How to shift the operating range of an amplifier while keeping the speaker volume constant?

[UltraNearFieldJock]

How to shift the operating range of an amplifier while keeping the speaker volume constant?

Imagine the following situation:
A Class‑D Hypex Ncore NC400 power amplifier receives its input signal from a DAC through a passive volume control (potentiometer). The amplifier output drives a Visaton AL180 (8 Ω) loudspeaker directly. According to the specifications, the amplifier’s output impedance is only 3 mΩ. My ears are only a few centimeters away from the speaker, so I need extremely low listening levels—basically like using headphones.

The problem is that the passive volume control is very sensitive in this low range, so adjusting the level accurately is difficult.
The optimal operating range (“sweet spot”) of the amplifier is approximately 5–20 W (see the figure below).

Is it possible to increase the input signal level while keeping the actual acoustic volume of the loudspeaker very low and constant?

My first idea was to place a low-value resistor (e.g., 2 Ω or less, 10 W) in parallel with the loudspeaker so that the amplifier’s output current increases, but with more current, (less resistance) the amplifier’s parameters become worse (the graph in the picture shifts upward)..
Another idea is adding a series resistor (e.g., 30 Ω or more, 10 W) to increase the amplifier’s output voltage, but this would severely worsen the damping factor.

Perhaps the best solution is simply to buy a more suitable, lower‑power amplifier, but maybe you know a hack to answer the question.

 

[Flaesh]

(JBL M2 crossover)

My ears are only a few centimeters away from

hat-phones?..

 

[UltraNearFieldJock]

View attachment 507701
(JBL M2 crossover)


View attachment 507702
hat-phones?..

My phones are here to see: UNF Ultra Near Field.
Yes, my next idea was to combine parallel and series resistors, but what about the damping factor?

 

[DVDdoug]

No resistors on the output!

An attenuator on the input is a better idea.

-3dB is half power (with nothing else changed) and probably not significant. -6dB is 1/4 power and -12dB is 1/16th the power. (With an attenuator you might still get full-power if you have a strong signal and turn it up, etc.)

My first idea was to place a low-value resistor (e.g., 2 Ω or less, 10 W) in parallel with the loudspeaker so that the amplifier’s output current increases, but with more current, (less resistance) the amplifier’s parameters become worse (the graph in the picture shifts upward)..

That's 4 times the current & power than is going to the speaker and you might damage the amplifier. Also, 2 and 8 Ohms in parallel is 1.6 Ohms and your amplifier probably isn't rated for that.

Another idea is adding a series resistor (e.g., 30 Ω or more, 10 W) to increase the amplifier’s output voltage, but this would severely worsen the damping factor.

The problem with increasing the damping factor is that you are making a Voltage Divider. That would be fine if speaker impedance was constant BUT speaker impedance varies over the frequency range so you get different attenuation at different frequencies and frequency response variations.

 

[UltraNearFieldJock]

The problem with increasing the damping factor is that you are making a Voltage Divider. That would be fine if speaker impedance was constant BUT speaker impedance varies over the frequency range so you get different attenuation at different frequencies and frequency response variations.

Thank you, that’s a very strong argument. I hadn’t considered it!

That's 4 times the current & power than is going to the speaker and you might damage the amplifier. Also, 2 and 8 Ohms in parallel is 1.6 Ohms and your amplifier probably isn't rated for that.

Yes, I should be careful, but 2 ohms at the output could be OK. The question is whether that would be enough to shift the operating range. In the graph shown in the picture, I believe it represents output power, not power loss?

An attenuator on the input is a better idea.

Yes, this could be a solution to the volume‑adjustment problem. I thought about using two potentiometers in cascade. I think this attenuator is nothing more than a resistive voltage divider, right?

 

[DVDdoug]

Yes, I should be careful, but 2 ohms at the output could be OK. The question is whether that would be enough to shift the operating range. In the graph shown in the picture, I believe it represents output power, not power loss?

The voltage output won't change unless you overload the amplifier so the signal to the speaker won't change. You'll just get more total current. Amplifiers are "constant voltage" devices which means the voltage is independent of load. Current depends on voltage and resistance (Ohm's Law). The source resistance of 3milliohms will give you almost no voltage drop. Power isn't lost but besides the power going to the speaker, additional power wasted in the resistor.

I thought about using two potentiometers in cascade. I think this attenuator is nothing more than a resistive voltage divider, right?

That would work. You could build or buy a variable attenuator. But sometimes the left & right channels don't track perfectly. With 2 stereo pots the issue is potentially worse.

I think this attenuator is nothing more than a resistive voltage divider, right?

Correct! If you know how to solder you can build your own and since resistors are cheap you can easily (and cheaply) experiment with different attenuation. Total resistance (for the sum of both resistors) of around 10K is usually about right for line-level signals.

 

[Flaesh]

what about the damping factor?

I never thought about that. Normally, speakers are connected directly to amplifiers outputs))
Placing an attenuator before the amplifier will worsen the signal-to-noise ratio, right?.
How much power do you need for listening?

 

[UltraNearFieldJock]

How much power do you need for listening?

That’s a good question—can this be estimated or measured? I only have a basic multimeter. Right now I’m listening at an normal level: the sound in the room is barely audible, and outside the room it’s completely inaudible. I guess: far down 1W.

 

[UltraNearFieldJock]

Placing an attenuator before the amplifier will worsen the signal-to-noise ratio.

This is exactly my problem: I need this attenuator for very low listening levels, and the signal‑to‑noise ratio is not optimal—the graph shows this clearly. That is the reason why I need a very low‑noise amplifier, such as Hypex.

 

[DVDdoug]

Placing an attenuator before the amplifier will worsen the signal-to-noise ratio, right?.

Yes. Any noise generated inside the amplifier remains while the signal is reduced. Just like turning-down the volume control or when a song fades-out, etc. The noise isn't worse but the signal-to-noise ratio is worse.

...You didn't complain about noise. If you can't hear it, it's not a problem.

 

[MaxwellsEq]

How to shift the operating range of an amplifier while keeping the speaker volume constant?

Imagine the following situation:
A Class‑D Hypex Ncore NC400 power amplifier receives its input signal from a DAC through a passive volume control (potentiometer). The amplifier output drives a Visaton AL180 (8 Ω) loudspeaker directly. According to the specifications, the amplifier’s output impedance is only 3 mΩ. My ears are only a few centimeters away from the speaker, so I need extremely low listening levels—basically like using headphones.

The problem is that the passive volume control is very sensitive in this low range, so adjusting the level accurately is difficult.
The optimal operating range (“sweet spot”) of the amplifier is approximately 5–20 W (see the figure below).

Is it possible to increase the input signal level while keeping the actual acoustic volume of the loudspeaker very low and constant?

My first idea was to place a low-value resistor (e.g., 2 Ω or less, 10 W) in parallel with the loudspeaker so that the amplifier’s output current increases, but with more current, (less resistance) the amplifier’s parameters become worse (the graph in the picture shifts upward)..
Another idea is adding a series resistor (e.g., 30 Ω or more, 10 W) to increase the amplifier’s output voltage, but this would severely worsen the damping factor.

View attachment 507692
Perhaps the best solution is simply to buy a more suitable, lower‑power amplifier, but maybe you know a hack to answer the question.

Does the power amplifier have gain settings? If so, set the gain to the lowest setting.

If not, put a resistor between ground and the start of the wiper on each channel of the passive attenuator pot.

 

[Flaesh]

can this be

Yes. Basic multimeter - that's exactly what's needed. Set the maximum desired volume, then feed a tone from the generator, for example, 100 Hz at -6 dB and measure the alternating voltage at the amplifier output. Let's say the result is 2 V, which means 1 watt into 4 (again, for example) ohms. Therefore, 0 dB corresponds to 4 watts into 4 ohms.

 

[obsessive9119]

This might be an incredibly dumb question - but wouldn’t the easiest solution be to use a DAC with digital volume control?

 

[somebodyelse]

Why not use a more appropriate amplifier for your use case?

 

[NTK]

OP is just sufferring from acute OCD. The NC400 has zero problem with noise. It's idle noise per spec is 25 μV max. Relative to the standard test voltage for speakers of 2.83 V, it is -101 dB. Which means if you have a super high efficiency speaker of 100 dBSPL @ 1 m 2.83 V, the noise SPL will be -1 dB at 1 m, or 17 dB at 12 cm. But why would anyone want to listen to super high efficiency speaker near field?

If there is audible noise, the problem is somewhere else.

 

[UltraNearFieldJock]

Thank you for the many inspiring answers. Meanwhile, I think I can answer the question myself. I will make exact measurements today. Here is only an example based on a guess:

 

[Cbdb2]

That divider on output will change the freq. response. If you dont hear the noise from the first circuit, probable, use it. The distortion is still way below audible.
If your measuring, measure the FR also.

 

[RandomEar]

This is exactly my problem: I need this attenuator for very low listening levels, and the signal‑to‑noise ratio is not optimal—the graph shows this clearly. That is the reason why I need a very low‑noise amplifier, such as Hypex.

I'm pretty sure you're chasing ghosts here. Yes, the SNR of the amp is technically better at 10 W than at 0.1W - because the output voltage is much higher. But in practice, for modern low-noise amplifiers, the SNR is limited not by the amplifier but by the room noise or human hearing itself. The NC400 idle noise is so low that it shouldn't be audible at all. If you don't hear any noise at your listening position right now, it's either being 100% masked by the room noise floor or is below your hearing threshold. In that case, you cannot improve the SNR by modifiying your signal chain in any way. Period.

If the noise is above the hearing threshold but way below the room noise floor, making the room more quiet could cause noise from the speakers to become audible. In that case, an even better amp or less sensitive speakers would be possible solutions. Some weird attenuation scheme could potentially work, too. But there's lots of pitfalls which could at the very least alter the frequency response, as pointed out by fellow members.

That is of course not adressing the problem of the fiddly analog volume controls. To solve this, I would attenuate digitally up to a point where the analog control becomes usable and precise enough. That's easy to do and free.

 

[Waveform Fidelity]

How to shift the operating range of an amplifier while keeping the speaker volume constant?

Imagine the following situation:
A Class‑D Hypex Ncore NC400 power amplifier receives its input signal from a DAC through a passive volume control (potentiometer). The amplifier output drives a Visaton AL180 (8 Ω) loudspeaker directly. According to the specifications, the amplifier’s output impedance is only 3 mΩ. My ears are only a few centimeters away from the speaker, so I need extremely low listening levels—basically like using headphones.

The problem is that the passive volume control is very sensitive in this low range, so adjusting the level accurately is difficult.
The optimal operating range (“sweet spot”) of the amplifier is approximately 5–20 W (see the figure below).

Is it possible to increase the input signal level while keeping the actual acoustic volume of the loudspeaker very low and constant?

My first idea was to place a low-value resistor (e.g., 2 Ω or less, 10 W) in parallel with the loudspeaker so that the amplifier’s output current increases, but with more current, (less resistance) the amplifier’s parameters become worse (the graph in the picture shifts upward)..
Another idea is adding a series resistor (e.g., 30 Ω or more, 10 W) to increase the amplifier’s output voltage, but this would severely worsen the damping factor.

View attachment 507692
Perhaps the best solution is simply to buy a more suitable, lower‑power amplifier, but maybe you know a hack to answer the question.

Your speaker distortion, if you have really good loudspeaker is around the 0.5-2% (maybe more at lower listening levels) and your amp has 0.005% - thats 1000X less. You are barking up the wrong tree if you are expecting any improvements from the amplifier. Even the distortion from the capacitors/inductors in the crossover will swamp any distortion improvements you can make in this and any good amplifier.

 

[Waveform Fidelity]

Your speaker distortion, if you have really good loudspeaker is around the 0.5-2% (maybe more at lower listening levels) and your amp has 0.005% - thats 1000X less. You are barking up the wrong tree if you are expecting any improvements from the amplifier. Even the distortion from the capacitors/inductors in the crossover will swamp any distortion improvements you can make in this and any good amplifier.

The other thing worth looking at with very low listening levels, is what is the minimum current to make the diaphragms move? I suspect that most speaker drivers have “stiction” effects which will causes un-smooth motion, for example as the roll-surround just starts to break away from its stationary position.

The most obvious question is why you don't use headphones?