Death By Audio - Robot  [traced]

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
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Glass_Hero
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Post by Glass_Hero »

commathe wrote:A question for people who built this: how did you ground the jacks if you used a power-supply?

I'm looking at the schematics and I see a virtual ground between +/-3.6V. I surely can't ground my jacks to that? I feel like that will create a ground loop. Am I right in assuming I have to ground my jacks to the power-supply ground? Also, since I am using a volume pot after the second op-amp, surely I have to ground that to the power-supply ground too?
Your jack ground should attach to the virtual ground.. Use isolated power NO DAISY CHAIN.. there are a few ways to use a charge pump style power supply to allow the use of a daisy chain power, but you need to regulate the power to +/-3.6V for the modulation chip.. The -3.6V is the ground from your power supply.. Using a daisy chain connects the signal ground and power ground nodes and prevents the unit from operating.. I think you might be able to connect the jack ground to the -3.6V (power supply ground) and you might be able to use a daisy chain power supply, experiment with this on a breadboard.. I built a few of these and separated the signal and power grounds and use a different power supply than the rest of the pedals on the board.. You can use a 9V battery and add a power switch or get and isolated switching jack..

The master vol control you are adding after the 2nd opamp should connect to the virtual ground node.. You can also add a Clean signal to the circuit like the project that was brought up in this thread for examples (General Guitar Gear, i think)..

I hope this helps..

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Martini Max
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Post by Martini Max »

Anyone have a verified layout and schematic for this bad boy. My want to build this beast spurred the pedal building addiction in the first place, and I have yet to build one. Already sitting on the chip and a rotary knob.. Just need a verified schematic and/or layout. Finally, haas anyone got the chip to do more than the Robot pedal can? Someone please make me a happy man.

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Post by B3ar »

Vero layout, with some changes in the switching:

http://tagboardeffects.blogspot.com/201 ... odded.html

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Post by commathe »

I breadboarded this one up but I am getting a very weird high pitch squeal that is adjustable with the degrade/robot knob. It totally drowns out any guitar signal going in. It also seems if I disconnect one of the three pins (12/13/14) from the little network it stops, but then the chip stops outputting any sound at all. Not sure what is causing this :evil:

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Post by bugg »

I'm attempting to rework this circuit to allow it to be powered from a daisy chained supply.

It appears that this can be achieved by converting the TL072 Input/Output stages to use a virtual ground and adding a simple 3-pin voltage regulator to supply the required voltage for the HT8950.

Here's a hack-job modification to the schematic showing the changes I'm proposing, any feedback would be appreciated!

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Post by commathe »

I'm definitely going to try out your method. Been having a nightmare with this pedal. Still haven't got it off the breadboard.

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Post by bugg »

Here's another more simplified approach... This doesn't require any additional parts from the original schematic.
You might consider doubling the value of the 43 ohm resistor in the power supply since I removed the one connected to the negative side of the battery.

The idea here is to power the HT8950 IC from the voltage across Z2 rather than Z1... This should be the same voltage, but now the circuit ground is no longer floating at 3.6V.

This is just a theory and may or may not work, my IC's haven't arrived yet so I'm working blindly. Let me know if it works (or smokes!). :wink:

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Post by commathe »

Interesting. I thought about trying out that method but wasn't sure if it'd work. I thought the resistor between "true" ground and -3.6v was needed for something special. I feel like a voltage regulator would provide slightly more reliable results though.

I can't even tell if my issues are arising from my power section though :scratch:

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Post by bugg »

commathe wrote:I can't even tell if my issues are arising from my power section though :scratch:
The original circuit has apparently worked for others, so I'd have to assume your problem isn't because of the weird power supply arrangement.

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Post by commathe »

Which makes it all the more frustrating. I have 8 chips, 3 of them are giving me the same oscillation issue which makes me feel like I must have made a mistake that my quadruple checking isn't catching.

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Post by bugg »

My parts have finally arrived, I've got a hacked up veroboard layout to test the new grounding scheme. (still using the zeners)
If that proves to be successful I'll add the regulator and do away with the zener regulation.

I have a long work day so might not get around to it today but will post results ASAP.

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Post by bugg »

In the end I just couldn't live with all the wasted current through those zener diodes....

I decided to omit the zeners and the Vref network and just use the 3.3V regulator to power the HT8950 and supply the Vref voltage.
I basically just pulled the two zeners and put the 3.3V regulator in their place.

This worked like a charm, and it now plays nicely with other pedals on a daisy chain.

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Post by commathe »

Awesome! Definitely going to try this out!

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Post by bugg »

Here is the layout I ended up with after some modifications, this is a bit of a hackjob but I'm not terribly good at PCB/vero layout....

This version will play nicely with other pedals and does away with the crude zener diode regulation. Any 3.3V (L78L33ACZ, LM78L33) regulator should work but pay attention to the pinout. Rather than redo the entire layout I just oriented mine so the pins fit correctly.

The layout could still be improved by adding the standard Vref network for the Input/Output opamp stages to increase headroom but I didn't bother... It works, it's insane, I'm done. :D
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01-DBA_ROBOT_VERO - FIX3.png

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Post by tone seeker »

http://www.vellemanusa.com/products/vie ... &id=522368

Hello there! I did some searching on the HT8950 chip, and I found
that Vellman makes a kit using it. It looks like it has more options,
Or ar least, more to the original pedal. If so, couldn't you just replace
the Mic, with an guitar input jack, and place the output jack, where the
Speaker comnects?
I know it might be cheating, to use a kit, instead of building it from
scratch, but I wonder if the DBA guys didn't just use this as a platform
to begin with. Plus you would need to add fuzz to the circuit.

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Post by Mrutg »

bugg wrote:Here is the layout I ended up with after some modifications, this is a bit of a hackjob but I'm not terribly good at PCB/vero layout....
Can't ground this beast. This is my first DIY with virtual ground. Should the Output lug in the regulator be left alone or need to be connected to something? Is it the virtual ground?

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Post by Bouncingbuddha »

Sorry if this has been mentioned before in this thread but I was looking at one of the original robots and for the life of me I can’t work out where or what the photocell (photo eye) goes or controls. Can you even put one of them into the new circuit and if you can, what would you remove or add?

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