JHS Pedals Overdrive Preamp

[jeffwhitfield]

I highly doubt this is a complex circuit so shouldn’t take long to trace. It’s a precursor to the DOD 250. Very interested in seeing what this circuit looks like. Sounds interesting.

JHS Pedals, Kansas City USA— Overdrive Preamp

www.jhspedals.info

 

[jesuscrisp]

Curious to hear how much Gain with a B250K?
DOD 250 uses 500K, Distortion + uses 1M!
Every modder seems to use a C taper!

The gain control in the 250 works the other way round, the bigger the less gain you get. Usually you have 1M resistor in the feedback loop + 500K pot to ground, the JHS has 510K + 250K pot. The resistance to ground also factors into the low end cut off, which is why the 250 is usually darker at low gain and brighter at high gain settings.

Jhs config should give you the same amount of gain, just somewhat brighter. The biasing of the op amp is also different in the JHS one, which is why I'm curious about how the taper interacts with it (if there is an effect). With a standard DOD250, linear taper gain pot is usually stupid.

Guess I'll just breadboard it myself too.

 

[jeffwhitfield]

The gain control in the 250 works the other way round, the bigger the less gain you get. Usually you have 1M resistor in the feedback loop + 500K pot to ground, the JHS has 510K + 250K pot. The resistance to ground also factors into the low end cut off, which is why the 250 is usually darker at low gain and brighter at high gain settings.

Jhs config should give you the same amount of gain, just somewhat brighter. The biasing of the op amp is also different in the JHS one, which is why I'm curious about how the taper interacts with it (if there is an effect). With a standard DOD250, linear taper gain pot is usually stupid.

Guess I'll just breadboard it myself too.

Keep in mind that the capacitor values posted are just estimates. The only accurate ones are the electrolytes, which you can see in the photos of the PCB. Definitely would love some folks to breadboard this and test it with me.

 

[cdwillis]

I love the assumption that it sounds good because it was a rare early version with parts they salvaged from junk. Maybe, just maybe, they settled on the basic design with the silicon clipping diodes and properly biased input on the op amp for a reason lol. The emperor has no clothes. It's sad that people would pay that much for one of these in earnest.

 

[mybud]

“Clothes make the man. Naked people have little or no influence on society.“ Mark Twain

 

[Coda]

“Clothes make the man. Naked people have little or no influence on society.“ Mark Twain

Tell that to Pam Anderson...

 

[jeffwhitfield]

I love the assumption that it sounds good because it was a rare early version with parts they salvaged from junk. Maybe, just maybe, they settled on the basic design with the silicon clipping diodes and properly biased input on the op amp for a reason lol. The emperor has no clothes. It's sad that people would pay that much for one of these in earnest.

Yup, that's my thoughts on it. I'm comparing it to my Irrlicht and for the silicon diodes I can pretty much match it with some extra compression and the Weight control. The Irrlicht is a pretty flexible version of the 250 circuit. Only thing it's missing is the germanium diodes...which might be an interesting addition to it.

 

[cdwillis]

I breadboarded a 250 circuit on one of my small breadboards a couple days ago to play around with. I put 3 1n4148s on it to start to see how the asymmetrical clipping would sound. I don't know that I ever really tried germanium diodes with it before. The 1n34a diodes I got from Tayda all have a strangely high vF that matches a silicon diode range, but they look like 1n34a diodes. I ordered some more from Cusack for a klon, so I'll see how those sound.

 

[jeffwhitfield]

I breadboarded a 250 circuit on one of my small breadboards a couple days ago to play around with. I put 3 1n4148s on it to start to see how the asymmetrical clipping would sound. I don't know that I ever really tried germanium diodes with it before. The 1n34a diodes I got from Tayda all have a strangely high vF that matches a silicon diode range, but they look like 1n34a diodes. I ordered some more from Cusack for a klon, so I'll see how those sound.

The ones I get from StompBoxParts and Pedal Hacker are good. D9E’s are another option

 

[jimilee]

The ones I get from StompBoxParts and Pedal Hacker are good. D9E’s are another option

I like D9Es better than the 1n34a, like a lot.

 

[music6000]

I breadboarded a 250 circuit on one of my small breadboards a couple days ago to play around with. I put 3 1n4148s on it to start to see how the asymmetrical clipping would sound. I don't know that I ever really tried germanium diodes with it before. The 1n34a diodes I got from Tayda all have a strangely high vF that matches a silicon diode range, but they look like 1n34a diodes. I ordered some more from Cusack for a klon, so I'll see how those sound.

Tayda 1N34A's are questionable, Look like Germanium but act like Silicon with similar forward Voltage????????????????
I Don't use them in my Builds!!!

 

[jeffwhitfield]

I like D9Es better than the 1n34a, like a lot.

I put D9E’s in my Klon Klone. Helps soften things up a bit.

 

[cdwillis]

Tayda 1N34A's are questionable, Look like Germanium but act like Silicon with similar forward Voltage????????????????
I Don't use them in my Builds!!!

Here's a pic of the diode and what it measured on my cheap tester. I'm not sure what mA this thing is feeding the diode to test, but the tester has been pretty accurate with everything else I've thrown at it. I actually left the diode sitting in the tester while I ate dinner so when I came back to measure it I knew the heat from handling it with my fingers didn't screw with the reading.

https://imgur.com/a/Zxuh9Ci

I've measured four of the ten I ordered and they were all like this.

 

[music6000]

Here's a pic of the diode and what it measured on my cheap tester. I'm not sure what mA this thing is feeding the diode to test, but the tester has been pretty accurate with everything else I've thrown at it. I actually left the diode sitting in the tester while I ate dinner so when I came back to measure it I knew the heat from handling it with my fingers didn't screw with the reading.

https://imgur.com/a/Zxuh9Ci

I've measured four of the ten I ordered and they were all like this.

Shouldn't it be reading around 300mV?
Test the Diode & write down the numbers.
Rub between fingers then test again & write down those numbers.
Are they the same?

 

[cdwillis]

I rubbed the glass in between my fingers for a few seconds and put a little hot breath on it then measured. Came out to 601mv vs the 624mv that I measured cold last night. Should be around 300mv if it was an actual germanium diode.

 

[Chuck D. Bones]

Unless you know the test current, that Vf reading is meaningless. Do you have a DMM? If so, try measuring Vf with that. The Ir number is consistent with Germanium. Silicon would read 50nA or less. I too was skeptical of the 1N34A and 1N270 I bought from Tayda a few years ago, so I put them on a curve tracer.

 

[giovanni]

What’s the verdict here? The tayda ones look different than others but are they too different?

 

[Chuck D. Bones]

The verdict is they're ALL different and that's to be expected with 1N34s.
If it's really a Distortion+ (DOD 250, et.al.), then the diode curve doesn't matter much. Some people prefer silicon diodes for more volume. You can also try schottky or LEDs. The Distortion+ drives just under 400μA max into the diodes, which puts the peak Vf for the diodes shown above in the 160mV to 270mV range.

 

[spi]

I had some from Tayda that I measured with my DMM at 310-325 mV.

Could be different batches though.

 

[Chuck D. Bones]

What's your DMM's test current?

 

[zgrav]

I like D9Es better than the 1n34a, like a lot.

I was looking at the 250 vero layouts on tagboardeffects site and one commenter recommended Bat46 diodes for the build over the reportedly thin-sounding 4148s.