Chuck D. Bones
Circuit Wizard
I built one of these a few years back and my only complaint is the hiss. I replaced the aluminum caps with tantalum and it helped, swapped out a couple of opamps, but none of it made much difference. With the GAIN & Tone turned up there was too much hiss. Other than that, it is a sweet sounding MIAB. Nice harmonic content, good dynamics and very smooth note decay.
So I built a breadboard, minus the charge pump, and ran it on +18V. Definite improvement. I used TL072 all around, although LM833 and the other low-noise dual opamps will all work. The last 3 stages have the least headroom, but with 18V power, there is more than enough.
Here's a quick circuit description. ROG gets into more detail on their website. IC1B, the TONE & GAIN controls emulate the Normal & Treble channel preamps wired in parallel. ROG used a "boutique modded Marshall Super Lead" as their reference. I don't have access to one of those, so I take their word for it that the 1st stage delivers the correct EQ. There is more gain when TONE is turned up, but overall, U1B barely makes up for the losses in the TONE network. IC1A boosts the signal and drives the asymmetric clipper made up of D1, D2 and surrounding components. As far as I can tell, the purpose of D7 & D8 is to limit how hard IC1A hits the D1/D2 clipper. According to ROG, the D1/D2 clipper circuit is the secret sauce that makes the tone and dynamics. IC2 provides some additional gain and drives the fixed tone stack (C4, C8, C14, R8, R9, R13 & R19). IC3 and the green LEDs (D3 & D4) emulate clipping in the Marshall's phase splitter. D6 & D6 provide a little more asymmetric clipping at the end to model imbalance and non-linearities in the pentode output stage and the output transformer. IC2B & IC3B form a cab sim and provide some EQ at the end. The flattest setting is TOP & BOTTOM at zero, PRESENCE at 10. It should be noted that all of the opamps are intended to run in the linear region; no opamp clipping. That's why ROG runs the power rail at +24V. My breadboard runs fine on +18V.
Knobs (L-R): LEVEL - PRESENCE - BOTTOM - TOP - TONE - GAIN
So that's one way to get rid of the charge pump. Another way is to put clipping diodes D1, D2, D7 & D8 in the feedback loops of the opamps. If we do that, then we can run it on 9V. It won't have exactly the same dynamics, but I got pretty damned close.
I used the existing first stage, but I increased the gain by 6dB and moved the low-freq cutoff down. I re-jiggered (technical term) the TONE network so that the response is pretty flat when TONE is at zero, just like a Marshall's Normal channel. When TONE is at 10, we get the usual bass cutoff of the TREBLE channel.
I moved the GAIN pot and D7/D8 clipper to the feedback loop of the 2nd stage. The red LEDs and R11 provide soft clipping to keep U1B from saturating and provide the right amount of drive for the next stage. The GAIN knob has a 40dB range.
The 3rd stage has a gain switch (MORE) at the input. With the MORE switch open, the overall gain is less than the stock Thunderbird and allows one to play clean with GAIN at zero. Closing the MORE switch adds 15dB of gain, putting the overall gain a little higher than the stock Thunderbird. D3-D6, R15 & R16 approximate the transfer function of the stock circuit's D1/D2 network and prevent U2B from saturating. I used a TLV2272 rail-to-rail opamp for the 3rd & 4th stages to maximize headroom. I also skewed Vref and flipped the polarities of all of the asymmetric clippers to maximize headroom.
Everything after that is unchanged except...
I moved the LEVEL control ahead of the cab sim to preserve headroom.
I retuned the PRESENCE control for more range.
Try it, I think you'll like it!
The 1st two stages are top right, middle two stages bottom center, last two stages top left. At the moment, I'm running JRC2068D for the 1st two stages. Other opamps on deck at the bottom right: TL072, LM833, NE5532.
Knobs (L-R) LEVEL - BOTTOM - PRESENCE - TOP - MORE switch - TONE - GAIN
So I built a breadboard, minus the charge pump, and ran it on +18V. Definite improvement. I used TL072 all around, although LM833 and the other low-noise dual opamps will all work. The last 3 stages have the least headroom, but with 18V power, there is more than enough.
Here's a quick circuit description. ROG gets into more detail on their website. IC1B, the TONE & GAIN controls emulate the Normal & Treble channel preamps wired in parallel. ROG used a "boutique modded Marshall Super Lead" as their reference. I don't have access to one of those, so I take their word for it that the 1st stage delivers the correct EQ. There is more gain when TONE is turned up, but overall, U1B barely makes up for the losses in the TONE network. IC1A boosts the signal and drives the asymmetric clipper made up of D1, D2 and surrounding components. As far as I can tell, the purpose of D7 & D8 is to limit how hard IC1A hits the D1/D2 clipper. According to ROG, the D1/D2 clipper circuit is the secret sauce that makes the tone and dynamics. IC2 provides some additional gain and drives the fixed tone stack (C4, C8, C14, R8, R9, R13 & R19). IC3 and the green LEDs (D3 & D4) emulate clipping in the Marshall's phase splitter. D6 & D6 provide a little more asymmetric clipping at the end to model imbalance and non-linearities in the pentode output stage and the output transformer. IC2B & IC3B form a cab sim and provide some EQ at the end. The flattest setting is TOP & BOTTOM at zero, PRESENCE at 10. It should be noted that all of the opamps are intended to run in the linear region; no opamp clipping. That's why ROG runs the power rail at +24V. My breadboard runs fine on +18V.
Knobs (L-R): LEVEL - PRESENCE - BOTTOM - TOP - TONE - GAIN
So that's one way to get rid of the charge pump. Another way is to put clipping diodes D1, D2, D7 & D8 in the feedback loops of the opamps. If we do that, then we can run it on 9V. It won't have exactly the same dynamics, but I got pretty damned close.
I used the existing first stage, but I increased the gain by 6dB and moved the low-freq cutoff down. I re-jiggered (technical term) the TONE network so that the response is pretty flat when TONE is at zero, just like a Marshall's Normal channel. When TONE is at 10, we get the usual bass cutoff of the TREBLE channel.
I moved the GAIN pot and D7/D8 clipper to the feedback loop of the 2nd stage. The red LEDs and R11 provide soft clipping to keep U1B from saturating and provide the right amount of drive for the next stage. The GAIN knob has a 40dB range.
The 3rd stage has a gain switch (MORE) at the input. With the MORE switch open, the overall gain is less than the stock Thunderbird and allows one to play clean with GAIN at zero. Closing the MORE switch adds 15dB of gain, putting the overall gain a little higher than the stock Thunderbird. D3-D6, R15 & R16 approximate the transfer function of the stock circuit's D1/D2 network and prevent U2B from saturating. I used a TLV2272 rail-to-rail opamp for the 3rd & 4th stages to maximize headroom. I also skewed Vref and flipped the polarities of all of the asymmetric clippers to maximize headroom.
Everything after that is unchanged except...
I moved the LEVEL control ahead of the cab sim to preserve headroom.
I retuned the PRESENCE control for more range.
Try it, I think you'll like it!
The 1st two stages are top right, middle two stages bottom center, last two stages top left. At the moment, I'm running JRC2068D for the 1st two stages. Other opamps on deck at the bottom right: TL072, LM833, NE5532.
Knobs (L-R) LEVEL - BOTTOM - PRESENCE - TOP - MORE switch - TONE - GAIN
