ARTA Measurement Box

All informations I'll provide are from and can be found in the LIMP Manual.

The principles underlying measurement in LIMP are the following (see figure 1) :

Voltage V1 (Line in, right channel) through reference resistor Rref and speaker, and V2 (Line in, left channel) via the speaker gives the impedance: Z = Rref × V2 (f) ÷ (V1 (f) – V2 (f))

In the original LIMP User Manual, reference is made to two test setups:

- Impedance measurement using the headphone jack of the sound card ;

- Impedance measurement with power amplifier.

You can decide to make your measurement without using an amplifier but I prefer to use one.

The second figure is quite simple because the "only" things you need is a reference resistor (of known value) and a voltage probe (= voltage divider) to reduce the voltage send in your soundcard inputs.

To calculate the value of R1 and R2, you need to know : maximum soundcard input voltage (V in max), soundcard input impedance (Z in), amplifier power (P), loudspeaker nominal impedance (Z hp).

Note : I'm using the initials HP because in France it's meaning "haut-parleur" = loudspeaker.

If you know the maximum soundcard input voltage in dBu or dBV, you can easily calculate the voltage.

V in max = V0 * 10 ^ (L(dB)/20) with V0 = 1 V for the dBV and V0 = 0.7746 V for the dBu.

The maximum output voltage is calculated for a load impedance Z hp as follows:

V out amp max = SQRT (P * Z hp)

The value of the voltage divider is V in max / V out amp max.

In dB : G in = 20 * log (V in max / V out amp max). "log" is the base-10 logarithm.

If G in = -20dB, you need to choose R1 and R2 to get a -20dB of attenuation.

The figure 3 shows the principle of the voltage divider.

To calculate the attenuation, you use this formula : Vout = (R2/(R1+R2)) * Vin.

To get a 20dB attenuation, I choose R1 = 6.8k ohms (blue grey red gold) and R2 = 820 ohms (grey red brown gold).

Which leads to : G in = 20 * log (R2/(R1+R2)) = -19.36dB.

This value fits with my configuration but you need to choose the one which fits with your soundcard and amplifier.

You need :

- 1 box ;

- 1 power resistor (Limp user manual recommands 27 ohms but I choose a 5.6 ohms, 5 W resistor that I picked up from an old printed circuit) ;

- 2 resistors R1 (6.8kΩ in my case) ;

- 2 resistors R2 (820Ω in my case) ;

- 1 6.35mm stereo female jack (or 2 mono or 2 RCA) ;

- 4 connectors for the loudspeaker (x2) and the amplifier (x2). I choose to use bolts and nuts with lugs but you can also use loudspeaker connectors, as you want ;)

- some wire ;

- heat shrink tube.

The tools you need are :

- soldering iron with solder ;

- drill press or drilling machine ;

- screwdriver ;

- pinches ;

- bolt cutters ;

- etc.

You can choose any box/case from grab. I choose an old Legrand distribution/junction box.

I drilled the box with my drill press and I set up the jacks and the nuts and bolts.

Note : I set up a second female mini jack but it's a mistake. I think I'll replace this by a switch in order to short circuit the reference resistor when I'm calibrating the soundcard.

I soldered 2 pieces of wire to the reference resistors legs and I protect it from short circuit.

I don't have all the soldering steps and the crimping of the lugs but you get the main idea.

Be carefull to connect the right leg of the jack to the amplifier side and the left leg of the jack to the loudspeaker side. But anyway if you invert the left and the right it's not a big deal because you can invert the cables and the software will tell you if you didn't wired correctly.

My test bench is composed of a Lexicon Omega Studio soundcard and a 8LM amplifier.

Result is very good because the software measure the impedance and phase versus frequency of the loudspeaker. With the menu Analyze you can calculate the loudspeaker parameters with the added mass method or the closed box method.

I get some noise in high frequency and I don't know where does it come from...

It can be due to the fact that it's an old speaker or maybe it's because I'm using a tube amplifier (the frequency response is maybe not very linear).

I hope you enjoyed this instructable which is not really complicated but can be really usefull.

It's my first instructable so do not hesitate to ask questions or make comments :)

See you !