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Power amplifier with remote sensing (speaker cable compensation)

Stefan1024

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So I'm running the AHB2 and I'm pretty happy with it. However, it has a relatively high output impedance:

1635601890850.png


The 23 mOhm at low frequency are not too bad, but it rolls of quickly above 500 Hz. For the speaker cable I'm using a high frequency litz wire with 1050 strands of 0.1mm wire (8.24mm^2) what leads to additional ca. 8 mOhm. So there is some room for improvement.
By the way, this high frequency litz wires offer great value for a relatively low price.

Now I'm thinking about building thw monoblocks with a Purifi 1ET400A that has only one 1 mOhm output impedance to start with. Additionaly, The sense lines for the amplifier are routed seperately. Usually they are joined on the output connector of the amplifer. But I'm planing on routing the sense lines up to the speaker and only connect the sense to the speaker rigth at the end of the cable. So the amplifier would compensate the impedance of the cable and (at least in theory) also EMI.
Basically using the 4 wire measurement system idea:

1635603844416.png


So here are my questions:

1) Do you think it is worth it to reduce the output impedance?
Given the crossover has much higher (DC) resistance or sometimes even straigth up series resistors of 1 Ohm or more it feels a bit silly to improve the amplifier impedance to a few mOhms.... thougths?

2) Instability caused by remote sensing

By adding some inductance / delay into the carefully calculated feedback loop of the 1ET400A, this may cause instability. I think I'm still on the save side, but if someone has experience to share I'm interesed.

3) Noise picked up by remote sensing
I'm planning on using star quad cable for the sense lines. So again I think I should be save.

Thank you
 

pjug

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Personally I wouldn't worry about what you have with the AHB2. But if you really want to go with Purifi monoblocks and cable resistance bothers you then why don't you just locate the amps right at the speakers?
 

egellings

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Put the amps near the speakers with a short length of wire (<1ft.) and send the signal instead. The only way I could see remote sensing working is if the speaker, cable and amplifier were bought as a combo, and the engineering kinks had been worked out of the group.
 
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KSTR

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JBL also had a remote load sensing amp series in the 80'ies.
I'm with @SIY here, no net benefit. But it's a nice engineering challenge notably when it has to be bullet-proof (sense wires not attached, partly attached, reversed polarity, etc).
 
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Stefan1024

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Hi all

Thank you for the input.
Missing sense wire or loose connection is not an issue as I can add a small resisotr (e.g. 10 Ohm) on the amplifier as a backup. Wrong polarity is also not an issue with SpeakOn connectors. Stability on the other hand.... well lets hope Bruna has build in enougth margin :)

However, overall I get the impression lowering the output impedance is not realy worth the trouble - or the 1k to 1.5k in materials for that matter.
Still, having some monoblocks placed close to the speaker with short cables as @pjug and @egellings suggested would look nice. Especially with with optimized cable routing (having the mains cable enter the case from a different side than the signal and speaker cable).

Maybe one day when I'm bored...
 

KSTR

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1) Do you think it is worth it to reduce the output impedance?
Given the crossover has much higher (DC) resistance or sometimes even straigth up series resistors of 1 Ohm or more it feels a bit silly to improve the amplifier impedance to a few mOhms.... thougths?
Coming back to this basic question, fixing mOhms differences is futile, notably with a multi-way speaker. The driver's large voice coil DC resistance will vary much more as they heat up and cool down, same goes for inductors and the impedance levels are generally high in crossovers (high compared to output impedance, in both cases). All this dominates the static and dynamic operating conditions for the drivers and produces errors way way larger than the truly microscopic impact on frequency response and damping from a non-zero and non-flat output impedance profile.
 

pma

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"Remote load sensing" from speaker is only good to get oscillations with high probability.
 
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Stefan1024

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Coming back to this basic question, fixing mOhms differences is futile, notably with a multi-way speaker. The driver's large voice coil DC resistance will vary much more as they heat up and cool down, same goes for inductors and the impedance levels are generally high in crossovers (high compared to output impedance, in both cases). All this dominates the static and dynamic operating conditions for the drivers and produces errors way way larger than the truly microscopic impact on frequency response and damping from a non-zero and non-flat output impedance profile.
Yes, this was also what I was thinking, but I'm allways open for new inputs or opinions. Well in this case confirmation.

BTW, I'm running high efficeincy speakers (99dB/W) with 15'' base, so I'm a bit fanatic on giving the amp the possibility to accelerate and slow down the large membrane as good as possible.

Tobian Sound System HC-15: https://tobian-soundsystems.com/en/hornspeakers/15hc-coaxial-hornspeakers
 

solderdude

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I'm a bit fanatic on giving the amp the possibility to accelerate and slow down the large membrane as good as possible.

Why would you believe this would be (audible) better using sensing ?

Why worry about 0.1dB variances in the FR caused by varying speaker impedances ?

Have you asked Purify if remote sensing is a good idea and what length they would specifiy as max ?

Do you have starquad cable for loudspeaker purposes ?
 
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Stefan1024

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Why would you believe this would be better using sensing ?

Why worry about 0.1dB variances in the FR caused by varying speaker impedances ?
Well, an ideal amplifier has a zero output impedance, and remote sensing compensates the wire impedance. So you are closer to the "ideal".

I was just thinking about how can my setup be impreoved. But you are totally rigth, 0.1db in FB is totally irrelevant, given there are much larger factors, e.g. room accoustics.
 
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Stefan1024

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Have you asked Purify if remote sensing is a good idea and what length they would specifiy as max ?

Do you have starquad cable for loudspeaker purposes ?
I have not aseked Purify yet. First I like to be sure that it is acctually a good idea before I bother them.

By now I use the HF litz wire for the speakers and starquad for line signals.
 

solderdude

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What would be the benefit of litze for speakers ?
What capacitance and inductance did that cable have ?
 
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Stefan1024

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What would be the benefit of litze for speakers ?
What capacitance and inductance did that cable have ?
Every stand of the wire is insulated individually with a thin layer of varnish like in a transformator. This gives you lower impedance at high frequencies due to reduced skinn effekt.
Last year I have been developing a small inductive heater running at 100kHz and we got by far the lowest impedance using HF litz wire. And 100kHz is not that for of the audio band. Using some leftover wire was rather cost effictive :)

Capacitance and inductance is not specifed, but I could measure that - well not today, as the measurement equipment is at my workplace and I'm at home.
 

preload

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Every stand of the wire is insulated individually with a thin layer of varnish like in a transformator. This gives you lower impedance at high frequencies due to reduced skinn effekt.
Last year I have been developing a small inductive heater running at 100kHz and we got by far the lowest impedance using HF litz wire. And 100kHz is not that for of the audio band. Using some leftover wire was rather cost effictive :)

Capacitance and inductance is not specifed, but I could measure that - well not today, as the measurement equipment is at my workplace and I'm at home.
The litz topology (assuming both + and - wires are interwoven, and the conductors are individually insulated) should have higher capacitance and lower inductance than a typical paired zip cord topology.

BTW what exactly is the problem you're trying to solve? I get that you want to reduce the output impedance further, but why?? The ahb2 is likely transparent without further modification, particularly since you're supposedly running low DCR and low inductance cable.
 

kchap

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Every stand of the wire is insulated individually with a thin layer of varnish like in a transformator. This gives you lower impedance at high frequencies due to reduced skinn effekt.
Last year I have been developing a small inductive heater running at 100kHz and we got by far the lowest impedance using HF litz wire. And 100kHz is not that for of the audio band. Using some leftover wire was rather cost effictive :)

Capacitance and inductance is not specifed, but I could measure that - well not today, as the measurement equipment is at my workplace and I'm at home.
I'm biased (no pun intended) toward Class D for reasons of efficiency but the AHB2 is a very good amp. Others have pointed out the very real dangers of stability issues so stick with what you have.

We can argue the improvements the litz wire makes will be inaudible but, it seems the litz wire did not cost you much. It's a great conversation piece.
 

RMD

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So I'm running the AHB2 and I'm pretty happy with it. However, it has a relatively high output impedance:

View attachment 162214

The 23 mOhm at low frequency are not too bad, but it rolls of quickly above 500 Hz. For the speaker cable I'm using a high frequency litz wire with 1050 strands of 0.1mm wire (8.24mm^2) what leads to additional ca. 8 mOhm. So there is some room for improvement.
By the way, this high frequency litz wires offer great value for a relatively low price.

Now I'm thinking about building thw monoblocks with a Purifi 1ET400A that has only one 1 mOhm output impedance to start with. Additionaly, The sense lines for the amplifier are routed seperately. Usually they are joined on the output connector of the amplifer. But I'm planing on routing the sense lines up to the speaker and only connect the sense to the speaker rigth at the end of the cable. So the amplifier would compensate the impedance of the cable and (at least in theory) also EMI.
Basically using the 4 wire measurement system idea:

View attachment 162219

So here are my questions:

1) Do you think it is worth it to reduce the output impedance?
Given the crossover has much higher (DC) resistance or sometimes even straigth up series resistors of 1 Ohm or more it feels a bit silly to improve the amplifier impedance to a few mOhms.... thougths?

2) Instability caused by remote sensing
By adding some inductance / delay into the carefully calculated feedback loop of the 1ET400A, this may cause instability. I think I'm still on the save side, but if someone has experience to share I'm interesed.

3) Noise picked up by remote sensing
I'm planning on using star quad cable for the sense lines. So again I think I should be save.

Thank you
Pardon me if I am missing the big picture, but isn't the loudspeaker impedance the big fish here?
For a fullrange single speaker, the impedance variation as seen by the amplifier can be huge (For an 8E speaker, it can vary from 5E to 32E!). A multiway speaker too needs to have a crossover that is designed for constant impedance else its effective impedance too can be nowhere close to a constant value and often bring undesirable results.
I agree that steps taken to ensure that the power delivered into the loudspeaker be proportional to the input signal and input signal only will yield fantastic results but cable impedance would be too small culprit compared to loudspeaker impedance
 

solderdude

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Using some leftover wire was rather cost effictive

That would be a good reason to use it. :)
Since it was developed do you have info on the cables tested ?
What length are you talking about ?

100kHz is over 2 octaves above the audible range (LF radio frequency). How much effect will there be at a more important 10kHz ?
 
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