Rockman Chorus Pedal

[Vivek]

That brings us to the Rockman's unusual BBD input biasing. This would be my chief suspect. Pin 3 needs to be at the right level or no signal will go through the BBD. Often there's a trimmer to adjust this, although it's possible to do it with fixed resistors instead. This schematic basically uses the 0V level as the bias, but shows an (apparently optional) single diode and a 10K resistor to push the bias away from ground by 0.6V.

I feel :

For a very large range, the DC bias on Pin 3 only determines THD. It does not determine work/no work unless you really go out of your way to apply seriously wrong DC there.


If the MN3007 gets +6 and -6 supplies, Pin 3 should be maybe roughly between -1.5 to +.6 V, at point where THD is lowest for that particular MN3007. But yet, there will be chorus (with higher distortion) if you bias anywhere from -2.5 to +1.5

In the Rockman schematic, the DC on pin 3 is either 0 or +0.6V. That's fair enough and both should work but at slightly different THD. One has to choose between these 2 settings. If one chooses wrongly, it still works, but at slightly more THD.

Both bias options are not so wrong a DC on pin 3 as to prevent operation of the BBD.

[Vivek]

I got the Rockman Chorus bread boarded right now. I am using a +9/-9 volt supply.

1 - +9.05v
3 - +.60v
4 - -7.06v
5 - -8.56v

Thanks
Rob

Brother Rob,

A) I see that your supply has slightly unbalanced outputs on each side. I dont know if that is the cause of the problem. Is your supply adjustable ?

B) Normally pin 4 should be 1V more than pin 5.
You could try to adjust R150 2.2K so that pin 4 is 1 volt more positive than pin 5. Try 1.5K or 1.2K there.

C) You could try to remove R164 temporarily and see if you get a delay type of sound after mixing Chorus out with 6mainsig.

[ElectricDruid]

Both bias options are not so wrong a DC on pin 3 as to prevent operation of the BBD.

No, true, as long as they're working correctly, it should be ok. I was proposing to test whether they are working correctly, but a voltmeter might be enough. I've just recently seen a case of the bias stopping the BBD from running, as I mentioned, so perhaps I've got it on the brain!

There's only a few things it can be: bad clock, bad bias, bad chip. Otherwise signals of some sort should go through the BBD.

[Fender3D]

B) Normally pin 4 should be 1V more than pin 5.

Actually pin 4 IS 1V more positive than pin 5 (pin 4 - -7.06v pin 5 - -8.56v)

Opamp output offset voltage adds up to bias voltage.
Definitely wrong BBD bias voltage CAN prevent it from functioning, as Tom correctly said.

[Vivek]

I calculated pin 4 to be 1.5V more than pin 5,

Spec sheet days the difference should be 1 V

The bias network for pin 4 has been calculated for +6/-6 but Rob is using +9/-9.



Yes wrong bias voltages can screw up everything, that is a general truth that cannot be denied

However, do you see any BBD bias error in the Rockman design or the voltages Rob posted ?

[caspercody]

Thanks everyone for your input! I am new to understanding BBD, and clock circuits. I found the following image to test a MN3007, but I do not have a 4047 chip. I ordered one from Mouser should be here in (2) days. But, could I use the Vbias going into Pin 3 on the tester image as ElectricDruid mentions in post #19? Would this replace the 10K and diode on IC106A? Or the 6.8K and  2.7nf connected to pin 3 of MN3007?

Is the 15PF the clock timing cap?

As for this statement: The MN3007 datasheet suggests a pair of 100K resistors and a 3u3 cap to make a midpoint bias supply, but suggests that the lower resistor should be "adjusted for minimum distortion". On a proper bipolar supply, the situation is slightly different, but you can still create a virtual ground for this bias point alone. I do not understand this statement? Is the "lower" resistor referring to the 2.2K?

I also found this block diagram of a chorus that helped me understand where the clock, LFO were in the circuit.

[ElectricDruid]

But, could I use the Vbias going into Pin 3 on the tester image as ElectricDruid mentions in post #19?

You could.

Would this replace the 10K and diode on IC106A?

Yes

Or the 6.8K and  2.7nf connected to pin 3 of MN3007?

No, this is additional single RC filtering stage, following on from the active filter around U2A.

Is the 15PF the clock timing cap?

Yes, the 15pF on the 7555 is the clock timing cap.

As for this statement: The MN3007 datasheet suggests a pair of 100K resistors and a 3u3 cap to make a midpoint bias supply, but suggests that the lower resistor should be "adjusted for minimum distortion". On a proper bipolar supply, the situation is slightly different, but you can still create a virtual ground for this bias point alone. I do not understand this statement? Is the "lower" resistor referring to the 2.2K?

The lower resistor would be the lower of the pair of 100Ks used to make a bias voltage.
With a bipolar supply (say +/-6V) we actually have a genuine midpoint ground voltage, so there isn't usually any *need* to create a "virtual ground" on a bipolar circuit like we would on a single-supply circuit. However, in this case, we would like to be able to tweak that voltage a bit, so it makes sense to create a "virtual ground" rather than using the genuine one.

I also found this block diagram of a chorus that helped me understand where the clock, LFO were in the circuit.

Yeah, that's a good diagram to get your head around the building blocks we need for a chorus. The Rockman design is basically the same thing except that it uses a 7555+4013 flip-flop to provide the clock.

[caspercody]

What does the 4013 do in the circuit?

[ElectricDruid]

What does the 4013 do in the circuit?

It provides the biphase clock signal that the BBD needs by dividing the clock frequency in half and providing both positive "Q" and negative "~Q" outputs.
The 4046 PLL includes this function already and has both outputs, so it doesn't need the extra chip. The 4047 astable is another chip you see used for this job (EH Small Clone, for example) since it also includes positive and negative outputs.

[Peetem]

The question was asked in a PM, but I thought it relevant to the discussion....

The Rockman chorus pedal has a unique sound from the Compressor, CLN2 and EQ2 aspects of the circuit (we can debate if the CLN1, EQ1 and other aspects are important to the Rockman sound later    ).  Therefore, those elements were included in the posted schematic. 

Without them, the rest of the chorus isn't a whole lot different from any other chorus unit.

[Peetem]

Hey -

Another poster noted it - there is an error on the schematic.

C16 should go to +V and -V.

I'll correct and repost the schematic.

[Peetem]

[caspercody]

I finally got some time to bread board the below MN3007 tester. By adjusting the 50K to about half way I get sound out of the board, anyway in either direction and no sound. So that must be it's bias point.

But adjusting the 25K pot (delay) there is no difference in the sound, maybe some less or more low level noise. Should I be hearing a delay sound?

[PRR]

> adjusting the 25K pot (delay) there is no difference

You can't hear short delays directly. One way to may delay apparent is to mix the raw and the delayed of a complex signal and listen for pasing and combing.

[caspercody]

I am not sure how to add clean signal on this circuit?

[Fender3D]

Should I be hearing a delay sound?

If you hear sound, that will be enough... the chip works!
Of course you have to set up bias correctly.

The Delay pot is useless, if BBD works (you do hear sound) it does not matter the delay it's working at

[caspercody]

Thanks for all your help, tonight I got a circuit working. Bias is a bitch on these chips.....

[ElectricDruid]

Bias is a bitch on these chips.....

It's like anything. The first time is difficult. After that, you know what you're aiming for and you've got more experience of how it behaves, so it gets much easier.

[caspercody]

Yes, I learned a lot from doing this. Now, I would like to find the best filters to put at front and end of circuit that will give the past in-colored sound. Does anyone have any ideas to look at?

[caspercody]

I found some good information on Electro Smash about the before and after filters.