This Week on the Breadboard: Animals Diamond Peak Overdrive (with some mods, of course)

[Chuck D. Bones]

C6 provides a little extra top-end cut. The cutoff freq varies with the VOLUME setting, but is in the neighborhood of 10KHz when VOLUME is at noon. It's subtle.

 

[MattG]

@Chuck D. Bones - what are your thoughts on BJT vs JFET for Q1? I see the Animals/Skreddy circuit, and your initial design, used BJT, but then you switched to JFET.

 

[Chuck D. Bones]

Either way works well. There's not much difference in tone. The JFET version has fewer parts, but we have to deal with biasing Q1.

 

[jessenator]

A good starting point is Vp between 1 and 2V

I've been looking through some datasheets and I don't see that particular spec, "Vp". Am I not seeing something? Is that a value you calculate based on other specs in the datasheet? Thanks.

 

[Chuck D. Bones]

I have written extensively about JFETs in this forum.

Vp is the pinch-off voltage, the gate-source voltage where the drain current approaches zero. Most datasheets refer to it as Vgs,off and it is expressed as a negative voltage for N-channel JFETs. As a form of shorthand, we usually express Vp as a positive number, with the understanding that it is actually a negative value for N-channel JFETs.
In other words, Vp = -Vgs,off.

The other thing you need to know is that the datasheet just gets you in the ballpark. The datasheet Vgs,off range is large and in nearly every JFET circuit, you will need to measure and select the particular JFET for Vp or Idss.

 

[temol]

Finally boxed.

 

[Chuck D. Bones]

Ooooooh! Love the machine-turned finish.

Pretty nice inside too.

 

[MattG]

@Chuck D. Bones - is it possible to adapt to adapt a design like this to a bi-polar power supply? I.e., what if we have +/-9V supply available? Is that something that can be done with minimal tweaking, or does it require a significant redesign?

 

[Chuck D. Bones]

What are you trying to accomplish here? This circuit is optimized for +9V power. Sure, we can alter a circuit to run on other rail voltages, but why?

 

[MattG]

Partially just out of curiosity. I haven’t been able to find any examples of single-transistor gain stages with bipolar supplies.

But more specifically, I am thinking about integrating this circuit with another. The other circuit is opamp based, and benefits from a bipolar supply.

 

[Chuck D. Bones]

Just run this part of the circuit on the +9V rail. This circuit contains single-ended gain stages, which not benefit from split supplies.

 

[Blackboarcult]

I haven’t been able to find any examples of single-transistor gain stages with bipolar supplies.

Here are a couple of examples using BJT (a tad... Old):

https://www.drtube.com/schematics/marshall/2099u.gif

http://www.lynx.net/~jc/SchematicMonoBlock-II.pdf

For Jfets run on bipolar supplies, Gallien-Krueger amps from 1980 to late 90s feature them in several very interesting topologies

 

[lowpitch]

Getting slightly off-topic, but couldn't you just reference the transistor to the negative voltage instead of ground in a common emitter amplifier for example? If not, why not? I don't think this case was covered in your BJT biasing series, Chuck.

 

[Chuck D. Bones]

For the MonoBlock II preamp section, that's the same as running it on +30V since there is no DC ground reference in the circuit.

Marshall used 741's in the preamp, so it's hard for me to take that design seriously.

Running solid-state power amps on split supplies, now THAT makes sense.

 

[Chuck D. Bones]

Getting slightly off-topic, but couldn't you just reference the transistor to the negative voltage instead of ground in a common emitter amplifier for example? If not, why not? I don't think this case was covered in your BJT biasing series, Chuck.

Sure, we can do that. It's really no different from running on a single supply rail.