Billyhank 's squeal. I am confident that it can be trimmed out.

As is typically the case; there are hidden gems of knowledge in this thread that I appreciate greatly. Thanks again to all!

EDIT: Almost forgot...someone on another thread suggested that naming pedals is a good idea. I named this one "Booger".