Good input impedance

Balthazar · Post by **Balthazar** » 20 Aug 2009, 19:43

Hi

I'm trying to design a DIY tube distortion pedal for a guitar. I haven't read anything specifically about guitar amps and effects, only Morgan Jones' book about Hi-Fi amps. In this book, a principle seems to be that you should keep input impedance as high as possible, while keeping output impedance as low as possible. However many guitar techs say soundcards, amps etc. should have an input impedance of about 1M for guitars, I have also seen preamps with a specified impedance of 2.2M. Does this mean it should be *at least* 1 Meg, or that an input impedance of 1 Meg is preferred, even over higher impedance? In other words, is higher input impedance always better, also for guitar preamps/effects? Since I have an extra triode available (using double triodes), I could use it as a buffer to raise the input impedance from about 1M to about 25M. Will that be better, or should I drop it?

earthtonesaudio · Post by **earthtonesaudio** » 21 Aug 2009, 03:32

I believe it is a compromise between fidelity and noise performance. Much higher input Z and you get overly sensitive to picking up interference, not to mention this often requires higher value resistors which add their own noise. Go much lower and you begin to lose volume and treble content from"most passive guitar pickups.

Balthazar · Post by **Balthazar** » 22 Aug 2009, 12:18

Hi

Thanks a lot for your help. This of course, begs the follow-up question: Output impedance, should I keep it as low as possible, or opt for a "preferred value" ?

bajaman · Post by **bajaman** » 22 Aug 2009, 13:56

I'm trying to design a DIY tube distortion pedal for a guitar. I haven't read anything specifically about guitar amps and effects

I would suggest that you attempt something simpler - for example: a super hard on, linear power booster (lpb) or fuzz face,
before attempting to "design" a tube guitar pedal

I know we all have to start somewhere, but it is best to start with simple projects - learn to walk first, running comes later.

cheers
bajaman

UZILSD · Post by **UZILSD** » 22 Aug 2009, 15:54

Generally speaking, 1Meg of Zin is a reference point to have a good matching with the guitar signal. The Zvex Super Hard On has 5Meg and when set to unity gain it preserves more trebles. And it is considered to have a "super HI-Z". 25Meg would be just too much...
Consider that a lot of good sounding pedals (expecially the older ones) have a Zin much lower than 1Meg. An old wahs, Univibes, and so on usually have a Zin below 100K and they sound a little darker compared to modern stuff. But they still suond great.

earthtonesaudio · Post by **earthtonesaudio** » 23 Aug 2009, 02:56

SHO input impedance is not 5M, except perhaps when you remove the power supply.
viewtopic.php?f=13&t=3971&hilit=crackle+not+okay#p43357

Good advice from Bajaman.

Balthazar · Post by **Balthazar** » 23 Aug 2009, 15:13

I know we all have to start somewhere, but it is best to start with simple projects - learn to walk first, running comes later.

Uhmmm... I guess you have a good point there...

Thing is, I have already been wading through hundreds of pages of theory stuff about tube amlifiers for HiFi (Morgan Jones). I feel I have a very good grasp on that theory. I'm a heavy theory man. Then I ended up not using it cause my tranny amplifier just got better and better...

Transistor and other semiconductor stuff I know very little about. Also, when I was a kid, I used to play with electronics and make PCB boards with etch resist pen and got pretty good at soldering, wich I still am, but YES I am a novice....

Also, I really NEED a tube pedal, I don't need the other stuff you mentioned so much. I recently got my hands on the guitar amp handbook by Dave Hunter, I just started reading it... I see guitar amp circuits are far more subjective than HiFi, with more things that can't be decided just by theory. So, I think I'll order some strip boards, and start experimenting. But I'll work with low voltages. I'll probably just try to build simple parts of the circuit and test and see how it works on the strip board. The first part is just a boost stage that is non-linear and hopefully will add som nice 2nd order harmonics, maybe I'll just stick with that, who knows.

If it turns out to be too hard, then you will have a written permission to laugh at me, in public!!

UZILSD · Post by **UZILSD** » 25 Aug 2009, 18:30

earthtonesaudio wrote:SHO input impedance is not 5M, except perhaps when you remove the power supply.
viewtopic.php?f=13&t=3971&hilit=crackle+not+okay#p43357

Good advice from Bajaman.

I read that, but since my theory is quite rusted I can't get it. Would you please me explain what calculations you did? For knowledge's sake, nothing all.

Thanks

earthtonesaudio · Post by **earthtonesaudio** » 25 Aug 2009, 20:16

I didn't do the calculations (those were by estragon), but just from a qualitative standpoint you can see the input Z can't be 5M. The top of the upper 10M resistor goes to the drain of the MOSFET, and when the input swings up, the drain swings down, making the 10M between drain and gate appear smaller than it really is (lower impedance). Neglecting all other details, this makes the input impedance less than 5M. For higher gains, the swing at the drain is larger, making this resistor appear smaller still.

UZILSD · Post by **UZILSD** » 25 Aug 2009, 23:42

earthtonesaudio wrote:I didn't do the calculations (those were by estragon), but just from a qualitative standpoint you can see the input Z can't be 5M. The top of the upper 10M resistor goes to the drain of the MOSFET, and when the input swings up, the drain swings down, making the 10M between drain and gate appear smaller than it really is (lower impedance). Neglecting all other details, this makes the input impedance less than 5M. For higher gains, the swing at the drain is larger, making this resistor appear smaller still.

OK, this is what I figured. BUT...
Assume that the upper 10M resistor would be tied to the V+ rail, say 9V, then the Zin would be 5M since the V+ rail is considered an AC ground.
The point that freaks me out is that in the SHO configuration, at the top end of the upper 10M we have a negative amplified version of the input signal... but anyway, this point isn't always between 9V and 0V? The most of the time it will be under 9V. So HOW this would decrease Zin?!?

I repeat, I haven't read an electronics book or article in these last months. Maybe is a simple principle that I miss but now is tricking my mind!

earthtonesaudio · Post by **earthtonesaudio** » 26 Aug 2009, 01:37

UZILSD wrote:Assume that the upper 10M resistor would be tied to the V+ rail, say 9V, then the Zin would be 5M since the V+ rail is considered an AC ground.

Absolutely correct.

UZILSD wrote:The point that freaks me out is that in the SHO configuration, at the top end of the upper 10M we have a negative amplified version of the input signal... but anyway, this point isn't always between 9V and 0V? The most of the time it will be under 9V. So HOW this would decrease Zin?!?

The key difference is that the drain presents a changing voltage, not a static voltage like ground or the + rail. This means for a changing input signal, more voltage is dropped across the resistor than if it were tied to a fixed reference. More voltage dropped means more current flows, which means the impedance is lower. Larger gains mean more voltage dropped across that resistor, more current, lower impedance. Hope this helps.

UZILSD · Post by **UZILSD** » 26 Aug 2009, 11:01

earthtonesaudio wrote:
The key difference is that the drain presents a changing voltage, not a static voltage like ground or the + rail. This means for a changing input signal, more voltage is dropped across the resistor than if it were tied to a fixed reference. More voltage dropped means more current flows, which means the impedance is lower. Larger gains mean more voltage dropped across that resistor, more current, lower impedance. Hope this helps.

OK, I got it. I missed the effect it would be have in its evolution in time compared to a fixed reference.

Thanks!