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Gain Controls Vs Volume Controls
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Author:  Cogito [ April 4th, 2021, 3:50 pm ]
Post subject:  Re: Gain Controls Vs Volume Controls
FerdinandII wrote:
I was just trying to disabuse the OP of the following statement he made:
"...gain Controls reduce the voltage by impedance matching..."

In order to dispute this false concept, I chose to go back the the basics, ala Thévenin.
This concept kept creeping back into the discussion above and it needs to GO and never come back.

tomp wrote:
FerdinandII wrote:
You need to just forget the concept of impedance when looking at this issue. It is irrelevant.
V=IR
The line level output is (generally) a voltage source.
Image


Actually, I am right.

There is a problem in the Thevenin equivalent circuit you posted. Thevenin equivalent circuit must have a single voltage source and single series resistor. Like this:

Image

That is a closed loop.
Kirchhoff's Voltage Law: The sum of voltages around a loop is zero

Suppose, preamp puts out 5v at 1000ohms and the amp input load is 10kohms
Voltage drop across First resistor = 5/(1+10) = 0.45v
Voltage drop across second resistor = 50/11 = 4.54v . Amp’s gain stage will see 4.54v with 10k input impedance

If the amp’s impedance is reduced to 2kohms
Voltage drop across First resistor = 5/(1+2) = 1.66v
Voltage drop across second resistor = 5x2/3=3.33v. <— Amp’s gain stage will see 3.33v with 2k input impedance

Amp’s’ gain factor being constant, lowering the voltage seen by its gain stage reduces its output voltage. As Roscoe and others suggested, gain controls are potentiometers. Pots control the volume by manipulating the preamps input impedance.
Author:  Roscoe Primrose [ April 4th, 2021, 3:56 pm ]
Post subject:  Re: Gain Controls Vs Volume Controls
Cogito wrote:
[As Roscoe and others suggested, gain controls are potentiometers. Pots control the volume by manipulating the preamps input impedance.


:angry-banghead: For the last damn time, no, they do not. If you need further clarification, see my signature line.
Author:  DaveR [ April 4th, 2021, 3:58 pm ]
Post subject:  Re: Gain Controls Vs Volume Controls
Cogito wrote:
Amp’s’ gain factor being constant, lowering the voltage seen by its gain stage reduces its output voltage. As Roscoe and others suggested, gain controls are potentiometers. Pots control the volume by manipulating the preamps input impedance.


I believe you mean output impedance.

Regardless, nothing manipulates any other impedance.

There are different ways to implement a circuit that controls gain. A passive circuit can only reduce amplitude. If the input to the amp comes off the wiper of a pot the preamp load is relatively constant, assuming the input impedance of the amp is high.

If the preamp goes into an active circuit that has adjustable gain, the preamp load is constant and gain can be positive and negative.

I understand that you're going to stick to your statement and insist impedance is how gain is controlled. But it really isn't at least 99.99% of the time.
Author:  Roscoe Primrose [ April 4th, 2021, 4:01 pm ]
Post subject:  Re: Gain Controls Vs Volume Controls
DaveR wrote:
Cogito wrote:

There are different ways to implement a circuit that controls gain. A passive circuit can only reduce amplitude.


Actually, a passive circuit can only reduce power. It can increase voltage at the expense of an increase in output impedance.

Roscoe
Author:  DaveR [ April 4th, 2021, 4:14 pm ]
Post subject:  Re: Gain Controls Vs Volume Controls
Roscoe Primrose wrote:
DaveR wrote:
Cogito wrote:

There are different ways to implement a circuit that controls gain. A passive circuit can only reduce amplitude.


Actually, a passive circuit can only reduce power. It can increase voltage at the expense of an increase in output impedance.

Roscoe


True, as with a transformer.
Author:  FerdinandII [ April 4th, 2021, 4:23 pm ]
Post subject:  Re: Gain Controls Vs Volume Controls
I'm out. I should have listened to Walt.
All these people trying to explain the Theory of Relativity because the wrong amp in their system has the highest gain...... :crazy:
Author:  Cogito [ April 4th, 2021, 4:27 pm ]
Post subject:  Re: Gain Controls Vs Volume Controls
Roscoe Primrose wrote:
Cogito wrote:
[As Roscoe and others suggested, gain controls are potentiometers. Pots control the volume by manipulating the preamps input impedance.


:angry-banghead: For the last damn time, no, they do not. If you need further clarification, see my signature line.


I suspect we are talking same thing in different languages.

Earlier in this thread you wrote:
Quote:
The variable resistor (aka potentiometer) changes the source impedance seen by the following stage, but not the input impedance. The "gain" control on just about every amp that has one is a no different than the "volume" control on just about every amp that has one.


Since the top of discussion is preamp-amp integration, for me:
source impedance = preamp's output impedance
input impedance = amps input impedance <-- part of amp's gain control
following stage = amp's amplification stage

Is this right? If so, how do amp's gain control (aka potentiometer) change preamps output impedance seen by the amp without manipulating amp's input impedance?
Author:  Jim G [ April 4th, 2021, 5:27 pm ]
Post subject:  Re: Gain Controls Vs Volume Controls
Cogito wrote:
...I was planning to use a an existing preamp between plllxo and the subs for level matching.. My goal is to avoid pots in the system. ...

This doesn't seem like a good idea? Just going to add noise I imagine and make it harder for whatever is driving the system, a passive or tube preamp?

FerdinandII wrote:
I'm out. I should have listened to Walt.
All these people trying to explain the Theory of Relativity because the wrong amp in their system has the highest gain...... :crazy:

I'm out of my league with the theory being discussed, but wouldn't one use an amplifier with a gain control to reduce the overall gain of the 4 way passive system to match the subs? This will reduce overall system noise as well. I can only imagine this system will be quite noisy?
Author:  SoundMods [ April 4th, 2021, 5:38 pm ]
Post subject:  Re: Gain Controls Vs Volume Controls
Roscoe Primrose wrote:
Cogito wrote:
[As Roscoe and others suggested, gain controls are potentiometers. Pots control the volume by manipulating the preamps input impedance.


:angry-banghead: For the last damn time, no, they do not. If you need further clarification, see my signature line.

Now you know why I didn't jump into this "Rabbit Hole." Like you say: "I can explain it to you -- but I can't make you understand it."
Author:  Jim G [ April 4th, 2021, 5:51 pm ]
Post subject:  Re: Gain Controls Vs Volume Controls
Jim G wrote:
...I'm out of my league with the theory being discussed, but wouldn't one use an amplifier with a gain control to reduce the overall gain of the 4 way passive system to match the subs? This will reduce overall system noise as well. I can only imagine this system will be quite noisy?

...or use a much more efficient passive sub with external crossover? Mine are about 100db.
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