Fluctuating waveform ~

[Gaz]

Sure - a well filtered supply helps. As for the added cap+resistor, you added a high pass filter that removes the low freq bobbing. The 1uf+1M has a roll off below 1Hz. The .022uf+1M rolls off ~50x higher. See if you can still observe the bobbing on one side of the .022uf cap, but not on the other.

Thanks, I appreciate your hypothesis about this, and will test it out. Only thing is that I only tested with a 1K sine wave, which makes me think it's not a low frequency filtering issue. I have also used this buffer a lot without issue, and the only difference is that it was going into a return stage (actually an LND150!). It wasn't necessary for this amp since it basically has the return stage built in

This is the schem I normally use:



I basically lifted the buffer from a thread that both R.G. and Merlin were helping with:

https://www.diystompboxes.com/smffor...topic=110137.0

The schem is gone in the thread, but I have it here (albeit a little hard to read):

[Gaz]

It doesn't matter if you use depletion or enhancement mode MOSFET's. The only difference is that one will idle with the Gate at a little lower voltage than the Source and the other will idle with the Gate at a little higher voltage than the Source.

There is no reason to idle the follower at such a high voltage because we're not dealing with a very big signal. If you use a lower idle voltage, the voltage divider for the Gate will reduce power supply noise at the same time. 10M resistors just looks like an invitation to pick up extraneous noise to me so I would spend a tenth of a mA in order to use smaller resistors.

I don't see anything inherent in your FX Loop that would cause the observed bobbing and settling.

I have attached a Twin FX Loop for your consideration.

Thanks a lot for drawing that up! I get your point about high voltage - I have used this before with much higher signal level (coming straight off the tonestack, for example in a Marshall type amp) and done the attenuation with a voltage divider on the source (I just made another post with it). It makes sense with the lower voltage here, I don't need to do that.

My feeling still is that the bobbing waveform is something else since I only used a 1K test signal...

[J M Fahey]

Something you should consider is that you are needlessly injecting supply ripple/noise into the signal.
100% or 10% of it is still way too much,

Just split the top 2M2 resistor into two 1M ones in series, and decouple the joint to ground with a 10uF capacitor ... instant bliss.

Works for both types of Fets .

[66 Kicks]

Something you should consider is that you are needlessly injecting supply ripple/noise into the signal.
100% or 10% of it is still way too much,

Just split the top 2M2 resistor into two 1M ones in series, and decouple the joint to ground with a 10uF capacitor ... instant bliss.

Works for both types of Fets .

I juggled the values a little so that the AC still sees 220K just like the AB763.

[catalin gramada]

Man, check the wiring track first, specially the input one no matter is shielded or not. Just tap with you finger the isolation of input wire from you loop return to see if get changes from fluctuating waveform artefacts.

[Gaz]

I juggled the values a little so that the AC still sees 220K just like the AB763.

Very cool, and added Juan's filtering trick. Can, I ask about a couple aspects of the schematic? First your choice of 220K for the of the output/input of the send/return jacks. Why not a larger value? Also, why the 100K on the gate?

And just so I understand correctly, by idling at 10%, it means the maximum swing would be 30Vp?

[66 Kicks]

Very cool, and added Juan's filtering trick. Can, I ask about a couple aspects of the schematic? First your choice of 220K for the of the output/input of the send/return jacks. Why not a larger value? Also, why the 100K on the gate?

And just so I understand correctly, by idling at 10%, it means the maximum swing would be 30Vp?

The 220K on the Send jack is 10 times the 22K Source resistor because it is in parallel with the 22K for AC purposes and I didn't want to load down the Source Follower any more than I had to. I didn't want to go any larger because it also drains the 2.2uF capacitor when you first apply B+ to the buffer. The resistor on the return jack isn't critical because most FX devices have a very low output impedance. The Follower is designed to drive a device with a 25K input impedance, so the return resistor is a non-issue there also.

The Miller effect with the reverse transfer capacitance got neutered when the Drain got connected to B+. The Gate to Source capacitance also got lowered with the gain of less than one. So it takes a much larger Gate resistor to get the usual roll-off. A 100K on the Gate of an LND150 is still flat out to 20KHz.

This driver won't swing 30V. It was only designed to swing 12.6Vp into a 25K load originally and it's not swinging a whole lot more than that with the changes. The grandchildren are over and tearing up the place, but I can explain it more tomorrow if you like.

[Gaz]

This driver won't swing 30V. It was only designed to swing 12.6Vp into a 25K load originally and it's not swinging a whole lot more than that with the changes. The grandchildren are over and tearing up the place, but I can explain it more tomorrow if you like.

Thanks, I appreciate that! I am now just trying to understand the effect on 'performance' by lowering the gate voltage to lower the noise.