Built-in Low Pass Filters in PT2399

[YouAre]

I'm trying to simulate the filtering in various PT2399 based delays, and the LPF pins on the pt2399 confuse me a bit.

Using the Rebote 2.5 Delay schematic, http://tonepad.com/getFileInfo.asp?id=98 , I can see that pins 16 and 13 are the inverting pins for an opamp, and pins 15 and 14 are outputs, with 15 being the input to the delay portion. I want to assume that the "positive inputs" to the built in opamps are grounded, but i'm not too sure....

I breadboarded a similar circuit portion from "delay out" to pin 14 (from the rebote schematic still) using a jrc4558. So it was 2 passive low pass filters (making up a single 2 pole filter) feeding into another funky active low pass filter, with a bump in gain. That's what it sounded like, simple enough.

Then I breadboarded the same circuit, just using pins 13 and 14 of the pt2399 for the filter. I feel like I properly powered/filtered all the necessary pins to the pt2399, not leaving anything floating. The result was a lot more gain, very distorted.


I tried to hook up a quick and dirty delay on my breadboard, using input/output stages of the echo base, omitting the passive low pass filters, and omitting feedback. The result was still loud and distorted. Might be a mistake in my wiring, or maybe there's too much gain from those active LPF's without using the passive filters to bring the signal down a bit....


So my question (sorry for the lengthiness...) is how are the LPF pins set up in the pt2399? Are they simply inputs/outputs of basic inverting opamps with the positive terminal being tied to ground internally?

[Earthscum]

That Rebote schem is weird. It screwed with me.

Check this out: It's a Data Sheet

You'll see what's going on with it.

Then download THIS AWESOME INFO from merlin, from This Thread

ETA: That should get you started, and hopefully help you answer your question. The PT2399 has Op Amps built in, and you design your filtering around them. The non-inverting input is connected to VR pin internally. Rick (frequencycentral) found that you could wobble the VR pin, this wobbling the entire reference railinternally, to cause the same effect as a small wobble in varied resistance to the delay pin. That's probably TMI for you at this point, but I figure use that as motivation to not get frustrated... the PT can do backflips if you whip it right! And, I made a couple play dead, too... they are still playing. 

[YouAre]

That Rebote schem is weird. It screwed with me.

Check this out: It's a Data Sheet

You'll see what's going on with it.

Then download THIS AWESOME INFO from merlin, from This Thread

ETA: That should get you started, and hopefully help you answer your question. The PT2399 has Op Amps built in, and you design your filtering around them. The non-inverting input is connected to VR pin internally. Rick (frequencycentral) found that you could wobble the VR pin, this wobbling the entire reference railinternally, to cause the same effect as a small wobble in varied resistance to the delay pin. That's probably TMI for you at this point, but I figure use that as motivation to not get frustrated... the PT can do backflips if you whip it right! And, I made a couple play dead, too... they are still playing. 

I checked it's datasheet, but it didn't really say anything about the filters except that they're LPF's, which is why i came to you guys.

Ah! So my assumption was right-ish, the non inverting is at Vref instead of ground. Maybe that's why mine was different. I'll look over that thread and that awesome datasheet.

Thank you!


That datasheet looks awesome, and I'm going to look it over more in the morning.

[merlinb]

So my question (sorry for the lengthiness...) is how are the LPF pins set up in the pt2399? Are they simply inputs/outputs of basic inverting opamps with the positive terminal being tied to ground internally?

Yes (well, the +ve inputs are tied to Vref, technically). They are usually set up as Multiple Feedback or MFB filters.

http://sound.westhost.com/articles/active-filters.htm#s4

[YouAre]

So my question (sorry for the lengthiness...) is how are the LPF pins set up in the pt2399? Are they simply inputs/outputs of basic inverting opamps with the positive terminal being tied to ground internally?

Yes (well, the +ve inputs are tied to Vref, technically). They are usually set up as Multiple Feedback or MFB filters.

http://sound.westhost.com/articles/active-filters.htm#s4

Wow! I'm not at my place right now, but I think you helped me figure out why my quick-rig delay was distorting! I omitting the first resistor and cap to ground (r1 and c1 in this picture), thinking it was just a simple passive LPF, that added another pole to the filter. Not the case! I simulated what I built (without r1 and c1) and what I should've built (basically that picture) and the difference in gain is MASSIVE!

That LPF that merlin showed gives a 10db boost, pretty moderate. Without r1 and c1, you get a 28 db boost. No wonder my chip was distorting!

Thanks merlin! Your datasheet is awesome!



Two questions....

The clock out at pin 5. What's that look like? Is it just a 0-5v square wave? Does it reset itself when the delay receives it's input? If it doesn't, then does the clock output even stay in phase with the delay output?




Secondly, your alternative to the 5v regulator...It's my understanding that we won't get a steady 5v if we use that instead, but we will get less noise. Does this mean that the 5v regulator (78L05 in my case) is noisy or is it that the rail in general is noisy? If it's the case of just the rail being noisy and the 78l05, why not use the two in tandem? Have your power "conditioner" feed into a regulator. Might be overkill, but why not? Also, I don't have a bc337 available. Will a bc550, mpsa18, or 2n3904 suffice?

Thanks again!

[merlinb]

The clock out at pin 5. What's that look like? Is it just a 0-5v square wave?

Yes

Does it reset itself when the delay receives it's input? If it doesn't, then does the clock output even stay in phase with the delay output?

Not sure what you mean. I've not really looked at it. But the clock must vary in accordance with the current sucked out of pin 6.

Secondly, your alternative to the 5v regulator...It's my understanding that we won't get a steady 5v if we use that instead, but we will get less noise. Does this mean that the 5v regulator (78L05 in my case) is noisy or is it that the rail in general is noisy? If it's the case of just the rail being noisy and the 78l05, why not use the two in tandem? Have your power "conditioner" feed into a regulator. Might be overkill, but why not? Also, I don't have a bc337 available. Will a bc550, mpsa18, or 2n3904 suffice?

The PT sucks pulses of current from the rail, which causes the rail voltage to jump about. The regulator works hard to try and resist those variations. It so happens that a zener follower is better at it than a 7805 at these high frequencies. The zener follower does not give such a 'precise' 5V output as a 7805, but you don't need that kind of accuracy for a PT. A 7805 is still good enough for most applications though.
It needs to be a moderately beefy transistor since the PT can suck >40mA at short delay times.

[Earthscum]

I've thought about this before, if you have no problem adding an extra 8 pin device, only 3 are used (+, -, out), you could thrown in a LM386 for a great Vr, or use a spare op amp for the same. I have tried the LM386 Vr trick on other projects and works great.

[murrayatuptown]

Cool info here! This is something I've been wondering about too...and what purpose the integrators serve (pins 9/10, 11/12 IIRC...and what happens if the value of those capacitors is varied...an earlier PTC datasheet showed 0.082 uF, then  a later one 0.1 uF, and there is a circuit out there that I think uses 0.01 uF...Clarinot?

I have some regulators that will be a hassle to use, but I have 'em...LT1962ESM, 'low noise LDO'. I'm curious how they'll do considering the advice above...

Murray

[YouAre]

Yes

Excellent....

Not sure what you mean. I've not really looked at it. But the clock must vary in accordance with the current sucked out of pin 6.

I figured that much. But to clarify what I meant...

Say you look at pin 5 on a scope, and the output of a pt2399 based delay as well. If you were to hit a quick quarter or eight note on guitar, the output "blips" on a scope would be the same frequency, but not in phase with the output from pin 5, correct?

This is relevant to use the clock of the pt2399 to sync with other effects require a 0-5v pulse as an input.

I've thought about this before, if you have no problem adding an extra 8 pin device, only 3 are used (+, -, out), you could thrown in a LM386 for a great Vr, or use a spare op amp for the same. I have tried the LM386 Vr trick on other projects and works great.

You're suggesting using a lm386 as a voltage regulator for the pt2399? Can you please explain? Thank you

Cool info here! This is something I've been wondering about too...and what purpose the integrators serve (pins 9/10, 11/12 IIRC...and what happens if the value of those capacitors is varied...an earlier PTC datasheet showed 0.082 uF, then  a later one 0.1 uF, and there is a circuit out there that I think uses 0.01 uF...Clarinot?

Definitely interested in knowing this as well. Rebote uses .082, Echo Base uses .1, and Little Angel uses .01.

[Earthscum]

I've thought about this before, if you have no problem adding an extra 8 pin device, only 3 are used (+, -, out), you could thrown in a LM386 for a great Vr, or use a spare op amp for the same. I have tried the LM386 Vr trick on other projects and works great.

You're suggesting using a lm386 as a voltage regulator for the pt2399? Can you please explain? Thank you

Not for the PT, but instead of using the 5V regulated rail as Vr for I/O buffers, mixers, amps, etc. that you're going to run on the 9V rail, you can just hook up the V+ and Gnd of an LM386 and the output works as a Vr. It's already internally biased. It's about the same footprint as a pair of resistors and a electrolytic, and only 2 more holes. A bonus is that you don't have to think about bias rail impedance as much.

[YouAre]

Not for the PT, but instead of using the 5V regulated rail as Vr for I/O buffers, mixers, amps, etc. that you're going to run on the 9V rail, you can just hook up the V+ and Gnd of an LM386 and the output works as a Vr. It's already internally biased. It's about the same footprint as a pair of resistors and a electrolytic, and only 2 more holes. A bonus is that you don't have to think about bias rail impedance as much.

I see your point. I like the idea of only using the 5v output to power the pt2399. I don't like the idea of using that 5v as vref for the in/out buffers, even if it's coupled with a small resistor.

So you're saying you just hook up the 9v and ground pints on a 385, and the output automatically goes to 4.5-5 volts? Why is that?

[anchovie]

Cool info here! This is something I've been wondering about too...and what purpose the integrators serve (pins 9/10, 11/12 IIRC...and what happens if the value of those capacitors is varied...an earlier PTC datasheet showed 0.082 uF, then  a later one 0.1 uF, and there is a circuit out there that I think uses 0.01 uF...Clarinot?

Definitely interested in knowing this as well. Rebote uses .082, Echo Base uses .1, and Little Angel uses .01.

They're used for filtering in the A/D and D/A converters. Lower the cap values for more treble, increase for less, with noise levels being affected also. The LA uses 10nF because it's set to such a short delay time range that fidelity is better in the first place. I've used 220nF for really long, lo-fi delays.

[YouAre]

They're used for filtering in the A/D and D/A converters. Lower the cap values for more treble, increase for less, with noise levels being affected also. The LA uses 10nF because it's set to such a short delay time range that fidelity is better in the first place. I've used 220nF for really long, lo-fi delays.

Awesome! My goal is to get the filtering to be as clean/bright as possible while still blocking out most of the noise. Let's see how it works to tweak those caps. Thank you!

[murrayatuptown]

Thanks