SOLVED Blue Breaker.... weird question / comparison

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GenoBluzGtr

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I built a Blue Breaker a few weeks back. Sounds fantastic... sounds very close to the original Marshall pedal... I plan to build more.

I had bought a BYOC kit several months before, forgot I had it. But I found it, and decided to build it this weekend. However, it sounds nothing like the Blue Breaker or the original pedal. More gain, but not in a good way. Sounds "cheaper" and less hi-fi. The Range of the Tone and Drive control seems off, and at some setting below halfway on either of those, the volume takes a big hit (won't get above unity.

I thought I had made some errors, but I re-examined it and the build docs and it is all correct. I compared the schematics and here's what I found.... (see attached .pdf). The biggest thing I can find is that the input / output impedances are WAY off. The Blue Breaker uses 2.2M on the input and 1M on the output, where the "L'il Breaker uses 470K and 100K respectively. A couple of capacitor differences but I don't think those have much of an impact on gain/clipping etc. Also the 1n914s / 1n4148s are pretty identical.

Any thoughts on why the two pedals would sound SO different? All other values are identical (pots are 9mm on the smaller version, but same tapers/values). What impact does input/output impedance have on the gain/clipping/etc??
 

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It's C5 causing the difference. You'll get more bottom-end hitting the 2nd stage at high DRIVE settings with the Li'l Breaker. At lower DRIVE settings, they should sound the same. 1N4148 & 1N914 are the same diodes with different part numbers. R10 doesn't matter. R1 might make a slight different in top-end sparkle with some pickups, but it will be very subtle. C9 doesn't matter. Which opamp is in the Li'l Breaker? Are you positive you didn't change something inadvertently?
 
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They both use a TL072. And, yeah, I triple checked that I didn't swap any parts - it's identical to the schematic.

Even at lower settings on the drive control the clipping sounds much different... "rattier" or less "controlled" is the best way tot describe it. The Blue Breaker sounds more refined and 'hi-fi'. The L'il Breaker seems to sound more like Op Amp clipping is happening.

My thought on the impedance is two fold... lower input impedance should mean more overall current, correct? Also, Op Amps are most efficient when they have higher input impedance and lower output impedance. the Blue Breaker uses the exact same component values as the original Marshall pedal, and I've compared the two they sound the same, but this one sounds more like a cheap overdrive.
 
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What's on those schematics and the way you describe the tonal differences, doesn't add up. Something else is going on that you're not seeing. A bad connection, bad part, something like that. Have you traced the Li'l Breaker board to confirm that the routing matches the schematic?

"Op Amps are most efficient when they have higher input impedance and lower output impedance." This statement makes no sense and in some circuits is completely wrong.
 
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The Blue Breaker sounds very much like this....
.

and the BYOC L'il Breaker I built sounds a lot like the one in this video, but I've never played a Marshall Blues Breaker that sound as harsh as this...
.

So, I WILL say, that it sounds pretty much like the one in the demo video, but that sounds nothing (in my opinion) like a Blues Breaker... and definitely not like the Pedal PCB Blue Breaker I built. Maybe I'm being nit-picky, but it's very different kinds of drive to my ears. Just wondering why the same essential circuit is so different with just a few components changed out.
 
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Okay... I just opened up both pedals and re-checked all the components, they are all as-listed in the schematic. I traced the board and as best I can tell, it DOES follow the schematic.

however, I read voltages on the TL072 on both... here's what I found:

BlueBreaker:

Pin: 4.58 Pin 8: 9.14
Pin2: 4.57 Pin 7: 4.57
Pin3: 4.16 Pin 6: 4.58
Pin4: 0. Pin 5: 6.89


L'il Breaker:

Pin: 8.46 Pin 8: 9.3
Pin2: 8.47 Pin 7: 8.46
Pin3: 7.71 Pin 6: 8.44
Pin4: 0. Pin 5: 8.4

Both pedals are being powered by the same DC power supply and I read 9.31V on the "+" coming in from the power jack.

So I DO think there is overall lower resistance (and Impedance??) causing higher current flow, resulting in higher operating voltages on the IC Chip. To my ears it is DEFINITELY a "Headroom" issue. The Blue Breaker cranked up to full on the gain pot sounds "similar" to the BYOC at about 9-10 O'clock on the gain control - re: a "clipping level".

My question is, why and is this "by design" on the BYOC board?
 
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and I think I actually figured it out. You were correct, Chuck, it is NOT the impedance causing the issue.

I completely forgot to check the power generation section of the two schematics....

The Blue Breaker (like the original) uses two 47K resistors with two 100uF Electrolytic caps to generate a V Ref of 4.58V

The L'il Breaker (BYOC) uses two 10K resistors and two 47uF Electrolytic Was to generate a V Ref of 8. 47V.

The near-doubling of the voltage on the IC is driving it harder. Causing more clipping. I can replace the components in this section, and very likely get them to sound really close. Thoughts??
 
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OK, let's talk about the Blue Breaker first.
All the opamp voltages are what I would expect. Pin 8 is power, should be close to +9V. Pin 4 is ground, should be zero. All the rest are biased at Vref, which should be close to 4.5V. Looks like your DVM has a 10Meg input impedance, because it is loading down pin 3 a little bit. The only time pin 3 is at 4.16V is when you have the meter probe touching it. As soon as you take the probe off, it's back at 4.57V. This pedal is working correctly.

The Li'l Breaker should have all the same voltages on it, but it doesn't. Not even close. Not sure what voltage you measured on pin 3, but it's not 47.71V, so please go back, measure it again and correct the typo in your post above. Pin 5 is at Vref, which should be around 4.5V. It's not even close. If the IC is in a socket, pull it out and measure pin 5 again. Here are the possible things that could be wrong:
1) R11 is the wrong value.
2) R12 is broken, not connected, or the wrong value.
3) IC1 is bad.
4) There is a manufacturing flaw in the PCB (unlikely, but not impossible).

The differences in C9, C10, R11 and R12 will make no difference in this circuit.
 
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You're correct, it was 7.71C, not 47.71. Sorry.

I am measuring 8.48V ref coming out of the R11/R10/C9 Junction. and no, the IC isn't socketed but I will remove it, socket it and try a new IC. R12/R11 are the correct values and are solder correctly, no cold joints. But I will triple check.

Thanks!
 
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Here is the Lil Breaker... just snapped these Post-Removing the IC. As I mentioned, with IC out of the circuit, the Vref is still 8.47V...

R11/R2 are the two resistors between the Electrolytics on the right side of the board
 
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