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Ultimate DIY debugger award

5/25/1999 1:19 AM
aron	Ultimate DIY debugger award goes to anyone that can make my stupid clone of the mystery pedal bias. A nice bag of Hawaiian cookies or some other really delicious goody goes to the person that can make this thing bias. Aron

5/25/1999 6:39 AM
GFR	Aron, I didn't have 2.7V zeners, so I used two strings of diodes with four diodes in it (4x0.7=2.8V). -->~p~--->~p~--->~p~--->~p~-- = --Z<-- I also didn't have a TL071 so I used a TL081 (it's from the same family). I used a BC557B. Here's what I measured: Base -> 4.30V Junction of the two "zeners" -> 2.37V pin 2 -> 2.37V pin 3 -> 2.16V pin 5 (collector) -> 140mV pin 6 (emitter) -> 2.37V I fed it with a sine wave and looked at the scope. The output swings above and below 2.37V, and when it gets down to 1.52V it clips (sharply). When the output reaches ~4.88V, it clips too but much softer. Here's how it looked: OUTPUT (pin 6) * * * 4.88 * * * * * * * * 2.37-------------------------------------- * * * * 1.52 *************** PIN 5 (COLLECTOR) from 0.5V to 6V **** (depends on input level) * * * * * * * * * * * * * * 140mV *********************** ** Highly assymetric, with one of the clipping thresholds being hard and the other soft. If you drive it VERY hard (3V on the input) you get the following: OUTPUT ** ********** ~6V * * * * * * * * * * * * * * * * * * * * 2.37---------------------------------- * * * * * * * * * * 1.52 ***** ********** * The output goes high while it should be low and then goes back to low (when the input is too "negative" the op-amp goes to the "wrong" direction). The RAT (LM308N) shows the same effect. It's unlikely that you'll feed it with such a hot signal, unless you use a booster or another pedal before it. GFR

5/25/1999 6:39 AM

GFR

Aron,

I didn't have 2.7V zeners, so I used two strings of diodes with four diodes in it (4x0.7=2.8V).

 
-->~p~--->~p~--->~p~--->~p~-- = --Z<--

I also didn't have a TL071 so I used a TL081 (it's from the same family). I used a BC557B.

Here's what I measured:

Base -> 4.30V
Junction of the two "zeners" -> 2.37V
pin 2 -> 2.37V
pin 3 -> 2.16V
pin 5 (collector) -> 140mV
pin 6 (emitter) -> 2.37V

I fed it with a sine wave and looked at the scope.

The output swings above and below 2.37V, and when it gets down to 1.52V it clips (sharply).
When the output reaches ~4.88V, it clips too but much softer.

Here's how it looked:

 
OUTPUT (pin 6)                   *
 
                                * *
 
4.88                          **   **
 
                             *       *
 
                            *         *
 
                           *           *
 
                          *             *
 
2.37---------------------*---------------*--
 
    *                   *
 
     *                 *
 
1.52  ***************** 
 

 

 

 

 

 
PIN 5 (COLLECTOR)
 
                                
 
from 0.5V to 6V                ******
 
(depends on input level)      *      *
 
                              *      *
 
                              *      *
 
                              *      *
 
                              *      *
 
                              *      *
 
                              *      *
 
140mV *************************      ******

Highly assymetric, with one of the clipping thresholds being hard and the other soft.

If you drive it VERY hard (3V on the input) you get the following:

 
OUTPUT
 
            ******          ************
 
~6V         *    *          *          *
 
            *    *          *          *
 
            *    *          *          *
 
            *    *          *          *
 
            *    *          *          *
 
2.37--------*----*----------*----------*--
 
      *     *    *          *          *
 
      *     *    *          *          *
 
1.52  *******    ************          *

The output goes high while it should be low and then goes back to low (when the input is too "negative" the op-amp goes to the "wrong" direction). The RAT (LM308N) shows the same effect. It's unlikely that you'll feed it with such a hot signal, unless you use a booster or another pedal before it.

GFR

5/25/1999 11:52 AM
aron	What????? Are you telling me you got this circuit to work and bias? This is the 3rd one I have built. None of them give me the correct voltages on the pins at all on the op amp. Frequently pin 2 is too high or the voltages on the transistors are way off. I can't believe it. -->~p~--->~p~--->~p~--->~p~-- = --Z<-- Are the diode cathodes pointing to ground? (opposite the direction of the zener?) What is ? Thanks, Aron

5/25/1999 11:52 AM

aron

What?????

Are you telling me you got this circuit to work and bias?

This is the 3rd one I have built. None of them give me the correct voltages on the pins at all on the op amp.

Frequently pin 2 is too high or the voltages on the transistors are way off.

I can't believe it.

-->~p~--->~p~--->~p~--->~p~-- = --Z<--

Are the diode cathodes pointing to ground? (opposite the direction of the zener?) What is
?

Thanks,

Aron

5/25/1999 12:12 PM
GFR	All diodes pointing to ground (opposite of Zener). I used 1n4148's. Every diode has two voltages at which they conduct - the forward conduction voltage and the reverse conduction voltage. The forward conduction voltage is ~0.7V for most Si diodes. The reverse conduction voltage in a 1N4007 is several hundred Volts. A Zener is simply a diode with a controlled reverse conduction voltage. If you bias them in the forward direction they will conduct at 0.7V like a normal diode. That's why the zeners are pointing upwards in your schematic - you want to use the REVERSE conduction voltage (2.7V) as a voltage reference. Since I don't have a 2.7V zener I just added normal diodes in series until the SUM of their FORWARD voltages was roughly equal to the REVERSE voltage of the zener. That's why the diodes are in opposite direction of the zener. The string of diodes will not work exactly like the zener, because the zener will conduct at 0.7V in one direction and at 2.7V in the opposite direction. The string of diodes will conduct at 2.8V in one direction and at a high voltage on the opposite direction (if the reverse voltage of each diode is 10V, that would be 40V). This circuit certainly uses the zeners conducting only in one direction, so this difference is not of importance. The circuit is working, biasing, amplifying and clipping. I skipped the tone switch and volume control, because I built it at work (to be able to use an oscilloscope) and I don't have a guitar and amp here So I know it works, but I haven't heard it yet! You may try RG's suggestion and try another opamp - you may have fried one in your previous experiments. The transistor is a big candidate for frying.

5/25/1999 12:12 PM

GFR

All diodes pointing to ground (opposite of Zener). I used 1n4148's.

Every diode has two voltages at which they conduct - the forward conduction voltage and the reverse conduction voltage. The forward conduction voltage is ~0.7V for most Si diodes. The reverse conduction voltage in a 1N4007 is several hundred Volts. A Zener is simply a diode with a controlled reverse conduction voltage. If you bias them in the forward direction they will conduct at 0.7V like a normal diode.

That's why the zeners are pointing upwards in your schematic - you want to use the REVERSE conduction voltage (2.7V) as a voltage reference.

Since I don't have a 2.7V zener I just added normal diodes in series until the SUM of their FORWARD voltages was roughly equal to the REVERSE voltage of the zener. That's why the diodes are in opposite direction of the zener.

The string of diodes will not work exactly like the zener, because the zener will conduct at 0.7V in one direction and at 2.7V in the opposite direction. The string of diodes will conduct at 2.8V in one direction and at a high voltage on the opposite direction (if the reverse voltage of each diode is 10V, that would be 40V). This circuit certainly uses the zeners conducting only in one direction, so this difference is not of importance.

The circuit is working, biasing, amplifying and clipping. I skipped the tone switch and volume control, because I built it at work (to be able to use an oscilloscope) and I don't have a guitar and amp here

So I know it works, but I haven't heard it yet!

You may try RG's suggestion and try another opamp - you may have fried one in your previous experiments. The transistor is a big candidate for frying.

5/25/1999 8:56 AM
Gus	aron did you get my email

5/25/1999 11:52 AM
aron	Yes I did. I tried it and I couldn't make it work. I will try again. Aron

5/25/1999 9:07 AM
R.G.	Aron - my ISP is on the fritz again, can't send email or receive it. Glad to see GFR has confirmed my first suspicion about the operation of the circuit. However, as to why you can't bias up your breadboard version, as GFR notes, the bottom zener is just a refernce voltage, so your 2.7V zeners should give you 2.7 and 5.4V at their cathodes. Since you are messing with the innards of the IC at the balance nodes, it is possible that some mishap has damaged the IC you're working with. The input protection may not be as good on the balance nodes. I did find my original notes and my circuit differs from yours in a few details. There is a pair of back to back diodes across the + and - inputs of the opamp, and I found a "cancel" circuit that opened up several nodes which you show as connected to ground in the "bypass" mode. I don't know if you simplified for ease of drawing or not. The other parts of the circuit look the same except for minor component value changes, principally in the capacitors. I told you that you got it right...

5/25/1999 9:07 AM

R.G.

Aron - my ISP is on the fritz again, can't send email or receive it.

Glad to see GFR has confirmed my first suspicion about the operation of the circuit.

However, as to why you can't bias up your breadboard version, as GFR notes, the bottom zener is just a refernce voltage, so your 2.7V zeners should give you 2.7 and 5.4V at their cathodes. Since you are messing with the innards of the IC at the balance nodes, it is possible that some mishap has damaged the IC you're working with. The input protection may not be as good on the balance nodes.

I did find my original notes and my circuit differs from yours in a few details. There is a pair of back to back diodes across the + and - inputs of the opamp, and I found a "cancel" circuit that opened up several nodes which you show as connected to ground in the "bypass" mode. I don't know if you simplified for ease of drawing or not.

The other parts of the circuit look the same except for minor component value changes, principally in the capacitors.

I told you that you got it right...

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