By Max.AV.Mezzomatto

The following guide was born from a request by a friend. He needed to measure the impedance of some loudspeakers and asked me for a simple and inexpensive method to do it. You will see that the only thing you may need to buy, if you do not already have it at home, is a simple USB audio interface, prices start at around 60 to 70 euros and go up, a few cables, and a simple interface box to build yourself. Keep in mind that the audio interface can also be used for many other purposes, so it is not a purchase made for a single task only.

I will try to guide you step by step in the simplest possible way. For some it may even seem boring, but online, in Italian, it is difficult, if not impossible, to find clear information about this measurement procedure. One last thing and then we really start, I promise. The measurements carried out with this system were compared with another, more expensive and reliable measurement system that uses the well known CLIO. The deviation is perhaps not even 5 percent, which is more than acceptable, considering that we are talking about a hobby level system made to play at home, intended for DIY builders and enthusiasts. Further on you will also see comparison photos.

Required equipment:

1. A laptop PC with a USB port and Windows installed.
2. At the following address www.artalabs.hr/download.htm you can find LIMP, the software required to perform the measurement, together with usage conditions, limitations, and costs if you decide to purchase it.
3. An external USB audio interface, in my case a Focusrite Scarlett Solo, but almost any interface will work. Recommended requirements are two inputs, usually Mic and Instrument or Line with adjustable level, and two mono outputs.
4. A mono or stereo amplifier of about 30 to 40 W, whatever you have in the basement is fine. It is preferable that it has a volume control to make adjustments easier. In my case I use a small DIY amplifier based on the LM3886 and equipped with a volume potentiometer.
5. A measurement box to be built yourself, it will be explained how to make it.

The cables listed below are suitable for my audio interface. If your interface connections are different, you will need to modify them accordingly.

• 1 signal cable RCA to unbalanced XLR
• 1 signal cable RCA to 6.3 mm mono jack
• 1 signal cable RCA to RCA mono
• 1 red and black cable terminated with banana plugs and alligator clips, measurement box to loudspeaker
• 1 red and black cable terminated with banana plugs on both ends, measurement box to amplifier

NOTE, which is quite important for measurement accuracy. Cable quality must be good. In practice, avoid poor quality cables. The system can be sensitive to shielding, contacts, and connection reliability. I am not talking about using hi end cables, but about using well made cables. Specifically, I use DIY cables with connectors costing 3 to 5 euros and standard OFC microphone type cable or similar.

Connections required for the measurement

Box to be built for performing the measurements

The schematic to be built inside the box

Summary table of the various cable connections, it can be useful as a reminder when building your cables

As you can see, the box is very simple to build. It uses two panel mounted RCA connectors, four red and black connectors, and a few cents worth of resistors. The schematic to follow is the one shown in the photo above and is taken directly from the LIMP Artalabs manual. I recommend having a look at it. The software is distributed under a shareware license. It can be downloaded for free and used in demo mode. If you find it useful, nothing prevents you from purchasing it.

The measurement, without going into too much detail, uses the known reference resistor method, Vref. The other resistors, together with the diodes, provide additional protection for the USB audio interface.

Once you have gathered all the material and made the connections, we are ready to start.

• Lower the output volume of the audio interface and set the input potentiometers to about one third of their travel.
• Make sure that the 48 V microphone phantom power is turned off.
• If the USB audio interface has an Inst or Line switch, set it to Line.
• Turn on the PC and launch LIMP, running it in DEMO MODE if it is not registered. The initial screen looks like the one shown below.

• Disconnect the loudspeaker under test to perform the initial settings required to carry out the measurement.
• Set the amplifier volume to zero and turn it on.
• In the menu at the top of the screen, open Setup, then choose Audio devices from the drop down menu. Check that your audio interface is recognized, in my case you can see the Focusrite Scarlett Solo in the photo below. If it is not recognized, you need to check in the PC properties that the audio interface is installed correctly, including drivers, and that it is set as the default device.
• Below I summarize the sequence of buttons to press and selections to make. From now on it will always be indicated in this way.
• Setup > Audio devices > verify that your USB audio interface is recognized > OK.

• Now in the top menu select Setup > Measurement. This screen opens.

• Set the following values. Reference channel LEFT, Reference resistor 27 ohm, this is the value of the Rref used in the box, Max Averages 10, then OK.
• In the top menu select Record > Calibrate. This screen opens.

• Press Generate, this is the internal signal generator. Increase the USB audio interface volume to about one third of its range, then slowly increase the amplifier volume. You will see the Input Level Monitor indicator rise. Increase until it shows about minus 40 dB. Now, gently adjust the input potentiometers, Mic and Line, so that the two signals are as equal as possible. Always correct using the audio interface master volume to stay around minus 40 dB, as shown in the photo above.
• At this point press Calibrate. The system performs the calibration test. In the box next to Status the difference between channels, Channel diff., will be displayed. If everything was done correctly, the difference between the two channels should be less than plus or minus 2 dB, otherwise the system will report an error. In that case return to Generate and repeat the procedure by adjusting the input sensitivity. This calibration is essential for the accuracy of subsequent measurements, so try to reduce the difference as much as possible. It is not difficult to achieve differences of plus or minus 0.5 dB. If the difference is within plus or minus 2 dB, press OK and continue. Note that the loudspeaker was still disconnected up to this point.
• Now connect the loudspeaker to be tested.
• In the top menu select Setup > Generator. This screen opens.

Check that Pink Noise, 0 dB, 1 KHz, 20 Hz are selected, as highlighted by the ellipses. At this point press Test. You should hear pink noise coming from the loudspeaker and the Level Monitor indicating its level, which in this case will be different for the two channels. This is normal and is fine, do not touch the input potentiometers. OK, we are ready for the measurement.

On the toolbar press the red PLAY button. The loudspeaker impedance curve will be displayed as in the following photo. Using the Max and Min cursors, located on the right side of the screen, you can move and adjust the scale of the obtained curve as you wish for better viewing.

If necessary, by right clicking the mouse a drop down menu opens in which it is possible to extend the frequency scale and bring it down to 5 Hz, as shown in the photo below.

On the lower left side of the screen the impedance value in ohm is displayed. To obtain the actual value, you must subtract the value of the reference resistor, which in our case is 27 ohm. Let us make an example. In the photo, if you place the cursor at the lowest point of the curve, at the bottom of the screen under Cursor, you will read the frequency and the impedance value. In the example we are at about 220 Hz and the indicated value is 34.17. From this you must subtract the 27 ohm reference resistor. 34.17 minus 27 equals 7.17 ohm.

On the right side of the screen you have the phase scale. Another example. If you move the cursor, as in the photo, to the maximum peak of the curve, where the phase crosses zero degrees, you can read the loudspeaker resonance frequency and its maximum value in ohm. Also in this case you must always subtract the value of the 27 ohm reference resistor mentioned above.

In the example shown in the photo, with the phase at about zero degrees, Fs, the resonance frequency, is 49.61 Hz and the effective maximum resistance value of the loudspeaker is approximately 64.67 minus 27, which equals about 37.67 ohm. Always read the Cursor section at the bottom left.

That is it, you have completed your first measurement of a loudspeaker impedance curve. This measurement allows you to understand the electrical behavior of the loudspeaker at different frequencies, information that is essential when designing a loudspeaker system. For now, let us stop here.

Finally, here are some photos taken to compare the two systems, as mentioned at the beginning of the tutorial. Above are the measurements made with CLIO and below those made with LIMP. Note the small loudspeaker break up modes present in both measurements.

And again the measurement of a small Sony loudspeaker, taken as is and used as a test mule.

The difference between the two systems is really minimal, do not be fooled by the slightly different scales, and it is of little importance for the purpose for which these measurements will be used. That purpose is, let us always remember it, the hobby of DIY loudspeaker building. It is only the beginning of a long path that leads to the final realization of a loudspeaker, but you have to start somewhere.

Hoping to have been useful and to have sparked your curiosity, I send you my regards. Finally, a special thank you should also go to my dear friend Stefano Bianchini of SB-LAB for allowing the publication of this small handbook on his website.

A white noise generator is a surprisingly useful tool in an audio lab. It allows you to quickly check the frequency response of a preamplifier or power amplifier, highlight resonances and colorations, verify the effectiveness of a filter or tone control, calibrate a simple RTA analyzer and, above all, “stress” a signal path in a repeatable way to track down hum, hiss and grounding problems. In this article I describe a small, fully transistor-based white noise generator, designed to be inexpensive, easy to build and suitable for direct connection to a line-level input. The output is buffered, the level is compatible with Hi-Fi electronics and the power supply is stabilized and filtered so that the unit itself does not become a source of interference.

Having a white noise generator can be very useful for performing various tests on audio electronics, so I designed a small transistor-based generator capable of delivering white noise with sufficient bandwidth for testing audio equipment and an output level compatible with a line input. The basic idea is to create a source that is “dirty in the right way”, meaning wideband and statistically random noise, but with a controllable level and an output capable of driving the typical impedance of a line input without trouble, even when it drops to relatively low values.

The circuit is deliberately simple and uses common components. The noise source is the EB junction of a BC337 reverse biased. Under these conditions the junction operates in breakdown and generates wideband random noise, a raw noise that is then amplified. This solution is practical because it does not require special Zener diodes or selected transistors, and in most cases already provides more than enough noise level for audio test applications.

The signal generated this way is very small and must be brought up to a usable level. The two following BC337 transistors therefore work as audio amplification stages, with a high overall gain, on the order of about 60 dB. Such a high gain is convenient because it makes it easy to obtain a line-level output, but it is also the reason why construction must be careful. Any disturbance physically picked up by the circuit, typically 50 Hz mains hum, would be amplified together with the noise, ruining the result and making the generator unusable for serious measurements.

To make the output stable and less influenced by the load, I added a BF256 JFET buffer used as a decoupling stage. In practice, the transistor stages generate and amplify the noise, while the JFET isolates the circuit from the outside world, providing a low output impedance and reducing sensitivity to load variations. This is important because, in the lab, the generator may be connected to different inputs, long cables, attenuators, or equipment with impedances that are not always ideal.

R8 and R5 form the feedback network whose purpose is precisely to compensate for the effect of the load on the output. The goal is not to achieve any “magic” distortion figure, which is irrelevant here since we are generating noise, but to maintain a reasonably constant level and predictable behavior when the input impedance of the connected device drops. In this project the output remains usable even with loads down to 22kohm, a value that covers many real line inputs and quite a few passive attenuators.

The power supply is another critical point, often underestimated in small bench generators. The circuit is powered at 15volt stabilized using an LM317L, more than sufficient to supply the roughly 7mA drawn by the generator. I deliberately chose a simple solution, a small salvaged external wall-wart power supply providing 24volt DC, so that everything directly connected to the mains stays outside the enclosure. This reduces risk and, if the wiring is done properly, also helps limit the coupling of mains-related interference into the high-gain circuit.

To prevent any noise or ripple from the external power supply from entering the generator, the RC cell R9/C6 acts as an additional filter upstream of the most sensitive section. In practice it is a “second wall” that attenuates unwanted components before they can reach the amplification stages. This is particularly useful when using inexpensive switching supplies, which can introduce high-frequency spurs, and when working with long cables or imperfect outlets.

Because of the high gain, it is essential to shield the circuit properly and set up grounding and wiring correctly. If left unshielded or mounted in a plastic enclosure, the generator would easily pick up the 50 Hz field and ambient interference, amplifying it excessively. A metal enclosure properly connected to ground, short connections, a well thought-out ground point that avoids loops, and an output connector firmly fixed make the difference between a clean generator and one that produces more hum than white noise.

In practical use, this generator can be employed in many ways: as a source to measure frequency response with a sound card and RTA software, to compare two signal paths, for example before and after a modification, to quickly check whether one channel “sounds” different from the other, or to verify the effectiveness of a noise filter or a shielding intervention. Even without sophisticated instrumentation, white noise is useful because it immediately highlights anomalies, such as superimposed hum, excessive hiss, a bandwidth that rolls off, one channel being more attenuated, or a contact or grounding problem.

If you want to be precise, you can also adjust the output level so that it is consistent with real sources, for example a few hundred millivolts RMS into a line input, using a simple attenuator or calibrating the level with a true RMS multimeter or via a sound card measurement. This is not strictly necessary, but it helps to repeat tests consistently and to avoid overdriving the input stages of the device under test. Here is the schematic, click to enlarge.

Looking at the schematic, the “lab-oriented” approach is immediately clear. A simple noise source, two gain stages to bring the level up to usable values, and a final buffer to make the output robust. It is a project that favors practicality and reliability, and it can be built on perfboard or on a home-etched board without difficulty. With neat assembly and good shielding, the result is a compact generator that becomes very convenient to keep on the bench. And here is my build.

In the practical build I tried to keep connections as short as possible in the high-gain areas, physically separating the power supply and filtering section from the amplification stages. Component placement also matters. The more compact the signal path, the less “antenna” area is exposed to ambient electromagnetic fields. If you plan to replicate this circuit, my advice is to think about the layout first and only then solder, because with 60 dB of gain an improvised layout often leads to disappointing results.

Here the compactness of the circuit is even clearer. If you use a metal enclosure, it is worth connecting the main ground point to the enclosure at a single point, near the output connector, to reduce the likelihood of loops. Alternatively, if the generator is connected to equipment that is already well grounded, it may make sense to include a small resistor or decoupling network between signal ground and chassis, but in most cases clean wiring is already sufficient.

Once completed, this small generator becomes a “Swiss army knife” for testing. You connect it, set the level, and in a few minutes you can tell if one channel is noisier, if an input has a problem, if a filter works as expected, or if the shielding of a device is insufficient. With very little expense and few components, it is one of those projects worth building because it turns out to be useful far more often than you might think.