Adding input and output buffer to DOD250

[Vlad123]

Hi there guys, I'm new on this site. This is my first topic. I realy like DOD250 overdrive, and I want to add input and output buffers to it. How can I do that? I saw a lot buffer schematics, but I don't know how to add them on exsiting DOD schematic.

[samhay]

Why?

More specifically, the input impedance is quite high, and the output impedance is at most 50k. What do you hope input and output buffers will achieve?

[Vlad123]

I want input impedance to be higer than 1M, a output lower than 10k. I just want to know is that going to make some changes in sound, also i would like to try ne5534 in this circuit, which has pretty low input inpedance and I was hoping that input buffer is going to help

[Mark Hammer]

I highly doubt that a 5534 is going to improve anything about the 250.  I mean, it's a decent op-amp.  But a decent op-amp is not what that circuit is about.  And, truth be told, you want input loading, because it will reduce some of the undesirable fizz that happens when one generates harmonics of harmonics.  As for output loading, use the other half of a 4558 for that.

[PRR]

"Lipstick on a  pig" comes to mind. That's not quite right, but is a thought.

Output impedance is zero to 28K depending on pot setting. In guitar-cord work, that is plenty low. If you must go into 10K studio gear, you may want to buffer the output.

Input impedance is essentially 1Meg and there are very-very few pickups where 2Meg or 3Meg makes any difference. In this circuit you could just increase the 1meg resistor. With '741 the DC bias point will shift off of 4.50v but probably not enough to do any harm. (Anyway this circuit is *intended * to "harm" signal.) With '5532 the offset will be worse, maybe enough to get some one-side clipping before the asymmetrical clipping diodes.

[amz-fx]

Paul,

Isn't the 1M input resistor in parallel with the Zin of the opamp?  On the 741, Zin is 2M. On the 5532 it is typically 300k (but only guaranteed to be above 30k).

For the 741, 1M // 2M is 667k, which is plenty.

On the 5532, a 1M input resistor with the Zin in parallel is 231k. Not good.

Best regards, Jack

[PRR]

> the Zin of the opamp?  On the 741, Zin is 2M. On the 5532 it is typically 300k (but only guaranteed to be above 30k).

When an opamp datasheet shows an input impedance, this is measured Open-Loop.

Put NFB around it, in our usual ways, impedance at the pin goes to infinity (or zero for inverter). Leverage is roughly the excess gain, 10X to 1,000X for most circuits over most of the guitar band. This suggests a '5532's input pin will look like >300K, probably much more because the '5532 has so much gain in the audio band.

[amz-fx]

Excellent info, Paul. Thanks!

Best regards, Jack

[PRR]

Idiot Assistant's stupid opinion:



I actually have a '741 in my antique SPICE-liar. The model is one of the very old "simple" models, but it does model the input as two BJTs at approximate operating current.

FWIW, open-loop (not shown) the input appears to be 1Meg (0.997..Meg), which is in-sight of what you quote from a datasheet.

Closed-loop the effective input is MUCH higher (as Mr Black promised).

Don't have a dummy '5532 here, but you say 30K, I stuck a 30K on, and got the second plot. Like this, the input approximates a 60pFd cap (2 feet of cable). On a real '5532 at reasonable closed-loop gain, the excess gain is higher and so is the input impedance (less pFd).

All these trends are approximately what I would expect, from years of hand-analysis and prototype measurements.

The open-loop Zin matters only for open-loop applications: comparators, and large over-drive.

[Guitar Poppa]

I want input impedance to be higer than 1M, a output lower than 10k. I just want to know is that going to make some changes in sound, also i would like to try ne5534 in this circuit, which has pretty low input inpedance and I was hoping that input buffer is going to help

Hello Vlad !

I think you have got realistic and sensible answers... I would just summarize them by taking the point of view of a guitarist...

A stompbox must have a high impedance input... OK : it prevents from level and treble losses.
But it ain't no use to have too high input impedance : Over 1Mohm, you'll get not much extra level and harmonics... What you will hear more will be the defects of the cable, the resonance caused by the interaction of the cable capacitance and the pick-up reactance... If the cable is high quality, the resonance frequency will be high (over 3KHz), and you'll get a sound that will be heard as badly sharp. You may try, changing the value of the resistor which polarizes the input...
Like PRR says, if you try to high values (>2M2), there is a risk of dissymetry on output DC voltage, and this dissymetry will damage the output signal.

1Mohm his a traditionnal value, because it's a simple number... But it can not be the best in real situation. By experience, I would advice you to try values of the input resistor between 470k, 680k, 1M and 1,5M... If you use mainly single pickups, 470k to 1M is quite enough. Personnaly I often use 680k. If you use mainly humbuckers, let 1M be. Try 1M or more if you have an old Gibson.

If you want try different resistances, and different guitars, keep always the same cable. The cable may have more influence on the sound character than the value of the resistor !

Low output impedance can be a good thing if you use more than 10 meters good cables between stompbox and amp, or more than 5m cheap cable.
In the present case, PRR said the output impedance depends of the volume potetiometer and his setting... In common situation, it will be about 25 to 30kohm. With 3 meters good quality cable (100pf/meter), you'll have treble loss only over 17 kHz, which is far over you need for electric guitar. With 10 meters, the band with will be limited to 5Khz, which is enough. Remember that too high and unnecessary trebles give a bad sonic impression. In electric guitar world, 5 kHz his high.

A remarkable case could be when you want to connect your stompbox directly to a mixer. If its inputs are low impedance, the mixer will suck a lot of level from your stompboxe. You'll try to set the volume higher, or the gain on mixer channel... Perhaps you'll get the level you need, but the impedance mismatch can damage the sound by making the diode stage of the DOD250 work abnormally. Ultimately, this is the only case in which an output buffer would be essential.

>>>> So...
If you use to play in common situations, you should let your DOD250 as it is... Just adjust the 1MOhm on input if you want to tweak the tonal balance for a particular guitar... Install an output buffer only if you'll have low impedance gears connected on the output.

Happy to give a hand !
Guitar Poppa
http://guitarpoppa.com/?page_id=69

[Vlad123]

This is realy helpful, thanks everybody  I also wanted to ask what do you think about adding input buffer to LPB-1?

[Pojo]

This is realy helpful, thanks everybody  I also wanted to ask what do you think about adding input buffer to LPB-1?

The input impedance on the LPB is low enough to benefit from a buffered input. Although some folks like how the stock version loads the pickups and warms up the tone depending on the cable, pickups, location in the pedal chain, etc. A buffer will isolate the circuit and make it behave in a more predictable manner. Best thing to do is just try it and see if you like what it does to the sound. Tinker and learn....that's what it's all about.

[Guitar Poppa]

This is realy helpful, thanks everybody  I also wanted to ask what do you think about adding input buffer to LPB-1?

The input impedance on the LPB is low enough to benefit from a buffered input. Although some folks like how the stock version loads the pickups and warms up the tone depending on the cable, pickups, location in the pedal chain, etc. A buffer will isolate the circuit and make it behave in a more predictable manner. Best thing to do is just try it and see if you like what it does to the sound. Tinker and learn....that's what it's all about.

I would have written exactly the same if Pojo was not the first to reply!

Guitar Poppa

[Vlad123]

Thank you guys