Influence of power amplifier output coil to frequency response

[PMA]

Influence of power amplifier output coil and speaker cable to frequency response

We like to measure our amplifiers into purely resistive, non-inductive load. Is it easy? Yes, it is. Is it right? I do not think so. I am developing a novel method to measure power amplifiers and as a byproduct I have found very obvious thing – the speaker cable and amplifier output coil both influence frequency response at speaker box terminals in a way, that is not negligible at all.

Let's first see a real world situation with speaker cable but no output coil. The amplifier is assumed as an ideal voltage source with zero output impedance for now. We add cable and terminals resistance (R99 = 0.02 ohm), cable inductance (L3 = 3uH) and cable capacitance (C49 = 50pF). Changing L3 to 2uH or so does not affect results much. The rest of the circuit are equivalent circuits of woofer and tweeter and C4 is the simplest high pass cross over possible.

Now let's measure behind the speaker cable, at speaker box terminals. The voltage measured is V(out1). Let's make a frequency response plot and see what is happening.

[PMA]

From the previous post we have seen that the influence of speaker cable to frequency response at speaker box terminals is not negligible in the real world speakers. It is quite different from frequency response into resistive load.

Now let's add a usual part of most power amplifiers, the output coil. The output coil L6 = 2uH (quite usual value) and it has damping resistor R100 in parallel. We shall step this resistor from 1 ohm to 10 ohm, again quite usual values.

In the image attached we can see the result. The output coil of 2uH affects frequency response above 10kHz substantially. The damping resistor R100 does not help much to change this situation.

[roender]

Hi Pavel!

Very interesting results, thanks to share it with us! Somehow I was aware about that but never simulated to see the results.
That's way I prefer to put the mono-bloc amps right at the back of the speakers and use long interconnect, high quality, cables.
By the way ... I never used output coil in any of my amps.

Cheers,
Mihai

[Juergen Knoop]

The output coil of 2uH affects frequency response above 10kHz substantially.

the cart is difficult to read. Are we talking about 0.02dB difference?
Regards

[PMA]

Originally Posted by Juergen Knoop

the cart is difficult to read. Are we talking about 0.02dB difference?
Regards

No. This 0.02dB is the difference between 1 ohm and 10 ohm damping resistor in parallel with output coil.

The difference between "output coil" and "no output coil" is about 0.2dB at 20kHz (in this particular case), it is the difference between images in Post no.1 and Post no.2

[PMA]

Originally Posted by roender

Hi Pavel!

Very interesting results, thanks to share it with us! Somehow I was aware about that but never simulated to see the results.
That's way I prefer to put the mono-bloc amps right at the back of the speakers and use long interconnect, high quality, cables.
By the way ... I never used output coil in any of my amps.

Cheers,
Mihai

Hi Mihai,
thanks for your comment. I was always aware of the fact that there would be "some difference", but never analyzed and quantified before. Few days ago I made a measurement comparing square impulse response before and behind speaker cable, with real loudspeaker box as a load. From time response it was obvious that there was an influence even in audioband, not only ultrasound band difference. So I started to simulate and simulations did confirm the measurements. I am attaching the time domain measurement, but the amplifier used has no output coil, so it is the influence of speaker cable (2m approx.) + speaker complex impedance.

Regards,

[PMA]

We can see the differences of frequency responses "with output coil" and "without output coil" if we put both plots into one image, as shown now.

Anyway, we can also see the influnce of speaker cable impedance to frequency responses - this would depend on cable used and overall speaker impedance vs. frequency response.

[roender]

Hi again Pavel!

What do you think about the choice of not using power cables nor output coils, putting the amps right behind the speakers and using the long interconnects instead?
The downside is the costly mono-bloc design ...

[PMA]

Hi Mihai,

your approach is really great. In case your amplifier is stable without output coil, you will suppress both influences of output coil and speaker cable impedance. I support your idea.

Regards,

[jan.didden]

Originally Posted by PMA

We can see the differences of frequency responses "with output coil" and "without output coil" if we put both plots into one image, as shown now.

Anyway, we can also see the influnce of speaker cable impedance to frequency responses - this would depend on cable used and overall speaker impedance vs. frequency response.

Wow! That's a whopping +/-0.02dB deviation in freq response! That can surely be cleary heard; I'll gladly sacrifice some stability and reliability for that big improvemment in sound quality Pavel!

jan didden