A Basic Audio Workbench

Old electronic test equipment can now be bought at a very sharp price. Gear which would have cost thousands 15 years ago, is now available for often less than 100€. This is a real bargain - the quality of equipment made by companies like Hewlett Packard and Tektronix back in the 70s and 80s was very high indeed, and it can be more robust than almost anything made today. When you buy it in good condition, it often works as well as it did when new.

There are also PC based tools available, some of which are very good. But to buy professional quality equipment - well, for a test set like this one, by Prism Sound, we are up at more than 5000 UK pounds. The brand leader in audio test is Audio Precision, but even an old second hand set typically sells for thousands on eBay. (Update - I finally managed to find one of these at a bearable price - you can see it in use in the picture at the topof this page.)

workbench

Don’t be fooled into thinking that a cheap sound card and free software will achive the same results. It won’t. Factors such as sampling rate, noise, and repeatability of measurements, can make observations made sith such equipment quite deceptive. And one thing we do sometimes need to do in audio is make measurements at very low signal levels. PCs are not exactly great at this. Any device which uses USB power or a “wall wart” (cheap switch mode) power supply is already generating unwanted noise. And USB power, at 5VDC, just makes it harder to achieve the high signal to noise ratios that we want in audio.

When I decided to set up a home workshop, I didn't initially want to spend a lot. So I started looking for old test gear. This is what I started with, and I think it was a very inexpensive and usable setup.

Oscilloscope

Tektronix 453

A scope is pretty much essential for any kind of electronics work. There are a number of types, you will see. For audio work, you want an old fashioned analogue scope, with a bandwidth of, say, 20MHz or more. (In fact it is hard to find one less than 10MHz.) In an audio circuit, we are mostly inetersted in what happens up to 20kHz, but what happens at higher frequencies is also important. In particular, if a circuit is unstable (oscillates) it will typically do so at anywhere between 100kHz or so, and some tens of MHz. An analogue scope is really the best tool to see this with. They are also useful for quick tests of equaliser cicruits (you feed a square wave to teh input and observe the output), checking power supply ripple and stability, DC offsets, and so on.

Some of the very best ones were made by Hewlett Packard and (especially) Tektronix. The 453 was made in great quantities and was made to absolutely amazing standards. You can still find the service manual for them, which specifies every detail of use, servicing and calibration in great detail. They can be found for around 100€ second hand. Their one drawback is that they are fan cooled, and the fan tends to be noisy. (I am looking at a mod for that, when I get time.) You can really feel the quality in every switch click of these fine instruments.

You can also find good second hand units made by Hameg, Kikusui, and other companies.

regulated, adjustable DC Power Supplies

DC Power Supplies

Every electronic circuit needs to be powered with clean DC, and so we need these. Again, older gear is great value. There are different types of power supply unit (PSU) and not all the same. What we want is a good lab PSU. There are numerous cheap Chinese ones on the new market today, but I don’t much trust them. PSUs dissipate heat, and so need to be solidly designed and built. Read around the net and you will come across stories of some of these products that are not encouraging, to say the least. Buyer beware.

Many power supplies available now are switch mode types. This means that they regulate their output by rapidly switching an internal circuit on and off. These tend to dissipate less heat, but are not generally good for audio work, because of the electrical noise generated by the switching. Make sure you get a linear type, which regulates its output in a smooth fashion, which does not generate noise. Almost all lab supplies from before about 1990 are linear.

Very often we need to power circuits with split supplies - typically +/-15V DC or so. So it is very good to have a dual PSU, like the one made by Farnell on the left in the picture. The one on the right, by Phillips, is a single PSU.

Other features that are very desirable : output voltage adjustable up to about 30V DC, current up to 1 or preferably 2A. Protection against excessive output current is essential, and if this current shutdown is adjustable, even better. Ability to read both current and voltage on a front panel meter is also needed.

(In the US there recently seemed to be a lot of old Hewlett Paxckard gear on the market, which looked to be of absolutely stunning quality. Unfortunately not many of them get to Europe.)

Audio Oscillator

oscillator

We need some way to feed a signal into a circuit, and that is done with an oscillator. We need at least sine and square waves, and from something less than 20Hz to at least 100kHz. (We need to go higher than 20kHz so that we can check that the frequency response of our circuit “rolls off” correctly above 20kHz. Contrary to what you might read in some places, an amplifier which displayes a flat frequency response to 200kHz is not a good thing!)

There are also function generators that produce other waveforms, but in my experience this is not needed. Sine waves are used to test frequency response, and square waves to check the stability, and that is enough.

If you can find one with a balanced output, so much the better, but these are not common. In a future page, I will show how to convert an unbalanced out to a balanced one, with attenuators so that the circuit can be used to test a microphone amplifier. A combination of switched and continuous attenuators is usually available, however, on most units.

Most boxes billed as audio oscillators will have these features, and use a Wien bridge circuit internally to generate the signal. This is good, as these types of circuit typically produce very low distortion.

AC millivoltmeter

oscillator

Our scope let’s us see the output of our circuits, but to accurately measure the signal level, we need a meter, one that measures AC signal level. Such a meter should be calibrated in both volts and dBu, and be capable of making accurate measurements from less than 20Hz to a few MHz.

By measuring signal level at both input and output of a circuit, we can calculate the gain at that frequency. By repeating at many frequencies, we can see how the frequency response is.

The ability of the meter to measure very small signals is also important. The one in the picture, by Leader, goes to -60dBu, around about 1mV, which is not bad, but not great. It does however have a nice feature, in that it has two channels, which makes it possible to check input and output at the same time, without switching. (And as I paid only 20€, I can hardly complain.) A superior model by HP, for example, might be expected to measure down to -80dB or so.

When a meter is more sensitive, it can also be used to measure the output noise of a circuit. When it is used in this way, we also need some kind of filter, so that signals above 20kHz are removed from the measurement. Some meters have this built in - if not, it is not hard to conctruct a simple external filter.

Even though the meter above only goes to about -75dBu at best, I have been able to use it recently to test some mic amps I was working on to the lower limits of their noise specifications. This is because these circuits have high gain, around 70dB, and at that setting output noise sits at around -60dBu anyway, under best circumstances. So even simple test gear can be used to make useful measurements, if you are careful.

An oscillator/meter combination like this can be used to do a lot of audio testing, but, by itself, cannot be used to make distortion measurements. For that a specialised distortion test set is needed. A set such as this has an oscillator and meter in one unit, and can be used for level and distortion.

Digital Multimeter

Fluke 77 multimeter

A simple but essential item. Better to buy a very good basic one than a fancy cheap one, in my opinion. I have had my Fluke 77 for about 20 years, it is still reliable and accurate.

This is what you absolutely must have on a multimeter:

Many meters come with other functions - transistor tester, frequency counter, temperature probe, capacitance meter, and so on. These are niceties, but not really essential.

A seperate current measurement terminal is essential, because the meter looks like a short to the circuit under test when you measure current. If you would be across (for instance) the DC power rails and accidentally switch to current range, you could easily damage the circuit, the meter, and yourself. (I am not sure if meters without this even exist - but if you see one, leave it right where it is.

You can find out how to make a voltage reference which can be used to calibrate a meter here

(to be continued)