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

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.
 
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.
 
@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?
 
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?
 
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.
 
Just run this part of the circuit on the +9V rail. This circuit contains single-ended gain stages, which not benefit from split supplies.
 
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.
 
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.
 
lowpitch said:
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.
Click to expand...
Sure, we can do that. It's really no different from running on a single supply rail.
 
Referencing the first two schematics shown in post #1: the first is the original Diamond Peak, and the second is Chuck's first-pass modified version. Specifically, I'm curious about what I think is the bias network for Q1 (R3, R6, R7; and C2 in Chuck's version). My naive take is that Skreddy's wiring would have considerably lower input impedance, and also looks like the signal wouldn't be biased at 1/2 VDC, since the voltage divider isn't symmetric (24k and 100k). Just curious about the thought processes behind the two different approaches.
 
Both circuits bias Q1 at around 5V. The v1.2 circuit biases Q1 a little colder than the stock circuit. Easily adjusted by changing R6 or R7. The symmetry of the voltage divider (R6 = R7) is not important because there are voltage drops in R3, R6 & R7 that depend on the resistance and Q1's base current. What matters is Q1's collector voltage.

The input impedance is mainly influenced by R3 and Q1's HFE (higher HFE = higher input impedance).

The input impedance of the stock circuit is around 150K. V1.2 is around 250K. Either one is sufficiently high.

V1.2 has filtering on Q1's bias (C2 on v1.2 sch) which I think is necessary for lowest noise. The effect may not audible if you have clean power to begin with. We could just build the stock circuit and connect a 10uF cap in parallel with R6 id we want a filtered bias voltage.

The final version ended up with a JFET for Q1 which saves a few parts. All three input stages are going to sound pretty much the same. The JFET stage might sound a little brighter with single-coil pickups because the input impedance is higher.
 
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