This Week on the Breadboard: Blues Driver BD-2 & BD-2w - part 3

Chuck D. Bones

Circuit Wizard
The CB mods
I like the idea of the S/C switch, but I want it to have a more pronounced effect. Dividing the S/C switch into a TIGHT switch and a FAT switch simplifies things and allows four tonal options instead of two.

The TIGHT switch
I find switching C41 in and out to be counter-productive. I split the difference, made C16 = 4.7nF and deleted C41 and its switch. When I first assembled my BD-2 breadboard, I built the 1st tone shaping network just like a Fender tone stack by including pots for TREBLE, MID & BASS. I found that the BASS pot was the most useful. Seems Roland came to the same conclusion when they added R42 and its switches. We can make the TIGHT switch have a little more leverage if we reduce R21 & R42 a bit. I ended up with 10K and 3.3K, respectively. Feel free to experiment. Now the TIGHT switch's freq response looks like this. Remember, the first stage cuts the bass. The blue curve is the same as the BD-2 and restores most of the lost bass.

BD-2 cb tight switch freq resp.png

When the TIGHT switch is on, we're getting an even tighter bottom-end compared to the BD-2w. I also bumped C13 up to 390pF for a little bit more treble. It's subtle. A single-pole toggle switch and a 10K resistor is all we need to implement the TIGHT switch.

The FAT switch
I kept the same tuning for the FAT switch as the BD-2w. I think it does enough. You'll need a DPDT toggle switch, a 5.6nF cap and a 22nF cap to implement the FAT switch. If you want it darker in FAT mode, increase C43. Or just turn the TONE down, duh.

The 3rd Stage
I could not abide the 3rd stage's crazy 125Hz bump. I wanted bass boost over a wider range. First, I retuned the gyrator to make a broader 5.5dB hump at around 60Hz. From here onward, I'll refer to the Cobalt ref designators. Try to keep up. Referring to the Cobalt schematic, I changed C17 to 390nF, C18 to 6.8nF and R20 to 5.6K. This plot compares the original and modified 3rd stage freq responses. Green is the cb mod.

BD-2 cb 3rd stage freq resp.png

The overall freq responses are compared below, with all controls at noon, BD-2w mods in place, TIGHT and FAT switches off. Green is the cb mod. The freq response is smoother (no bumps) and the bass extends almost an octave lower. We have to be careful how much bass we add in the 3rd stage because the compression of the 1st and 2nd stages brings the bass content up to equal everything else at higher GAIN settings. But if you want even more bass, that's doable too!

BD-2w cb freq response.png

I have one more mod up my sleeve. I liked the idea of no ICs on this board and I knew that the bass boost filter and output buffer could be realized with a single MOSFET. It's a breadboard, so anything goes. I deleted IC1, Q7, R19-R23, C16-C19 and replaced them with this (ignore the ref des here, it's an LTSpice schematic):

3rd stage - cb mod.png

I also changed C20 to 1μF and R24 to 100K (optional). This mod does a few things.
  1. Gets rid of an IC and two capacitors.
  2. Implements the same freq response as the 3rd stage cb mod.
  3. Has a slightly higher output impedance, but it's under 600Ω so no problem driving cables.
  4. Reduces the gain of the 3rd stage just enough so that we can dime the LEVEL control without saturating the 3rd stage. It still gets plenty loud.
  5. Inverts the signal. Some people might object to this for religious reasons.
  6. Sounds great.
This next plot compares the freq response of the cb modded opamp 3rd stage with the MOSFET 3rd stage. The gain of the MOSFET design is 2.5dB lower to preserve the headroom. The curves match down to 40Hz, then the MOSFET circuit drops off more steeply below that.

BD-2 cb MOSFET 3rd stage freq resp.png

I'm figuring out a way to kludge the MOSFET 3rd stage onto the BD-2 board. Should be pretty easy to hijack the unused pads & traces once the original 3rd stage is deleted. The final details of the Boneyard edition will be the subject of part 4.

The -2w "C" mod
We can easily mod the Cobalt to get the -2w "C" tone by changing the parts listed in the table below. This is NOT the Boneyard mod and there are no switches. Bear in mind that this mod makes a subtle change to the BD-2's tone, just like the BD-2w's S/C switch.

Cobalt Ref DesBuild DocsC Mod
C1100nF2.2uF film *
C22.2nF12nF +
C8100pF47pF
C105.6nF6.8nF
C135.6nF10nF
C1418nF39nF
C1756nF68nF *
D51N914not used
D61N914not used
R2220K2M &
R81.5K1.2K
R181M10K
R201.2K820R *
* I doubt anyone will hear the difference by changing these parts, so don't bother. I put them on the list for completeness.
+ 10nF or 15nF are plenty close if you don't have 12nF.
& Use 2.2M if you don't have 2M.
To go the whole nine yards, make C3 and C20 tantalum.

That's all for part 3. It will be a few days until part 4 is ready.
 

Feral Feline

Well-known member
Bass is my religion.

I suppose I'd have to invert the signal again to include a clean blend (bass-guitar in mind).


I'm going to have to re-read the series a few times to try to get it all to sink in.

Come on part 4!

🙏
 

Chuck D. Bones

Circuit Wizard
This is a "dirty boost" and unless you dime the GAIN control there is plenty of clean bleed built in. You may not need a clean blend. Try it as-is and then decide.
 
Top