Master volume or static resistance between Decouple cap out and next input.

[Thecomedian]

its just a quick question. Will a circuit fail to work without any resistance in this spot, or will the full signal go directly to the next input? Does a volume control or resistor only matter if you wish to attenuate certain frequencies with the RC pass-effect the decouple cap and resistance has, or to control the absolute volume of the output signal, or will it's absence cause a circuit not to function at all?

[PRR]

Not sure if you mean coupling cap or de-coupling cap.

I think you need to draw a picture.

Or.... just try it?

[Thecomedian]

http://www.beavisaudio.com/techpages/Buffers/Buffer_OrmanTransistor.gif

something like this. It has no resistance between "out" and the decouple cap (10uf). It has the regular bias resistor on source, but nothing after that, like a pot or static resistor.

[tubegeek]

In the schematic you referenced, there is a bias resistor for the SOURCE. There is nothing in the DRAIN lead but the 9V, FYI.

There is often a resistor in series with the output of a device (called a "build-out" resistor) - this isolates the device from the cable capacitance and helps with stability. May or may not be needed, may or may not depend on the length of the cable to the next item in the signal path.

There is also often a resistor between the output of a device and ground (called a "pull-down" resistor.) This pulls the output end of the output coupling cap to 0V, so that the operation of a bypass switch will not cause a "pop." It can be omitted so long as the "pop" is not going to be a problem, assuming that the next input being fed has some way for its input node to get the proper DC level. This assumption is a pretty reasonable one to make, it would be almost unheard of for an input to depend on the previous circuit for its biasing. Unless of course there were some sort of design mistake or unschooled circuit-hacking going on.

A coupling cap blocks DC voltage from one circuit block to the next circuit block. A decoupling cap goes from a power supply node to ground and protects circuit blocks from unwanted coupling via the power supply.

One of my mentors, Stephie Bench, has explained that the power supply coupling creates a "relaxation-mode oscillator" topology, but I still haven't learned enough about relaxation-mode oscillators to understand that advice. A relaxation-mode oscillator sounds to me like what they put in those vibrating massage chairs at the airport, but what the hell do I know?

[GibsonGM]

I think it might be wise to also mention that the input signal for the next device in the chain is developed across that device's input resistance and/or impedance...I think I see what the OP is getting at...there is a "missing way for the final signal to develop", in a way.  But it actually does have a resistance to 'build across', if you will, in the next device, which by its own nature must draw current from this preceding one...

[PRR]

TC is not talking about a DE-coupling capacitor, as I understand it.

However.......

> power supply coupling creates a "relaxation-mode oscillator" topology, but I still haven't learned enough....

Bah. Bench knows too much.

An "amplifier" is usually a *chain* of stages. (The row-of-bottles in the back of a tube-amp is a good image.)

First stage takes teeny signals and makes them larger, next stage even larger, and last stage makes them BIG enuff to slap a speaker.

For convenience we have just one battery or other power-source for the whole chain.

The last, speaker-slapper, stage will cause some wobble in the power supply. (Unless the power supply is infinitely strong, but it never is.)

Wobble in the power feed to the first, teeny-signal, stage will be amplified through the 2nd and power stages, and cause more wobble.

It is the same as a mike, amp, and speaker. If the gain is high, you WILL have a howl-around.

(Actually there is a Phase requirement too. But any sufficiently complex system "will" find a phase-point and howl.)

The straight-forward way to fix this....

Read Radiotron Designer Handbook page 28-30 3rd Edition.

If an amplifier chain is highly unstable, it will leave the Linear zone. Tube amps (and others) *can* get so far-out that they become "relaxation oscillators", a fertile field of study.

[tubegeek]

The straight-forward way to fix this....

Read Radiotron Designer Handbook page 28-30 3rd Edition.

Will do. A favorite book at any rate. Thanks!