A noob question from a complete noob (about a/b switches & dropdown resistors)

[Sir_Ian]

Ok. I made a topic a while ago about building a simple a/b switch. And I was getting popping when I inserted the input jack. So, I'm pretty sure the dropdown resistor is my solution. But the question is....

If I put a 1.5 meg resistor between my imput and ground, how will that impact impedence? (remember this is just a switch, so I'm not worried about previous effects coming into the switch, I'm more worried about this switch going into a expensive amp).

I've don't more reading and I'm worried that it could be bad for my amp because the a/b switch would actually be at the end of my effects chain.

Would I need to put an output buffer on it so that I have a low outup impedence?

Another thing. I am about to build my first pedal. I am going to build a Tubescreamer 8. I'm doing the schematic from general guitar gadgets. I'm actually building it as a wedding gift for my brother. But anyways, do I need input or output buffers on it? The concept of buffers is kinda new to me. I know that I need it to have a high input impedence and low output impedence, and the question is whether the Tubescreamer naturally does this. Maybe somebody could point me to an article about how to proactively CHANGE impedence. I've read a bunch of posts using the search function and some online articles about the philosiphy of impedence and why i want low output and high input. It makes sense and all. But I don't quite know how certain parts affect it...if you get what I mean.

And lastly. For the life of me, I have no clue what the input capacitor does. I know it stops DC just like all caps, but is that the only reason its there? I had put a .1uf cap on the input of my a/b switch, cuz somebody said it might help the popping, (but it didn't, but I'm not mad). So I'm curious of what effect it is having on my a/b switch and what purpose it serves in pedals.

Once again. Thank you all very much. You guys are very patient with me, and do know I try to do as much homework as I can, but sometimes, I must resort to begging for answers. Thanks.

[petemoore]

http://www.muzique.com/lab/imp.htm
  Jack Orman explains pedal impedance at AMZ.
    IIUC it's the plugging into jacks which is causing the popping.
  Cures for that include..
  Using the standby switch, zeroeing a volume control downstream, and [dunno if they still make] the 'auto-mute' switched cables, a little momentary switch built into the larger plug body grounds input when plug is pulled out about 1/8''.
  Standby always works.
   Zeroeing volume controls may lead to the debugging of zero volume problem.
   Those switch-mute cables had their set of problems also.

[Sir_Ian]

I think that putting in a standby switch is probably the best solution to my problem. So that's probably what I will do. Good Idea, thanks. But I still want to know what my other options are in order to further my understanding of pedals.

The article ends with "In general, the ideal situation is to have a low output impedance connected to a high input impedance in order to pass the best fidelity signal across the audio frequency band. A unity gain buffer, which has a high input and low output Z, will sometimes be used as a line driver to preserve the full frequency response of the signal."

So is a unity gain buffer something I should consider using. (can someone link to a good scematic?) Would I need to still add a dropdown resistor resistor in front of the buffer? If so, I'm guessing this would somewhat compromise the high input impedence of the buffer.


So, do anyone here personally use buffers before each effect on their pedals? Should I be doing this?

And lastly, whats the input capacitor for?

thanks

[Sir_Ian]

can anybody else give me a lil help?

Are you guys using buffers in all your pedals, and what is the function of the input capacitor?

[Sir_Ian]

I feel like its bad etiquette(sp?) to keep bumping my own topic. By I do kinda feel like I want an answer. Any help will be very appreciated.

[frokost]

Buffers in the tube screamer: http://www.geofex.com/Article_Folders/TStech/tsxfram.htm

Buffers in general - search. Sometimes they are necessary, sometimes not. It depends on the circuit.

What does the input capacitator do: Block DC, yes. But also, in the FAQ there are some "Simple mods" that involves changing the input (and output) capacitator: http://www.diystompboxes.com/wiki/index.php?title=Simplemods#Simple.2C_Easy_Mods.2FTips_.26_Techniques

[R.G.]

If I put a 1.5 meg resistor between my imput and ground, how will that impact impedence? (remember this is just a switch, so I'm not worried about previous effects coming into the switch, I'm more worried about this switch going into a expensive amp).

I've don't more reading and I'm worried that it could be bad for my amp because the a/b switch would actually be at the end of my effects chain.

The amplifier has an input impedance, probably 1M+68K to ground, and probably no input capacitor. Some do, most don't. That is because the "natural" bias point for preamp tubes is with their grids grounded through a large resistor.

The effects have a low output impedance, probably in the range of 10k or lower. Almost certainly less than 100K.
A guitar, all by itself, has an output impedance that is variable, from about 4K resistive up to perhaps 1M or so inductive as frequency rises, reflecting the inductive nature of the pickups themselves. This is the reason for high input impedances. Input impedances must be higher than the guitar's output impedances or treble will be lost.

Your adding a 1.5M resistor to ground adds more loading to whatever drives the amplifier. It lowers the impedance and might start some small treble loss by loading a guitar if a guitar drove it directly. But it will not cause problems with only effects driving the input. It will also not harm the amplifier in any way.

Would I need to put an output buffer on it so that I have a low outup impedence?

No, not unless the cable between the AB and the amp is very long. This is an effect of the low impedance of the effects, not the AB box, which largely plays no part in this.

Another thing. I am about to build my first pedal. I am going to build a Tubescreamer 8. I'm doing the schematic from general guitar gadgets. I'm actually building it as a wedding gift for my brother. But anyways, do I need input or output buffers on it?

The schematic from GGG already has all the buffers you need.

The concept of buffers is kinda new to me. I know that I need it to have a high input impedence and low output impedence, and the question is whether the Tubescreamer naturally does this.

It does.

Maybe somebody could point me to an article about how to proactively CHANGE impedence. I've read a bunch of posts using the search function and some online articles about the philosiphy of impedence and why i want low output and high input. It makes sense and all. But I don't quite know how certain parts affect it...if you get what I mean.

Try the GEO article on impedance.

And lastly. For the life of me, I have no clue what the input capacitor does. I know it stops DC just like all caps, but is that the only reason its there? I had put a .1uf cap on the input of my a/b switch, cuz somebody said it might help the popping, (but it didn't, but I'm not mad). So I'm curious of what effect it is having on my a/b switch and what purpose it serves in pedals.

Capacitors (a) block DC and (b) pass high frequencies better than lows. In fact, (a) is a consequence of (b), as the blocking is nominally perfect at zero frequency.

In effects, both a and b are used. All single-supply (i.e. 9Vdc)  effects need the DC blocked front and back, so there will be a cap at the input and output. Caps are not perfect, so there is always a little leakage. That's the function of the pull-down resistors, to eat the leakage. There is an article on that topic alone at GEO.

Beginners are mystified by buffers, unnecessarily. A buffer is no more or less than a circuit which presents a very high input impedance and a low output impedance. It's gain is often, but not necessarily unity. A buffer is a "power lifter". It accepts signal from a weaker circuit and makes it strong enough to drive whatever comes later. Buffers separate loads from signal sources and make both of them happy. And that's the total story on buffers that beginners need to know.

[Sir_Ian]

Wow, Thank you so much guys. This is greatly appreciated. I understand better, and willl definately be reading those articles. Gracias.