This Week on the Breadboard - ROG Britannia

Chuck D. Bones

Circuit Wizard
Another great pedal from the guys at Runoff Groove. This one is designed to mimic the AC-15 tones. I built it 98% by the book. For JFETs, I used 2N5457, SK193 & SK193. Q1's drain resistor ended up at 3.9K. Q2's source resistor is 18K. Q4's source resistor is 1.2K and the drain resistor is 10K. All of the drain voltages ended up with 100mV of the values on the schematic. I added a 2.2K resistor at the bottom of the GAIN pot. I ditched the 100nF cap after the TREBLE control, it was unnecessary because there is no DC to block and it is not part of a filter. I ended up changing the BRILLIANCE pot to B100K, it had a better feel. It doesn't do all that much and could be an internal trimmer instead of a front panel control (I can't believe I just said that). I also got rid of a redundant resistor in the last stage. I changed the 220nF cap to 22nF, the 4.7K to 47K, the 6.8K to 68K, jumpered the first 47K, changed the 2nd 47K to 27K, bumped the 1nF cap up to 1.5nF and the 330pF cap to 560pF. The filter is tuned the same and uses one less resistor. Not counting the LEDs up front, this thing has four separate diode clippers and Q2 can be driven to saturation & cutoff. All that makes for a lotta compression and distortion when GAIN is dimed. Setting gain below 10:00 provides very smoother distortion and easy volume clean-up. The bottom-end is rolled-off by the 2.2nF before the GAIN control. This preserves the clarity, even at high GAIN settings. Plays nice with single-coils or humbuckers. ROG did a very good job apportioning the gain and clipping at each stage. TREBLE & BASS controls are effective. Maximum output is just under 3.5Vp-p, so it can hit your amp pretty hard it you want it to. I used a TL072 opamp, but any dual FET input opamp would work. The things I changed had no effect on the tone, they just reduced the parts count a little bit. This one is definitely a winner.

ROG britannia.png
I left out the input protection LEDs. Controls are (L-R) VOLUME, BRILLIANCE, BASS, TREBLE, GAIN. Vref is on the bottom red rail.
ROG Britannia breadboard 01.jpg
 
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I thought the same thing. D1 & D2 don't do anything unless you're overdriving the Britannia with a boost. D3 & D4 conduct only at high GAIN settings. D5 & D6 provide very light clipping because the tone stack eats most of the signal and there is barely enough left to turn on D5 & D6. D7 - D10 and Q2 provide the majority of the clipping.

The guys at ROG saw fit to do it that way and I did not try altering the clipping. If you breadboard this, you could experiment with alternate diodes or no diodes in some of the places. D7 & D8 interact with D9 & D10 in that D7 & D8 limit the amount of signal that is available to drive D9 & D10. You could tweak the gain of the first opamp (R19 & R20) to control how hard D9 & D10 are driven.
 
I have not. The 2nd stage is based on the Omega. Personally, I think the Britannia has more than enough gain already. I usually set the GAIN control below noon.
 
I was comparing your schematic to the original and it looks like the terminals on the second opamp are flipped. From the picture of the breadboard it's hard for me to make out if that was intentional, but I would think not.
 
Stupid question: why are there 5 sets of clipping diodes? Seems like overkill to me
The guys at RoG believe that JFETs clip nicely when they're doing light clipping, but sound bad when hard clipping.

The LED's hard-clipping to ground were implemented to prevent the JFETs from hitting their saturation point. Notice that they're all at the transistor's Gates, rather than after the Drain -- pre-amplification rather than post-amplification. I think they detailed this theory when they released the Azabache (their Fender Amp-in-a-Box).
 
Another great pedal from the guys at Runoff Groove. This one is designed to mimic the AC-15 tones.
Is it noisy?

I've built quite a few RoG circuits, but avoided their final few amp-in-a-box designs, because I heard the cascading gain stages lead to a lot of noise.
 
I heard the cascading gain stages lead to a lot of noise.

That's kinda painting with a broad brush. Done right, 4 or 5 gain stages doesn't have to be noisy. My Britannia breadboard is very quiet. It's all about part selection, how the impedances are scaled, how the gain is apportioned, quiescent current, bandwidth, etc. I've been battling excess noise in my ROG Thunderbird. One source was a leaky cap. Still looking for the rest...
 
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