PT2399 pin 6 quandary - using an NPN transistor as a control element

[jimladladlooklike]

Hi folks,

I know there are plenty of pedals out there that implement an envelope detector circuit to adjust the current flow from pin 6 of a PT2399 in relation to how hard you play, but I was wondering if it was possible to use an NPN transistor as the control element. I've breadboard a schematic that I've put together and am not having much luck so far. Here is said schematic:



The circuit has a unity gain input buffer, which then feeds both the input of the 2399 and the envelope detector circuit. After the ED, the signal is sent to the control element of choice, one of two 2N3904 transistors. From my (rudimentary) knowledge of BJTs, the voltage from the ED circuit fed into the base of the 2N3904 should affect the current flow from pin 6 to ground via the collector to the emitter. The second 2N3904 (the collector of which is connected directly to pin 6) is for the anti-latch up circuit from ElectroSmash, although no resistor or cap values were specified for this so I'm not sure if they will work? I'm also unsure whether this will affect the first 3904s behaviour.

I am getting no sound at all at the circuit output. When not playing my guitar I measured about 4.3VDC at the node where the 330R, 4.7uF and 47K resistor connect. When I play this goes up to about a maximum of 5.1/5.2VDC, so the envelope detector circuit is working correctly I think... I also get a very loud audio signal at this point when using an audio probe, which I guess is due to the relatively large 1M feedback resistor. However, is it a problem that this audio signal exists here? If the aim is to create a DC signal, then there should be no audio right?

I also get signal at pin 15 but not pin 16 of the PT2399, which seems strange as they are both fed by the same signal source.

After doing some reading of the ElectroSmash article I noticed that the pin 6 voltage should be a constant 2.5V. However, this wavered at around 3.4 volts and got higher when I played through the circuit.

I apologise I've not had chance to take a full reading of voltages as of yet, but any pointers at this stage would be much apreciated!

[duck_arse]

the voltage oin the 330R//4u7 will try and turn the transistor on, very hard. there is no base resistor to drop the voltage, so the emitter w/ the tiny resistor will force the base to a voltage a little above 0V7, I think. a fight ensues.

you can try and rebias the rectifier opamp, or replace it with an LM358, which will run down to about zero volts. baias it w/ two diode drops, and add a series base resistor of 10k about, see where that gets you. or AC couple the base - still w/ a series resistor.

[antonis]

I also get signal at pin 15 but not pin 16 of the PT2399,

Check 10k resistor left leg for proper connection..
(without signal coming into pin 16, MFB LPF can't work hence succeeding comparator can't compare..)

[Ripthorn]

One thing I see is that you have a 3.3 Ohm resistor on one of your transistors. When this transistor is fully on, you don't have enough resistance on pin 6 and this will result in the chip not outputting any delayed signal. Pin 6 is very sensitive to this.

The reason you don't hear sound on pin 16 is because it is the input to an opamp with virtual ground summing, so an audio probe here won't have any sound. It seems like magic, but summing amps do this.

[anotherjim]

Depends on what you're aiming for. Envelope chorus (quite nice actually) or funny wobbly delays?

[merlinb]

I was wondering if it was possible to use an NPN transistor as the control element.

Yes.
Connect your 22k resistor between pin-6 and ground, so the pin always sees *some* resistance.

[jimladladlooklike]

Hey everyone, thanks for all of your advice, super helpful as always. Here is the schematic I have put together for those interested:



Excuse my schematic drawing if it's a bit unorganised, first time I've attempted something such as this.

There are pots controlling a JFET blend and the general sensitivity. There are also three switches: one for controlling the fq response (creates a mid boost for lo fi "telephone" sound); one for switching out smoothing caps (goes from gritty to smooth); and one that connects the base of the 2N3904 to ground (when removed from the circuit it seemed to make the repeated sound from the PT2399 stay more wobbly for longer.)

Anyway, very happy with the sounds this thing makes so I boxed it up. If anyone want the layout let me know and I will post. It's very much more a chorus type effect than a delay, maybe I will try a similar thing with a delay circuit further down the line.

I do have a couple of issues however. The effects output is quieter than my bypassed signal and when engaging/disengaging the effect there is a loud pop, despite the 1M pull down resistor. Any ideas on how to fix the former and would a larger pull down resistor fix the latter?

Thanks again

[antonis]

Both output caps (10μF) aren't DC grounded..

You may try 1M resistor from mix pot wiper to GND or better, wire your 3PDT switch with both IN & OUT grounded when effect is bypassed..

P.S.
Pull down resistor value is inversely proportional to cap discharge time..

[anotherjim]

In the schematic, the JFET gate has no DC. I think the 10n cap is meant to go the other side of the 2M2 bias resistors?

A log taper can be an easy way of mixing between two signals of different levels and get a balance somewhere around 12 o'clock, but as Antonis said, you need a pull-down resistor on the pot wiper. If it's a 50k pot, then 500k is the lowest value resistor that won't change the balancing point (10 to 1 rule). Since the load impedance of whatever the output is driving is probably 1M, then a 1M pull-down will be ok as the total load is then 500k. Personally, I'd prefer to buffer the output to make it immune from the external loading.

[antonis]

In the schematic, the JFET gate has no DC. I think the 10n cap is meant to go the other side of the 2M2 bias resistors?

Eagle eye, Jim..

Also, 2M2 bias resistors together with 10nF cap could be omitted, in case of no HPF needed..

[jimladladlooklike]

Hey guys, thanks for your help as always.

My bad regarding the no DC to the gate of the FET issue. Will sort that out.

When it comes to the 10uF output caps, I'm guessing they should be connected to ground and the signal should then be taken from the positive side of each? If so, can the 10nf cap on the output of the 2399 to ground be omitted? Also would this create a need for another coupling cap?

[niektb]

oooh no, you don't want to ground the - pin on the elco's in this case! that would filter away everything but maybe a tiny bit of bass. You want to leave the signal path as is (so take the signal from the -) but álso put a 1M resistor from the - to ground (to bias the signal around ground)

[antonis]

When it comes to the 10uF output caps, I'm guessing they should be connected to ground and the signal should then be taken from the positive side of each?

To be more precise, it's cap's "floating" side the one which has to be grounded..
(it has nothing to do with signal in or out.. - see 1μF input cap & 1M pull down resistor respective configuration)

The idea is to prevent an incoming (or outcoming) signal from hitting on some DC voltage obstacle creating an audible pop..

[duck_arse]

how come you have only 33k at the input of the input buffer? I'd think this would loose a fair bit of guitar signal. 330k?

is your "SPDT" [either one of the two] with the two caps a centre off type? if not, you should probably hardwire the lesser value to ground and switch the higher value in in parallel with.

where are you connecting pin 4?

[jimladladlooklike]

Thanks guys.

I've grounded the two output caps as instructed, adjusted the layout so the FET drain is connected to +9v and replaced the 33k from pin 3 of the IC to ground with a 1M resistor.

That note on the schematic regarding pin 4 was old (think I went deep on some forums regarding getting rid of noise and used it as a reminder to do more research); it's simply connected to GND.

The cap selector switched aren't centre off. Would wiring them up as suggested eliminate popping when using said switches?

Thanks

[duck_arse]

do those cap select switches pop now? wired as I suggested, you could add a 1M across the switch contacts to the larger value cap, then the switch shorts across the resistor. this would keep the cap referenced to ground at all times, which would be the best way to not pop.

[jimladladlooklike]

Hey everyone! Have been meaning to share this project with you all but life seemed to keep getting in the way...

Anyway, here's a video of the thing.

https://youtu.be/bB02ntdM6Ho

I love the "lofi" sounds you can get from it, but the output could probably to with a buffer or even a slight boost as it seems like theres a slight volume drop. Either way it's a fun first attempt I reckon

[Locrian99]

Sounds awesome man.  Reminds me a little bit of the sea machine from eqd

[jimladladlooklike]

Hey Locrian, thank you v much!