Hardware required to do the measurements

From DRC

Contents 1 Microphone 2 Microphone amplifier 3 Sound card leads 4 Phono Leads 5 Adaptors 6 Attenuator 7 Channel shorting plug 8 Putting it all together 8.1 reference channel 8.2 measurement channel 8.3 shorted channel

Microphone

You will need a microphone to do your measurements.

Shown here is a inexpensie tie clip model that uses a small battery in the plug to polarize the measurement capsule.

Tie clip mic. Click for larger image

The unit has been given a wand by fitting a plastic straw which was cut down the edge. Once the straw was fitted over the cable it was filled with silicon sealant/mastic which stiffened the assembly. This was then fitted to a 'helping hands' unit, which are sold as a soldering aid, to form a stand.

Another very popular option is the Behringer ECM8000 Behringer homepage. Price is around £40 in the UK.

A comparison test between different class microphones link.

Microphone amplifier

You will need an amplifier to boost the signal from your microphone. You may consider using the microphone input of your sound card, but if you loop back one channel from the sound card's output back to it's input to provide a reference measurement then you cannot use the microphone input as the levels will not match, the loopback signal will be too large. Hence you need a microphone pre-amp. This picture shows the front of a microphone pre-amp.

The rear of the microphone pre-amp shown above.

Again, Behringer make a cheap and cheerful pre-amp called the Behringer UB802. Worth checking out at around £40.

Sound card leads

Your sound card will probably have a 3.5mm connector for two channels while your HiFi probably has a phono connector for each channel. Because only one channel is measured at a time the channels have to be split apart so the connections can be routed seperately. This picture shows examples of 3.5mm to phono adaptors.

You will need two of these adaptors. One adaptor splits the sound card output where one channel goes to your HiFi system while the other channel is looked back. The other adaptor goes to the line input of your sound card and joins the lookback channel with the single channel from the microphone pre-amp. This picture shown an example of an adaptor lead.

If your sound card has a digital output and you decide to use an external DAC instead of the analogue output from your sound card you will need a TOSLink cable to connect your sound card to the DAC.

Phono Leads

You probably have some phono leads in your HiFi system already. You will need one more to loop one channel from you sound card's output back to is input, and another to connect the output from the microphone pre-amp to the sound card input.

Adaptors

Depending on you hardware you may need additional adaptors. Shown here are a 3.5mm to 1/4 inch adaptor used to connect the microphone to the pre-amp, and a phono-phono adaptor used to connect a phono lead to the 3.5mm to phono adaptor shown above.

Attenuator

If you use the loopback arrangement where one channel (the reference channel) output from the sound card is looped back to the input (to compensate for the characteristics of the sound card) you may find that you have to set the input gain very low to avoid overload. Depending on the gain of your microphone amplifier you may find there is not enough gain on the other channel (the measurement channel) to give a decent sized signal. What do you do?

In such cases an attenuator is needed to reduce the signal on the measurement channel so that more gain can be applied to both channels at the sound card input. The picture shows a 5k multi-turn variable resistor wired between two phono sockets to produce an inline attenuator. Once an appropriate level of attenuation is been obtained (in this case 1:16) the unit is sealed with heat shrink tubeing.

Channel shorting plug

When doing your measurments one channel will be driven from your sound card, so what about the other? If it can be muted on your pre-amp or power amp do so, otherwise you should short out the channel on the input to your pre-amp or power-amp. The picture shows an channel shorting plug made from a phono socket.

Putting it all together

When it is all connected the channels should be as shown below (for a 2 channel setup). If your system has more channels/speakers just make them shorted channels.

reference channel

sound card output -> 3.5mm-phono adaptor -> phono lead -> attenuator or phono-phono adaptor -> 3.5mm-phono adaptor -> sound card input

measurement channel

sound card output -> 3.5mm-phono adaptor -> phone lead -> pre-amp -> power amp -> speaker -> microphone -> mic pre-amp -> phono lead -> 3.5mm-phono adaptor -> sound card input

shorted channel

channel shorting plug -> phono lead -> pre-amp -> power amp -> speaker