Double Mu-amp/mini-booster distortion keeps dying!!

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Diodes3Ground
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Joined: 05 Feb 2018, 01:34
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Post by Diodes3Ground »

I've been going through a bunch of designs that I've drawn up in the past and didn't quite work out. Many of them are either modulation effects or involve some type of blend circuit (and I may make a separate post about them in the near future), but one of the most aggravating ones is this FET-based distortion that uses two mini-booster/mu-amp type gain stages to produce its dirt.

I've built similar types of circuits before, such as straight up mini-booster clones and Wampler Pinnacle work-alikes, but lately, every single circuit I build with a mu-amp has had the same issue. Too much gain on tap (regardless of transistors used and with little impact from values chosen for the source resistors and nearby components), to the point of being extremely noisy. And if the front end is hit by a signal that's even slightly stronger than just normal pickups (I'm talking tube screamer or treble booster levels), the transistors in the first stage start fizzling out and die. In this particular design, I tried creating something like a stripped down Catalinbread DLS with some elements borrowed from the Pinnacle and Zvex box of rock for a 70's Marshall voiced distortion. Instead, the gain was at extreme levels, with nasty hum even at the lowest gain setting and way too much saturation and compression to sound anything like I was going for. Increasing some of the source resistor values reduced the gain a little bit, but it was still pretty high. Then, I ran my tube-screamer style build into the front of it, and the circuit died on me. I built it with pad for surface mount J201's when I first built the prototype, but also included pads for through hole JFETS, so I removed the smd's and put 2n5457's into the circuit. Exact same thing happened again, both with the gain and the transistors dying in the front end when boosted.

Any idea what I'm doing wrong? I didn't get a chance to record voltages before the transistors gave out, but might be able to try again in a few days when I get a chance. In the meantime, here's the schematic for the distortion, as well as my take on the mini-booster circuit. I've been over the schematics again and again (both the ones I've drawn and those of the AMZ mini-booster and other distortions using mu-amps), and I'm pretty sure I have everything drawn correctly, but obviously something is causing the issue. I've used various JFETS from various sources, so I also know it's not likely I just got a bad batch. Big thanks to anyone who can help me figure this out, between this project and a couple of similar past ones it's been frustrating me.

(Note: The backwards bjt's in the schematics represent the surface mount JFET footprints - I made sure all the pins were transposed properly, I just haven't gotten around to finding a decent smd library for Eagle yet).
Attachments
Deluxe Mini-Booster Schematic.jpg
Double Mini-Booster type distortion schematic.jpg

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