Using LEDs to prevent a PT2399 from distorting [renamed]

[samhay]

I think it just needs to be a single, reversed Zener (anode to ground) -- just like what we do with a zener to protect a charge pump from overvoltage with a 9.1 to 12v Zener.

Ahh, I see - sorry, I guess you are playing with a DC voltage, not the signal input. In that case a single LED (or Zener) will do nicely; but then you already knew that.

Looking at Merlin's (Valve Wizard) PT2399 data sheet, it looks like pin 7 talks to the (-) input of the pin 11/12 op-amp, which is usually ignored (cap across it). I'm intrigued as to what you're up to now.

[midwayfair]

Ahh, I see - sorry, I guess you are playing with a DC voltage, not the signal input. In that case a single LED (or Zener) will do nicely; but then you already knew that.

For some reason I couldn't get a single LED to do it last night, it had to be back to back before it worked right. At least, I think it did. Only one LED lights up (the one you'd expect), I know that, so that indicates that it ought to work with just one diode. I'll experiment more tonight and report back.

[samhay]

Is it an audio signal you are sending to pin 7? If so, it might be biased up to Vref (2.5 V) and then clipping both the op-amp rails at Vref +/- 2.3V (if you are listening at pin 12).
Back-to-back LEDs is a bit odd as, if I have got this right, you should have a voltage drop of about 7 V across them - it would have to be conducting through the reverse voltage of the reversed LED (not sure, but I have seen about 5 V for this; don't think it will cause it to light up) + the forward voltage of the forward LED (another ~2 V). The PT2399 is running on 5V, so unless there is some voltage division happening, this should still be clipping. If the reverse voltage of the LED is much lower, then it will work. Either way, having back-to-back LEDs is probably not a great idea.

[slacker]

I don't think he means back to back in the proper sense, I think he means anti parallel, like we use for clipping.
I'm not sure what the LEDs are doing, from memory pin 7 normally sits at a bit below 1 volt, don't think I've ever measured it with signal present though. I'll have to have a play and see if I can figure it out.

[midwayfair]

Okay more experimentation. One LED DOES work. The other one just seems to do nothing. I guess I just wasn't listening at volume last night. It's oriented cathode to ground.
(and yes, I meant "anti-parallel" like in clipping)
slacker: Pin 7 idles at .63v, and increases with signal.

[samhay]

Okay more experimentation. One LED DOES work. The other one just seems to do nothing. I guess I just wasn't listening at volume last night. It's oriented cathode to ground.
(and yes, I meant "anti-parallel" like in clipping)
slacker: Pin 7 idles at .63v, and increases with signal.

OK - that makes sense. You will only need one diode to ground as pin 7 (or any others) will not be able to pull below ground.
The PT2399 sets its own internal Vref to ~ +2.5 V at pin 2, so you could do anti-parallel diodes to pin 2 if you have some sort of AC signal output. If it idles at 0.63 v then that is something else entirely. Sounds like fun.

[EATyourGuitar]

what you are doing with the clipping diodes before the delay is exactly how some big manufacturers do it. I was never sure if it was intended to prevent damage or maybe have distortion on some settings as a new feature.

But it's not before the delay, exactly. It's on the current control chip of the PT2399 (at least, I think that's what CC stands for in the datasheet). Do you have a schematic where it's used? I'm genuinely curious to see if it's done any other ways.

here is one example. after the feedback knob the signal gets clipped by a pair of 1N4148.

if you ever need clipping to happen at a specific voltage, you can use TVS diodes. for example, http://www.mouser.com/Semiconductors/Discrete-Semiconductors/TVS-Diodes-Transient-Voltage-Suppressors/

you can even get a bidirectional transient voltage suppressor in a two lead package 

[samhay]

if you ever need clipping to happen at a specific voltage, you can use TVS diodes. for example, http://www.mouser.com/Semiconductors/Discrete-Semiconductors/TVS-Diodes-Transient-Voltage-Suppressors/

you can even get a bidirectional transient voltage suppressor in a two lead package 

Haven't seen those before. Are they back-to-back avalance diodes?

[midwayfair]

Thanks for the tip, EATyourGuitar! There's a 2.2V one, which would probably be perfect. It's SMD, but the dimensions say it would fit on an LED pad with some clever soldering. I think I'll pick one up with my next mouser order to mess with it and see if it works like the LED. I would want the unidirectional one, right? Or does that not matter because it's only ever going to conduct one way regardless?

[EATyourGuitar]

if you ever need clipping to happen at a specific voltage, you can use TVS diodes. for example, http://www.mouser.com/Semiconductors/Discrete-Semiconductors/TVS-Diodes-Transient-Voltage-Suppressors/

you can even get a bidirectional transient voltage suppressor in a two lead package 

Haven't seen those before. Are they back-to-back avalance diodes?

I would assume so. I just noticed it yesterday and I was all "OH Hello bidirectional transient voltage suppressor diodes in a two lead package"

[samhay]

I would assume so. I just noticed it yesterday and I was all "OH Hello bidirectional transient voltage suppressor diodes in a two lead package"

Yeah - that was pretty much my reaction. Not sure that they will stop me using/abusing Zeners as transient voltage suppressors, but good to know that there are alternativesand/or options if space is really tight.

Jon - good luck with the SMD.

[midwayfair]

Jon - good luck with the SMD.

hehe.

This one doesn't look horrible for one part. There are flat leads bent under the package that I could unbend, and the pacakage is 4mm, about the size of an 1/8W resistor. I can solder leads to the flat leads to plug into a socket. It's certainly worth picking up along with the 1.8v zener to see if there's a difference, though honestly I might need a scope and decibel meter to figure out if there's a difference between the three types of diodes.

[kingswayguitar]

hope this thread isn't too cold
i have 0.77V on pin 7
when you say back to back diodes, what does that mean exactly?
http://www.flickr.com/photos/77164433@N04/8538616199/in/photostream
i managed to attenuate my input a bit with a jfet source follower, then boost the whole thing afterward...less noise and distortion but always looking to make it better

[midwayfair]

hope this thread isn't too cold
i have 0.77V on pin 7
when you say back to back diodes, what does that mean exactly?
http://www.flickr.com/photos/77164433@N04/8538616199/in/photostream
i managed to attenuate my input a bit with a jfet source follower, then boost the whole thing afterward...less noise and distortion but always looking to make it better

"A" in your picture is what I meant, and I think avalanche diodes are "B" -- but in any case it's not necessary. One LED is sufficient, because, as Samhay noted, the pin will never swing negative. You just connect the anode to pin 7 and the cathode to ground.

In your case, .77v isn't high enough to distort, but if you measure that pin while playing a guitar chord, you'll see it jump up. Also, note that clamping pin 7 sets an upper limit for the output of the PT2399. Keep that in mind in whatever design you're using; you may need to change the gain of the input and mixer stages.

[kingswayguitar]

thank you very kindly
i'll admit this design doesn't distort much anyway