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PostPosted: April 4th, 2021, 8:25 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.


In the simple circuit, of course current is flowing and is constant. In audio, we are primarily interested in voltage.

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PostPosted: April 4th, 2021, 8:34 am 
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Roscoe Primrose wrote:
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


Thx for explaining sensitivity, which can also be seen as efficiency of the amp.

But Dayton’s literature is claiming matching input sensitivity of the amp with output sensitivity of the preamp. That could only mean impedance matching to me.

Rewording my original statement for clarity.
When the impedance difference between preamp and amp is very high, amp’s gain stage most of the voltage put out by preamp. As the input impedance of the amp is reduced, voltage seen by the amp’s gain stage also reduces.

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PostPosted: April 4th, 2021, 9:07 am 
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tomp wrote:

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.


Off topic:

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.

The preamp requirements:
1. Solid State is OK (for the sub)
2. Fixed input load with quality resistor
3. Input load between 50k to 100k
4. Output impedance of 500ohms or lower
5. Variable amplification
6. Simple to build

Can anyone think of such a preamp circuit?

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PostPosted: April 4th, 2021, 9:55 am 
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Cogito wrote:
In the simple circuit, of course current is flowing and is constant. In audio, we are primarily interested in voltage.


This is mostly true at line level. If voltage changes across an impedance current changes with it. At line level current capability is generally not a factor. Current is a factor at speaker level.

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PostPosted: April 4th, 2021, 9:57 am 
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Cogito wrote:

Thx for explaining sensitivity, which can also be seen as efficiency of the amp.


Sensitivity, which is essentially a statement of gain has nothing to do with efficiency. A pure class A amp and a class D amp can have the same power output and sensitivity but are radically different in efficiency.

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PostPosted: April 4th, 2021, 10:01 am 
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Cogito wrote:
But Dayton’s literature is claiming matching input sensitivity of the amp with output sensitivity of the preamp. That could only mean impedance matching to me.


Impedance matching has nothing to do with sensitivity. Sensitivity relates to voltage and voltage gain. Impedance does not come into the picture for this specification. Impedance can be an issue if the source of the signal has a high output impedance and the device it's driving has a low or lower impedance. If the impedance is the same in both devices the device will receive half of the voltage the source is delivering. So, while impedance can affect sensitivity, the sensitivity specification does not take impedance into account.

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PostPosted: April 4th, 2021, 10:05 am 
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As stated in other replies gain can be positive or negative or zero. While some or all of Dayton sub amps may only attenuate the input signal providing negative to zero gain, which I would find very surprising, there are amps that can provide positive gain.

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PostPosted: April 4th, 2021, 10:30 am 
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Shashi,

There is another factor in play here, that might explain why Dayton worded things the way they did.

The output sensitivity is the voltage output the stage is designed to produce at maximum level.

The input sensitivity of the amplifier is the voltage input that is designed to produce full power output.

If the voltage output of the preceding stage is LOWER than the rated input sensitivity, then you will not be able to achieve full power output from the amplifier. You need an additional preamplifier stage or more gain in the amplifier. Sensitivity cannot be matched without this.

If the voltage output of the preceding stage is HIGHER than the rated input sensitivity, then you need to attenuate the signal via a potentiometer.

There is nothing wrong with using a potentiometer for making the adjustment in sensitivity. Ideally, you would like the gain structure to be as close as possible for minimal adjustment, obviously you have a master level control for the entire system. If you know the sensitivity, rated power output, and efficiency of the speakers, you can build in the requisite amount of attenuation at each amplifier (start with the most efficient), by using a fixed resistive voltage divider (or inductive device as an autoformer or transformer) at the front end so each amplifier/speaker combination produces the same SPL at the same input signal voltage. Once you do that, then the master volume is all you need to worry about and you do not need to add gain. However, the output at the preamp driving this needs to provide enough gain to drive the least efficient amp/speaker system to full power, and it is possible that a passive preamp is not practical in this case and an active preamp is required at the front end.

Alternatively, as you are exploring, another amplification stage can be inserted between the existing preamp to boost the signal level into the subwoofer amplifier to match SPLs. The problem with this approach is the potential for overloading the subwoofer amplifier input (exceeding its sensitivity). The amount of gain required is quite small (perhaps 6-10 dB) to match levels, and it is hard to find a circuit that will do this without alot of attenuation.

Considering that others (myself included) used a plate amplifier+subwoofer in a horn setup with success, tells me that this might not be a problem in practice, rather than theory. Unless you ARE running into a problem where you cannot get the bass output high enough to match the horns, I would experiment with adjusting the level at the plate amplifier and just forget about the technical details. If it works, it works. If not, then worry about other means of level matching as described above.

Do not pay much attention to what Tom was saying with respect to potentiometers, input capacitance, inductance, etc. Usually, the effects are well above the audio frequency range. though Miller capacitance with high gain input stages can be an issue with high frequencies in extreme cases (and particularly with MM phono input), or with preamps that do not have a low output impedance and output drive capability. Here we are talking about a SW amp, and even the most extreme case you can imagine will have effects several octaves above the subwoofer frequency range. Not a problem.

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PostPosted: April 4th, 2021, 11:13 am 
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I was almost tempted "to jump into this Rabbit Hole." Common sense prevailed. :crazy:

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PostPosted: April 4th, 2021, 1:53 pm 
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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
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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