Using a graphic equaliser for open loop current control.

I was going to set up a sound system using the following method:

1) Use a RMS current measurement probe to measure the current to the loudspeaker transducer with pure sine waves at spot frequencies from 20hz to 20khz at constant signal voltage. That’s the loudspeaker current only and NOT the input to a passive crossover network. 2) Draw a graph of frequency V current. 3) Use a graphic equaliser to modify the signal to try and get the loudspeaker current as flat as possible for the midrange at the chosen spot frequencies with smooth roll-on and roll-off.

4) Does this set up sound better? Let me know?

  • Hey Gary. Whilst Edwin has posted a detailed response below, this may be more suited to the Electrical Engineering StackExchange at electronics.stackexchange.com . This is more end-user-focused ('sound design', as in 'creation and manipulation of sound effects'), rather than electronic system design. – Skarik Sep 13 '16 at 14:21
  • @Skarik, I'm don't this this is necessary off-topic. Unless I'm mistaken, this exchange hasn't been strictly end-user focused since the merge with Audio/Video Production. It's fairly high-level, so electronics might still be a better fit. – user9881 Sep 14 '16 at 22:04
  • Question 4 does make this off topic as you get into opinion based. Gary - can you edit this onto something that works as per How to Ask then we can reopen – Rory Alsop Sep 18 '16 at 14:31

Lets start with a picture describing your setup:

enter image description here

On the left we start with a Signal Generator, its output measured in V1. From there we input in an EQ, its output measured in V2 From there into the Power amplifier, its output measured in V3 Onwards to the passive X-over, its output measured in V4 Then finally into an Amp-meter (A) to the speaker.

(apologies for not using the correct component symbols, I just hacked up this picture)


  • Even when the signal generator has a constant voltage across the whole frequency spectrum, the equalizer most likely has not, hence V1 and V2 will be different. Even when you set the equalizer to "flat - 0 dB".

  • Even when the input of the power Amplifier is a constant voltage across the frequency spectrum, the output voltage most likely is not, hence V2 and V3 will be different. You would expect this as it is a power amplifier, but what I am trying to say here is that the relation of V2 to V3 will not be linear, nor flat.

  • Even when the input of the passive X-over has a constant voltage across the frequency spectrum, the output voltage most likely is not, hence V3 and V4 will be different. (again, seen as linear/flat relationship)

What you cannot do:

So because the electronic components are likely not linear in behaviour across the frequency spectrum, you cannot measure Voltage vs Amp relationship as you have intended in your post.

What you can do:

You can do two measurements:

  • By varying the equalizer you can preset a V4, and measure A
  • By varying the equalizer you can preset A, and measure V2, V3, V4

But is unclear why you would do these measurements, and what the interpretation of the results will get you. The question really is: "What are you trying to accomplish"

What equal current does not mean:

Even when you "equalize" A, to be the same across the frequency spectrum, it does not mean that the speaker "produces" the same level of "sound". This due the various (mechanical) properties of the speaker itself.

Bottom line is that none of the components in the signal path have a linear (nor flat) response accross the frequency spectrum, and you need to be carefull interpreting the measurements in that respect. Can you update your post, to state what you are actually trying to accomplish with this measurement, and we can answer you better.

PS: please ensure you have the proper measuring equipment, standard Volt-meters and Amp-meters may not work in the frequency ranges indicated.

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