GK rails in a pedal?

[johnk_10]

Do you have the foldback off for these listening tests?

-Frank

actually, thru my amp, i'm not hearing that much of a difference with the foldback jumper in or out.

 

[boomertech]

actually, thru my amp, i'm not hearing that much of a difference with the foldback jumper in or out.

We may need to adjust the voltage levels of when the foldback is activated. When you measure the clipping voltage level (with foldback off) at the junction of R1/R2 we can then determine what the values for R19 and R20 should be for the clipping level that you want with foldback on. This is a great place to make the clipping asymmetrical for when the foldback is on, by making R19 and R20 different values.

Time for me to go analyze that tranny now.

-Frank

 

[boomertech]

Here is the plot for the TY-250P transformer testing shunt attenuation:

The green trace is the 0dBu reference input signal.

The purple trace is the transformer output without a load connected across its output terminals.

The yellow trace is the output with a 1K ohm load across the output terminals… I was expecting at least a 6dB drop because the secondary impedance on this transformer is 1K ohm. So, it’s only a little bit more than expected.

The orange trace is the output with a 500ohm load across the output terminals.

The blue trace is the output with a 250ohm load across the output terminals.

I am very, very impressed with this $5 transformer. It accepts a shunt attenuator very well and the frequency response is very good and stable with all of these loads.

BTW John, you will have to let me know what taper pot feels best for VR2.

-Frank

 

[Ewo]

Very cool project, Frank and John!

 

[tallboybass]

Yeah, this is like being in the laBORatory with the mad scientists!!

 

[johnk_10]

i built a separate input buffer with the output level control (using Frank's schematic) and ran it in front of my prototype V3. once I turn it up past 1/2 way the circuit pulls more current (normally it's 20ma and it goes up to 50ma) and shuts off with no output. I tried two other buffers that I built and tested with the same result. i'm currently trying to figure out why it's doing this.

 

[johnk_10]

after having issues with several buffered attenuator inputs, i found a very simple way to adjust the level at the input, and it works great. I just added a C500K pot in series before the input to the V3 power amp.
I've done this on several other distortion pedals and I has always worked well, and it works great on this one too.

 

[Ewo]

I'm not what you'd call a pedal hound, but I'd be sorely tempted to buy one of these if they're available for sale...

 

[boomertech]

i built a separate input buffer with the output level control (using Frank's schematic) and ran it in front of my prototype V3. once I turn it up past 1/2 way the circuit pulls more current (normally it's 20ma and it goes up to 50ma) and shuts off with no output. I tried two other buffers that I built and tested with the same result. i'm currently trying to figure out why it's doing this.

Have you looked with your o-scope to see if the output of U1a starts to oscillate when you turn up the level?

EDIT: Also check the output of the input buffer for oscillations.

-Frank

 

[boomertech]

I added filtering to prevent oscillations from the input buffer. Make C10 10uF or larger and I think that issue will be resolved.

REMOVED IMAGE... SEE NEXT POST

-Frank

 

[boomertech]

John, its looks like we need to dampen the simulated HV power rails. I added R29, R30, C11 and C12 to keep the circuit stable.

IMAGE HAS BEEN REMOVED. UPDATED SCHEMATIC LATER IN THIS THREAD.

-Frank

 

[boomertech]

This might be a fun experiment.

A lot of pedals have speaker simulation EQ curves. Although they seem to work well for most people, I had this idea a dozen years or so ago to put REAL speakers in a pedal. I have never tried to do this, but this GK project seems like the perfect place to try it out.

My idea was to take the speakers from a cheap pair of earbuds and couple them face to face in a tube. Then feed one side the signal from a low power amplifier and take the output from the other speaker (acting like a microphone) and feed that off to my rig or another pedal… acoustic coupling. It is a similar principle as a transformer, so I was going to call it an Acoustiformer. Here is a rough image of this idea installed in the GK amp sim:

This would replace R7 in the amp simulator and can be turned on or off by a DPDT switch on the pedal. The Acoustiformer would need to be mechanically decoupled from the pedal enclosure to prevent mechanical pickup. Maybe surrounding it in soft foam or suspending it by mounting it to the center of a stretched rubber band anchored by two posts may work well.

The vent hole would be very tiny, maybe melt a hole with a hot pin. This is to prevent pressure changes in the atmosphere from stressing the drivers.

Some other ideas I had with this were to try different materials in the tube between the two drivers to create other distortions, or gluing the drivers face to face for direct coupling.

It could be something really cool or a foolish idea… gotta try it to know for sure.

-Frank

 

[nshuman]

Not sure if there are Talkbass Awards. But, you two need to be nominated!

 

[johnk_10]

John, its looks like we need to dampen the simulated HV power rails. I added R29, R30, C11 and C12 to keep the circuit stable.


-Frank

I think that it might need to be stabilized too so i'll give those additions a shot.
it almost seems like it needs some sort of limiter in front of it since when the signal is too hot, it collapses into a really ugly distortion.
I tired a bearfoot blueberry in front of it without the input buffer and it sounded realy good up until a point, but too much gain and it just started fizzling out.

 

[boomertech]

I think that it might need to be stabilized too so i'll give those additions a shot.
it almost seems like it needs some sort of limiter in front of it since when the signal is too hot, it collapses into a really ugly distortion.
I tired a bearfoot blueberry in front of it without the input buffer and it sounded realy good up until a point, but too much gain and it just started fizzling out.

When that happens would you be able to measure the positive and negative supplies?

-Frank

 

[boomertech]

I think that it might need to be stabilized too so i'll give those additions a shot.
it almost seems like it needs some sort of limiter in front of it since when the signal is too hot, it collapses into a really ugly distortion.
I tired a bearfoot blueberry in front of it without the input buffer and it sounded realy good up until a point, but too much gain and it just started fizzling out.

John, I looked deeper into the LF353 data sheet and read this:

**********************************************
The maximum differential input voltage is independent of the
supply voltages. However, neither of the input voltages
should be allowed to exceed the negative supply as this will
cause large currents to flow which can result in a destroyed
unit.

Exceeding the negative common-mode limit on either input
will force the output to a high state, potentially causing a
reversal of phase to the output. Exceeding the negative
common-mode limit on both inputs will force the amplifier
output to a high state. In neither case does a latch occur
LF353 since raising the input back within the common-mode range
again puts the input stage and thus the amplifier in a normal
operating mode.

Exceeding the positive common-mode limit on a single input
will not change the phase of the output; however, if both
inputs exceed the limit, the output of the amplifier will be
forced to a high state.

The amplifiers will operate with a common-mode input voltage
equal to the positive supply; however, the gain bandwidth
and slew rate may be decreased in this condition.
When the negative common-mode voltage swings to within
3V of the negative supply, an increase in input offset voltage
may occur.
************************************************

Try changing the opamp to an OPA2134 to see if things change for the better. If they do, then the LF353 in position U1b is not a good choice... then we would have two options to go forward. Use the LF353 in the first position and change U1b to a better opamp for this position OR just use the OPA2134 for both positions. My vote goes for the OPA2134.

-Frank

 

[boomertech]

Not sure if there are Talkbass Awards. But, you two need to be nominated!

Thanks! This is just fun stuff and I needed to do something interesting before the end of 2014. I am just happy John has an interest in making this work.

-Frank

 

[johnk_10]

yeah, that sounds exactly like what the LF353's are doing (I've tried 3 of them so far). at first they seem to be fried, but if I let them sit a while they seem to work again, although they don't seem to sound as good as before they got overloaded. they do end up I think that I tried a OPA2604AP and it still did it though.

 

[johnk_10]

I like the challenge of building pedals that no one else has built before so I can't thank Frank enough for drawing up this one.
IMO, it's actually almost there, and it does have a thick & burly growl to it.

 

[blindrabbit]

I'm not what you'd call a pedal hound, but I'd be sorely tempted to buy one of these if they're available for sale...

I'm not really a fan of the GK sound, but watching this process unfold is so fun, I'm quickly becoming tempted to pick one up too.

+1 to the "TB Awards" nomination. Talk about watching two stars at work. Impressive work fellows. This is really something to see.