Can we use opamp as Output Buffer?

[Izzy]

I was wondering if we can substitube Transistor with Opamp for output buffer?


Will there be any side effects?

[R.G.]

Probably yes.

In what??

[Izzy]

In my small High gain pedal.

http://img362.imageshack.us/img362/3743/schematicsnu4.jpg


When I was using transistor for output buffer it gave me weak signal at the emmiter, and some crackling too.

And one more thing what is the difference between shunting diode to ground and keeping 2 diodes inopposite direction  across in series in the signal?

[zyxwyvu]

Here's a great page on buffers:

http://www.muzique.com/lab/buffers.htm

They are all pretty much interchangeable, though there are some differences.

About the diodes, 2 opposite diodes to ground will clip the signal at whatever the forward voltage of the diode is (silicon=0.6-7V). Putting the diodes in series will only allow signal through when it is more positive or negative than that voltage.

[puretube]

I was wondering if we can substitube Transistor with Opamp for output buffer?


Will there be any side effects?

ahh: I like that word... 

[JimRayden]

Ooh, ooh!!! I'm SO going to use that word on one of my tube pedals.

Thanks for pointing that out, Puretute.

----------
Jimbo

[Izzy]

Hmm I guess so. I was bread boarding my circuit and mistakely I  had those diodes in series instead of grounding, and the pedal was damn quiet! I was amazed how can this be possible?
How can a pedal with so much gain can be this quiet. (Quiet as in no Hiss, no noise)
But I had also notice it had less sustain.
Then I later found out  I had actually bread boarded diodes in series . LOL 
So I was only allowing the signal to pass when the signal were more than 0.6-0.7 v. haha


So Is using Germanium diodes in series a good idea to keep hiss, noise low?
But it will surely cut some sustain and might act as gate. but Is it a good idea?

[Izzy]

One more thing, In the abive schematics, when I shunted diodes to ground the signal was  too low and when I shunted to 4.5 V it crackeled and had some signal was on and off...on and off....

I dont get it why is this happening?


And when I used the transistor for output buffer , and checked with Audio probe the signal was weak and crackling, why is that?


I would really appreciate if any one can answer my questions.

Thanks iN advance.

[Izzy]

Anyone?

[slacker]

You need a DC blocking capacitor in between R26 and the clipping diodes. Without one DC has a path to ground through R9, R21, R22 and R24 which will mess up the biasing of U4A and probably U3B.
The tonestack will also reduce the volume quite a lot so you might need and amplifier stage after it not a buffer.

[Izzy]

Thanks Slacker! That makes sense!

So the key was Putting caps before diodes I really didnt thought about that.

And actually those tone stack were connected to Vb. Sorry I had connected to ground in those schematics.

And yes I was thinking the same, I thought there will be volume loss after the tone stack. But  the signal at the output was too loud compared to other pedals.

May be that was because diodes were not operating. I will try again and see what happens.


Thanks a lot  again.

[Izzy]

One more question:

What are the thing that we need to take care while making PCB layout?
I have heard that we shouldn't  keep certain traces close to each other?
Like  9v trace and drive trace? etc etc?

[Izzy]

Anyone?

[heyniceguy]

as far as keeping traces separate, it really only applies to the most sensitive parts of your circuit. In your case, the most sensitive are the signal traces at the very front of your circuit, generally, anything from the input jack to the input of your first gain stage. those are the traces that will be affected most by interference. keep them away from every other noisy trace: typically, unfiltered 9V traces and anything at the end of your signal chain. another rule of thumb is to use shielded wire for the wire going from the input jack to the board. connect only one end of the shield to ground.

something else to remember: interference is most easily caused when the two wires/traces are parallel and in close proximity with each other. in some situations, you just cant separate traces/wires from crossing. you can over come this by limiting any parallel routings and instead use 90 degree / perpendicular crossing. almost no interference is introduced with these kind of "t" junctions.

i wired my house with RJ45 ethernet (very sensitive to interference) when it was being built, and in some situations, i was forced to cross my LAN lines with my 120VAC power lines. but i made sure to keep their parallel routings apart and use strict 90 degrees when i had to cross their paths. no noise at all, and no lost connections.

[Izzy]

Thanks a lot dude! that helps a lot! 

[Izzy]

I have one more question:

>> How do you decrease the volume of a circuit and bring it down to normal level.

The volume is too loud in my design.
Is there anyway to do that ?

[Ge_Whiz]

If you have an output level control, put a fixed resistor between the rest of the circuit and the control. For example, if the fixed resistor is the same value as the pot, your maximum level will only be half the maximum you get without the fixed resistor. If the resistor is three times the pot value, you will only get one-quarter the maximum level when turned right up, etc.

[Doug_H]

When I was using transistor for output buffer it gave me weak signal at the emmiter, and some crackling too.

There's no reason a transistor buffer shouldn't work and the voltages in your schematic look ok (although the emitter voltage is a little low). Your problem has nothing to do with whether it's an op amp or transistor buffer. It may be the low emitter voltage is enough of a mis-bias to cause the sound problem. Try connecting r32 to vcc and add another resistor between base and ground. (Try between 820k and 1M.) Adjust until you get 4.5v on the emitter.

The biasing method in your schem is fine if you want to use a FET, in which case you would set the gate at 1/2 Vcc to get 1/2 Vcc on the drain. But it's different for BJT's.

[Gus]

I have one more question:

>> How do you decrease the volume of a circuit and bring it down to normal level.

The volume is too loud in my design.
Is there anyway to do that ?

You do ask a lot of basic questions and use the words "my design" 

[Izzy]

I know Initially I was going to use voltage divider but the simulation wasnt showing right. I tried brad boarding it was cutting the highs. So I wanted to make sure I was doing it right and also wanted to know is there any other way to do this.

But probably that was because I was changing the resistor before the diodes. Sorry for the inconvinience.