Call for comments: Blues Driver clone layout at gaussmarkov.net

[stobiepole]

Well, Gaussmarkov and I have been busy working on a BD-2 Blues Driver clone layout (Gaussmarkov insists it's a co-production , but really I'm just a beta tester)...Since the call out Gaussmarkov made for his Dr. Boogey layout was so successful, we thought we should take the same approach with this one and offer it up for comment.

The layouts can be found here:

http://gaussmarkov.net/index.php?page=layouts#bd2

There's two layouts: one has an extra opamp. It has to do with power supply buffering, or something...it was on the original schematic. The original ran at eight volts, the simpler version runs at nine. I've built a working pedal using the simplified schematic. I haven't got an original BD-2 to compare it to, unfortunately, but it sounds pretty good. I think it's got a lot more volume than a tubescreamer, and it actually reminds me of the Shaka Tube.

One thing we considered trying is adding an 8V zener diode to the circuit, and possibly some form of clipping diode switching. Perhaps make room for some of the more popular BD-2 mods, like silver mica caps...

Anyway, what do people think?

Chris

[gaussmarkov]

i think the boss blues driver (bd-2) is a good playground for diy.  the fact that mods for it are so commonly discussed is evidence enough.  you can change the clipping diode section and you can change the tone shaping with capacitor substitutions.

in addition, this pedal has two gain stages that are built around discrete op amps.  this basic building block consists of three transistors like this:



the two (facing) JFETS are called a "differential pair" and their gates are the analogues of the noninverted and inverted inputs of an op amp.

i have seen several recommendations from knowledgeable members to try working with this.  for example, Jay Doyle's discrete boost which has this element



Jay said

Use this as a clean boost to cut through the mix, to overdrive your amp, or push the gain up and utilize this circuit's smooth distortion. More importantly, try implementing this design in place of IC op amps and see if you can achieve some new and unique sounds!

another example, is Joe Davisson's Diode-Compression Op Amp

Joe said

This circuit was designed because the diode trick has to be done within the op-amp itself, therefore it was made as simple as possible. Use it in the distortion sections of your favorite overdrive/distortion unit.

and since then several folks have thought about making this change to the bd-2.  but i am not sure how far that has gotten.  it would be easy to add this to our layout.

R.G. gave a nice tutorial on discrete op amps in this thread.  Jay also gave some helpful links there.

our layout clones what is on the boss bd-2 schem that has been around for so long.  aron announced it's presence on the schematics page in 1999.  without even considering additional elements like Joe's compression diode, i thought there might be interest in sharing experiences about different transistor and op amp subs.  i am also wondering if there is interest in designing in additional features like Joe's diode and the ability to subsitute other transistor types.  the differential pair does not have to be JFETs, nor does the transistor providing the gain need to be a BJT.  Nelson Pass wrote an extended piece that is worth consulting called DIY Op Amps.

[gaussmarkov]

and here's a specific question about the boss schem that i hope someone will answer.  there is an op amp at the output of the circuit with two diodes (D5 and D6) across the inputs:



what are those for?  my conjecture is that they have no audio effect because when the op amp is working normally the voltage difference between the inputs is essentially zero.  hence the diodes would not be conducting.  are they there as some kind of protection for the op amp?

cheers, paul

[jrc4558]

and here's a specific question about the boss schem that i hope someone will answer.  there is an op amp at the output of the circuit with two diodes (D5 and D6) across the inputs:



what are those for?  my conjecture is that they have no audio effect because when the op amp is working normally the voltage difference between the inputs is essentially zero.  hence the diodes would not be conducting.  are they there as some kind of protection for the op amp?

cheers, paul

I think its a limiter that protects the IC from being overdriven. So that the IC only amplifies the signal without adding any saturation of its own.

[spudulike]

Diode clamp.
Isnt that final opamp stage an inductor simulator (gyrator) ? ?

[ulysses]

looks good - nice work

cheers
ulysses

[gaussmarkov]

Diode clamp.
Isnt that final opamp stage an inductor simulator (gyrator) ? ?

thanks for this conjecture.  i looked up op amp gyrators because i knew nothing about them and i think the answer is "no" but it may be that my schem is confusing.  the only connections/junctions are where the dots appear at the some place as wires cross or join.  i think this op amp is set up in the usual negative feedback configuration.  what's in the feedback loop is different from what you find in something more familiar, like a tubescreamer/ts808.

like a ts, there's a resistor and a cap in parallel.  but there are no clipping diodes.  and the differential gain caused by the series .047uF-cap-4.7K resistor in a ts (see geofex.com technology of the tubescreamer) is replaced by a section with an NPN BJT in it.  i haven't tried to figure that one out yet.

"diode clamp" is a good term to learn!  thanks for that.

I think its a limiter that protects the IC from being overdriven. So that the IC only amplifies the signal without adding any saturation of its own.

o.k.  it sounds like we are thinking along the same lines.  do you have some idea of how the two inputs could get so far off from each other that the diodes actually have an effect?  perhaps that NPN BJT would have something to do with that possibility?

looks good - nice work

cheers
ulysses

cheers back atcha!   

[beatstrat]

Good stuff; thanks for sharing.

[gaussmarkov]

here's a follow-up on the final stage of the boss blues driver bd-2.  my circuit analytical skills are limited so i asked the LTSpice/SwitcherCad simulator to tell me what this stage is doing.  here is my LTSpice schem:



and there is the AC analysis (the diode clamp makes no difference here, of course):



so this stage is giving the circuit a fairly narrow 6db boost at around 120Hz (the dashed line is the phase shift).  if you want to move this around it seems like the 1.2K resistor is a convenient place to start.  if you double that to 2.4K then you get about 5db at about 85Hz and if you halve it then 7.2db at 166Hz.  if this isn't already a bd-2 mod, it should be. 

[Stratomaster]

I looked at the layout, and I'm a bit confused.  There are 4 pads for the transistors, and one of them is supposed to be labeled as a square, but I don't see that.  Also, is one of the pots dual-ganged?  I just wanted to see if I could understand what was going on your clone.  Still learning the ropes in terms of circuit mapping, schematics, and layouts. 
Thanks.

[gaussmarkov]

I looked at the layout, and I'm a bit confused.  There are 4 pads for the transistors, and one of them is supposed to be labeled as a square, but I don't see that.  Also, is one of the pots dual-ganged?  I just wanted to see if I could understand what was going on your clone.  Still learning the ropes in terms of circuit mapping, schematics, and layouts. 
Thanks.

huh!?!  you are right.  and i see what happened.  i changed only the second version of the layout.  so if you look in the pdf project file and compare the two you can see where the square should be.  but i will fix that as soon as i can so that it is consistent everywhere.

and yes, the gain pot is dual-ganged.

[gaussmarkov]

I looked at the layout, and I'm a bit confused.  There are 4 pads for the transistors, and one of them is supposed to be labeled as a square, but I don't see that.  Also, is one of the pots dual-ganged?  I just wanted to see if I could understand what was going on your clone.  Still learning the ropes in terms of circuit mapping, schematics, and layouts. 
Thanks.

huh!?!  you are right.  and i see what happened.  i changed only the second version of the layout.  so if you look in the pdf project file and compare the two you can see where the square should be.  but i will fix that as soon as i can so that it is consistent everywhere.

and yes, the gain pot is dual-ganged.

ok.  i made corrections.  i am hopeful that in the process i did not introduce errors.  just for more clarity, the issue is that we laid out several transistors so that one lead straddles an underlying trace.  and we put in drill holes (and associate pads) so that socketing the transistors with inline sockets would be easier:  4 holes in a row instead of 3.  so we also put a square pad for the socket that is not supposed to be used.  here's a pic showing Q1 normal and Q2 with the extra square pad:



the project file explains all of this--but there is bound to be some confusion.  here is another colour palette where the difference between the leads (grey) and the pads (green) is clearer:

[gaussmarkov]

dang.  i did introduce an error.     i hope nobody picked it up yet.  i had the wrong pcb image for version 1.  now that is fixed.

rock on ...

[spudulike]

gaussmarkov - nice work 

I was alluding to the fact gyrators are used to simulate an inductor in a parametric equaliser. Check the article at GEO (http://www.geofex.com/Article_Folders/EQs/paramet.htm). The last stage resembled a parametric section to me.

[gaussmarkov]

gaussmarkov - nice work 

I was alluding to the fact gyrators are used to simulate an inductor in a parametric equaliser. Check the article at GEO (http://www.geofex.com/Article_Folders/EQs/paramet.htm). The last stage resembled a parametric section to me.

got it (i think).  i was focusing on the op amp in the final stage of the bd-2 and i should have been looking at that BJT as a replacement for an op amp buffer.  after moving things around a bit, the gyrator you reference and the stuff hanging off the feedback loop of the op amp in the BD-2 are comparable:



thanks hugh!!   

edit:  i guess that suggests that the 470K is the more natural resistor to use for changing the frequency of the peak?

[gaussmarkov]

so ... the diodes across the op amp inputs are for protection ... see R.G.'s What are all those parts for??.  also, this has been asked before:  Diodes across op amp input thread. 

i still have not found another example of an op amp in the power supply.  here's the basic setup:



my guess is that the op amp is in there to hold Vref steadier.  is this probably another example of the sort of precautions that commercial designers take that we can generally omit?

[markm]

So,
Would the op-amp then be considered unneccessary?

[gaussmarkov]

So,
Would the op-amp then be considered unneccessary?

hi mark!  that's my conjecture.  and the application of a unity gain op amp buffer
to power supply was new to me.  so i thought others might be interested, too. 

for the moment, i have laid out the bd-2 in 2 versions, one with and one without this feature.

[markm]

Interesting.
I have to say gm, you've done a superb job with this.....Excellent as always. 

[TELEFUNKON]

so ... the diodes across the op amp inputs are for protection ... see R.G.'s What are all those parts for??.  also, this has been asked before: 

http://i41.photobucket.com/albums/e270/gaussmarkov/Forum%20Posts/opampinps.png

...  is this probably another example of the sort of precautions that commercial designers take that we can generally omit?

it has been explained on this board many a time, and...