DR Boogie - I fixed the squeal in my DR

[ubersam]

Background: I breadboarded the DR boogie circuit based on electritab's schematic, using some variations (1M on Q1 gate tied to Vref instead of ground, drain trimmers replaced with fixed value equivalent resistors, miller caps on all Qs, Q3 bias switchable between 4.5v and 6.5v, 68k input resistor). I made up my own layout, etched the pcb, and built my circuit. That one squealed. I tried all the ideas that I found here and it still squealed. I got tired of it after a while and the thing sat around for months.

Fast forward to Sunday night:

I have been thinking about giving the DR Boogie another go, so I dug it out. In my layout, the tone stack solder pads are right next to the input. So the first thing I did was to relocate the tone stack to a separate board = Still squeals. So much for that idea. Then I thought, maybe I should try a different JFET. I have heard of others subbing in an MPF102, which IIRC, helped control the squealing. I had both the 2N5457 and the MPF102 but decided to try the 2N5457 first and I wired in a pot for the Q1 drain. This is when I made an interesting discovery: the squeal goes away when Q1 drain is @ 8v. The circuit still sounds just like how I recall it sounding when i breadboarded it, and it is not squealing. So, I decided to put the J201 back in and set its drain voltage to 8v, just to see whether or not my results was a fluke. Well, it wasn't. The J201 in Q1, biased at 8v did not squeal. I liked how the 2N5457 sounded in Q1 so I swapped it back in.

In some of the earlier discussion re: miller caps in the DR, it was suggested that only Q1, Q2 & Q4 needed miller caps -- I had them in all Qs (180pF). So, I decided to take out the miller caps from Q3 & Q5. I made another interesting observation after doing this. As I mentioned earlier, I made Q3's drain voltage switchable between 4.5v and 6.5v. With the Q3 miller cap removed, I get a bit of squealing on higher gain setting, when Q3 drain is @ 4.5v, when I switch it over to 6.5v, the squealing goes away. I have two choices, put the miller cap back on Q3 or leave Q3 drain on 6.5v. There is a difference in character between 4.5V and 6.5V, the latter sounding a little cleaner, otherwise I'd just leave Q3 drain at 6.5v. I guess the only choice is to put the miller cap back on Q3, maybe a smaller value than what I originally had in there (180pF).

More experimenting: I read in the other DR post some of John's observations. With the 1M volume pot, the DR loads down succeeding pedal causing tone loss. I have been wondering about this myself. To get around the loading issue, I am thinking about adding a simple buffer after the tone stack. I think it would work, at least in theory.

[gaussmarkov]

nice.  thanks for sharing.  it sounds to me like you have isolated a principle source of the dr. boogie squealing.  sweet.

[ubersam]

That solution worked on my layout which was not optimal to begin with. I had the circuit going in a "U" kind of path. It will be interesting to see if the same solution will work on squealing DRs built on other layouts.

I'll eventually breadboard another DR just to see if there is a big difference in sound/feel between 4.5V and 8V Q1 drain voltage.

[John Lyons]

Gaussmarkov's last revision of the DB with the scaled tone stack works well and has been built by a couple folks at least with no squealing or noise. Build that and you're good to go. No need for a buffer...

John

[km-r]

i was really frustrated with my boogie because of the noise and occasional squeels... i thought i was having problems with my switching that caused squeeling...

thanks,


hey, but for the noise, is there really nothing i can do?

[mattpocket]

Distortion pedals are bound to have a bit of noise, as are guitars, and amps.... Thats why god built noise suppressors! haha

Matt

[km-r]

Distortion pedals are bound to have a bit of noise, as are guitars, and amps.... Thats why god built noise suppressors! haha

Matt

do i buy? or build one?

[mattpocket]

Thats a question that caused me concern when I started building... One of the first pedals I ever wanted was a noise suppressor. When I started DIY I thought why not build one. But then I thought, my skills are fairly new to me, my pedal builds arent great, and I thought that maybe I could introduce noise to the pedal. The last this you want a noise suppresor to be is noisy. So I brought one. However, I think now I am a bit further down the line (plus I already have one) I would build one out of curiosity. To see if its any good.

Go for it! They are definately useful tools.

Matt

[km-r]

Thats a question that caused me concern when I started building... One of the first pedals I ever wanted was a noise suppressor. When I started DIY I thought why not build one. But then I thought, my skills are fairly new to me, my pedal builds arent great, and I thought that maybe I could introduce noise to the pedal. The last this you want a noise suppresor to be is noisy. So I brought one. However, I think now I am a bit further down the line (plus I already have one) I would build one out of curiosity. To see if its any good.

Go for it! They are definately useful tools.

Matt

you go for it matt! youve got the stuff... try out the LM1894 for a start!

wee!

[mattpocket]

Which noise suppressor are we talking about here?

[ubersam]

Gaussmarkov's last revision of the DB with the scaled tone stack works well and has been built by a couple folks at least with no squealing or noise. Build that and you're good to go. No need for a buffer...

John

I printed out Paul's schem and layout for studying. I noticed the addition of the power supply filtering, but I did not notice the scaled down tone stack. Thanks for pointing that out. I'll give his layout a go when I build another DrB. Right now, I am focused on conquering my layout, and hopefully learn a thing or two in the process.

[km-r]

Which noise suppressor are we talking about here?

could be any... that IC is for noise reduction...
is there a schem of that noise supressor you are talking about..?

[John Lyons]

Sam B

In the monster DB thread started by gaussmarkov there is a list of the scaled down values for the Tone stack.
Sorry to make you look it up but I don't have it here. Basically the values are scaled down by 10. So 1M would be 100K etc.

John

[gaussmarkov]

Sam B

In the monster DB thread started by gaussmarkov there is a list of the scaled down values for the Tone stack.
Sorry to make you look it up but I don't have it here. Basically the values are scaled down by 10. So 1M would be 100K etc.

John

... and the cap values are scaled up by 10 so that RC is held the same. 

[mattpocket]

I think there is a noise gate on GGG if I remember correctly. 

[John Lyons]

Here are the scaled tone stack values.

Slope Resistor:        was 47k    now 4k7
Treble Cap:             was 680p  now .0068
Mid Cap:                 was 22n    now .22uf
Bass Cap                 was 22n    now .22uf
Treble Pot:              was 250k  now 25k
Mid Pot:                  was 25k    now 2k5 (Use 5K with a resistor across the outside lugs to get close to 2.5K)
Bass Pot:                was  1M   now 100k
Volume Pot             was  1M   now 100k
Presence Resistor    was 22k   now 2k2
Presence Pot           was 100K now 10k
Presence Cap          was 3n   now .03uf

Thanks to George giblet for this suggestion!

John

[ubersam]

John & Paul - Thanks. I looked the schem over again last night and finally noticed the value changes in the tone stack. Except for the 2.5K linear pot, I think I have all the other values. I'm gonna head over to geofex to read up on the secret life of pots. I think that I can just add a 5K across the outer lug of a 5Kpot to get 2.5K, but I just want to make sure.

[gaussmarkov]

great.  R.G.'s article helped me a lot.  if you'd like to read another version, you could also look at

resistors: mods on gaussmarkov.net

it's part of a larger article on resistors.