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Not much to add to what has already been stated here. Here's how I break this stuff down in my own way:

The Voice pot is a variable resistor, i.e. it changes resistance as it is turned. Since lugs 1 and 2 are connected, this means that when it is fully counter-clockwise it produces the full resistance of the pot, 10k. When fully clockwise it is 0k. Since it is linear, then you know the "in-between" points more or less fall in even steps. IOW, turning the pot 10 degrees in the bottom part of the sweep produces the same rate of change in resistance as it does in the top part of the sweep, or any other are for that matter.

There are two "states" to consider with the variable pot:

One state is the amount of gain produced by the negative feedback loop which is all the stuff between pins 1 and 2 of IC1. It's helpful here if you just forget the amount of feedback there is variable by the Drive pot and consider it fixed. So, think of Drive as just a 500k resistor for the moment and ignore R9.

When the Voice pot is all the way up, it produces 0 resistance and therefore we can consider it removed from the circuit. Using the formula for calculating gain in a non-inverted feedback loop, we see

Gain = 1+ (Drive/R3) = 1+(500/1) = 501

When the Voice pot is fully counter-clockwise, we get the full resistance of 10k, so the formula for gain becomes

Gain = 1+ (Drive/(R3+Voice)) = 1+(500/11) = 46

So, just by turning the Voice pot we've varied the gain output by a factor of about 11. IOW, our gain goes DOWN when the Voice pot is all the way down and vice-verse and it will be quite noticeable since it the difference between the two is an order of magnitude.

Now consider the second "state" which is the filter created by connecting R3, the Voice pot and C3 to Vb. Since Vb is virtual ground, just consider it as a simple low pass filter. The cut-off point of this filter depends on two values: the total resistance and total capacitance connected to ground. You should also consider impedance, but since C3 is fixed I don't think you have to worry about it (maybe someone knows better). Anyway, we already know that Voice is a variable resistor, so that means this filter is also variable.

When the Voice pot is up, we have a 1k resistor and 100n cap. Using the formula for single pole HP/LP type filters we have

f(cutoff) = 1/(2*pi*R*C) = 1/(6.28*.001*.1) = 1592 Hz

When the Voice pot is down, the formula becomes

f(cutoff) = 1/(6.28*.011*.1) = 145 Hz

Keep in mind that the "cutoff" here is not a sharp knee. Rather, it is -6dB per octave. So, the further away from the cutoff the more drastically the frequency response is reduced. This means when our cutoff is 1592 Hz, we have more mid-frequency content with the high frequencies being steadily reduced by the filter. When the cutoff is at 145Hz, we reduce both mid and high frequency content.

The two states here are what make the Voice pot what it is. It simultaneously changes the mid-frequency content and overall gain. Remember this example kept the Drive pot as a fixed resistor. When you remove this restriction, you can see now that the Drive pot also influences the total amount of gain produced it also works with C2 to produce another variable filter but that is for high frequency content only) . IOW, the two controls are interactive, as mentioned in earlier posts.


Here's a much better (and probably more correct) technical explanation of how this works: http://www.geofex.com/Article_Folders/TStech/tsxtech.htm

Look at the "clipping" section.

Yes, that is the last etchable version I did for the Pork Barrel. It has more features than the current version (the Tonepad mods). If you want to do an etchable 1590B version, then do the Tonepad. This particular one requires a 1590BB.

500kC would work just fine in this build. It might even be a better choice, although I have not tried it myself.

I do not have experience with the NTE versions of JFETs but I have used NTE replacements for other parts several times. Try Googling the NTE part to get the datasheet to see if the pinout is different. Q1 is a safe bet since it is only a buffer. You can socket Q2 and see how it does. If it does not work out you can easily replace it: onsider posting a request in the "Parts Bin" part of the forum for some J201s in that case.

The Fail switch on the demo of the Zero Point is basically the same thing as the Loophole. Add a 22R between regulator output and ground with the switch toggling the connection to ground. You'll get about 15 seconds of detune effect before it stops working...then just let go of your momentary switch for an instant reset.

http://www.muzique.com/lab/tone3.htm

It does not have a huge amount of output but it should be over unity for sure as you can hear from the demo on the MBP youTube page. I'll check my build ASAP to make sure I did not put the wrong value somewhere in the build doc.

From what I gathered he uses two mosfets as ground clippers at the delay output and an inductor as a double pole low pass filter after that.

Yeah, Jack is a member here. He may have posted a little about the Dark Echo on FSB. I know the Wampler one has also been picked apart some---IIRC, there is a vactrol in parallel with the Delay pot which is keyed off dynamics, like you mentioned.

Also, midwayfair-- I don't know how Skreddy does his, but you can definitely clamp oscillation with a couple of back to back diodes to ground after the repeat pot in a delay. I did this with my old Rebote. I had a switch that simultaneous turned the repeats to max and kicked in two 1N4001 diodes which keeps the volume under control.

I think I failed to show that is the case in the build doc---I only showed one LED lit in the illustration, but it should be both.

So, yours sounds like it working properly.

One thing I will be working on in Loophole 2 is how to make that detune/fail thing work better. I got something mapped out a while ago but nor finished.

Are you using a 308 for the IC? Some of those voltages are looking pretty off. Ti start, you should be seeing about 9v on pin7. I don't know how you could be getting 4.5 there but 9v on pins 1, 2 and 8.

You should expect to read 0v on the front end of the input cap and the back end of the output cap. These caps decouple the DC portion of the circuit from the AC guitar signal. You would actually be in trouble if you were reading voltage there!

What are your voltage readings for the transistors?

I would double check your resistor values for R13-R16 and R20-23, too. One incorrect value on any of these might cause a significant drop in volume or fuzz.

As I mention in the project doc, and has been explained here in the forum: this is a minimally filtered PT2399 delay. Some amount of aliasing and artifacting is normal from the second half to max delay. This is present to some degree in all PT2399 designs, although more heavily filtered designs do tend to tune that noise out more succefully. It is possible to filter the Cave Dweller a bit more by increasing that 4n7 cap (C3) to a higher value. However, this will darken the delay and your regular guitar tone quite a bit more, so it is a trade-off.

You can also try swapping out different PT2399s. Some are just noisier than others and it's possible you might find a better one. But, you will also never eliminate the zipper noise completely. It's just a downside of this chip, unfortunately.

That's what is supposed to happen. The Detune acts as a current sink on the regulator which puts the ISD chip in a semi-failed state which causes the pitch to detune.

Make sure you are testing/probing with the detune off. It only works for 10-15 sec. and then the ISD will stop functioning. However, function returns to normal once you switch off the Detune.

Bad parts happen less frequently than you might think. Of course, it is a possibility and therefore do not eliminate that as part of your debugging. But, consider other possible potential areas: incorrect values, incorrect transistor orientation or subs, bad power supply, etc. Most of what you are describing is pretty typical of "builds gone wrong" so I think you will be able to track down the issues with enough patience and thoroughness.

If the LED is dimming when you turn the volume knob, I would double check your wiring. Are you sure you have the PCB, DC jack, 3PDT and one of the jacks all grounded?