Dual OpAmp questions - stacked gain stages - and a "new" OD circuit

[oliphaunt]

I am continuing to experiment with opamps, and have heavily modified an MXR Distortion + into this circuit which has much less gain and harshness.



I have built this using two single opamps (a TL071 and a JRC741) and it works.   I know the tone control is very rudimentary and accounts for a huge amount of signal loss.  I have tried other tone stacks, but this simple design helps smooth out the midrange and make this sound a lot less like a mid humped Tube Screamer.  I lost so much signal though that a make up gain stage is necessary, so I added a simple non-inverting stage after it.

My questions are:

- Is it ok combine this into one dual opamp such as a TL072?  Will there be some interaction between the two amps?  

- Gain stage 2 has a connection to the bias point of 4.5v through a 1M resistor which I simply copied from the first stage assuming it was needed.  I have disconected this and it seems to make no obvious difference.  Is it really necessary, or should it be implemented a different way?

- Does the amp need an output resistor( I currenty have 1k in there), and if so what would be an ideal value?

- Does anyone see any other glaring errors or obvious missing or unnecessary parts?

[oliphaunt]

- Is it ok combine this into one dual opamp such as a TL072?  Will there be some interaction between the two amps?  

I breadboarded the above schematic and updated it as I went to reflect what I have learned so far.  It seems to work very well, and there is no obvious issue with using a dual opamp.

A TL072 was ok, but the sound is less pleasant than the two single op amps.  I also lost quite a bit of max gain compared to the single opamps, but that doesn't really bother me, and is probably easily made up for by increasing the value of the gain pot.  I also gained some low end, a very sub low "umph" that is nice.   I also tried a 4558, 4559, and an OPA2134.  The 4558 is much better sounding, same gain and low end, but it has a creamier, smoother overdrive and slightly more mids.  The 4559 is a little more open sounding.  The OPA is my choice so far.  It has a bit less creamy OD, but is the most natural sounding.

- Gain stage 2 has a connection to the bias point of 4.5v through a 1M resistor which I simply copied from the first stage assuming it was needed.  I have disconected this and it seems to make no obvious difference.  Is it really necessary, or should it be implemented a different way?

In this configuration it must have the bias connected.

I would love any other thoughts on this project.

[amptramp]

To answer your questions in order:

You should add a bypass capacitor across the Vcc and Gnd leads of the op amp regardless of whether you use singles or a dual.  The TL072 should be satisfactory although others may sound different.

Yes, the gain stage does need the 1M resistor to 4.5 volts.  If it is not there, the amplifier input will have no DC path to ground.  You may be getting satisfactory results due to leakage current through the 0.33 uF cap from the first stage, but this is not repeatable.  Check the DC voltage at the output stage at pin 7 which has a DC gain of 1 and an AC gain of 3.  This should be at 4.5 volts, same as both inputs (pins 5 and 6).  When you disconnect the 1 M resistor, there may be little change since the input current is low, but with no DC return, the voltage will drift with temperature and the output stage may drift high or low and start clipping by itself.  You do not want erratic or non-repeatable performance.

The output resistance is fine for feeding a high impedance next stage.  The output impedance is zero at the zero level setting, 1K at the high level but goes over 25K in the mid setting.  I would reduce the pot to 50K.

Now for the nit-picking:

1. The 10 uF capacitor which sets the gain in the output stage gives a low frequency turnover of 1.59 Hz.  This is way lower than you need, so this value can come down to under 1 uF where you can get good film capacitors - much more stable and longer life than electrolytics.  To compare, you input turnover is about 33 Hz (from the input to the first stage), the ouput coupling is about 7 Hz (so the output volume control can afford to come down to 50K).  Turnover is the frequency where the loss is -3db compared to the midband.

2. I would interchange the positions of C3 and the 1K resistor that is in series with it to reduce the capacitance to ground from the non-inverting input of the op amp.  This will improve stability.  You will notice that some distortions like the tube screamer have a capacitor across the gain control - this improves stability and reduces the clipping of high frequencies where the distortion products would be in the very high frequencies.  Your dog will thank you.

3. You have symmetrical clipping, so you will get only odd harmonics of the signals you put in.  If you select different diodes - say the 1N34A and 1N4001 and a red LED the other way, the imbalance will give some even harmonics as well and may sound "fuller".

4. The 1N4001 is a rectifier diode with a fair amount of capacitance across it.  This means high frequencies will bypass the diode junction and clip at the 1N34 level - the opposite of what you want.  A good silicon diode for this use is a 1N914 or a 1N4148.

5. If you include a small capacitor across the 200K in the output stage, you may not need as high a capacitance for the tone control.

[PRR]

If you omit the 0.33u, you do not need the 2nd stage 1Meg.

10uFd does seem overkill; historically a 10uFd electro was the cheapest cap above 0.1uFd and we used them many places where smaller could have worked.

Gain really should not change between 2-single and 1-dual; gain here is all about the feedback networks (resistors, caps, and those diodes). If you heard a change with a dual, maybe parts were different, maybe your mood or energy changed. (It's all subjective.)

[oliphaunt]

You should add a bypass capacitor across the Vcc and Gnd leads of the op amp regardless of whether you use singles or a dual.

Such a s a typical 100u power filter cap?

1. The 10 uF capacitor which sets the gain in the output stage gives a low frequency turnover of 1.59 Hz.  This is way lower than you need, so this value can come down to under 1 uF where you can get good film capacitors - much more stable and longer life than electrolytics.  To compare, you input turnover is about 33 Hz (from the input to the first stage), the ouput coupling is about 7 Hz (so the output volume control can afford to come down to 50K).  Turnover is the frequency where the loss is -3db compared to the midband.

How do I calculate a more appropriate value?  I will try lowering and just see what happens.

2. I would interchange the positions of C3 and the 1K resistor that is in series with it to reduce the capacitance to ground from the non-inverting input of the op amp.  This will improve stability.  

Done, I see that is the typical way to do it, not sure how they got reversed.

You will notice that some distortions like the tube screamer have a capacitor across the gain control - this improves stability and reduces the clipping of high frequencies where the distortion products would be in the very high frequencies.  Your dog will thank you.

I  originally had a 10pf cap there, but the tone control I am using really does a job on the high end, and the small cap doesn't make any audible difference .

4. The 1N4001 is a rectifier diode with a fair amount of capacitance across it.  This means high frequencies will bypass the diode junction and clip at the 1N34 level - the opposite of what you want.  A good silicon diode for this use is a 1N914 or a 1N4148.

Very interesting, this could account for the difference in sound of 1N4001 diodes vs other silicons.  I have often seen that they have similar forward voltages and seen posts where people say they should therefore sound ther same but I always heard a very distinct difference in sound.  Thanks!

5. If you include a small capacitor across the 200K in the output stage, you may not need as high a capacitance for the tone control.

Do you mean put a cap in paralled with what is now the 47K resistor?

If you omit the 0.33u, you do not need the 2nd stage 1Meg.

I will try this, I love removing parts from a circuit!  

Gain really should not change between 2-single and 1-dual; gain here is all about the feedback networks (resistors, caps, and those diodes). If you heard a change with a dual, maybe parts were different, maybe your mood or energy changed. (It's all subjective.)

I thought that was odd also, and I can assume I made some accidental change in the breadboarded circuits.  I modded the two single opamp circuit for days, so I may not have documented every change I made correctly.  In the end it sounds better now, so I am happy, I just need to confirm my circuit on the board.


Thanks very much to both of you for these suggestions!  I will try them all as soon as get some more time to work on the circuit.

[familyortiz]

great stuff...
of course of the many offerings, I've liked working with the Analog Devices stuff, particularly the OP270 dual amp which is a low noise precision part well suited for this.

[rogeryu_ph]

Maybe you need a pull down 1M resistor after .22 cap to the ground so that when you switch ON and OFF with your bypass switch it would not POP.