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PostPosted: April 3rd, 2021, 10:08 pm 
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Going by the name, "Gain", I always assumed it would amplify the signal. I started building the Passive Line Level XO in my 5-way active system. The subs I am using (Tomp built) have an efficiency of about 93dB where as all other channels are much more efficient. With the active DSP crossover, matching levels is a breeze, but with PLLXO, I need to pump more signal into the subwoofer. This is where I was hoping subwoofer amp's gain control feature would come in handy. Big disappointment.

Dayton Audio explains how "Gain Control' works in their product literature.

Quote:
Low Pass Gain Will match the amplifier’s input sensitivity to the output of the pre-amp source


In essence, the Gain knob reduces the signal voltage by reducing the amp's input impedance.

Here is the question for the Gurus.
Both Gain Controls (in Sub amps) and Pots reduce the signal voltage. Gain Controls reduce the voltage by impedance matching where as Pots do it by leaking part of the signal to the ground.

Is there a sonic advantage to one method over the other?

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PostPosted: April 3rd, 2021, 10:49 pm 
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I believe it may be two ways of explaining the same thing. A pot is a voltage divider, the input signal is grounded through the resistive value of the pot, and the wiper is tapping off this resistor at some point that results in a lower signal voltage into the input of the amplifier. The impedance that the amplifier input will see is different depending on wiper setting. If we look at a circuit with a large resistor at the output of the preamp, say 1M, a 100K pot, then the input impedance the amplifier input will see with teh wiper at 100% volume is 1M||100K = 90.09K. However if you set the wiper on the pot to -3dB (50K), then the impedance at the amplifier input is 1M + 50K || 50K = 47.7K. So the impedance at half voltage level (-3dB) is approximately one half of the full voltage impedance. So it is the same. Now, if we are using inductive volume controls, it is a different matter.

Oh, attenuation is negative gain. Although strictly speaking gain does imply some amplification or transformation. A transformer may provide positive AC voltage gain at the expense of negative AC current gain. Obviously, there are impedances involved as well.

David


Last edited by David McGown on April 3rd, 2021, 10:57 pm, edited 1 time in total.

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PostPosted: April 3rd, 2021, 10:53 pm 
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Cogito wrote:

Dayton Audio explains how "Gain Control' works in their product literature.

Quote:
Low Pass Gain Will match the amplifier’s input sensitivity to the output of the pre-amp source


In essence, the Gain knob reduces the signal voltage by reducing the amp's input impedance.


I'm not sure how you make the leap from the Dayton quote to your statement....

Roscoe

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PostPosted: April 3rd, 2021, 11:21 pm 
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Roscoe Primrose wrote:
Cogito wrote:

Dayton Audio explains how "Gain Control' works in their product literature.

Quote:
Low Pass Gain Will match the amplifier’s input sensitivity to the output of the pre-amp source


In essence, the Gain knob reduces the signal voltage by reducing the amp's input impedance.


I'm not sure how you make the leap from the Dayton quote to your statement....

Roscoe


I am reading the phrase “amplifiers input sensitivity” to mean amp’s input impedance.

If that is the case, of system as a voltage divider network with output impedance of preamp as the first resistor and input impedance of the amp as the second resistor. The difference between these two impedance is huge (something like 200:10000ohms) allowing very little voltage drop. As the impedance of the amp is reduces, the voltage drop increases.

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PostPosted: April 3rd, 2021, 11:30 pm 
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David McGown wrote:
I believe it may be two ways of explaining the same thing. A pot is a voltage divider, the input signal is grounded through the resistive value of the pot, and the wiper is tapping off this resistor at some point that results in a lower signal voltage into the input of the amplifier. The impedance that the amplifier input will see is different depending on wiper setting. If we look at a circuit with a large resistor at the output of the preamp, say 1M, a 100K pot, then the input impedance the amplifier input will see with teh wiper at 100% volume is 1M||100K = 90.09K. However if you set the wiper on the pot to -3dB (50K), then the impedance at the amplifier input is 1M + 50K || 50K = 47.7K. So the impedance at half voltage level (-3dB) is approximately one half of the full voltage impedance. So it is the same. Now, if we are using inductive volume controls, it is a different matter.

Oh, attenuation is negative gain. Although strictly speaking gain does imply some amplification or transformation. A transformer may provide positive AC voltage gain at the expense of negative AC current gain. Obviously, there are impedances involved as well.

David


I don’t quite follow. Shouldn’t the wiper be at the input of the preamp?

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PostPosted: April 3rd, 2021, 11:37 pm 
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Cogito wrote:

I am reading the phrase “amplifiers input sensitivity” to mean amp’s input impedance.

If that is the case, of system as a voltage divider network with output impedance of preamp as the first resistor and input impedance of the amp as the second resistor. The difference between these two impedance is huge (something like 200:10000ohms) allowing very little voltage drop. As the impedance of the amp is reduces, the voltage drop increases.


Input impedance and input sensitivity are two totally different things...

Roscoe

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PostPosted: April 3rd, 2021, 11:51 pm 
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Roscoe,

Found the link below which describes input sensitivity adjustment as padding down the signal. That sounds like lowering the impedance with a variable resistor.

What am I missing?

https://www.sweetwater.com/insync/input-sensitivity/

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PostPosted: April 4th, 2021, 12:03 am 
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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. Sensitivity is just a measure of how many volts in for rated output (typically), doesn't matter what the input impedance is. Take an amp with a 10K input impedance that amp delivers rated output with 1Vrms input. If you put a 10ohm resistor across the input jack, the input impedance will be 10ohms, but it'll still take 1Vrms to generate rated output...

Roscoe

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PostPosted: April 4th, 2021, 1:41 am 
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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
The line level input is primarily straight resistance.
The only thing that changes when the input resistance of the following stage goes up or down is the amount of current that the (voltage) source device has to put out to maintain it's behavior as a voltage source.


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PostPosted: April 4th, 2021, 8:05 am 
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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
The line level input is primarily straight resistance.
The only thing that changes when the input resistance of the following stage goes up or down is the amount of current that the (voltage) source device has to put out to maintain it's behavior as a voltage source.


Not quite true. The line output of a preamp or other input device to an amplifier input is not a pure (infinitely low impedance) voltage source. It may range from a few ohms to over 100K ohms. The input impedance of the amplifier is also not a true resistive load although it might be close to that. There is usually some capacitance and perhaps a very small amount of inductance at the input. That is why it is called input impedance, not input resistance. The effect of the input capacitance (and interconnect cable capacitance) reduces the delivered output voltage of a preamp that has a high output impedance. If the preamp has a high output impedance, the combination of that high output resistance interacting with the parallel capacitance of the cable and amplifier input result in a 6dB/octave low pass filter, much as what you would see in a speaker crossover. The crossover point and magnitude of that drop depends on the circuit values.

The net effect of using a device to attenuate the input voltage to the amplifier depends on a number of factors discussed above. However, deviations from the ideal components such as a potentiometer that may have some capacitance or inductance leads to a non-linearity in the ability of the device to attenuate the signal across its range. For example, if a wirewound potentiometer is used, when the wiper is at the top of the pot, ie the wiper is in contact with the output of the preamp, there is little if any series inductance between the preamp and amp to affect the signal. As the wiper moves away from the input side, in addition to a resistance increase there will be a series inductance increase. The impedance change vs the resistance change will make the pot act like a low pass filter. Add that to amp and cable capacitance, you now have a 12 dB/octave low pass filter.

You might say so what if the wiper is at the top of the pot. That depends on the values in the circuit. Even if the pot is a perfect resistance it will load the output of the preamp even if the wiper is not connected to anything resulting in some attenuation of the output of the preamp. Just for the sake of discussion, although very unlikely, let's say the inductance of the pot is very high in relationship to it's resistance. At low frequencies, because of the ratio of inductance to resistance, the pot itself will be lower in impedance, loading the output of a high impedance preamp more than a low impedance preamp resulting in essentially a high pass filter. Again, this results in a non-linearity of the attenuation of the supplied voltage vs frequency.

So, at the end of the day, unless there is an intervening gain stage, anything between the output of the preamp and the input of the amp will reduce the voltage to the input of the amp and perhaps not in a linear fashion. It is important to remember as Roscoe mentioned that the voltage output of the amplifier is only dependent on the input voltage and the internal gain of the amplifier, although the amp itself may have a non linear frequency response which is another matter. The takeaway is that any device that is between the output of the preamp designed to attenuate the signal should be as close to ideal as possible. That means when a pot is used, it should have low inductance, low capacitance, very linear change of resistance with movement, and the best compromise of resistance value taking into account the output impedance of the preamp and the input impedance of the amp. Also consider those pesky non linear elements like capacitance and inductance in the interconnects which may have a greater or lesser effect depending on circuit impedances.


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