Can someone tell me why a lot of buffers end in an electrolytic cap?

[chumbox]

Hi All

Just wondering why when I look at buffers they tend to end in a 1uF or 10uF electrolytic?  Is this to do with impedance?  I was once informed that its best to keep electrolytics out of the signal path as it can interfere with tone (but I guess that's a separate debate... and thread).

A good example are most AMZ buffers such as these below (and since Jack know's his gear I figure there is a great reason for this and thus my question):

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

Thanks in advance

[R.G.]

They're there to solve two problems:
(1) keep DC from going out to the next thing connected to the output

and

(2) not knowing how big or small an impedance the output will drive.

Both are a result of not knowing what will be connected to the output. If whatever will be connected has its own DC blocking cap, then a cap for the buffer isn't necessary and you get frequency response down to whatever the cap in the thing connected to the output will allow.

The large capacitance, 1uF or 10uF or whatever, reflects that the output cap forms a high-pass filter with the input impedance of whatever is connected. A lot of hifi stuff has an input impedance of 10K. So a 1uF cap gives a lowest frequency passed of F = 1/(2*pi*R*C) = 1/ (6.28*10,000* 1E-6) = 1/(62.8e-3) = 15.8Hz if I did the math right. If what you're driving has a 1K input impedance, you need that 10uF to get down to 15Hz. If you have a pedal or guitar amp with a 1M input impedance, you can get by with a 0.01uF cap.

The cap value sets where you start losing  bass.

[chumbox]

Thanks R.G. 

Oddly enough I even understood that maths.  Really helped.

[PRR]

Two reasons to buffer:

1) you need to take a delicate signal gently

2) you have a brutal load to drive

Since the difference may not be clear, many "buffers" do *both*.

If you have to drive a heavy load, R.G. has shown that you may want over 1uFd.

Caps over 1uFd "tend" to be electrolytic. While large film-caps can be had, the cost and size may be an issue. Even at 1uFd an electro is typically far smaller and cheaper than a film-cap.

The distortion effects in electros are small (smaller than forum-chat would have you think), and smaller yet if the cap is way over-size. And the cost/size does not vary much up to many dozen uFd.

Also you may need to drive a long cable near 50Hz/60Hz AC power lines. For this you want the lowest impedance possible (or practical). While discrete buffers have some internal impedance, cheap op-amps have "zero" impedance at power frequencies. Hundreds of uFd may be advantageous.

[chumbox]

Thanks Paul