Tonepad Pulsar Tremolo, little variation between waveform modes

[dcjim]

Hi

Tonepad EHX Pulsar tremolo clone

http://www.tonepad.com/project.asp?id=14

The only mod is 10k pot for rate - recommended as stock value gives a less desirable range

Getting little variation between waveform modes, I was expecting square wave versus triangle wave, instead I'm getting this



Audibly the difference is negligible.

Traces are with rate set to maximum. Interestingly with rate set to minimum they look a little more like square waves but again this isn't right as they should look dramatically different no?

Thoughts?

Jim

[drolo]

I had a similar experience with another build using a similar LFO. I think it has to do with the latency in response of the LDR which cannot follow the abrupt changes of the LED (or is it the LED as well ...?)
Anyway i could not get a square either.

If anyone has a better LFO that will get the LDR/LED to respond faster, i would be interrested too :-)

[dcjim]

I don't think there's any LDR/LED in the LFO circuit is there?

[Jazznoise]

Looking at the schematic, I don't see why it should give you much of an audible difference. The wave shaping of the Pulsar is alot more complicated.

A cheeky thought, but what are the voltages of the LFO signal? Couldn't you just have the switch change between a cap and a pair of clipping diodes? They do a similar thing in the original Pulsar, I think.

[dcjim]

Looking at the schematic, I don't see why it should give you much of an audible difference.

I did wonder about that.

A cheeky thought, but what are the voltages of the LFO signal? Couldn't you just have the switch change between a cap and a pair of clipping diodes? They do a similar thing in the original Pulsar, I think.

I'll give it a whirl

[drolo]

I don't think there's any LDR/LED in the LFO circuit is there?

OOops i stand corrected i was reading this at work in between calls and my brain confused Pulsar with Tremulus ...

[dcjim]

Looking at the schematic, I don't see why it should give you much of an audible difference.

I did wonder about that.

A cheeky thought, but what are the voltages of the LFO signal? Couldn't you just have the switch change between a cap and a pair of clipping diodes? They do a similar thing in the original Pulsar, I think.

I'll give it a whirl

Square wave ... gained! Yay! Signal ... lost. Boo!

On the plus side it's my first debug using a scope so that's quite exciting.

I'm new. Why does the signal get lost?

[dcjim]

I'm new. Why does the signal get lost?

Ah, I think I know why. Because each diode has a voltage drop of .7v?

Solution, gain? Not practical to stick an op-amp in there. Alternatives?

[Jazznoise]

Hmm, you did swap either C6 or C7, right? And you used 2 diodes, not one? What voltage is the "clipped triangle" in comparison to the Triangle? Does it still function when the switch is set to triangle?

Check Q1's orientation. The base of it should be driven by your LFO, maybe play with your trimmer and see if changing the setting helps. Write the current setting down now!

[dcjim]

Hmm, you did swap either C6 or C7, right? And you used 2 diodes, not one? What voltage is the "clipped triangle" in comparison to the Triangle? Does it still function when the switch is set to triangle?

Check Q1's orientation. The base of it should be driven by your LFO, maybe play with your trimmer and see if changing the setting helps. Write the current setting down now!

Yup two diodes, I understand the principal of symmetrical clipping to get a square wave. Yup, diodes replaced C6 or C7. No trimmer, only rate and depth pots. My scope is very very old and neither cap is in circuit right now but voltages are about .1v peak to peak and 1v peak to peak for diode pair and missing cap respectively. Still functions set at former but signal is very quiet.

[Jazznoise]

.1V? That's a very large voltage drop. What diodes did you use? A pair of Si should be leaving you with your .7V. Something else is afoot.

I've still got a funny feeling, but I don't want to offend so please take this as an indication of my illiteracy! You swapped C6 for the 2 diodes and not each capacitor for a diode, and the 2nd capacitor is still left untouched and in circuit, yes?

[dcjim]

.1V? That's a very large voltage drop. What diodes did you use? A pair of Si should be leaving you with your .7V. Something else is afoot.

Two 1N4148s. Don't forget my scope is an absolute howler so not easy to be too accurate with that value but certainly there's a high ratio between cap and diode signal paths (if you will).

I've still got a funny feeling, but I don't want to offend so please take this as an indication of my illiteracy! You swapped C6 for the 2 diodes and not each capacitor for a diode, and the 2nd capacitor is still left untouched and in circuit, yes?

No offense taken. I am indeed capable of the upmost stupidity when the fancy takes me including but not limited to failing to read properly But not on this occasion sadly. C6 was replaced with two diodes head to tail so to speak in parallel connected between ground and ... wherever the other leg of C6 was. C7 is currently out of the circuit but I'm figuring that doesn't matter as you just get a harsher waveform with a click as it's unshaped.

What about the observation that the waveform varies in shape when rate is adjusted - and I don't just mean frequency?

[Jazznoise]

Does it go kinda rampy- to triangle? That wouldn't be too unusual and you may have seen that before you did this, though I'm not experienced with analog oscillators. Any pictures would help understand the problem better.

Perhaps the voltage drop is also an impedance thing, perhaps try putting a 1k resistor in series with the diodes. If you have any 10K pots, try wiring that in for ease and varying the resistance to see if you can find a decent ratio between the amount of clipping and the signal size. Getting it close to 0.7 is ideal, but 0.5 could be functional as long as you either incorporate compensation for depth changes or simply remember to turn the depth up on the square setting.

[Mark Hammer]

Try a pair of back-to-back LEDs instead of 4148s, or perhaps a 2+2 set of 4148s.  They will provide for a larger peak voltage reaching the transistor base.

Another thing to consider is that the current from the LFO not only has to fight your diodes, but also has to fight its way through 400k of series resistance (R12 + R13).

It,s funny, you know.  There are so many modulation waveform possibilities for this simple classic pedal, and we have explored practically none of them.

For example, what if R13 was replace with a diode of some type and 47k resistor.  Would it not provide a kind of ramp or sawtooth function (depending on diode orientation)?

[Mark Hammer]

And now that I think of it, if you stuck two caps in series with each other (I'm gonna say 3300pf each), placed that in parallel with R4, and then connected the collector of Q1 to the junction of those two caps, instead of to the junction of R6 and R10, wouldn't you have an LFO-modulated wah?

[Jazznoise]

LED's is a great idea, hadn't even thought of it!

The original Pulsar LFO can do PWM Square and Triangle/Ramp/Reverse Ramp waveforms, I actually find it a little strange that they removed it for this version, as it looks like a great little LFO.

The wah idea is also great - with a little work you might even be able to do the "Harmonic Vibrato" blondeface trick? This forum is dangerous sometimes! 

[Mark Hammer]

Hah!  I was right!

Once again, working my way through the big bin-of-incomplete pedal boards, I pulled out a Tonepad Pulsar board to try firing it up.  It had the ticking problem, so I did a search through older threads here for a cure.  Former member "Vanessa" had an excellent response that noted an error in the board layout of the time.  I did the fix, and all was well.  But I found the modulation a little choppy for my tastes, and started poking around to see if anyone had any recommendations, and stumbled upon this thread.

Since the board wasn't boxed up and it seemed like an idea worth trying, I followed up on the modulated wah idea I proposed a couple posts above this.  I desoldered the collector of Q1, soldered a 100nf (.1uf) cap to each end of R4 (just a shot in the dark as far as value, but a very lucky shot), and soldered their free ends to the newly liberated collector of Q1.  Sonuvabitch if it doesn't provide a great little modulated wah.  100nf seemed a little dark so I tried out 47nf, and that seemed a little more musical.

I won't claim that it is a fantastically musical modulated wah, but it sounds decent and has its uses.  At medium fast speed, it does what older effects like the PAiA Synthespin attempted to do.  I still wish the LFO was a little "rounder", but it'll do nicely.  Worth trying, if you have a Pulsar sitting around doing nothing important.

Now I just have to figure out a simple way of being able to select between tremolo and wah with a toggle.