wintercept
Well-known member
Ok so between this and the general questions forum, I think (I hope) I'm in the right place after browsing the threads here. This whole project came about because I got some of these Mazda AA120 diodes from Small Bear ages ago. These are not made by the car company but rather a semiconductor manufacturer from the UK. Anyways, I have a buddy who races spec Miatas— MX-5, MX Drive, get it? He is also a fantastic shredder/metal player, so this project is inspired by him.
This design is aimed at boosting high gain tube amps, like how a lot of people run a tube screamer with the volume up and the drive down to get a tight metal tone. I use my Zendrive the same way. I wanted to work outside of the box and decided to build around a discrete opamp. I drew up an entire schematic, whipped it up on the breadboard and it worked right out of the gate. That felt great, and it sounded pretty good too. I swapped the original Rat/Zendrive style tone control for a SWTC and this is how it currently sits:
Disclaimer: this is a WIP! The schematic shown works but I wouldn't recommend building it.
Some of the resistor and cap values were decided at the breadboard based on what I had out already. The input and output buffers are your standard lot, and they work great. The TL072 buffer (U2A) will be reconfigured as a gain stage to raise the output a bit and add some filtering as needed. Where I'm struggling the most is refining the discrete opamp. I studied a number of pedals that use them like the Boss BD-2, OD-2, and OD-3 among others, the Seymour Duncan Pickup Booster, the Clapton mid boost, and the Ibanez CR-5. These designs all vary quite a bit, most use a larger value resistor (R7) for the emitter side of the long-tail pair; whereas the Pickup Booster uses matching resistors for both (R6 and R7). The 500K Drive pot (R10) is definitely too large, feels like I'm maxing out on gain from the opamp and I want less anyways. Then there's the resistor (R21) on the collector of Q4, I've yet to try anything else there.
So, if anyone has any advice or can point me towards some literature on simple discrete opamp design, I'd love to hear it. Most of the stuff I've found is about more complex HiFi designs with current sources and stuff. I don't need any of that! I mostly need to figure out what R6, R7, and R21 do.
This design is aimed at boosting high gain tube amps, like how a lot of people run a tube screamer with the volume up and the drive down to get a tight metal tone. I use my Zendrive the same way. I wanted to work outside of the box and decided to build around a discrete opamp. I drew up an entire schematic, whipped it up on the breadboard and it worked right out of the gate. That felt great, and it sounded pretty good too. I swapped the original Rat/Zendrive style tone control for a SWTC and this is how it currently sits:
Disclaimer: this is a WIP! The schematic shown works but I wouldn't recommend building it.
Some of the resistor and cap values were decided at the breadboard based on what I had out already. The input and output buffers are your standard lot, and they work great. The TL072 buffer (U2A) will be reconfigured as a gain stage to raise the output a bit and add some filtering as needed. Where I'm struggling the most is refining the discrete opamp. I studied a number of pedals that use them like the Boss BD-2, OD-2, and OD-3 among others, the Seymour Duncan Pickup Booster, the Clapton mid boost, and the Ibanez CR-5. These designs all vary quite a bit, most use a larger value resistor (R7) for the emitter side of the long-tail pair; whereas the Pickup Booster uses matching resistors for both (R6 and R7). The 500K Drive pot (R10) is definitely too large, feels like I'm maxing out on gain from the opamp and I want less anyways. Then there's the resistor (R21) on the collector of Q4, I've yet to try anything else there.
So, if anyone has any advice or can point me towards some literature on simple discrete opamp design, I'd love to hear it. Most of the stuff I've found is about more complex HiFi designs with current sources and stuff. I don't need any of that! I mostly need to figure out what R6, R7, and R21 do.
Last edited:
