DH Pitch Shifter at GGG

[newperson]

Hi,

Thinking about building this next.  I have been looking at the past posts and see several people have had noise issues with this one and a couple people have not.  Is there anyone around now that could tell me anything about this build?  Problems or a happy good build?

Thanks,
Paul.

[rocket]

I am currently working on it and have noise and distortion issues.
noise is worst as the signal fades. with no input it's quiet.

[newperson]

Hi,

If there was not any noise would you be happy with the sounds?  Why are the two switches for the robot and vibrato momentary and not regular on/on switches? 

-Paul.

[newperson]

Hi,
I put this pedal together today and it is very noisey at higher volumes.  It sounds like it is clipping the signal when I play harder on the guitar.  Otherwise is makes lots of fun sounds.

What could cause the real low headroom/ clipping distorted sound?

-Paul.

[newperson]

I have looked around more and see others with the low head room issue.  Is this just the case for this pedal because the chip cannot see more voltage?  It seems to be around 3.5 volts.

Thanks,
Paul.

[Dean Hazelwanter]

Hi Paul...

Yes, it's largely a function of low headroom from the HT8950. It's max voltage is 4v, so I chose a 3.6v zener for its supply. Now if I would have been smarter, I would have put a pot where R4 is, to limit the signal the signal going into the HT4950 to sub-clipping levels. You can try this if you want. Replace R4 with a 10K pot. Pin 1 of the pot to ground, pin 2 to C3, and pin 3 to the U1 pin1/R3 junction.

Of course the right way is to use a compander chip like the 571. Compress the signal before going into the chip, and expand it back after the chip.

[newperson]

Hi,

Thanks for the reply.  Would putting on a clipping pot put any stress on the chip and perhaps burn it up?  I would not want to risk that.  Also, how would a "compander chip like the 571" be put into this circuit?  I am fairly new.  Would that chip take care of the clipping?  I would be interested in learning how to add this for educational reasons. 

Your design has lots of cool sounds on it.  I mainly play with strange sounds, so the low-fi-ness part of the chip it makes it better to me. 

I now see that the two momentary switches are momentary for a reason.  I used on-on switches, so I have to click it on-on twice to get it to turn on and off.  How does putting the ground to the signal make the change on the momentary switch?

Thank you,
Paul.

[Dean Hazelwanter]

Hi...

The pot would simply act as a volume control to turn up/down the signal coming out of the 1st opamp stage to be fed into the input of the pitch chip. So no, if done like I suggested, it wouldn't hurt anything.

If you look at Scott Swartz's excellent PT-80 design (also on JD's site), it will give you an idea of where the 571 would go in the circuit. Would it 'take care of the clipping'? I don't know if it would cure it completely, but it *would* make it better. But remember that the headroom with the pitch shifter chip will be poor because of the power supply limitation.

Glad you like it.

Now I understand the confusion. There are 2 ways to control the pitch. The TGU/TGD (ToneGoUp/ToneGoDown) inputs connect to momentary switches, and literally switch the pitch to the next higher/lower interval. The ROBot and VIBrato pins are similar, but I didn't find them that interesting so I merely left pads on the PCB in case anyone wanted to use them. I preferred the other option - *explicitly* setting the pitch via SW0-SW2. Using this method, you always know where you are. The diode-decoding just makes it so you don't have to switch 3 SPST switches. Connect a rotary switch to select through the pitch intervals. I originally did the PCB layout as per the schematic, and had a separate PCB (not represented on the schematic) to diode-decode the logic to *explicitly* set the interval (pitch). JD suggested that I incorporate the diode decode onto the main board, and that's the resultant PCB layout. I had sent some build notes expaining this but they seem to have gotten lost.

[newperson]

Ok,
I will look at the pt80.  I have built one of this a couple/few years ago.  I did use the rotary switch for the pitch changing.  It works just fine.  But I also used the 2 switches to for the Vib and Robot.

Also, can you point me to an article explaining how voltage controls headroom?

Thanks for the response,
Paul.

[Dean Hazelwanter]

Hi Paul...

No article required. This chip's app note  shows the maximum input voltage as 580mV. If the chip's input voltage (the guitar signal) is larger than this (and it often is), the chip *will* distort.

[rocket]

I would like to hear sound samples - are there any online?

[Dan N]

I can't get the page to open, but <moosapotamus.com/> had samples. Charlie added an extra yahoo by using a ldr for the vibrato resistor controlled by an led driven by the chip. Pretty cool!