Tonepad Coral Chorus bias adjustment

[still4given]

Hi Guys,

I'm new here. I have been building Hi-Fi amps for a few years. I been a guitar/bass player since the 60's and never thought about building my own stomp boxes until about a month ago. I found Tonepad and started building them. I built the TS-9 clone and  the BluesBreaker first and both worked great. Then decided to tackle the Corrrrral Chorus. I have it working but I'm not sure how to go about setting the trimpot. I've read that it controls the bias but have been unable to find instructions on how to adjust it. I'm hoping one of your can give me some pointers.

Thanks, Terry

[bluebunny]

Welcome, Terry.

Looks like that trimmer is only used to set Vb, which in turn is used for biasing the opamps and transistors.  So I would adjust until you see half your power supply appear at the wiper - 4.5V.

[12Bass]

Generally, BBD bias is set by finding the lowest distortion from the output of the delay path and found within a very narrow range on the trimpot.

[bloxstompboxes]

I just set mine by ear until it sounded best. That seems to be the same as what 12bass stated. Though, you could use a scope to get it just right.

[Mark Hammer]

1) Feed your chorus the absolute cleanest signal you can, such that distortion from the BBD is not confused with any other distortion.  Running your amp clean and bright is also part of this equation.

2) If you have the means, use headphones, rather than listening in a way that confuses guitar with other sound sources.

3) Lift one end of the resistor that mixes dry in with wet, so that you hear only wet signal - you can resolder it when set up.  Set the Rate/LFO control for slowest speed and Depth for minimum.  This will give you a delay-only signal that comes closest to being a "clean" signal...except delayed.

Trimpot settings will pass through several "zones":

• Set either way too high or way too low, you will not hear any sound at all.
• Set a little closer to the sweet spot, on either the high or low side, you'll hear a distorted sound.
• As described, there will be a narrow-ish zone where the cleanest delay will be found.

So, to sum up, give yourself maximum opportunity to hear ONLY the delayed sound, so that as you adjust the trimpot you can easily hear audible differences.  Using a scope  is probably faster, in production, but as a one-off exercise, what has been described is certainly good enough.

[still4given]

Thanks Guys,

Pretty clear instructions but I don't know which resistor is responsible for mixing the wet and dry signal. Can you be more specific?

Thanks, Terry

http://www.tonepad.com/getFileInfo.asp?id=101

http://www.tonepad.com/getFile.asp?id=101

[PRR]

Q1 IC1a buffer and boost the input. Still dry.

Q2 IC3(BBD) Q3 delay it (a variable amount for Chorus). "Wet". ("Wet" term really comes from reverb, with multiple delays and a "wet" sound. The very small delay for chorus is not really audible until you mix-back with dry signal.)

R21 and R22 mix the Dry and the Wet. IC1b buffers and de-boosts to the output.

> Lift one end of the resistor that mixes dry in with wet, so that you hear only wet signal

He means "one end of R21", a 47K sitting just off the dot/notch end of IC1.

Start with trimmer centered. That's not exactly right, but is very close. (At least for the BBDs which I have worked with.)

[12Bass]

My approach is to send a constant test tone from my computer to the input, e.g. a 0.5V peak-to-peak 200Hz sine wave, and use headphones to listen carefully for audible distortion in the delay path.  Added harmonic distortion can be heard on either side of the "sweet spot" and will disappear when optimal bias voltage is reached, leaving a relatively pure sine tone.  This phenomenon can also be observed with an oscilloscope or spectrum analyzer.   

[Mark Hammer]

Q1 IC1a buffer and boost the input. Still dry.

Q2 IC3(BBD) Q3 delay it (a variable amount for Chorus). "Wet". ("Wet" term really comes from reverb, with multiple delays and a "wet" sound. The very small delay for chorus is not really audible until you mix-back with dry signal.)

R21 and R22 mix the Dry and the Wet. IC1b buffers and de-boosts to the output.

> Lift one end of the resistor that mixes dry in with wet, so that you hear only wet signal

He means "one end of R21", a 47K sitting just off the dot/notch end of IC1.

Start with trimmer centered. That's not exactly right, but is very close. (At least for the BBDs which I have worked with.)

If I was designing a product from scratch that required fine-tuning, I would probably use a trimpot where the ideal value was somewhere near the middle of that pot's range, so that one has assurances of equivalent adjustability in either direction.  Of course few pots, trimmer or otherwise, come with ironclad assurances of conforming precisely to some taper, so "the middle" is the safest bet for a starting point.

My approach is to send a constant test tone from my computer to the input, e.g. a 0.5V peak-to-peak 200Hz sine wave, and use headphones to listen carefully for audible distortion in the delay path.  Added harmonic distortion can be heard on either side of the "sweet spot" and will disappear when optimal bias voltage is reached, leaving a relatively pure sine tone.

Very sensible, from the perspective of being able to easily compare best and second best.  My signal generator is in the garage,  I need to get that sucker out more regularly.

[still4given]

Hi Guys,
Very kind of you. My son borrowed the pedal for use at worship concert this weekend but as soon as I get it back I will go through the techniques you have outlined and see if I can optimize it. I was quite pleased at how good it sounds. I used a Visual Sounds H20 for quite a while and this is much quieter and has a very pleasant chorus. My son's other guitar buddies have been asking if I can make more.
I was able to fit it into a 125B case including the battery. Pretty cool little unit.

Blessings, Terry