To briefly recap, the RIAA preamplifier was presented for those who have a modern sound system. More often than not these days, modern systems don't include a "phono" input, which precludes the use of a turntable to listen to records (as distinct from CDs!).

The preamp not only provides the necessary gain for a magnetic cartridge, it also "tailors" the signal to the RIAA standard, allowing you to plug into any "aux" or line-level input.

The idea of our preamplifier was to make it fully self-contained, with its own 240V to ±15V DC supply built on the PC board. To be honest, this didn't work out quite as well as we had hoped due to noise being induced into the preamp from the mains supply. But we were one step ahead on this one (just in case!), making the two sections of the PC board separable.

The 240V to power the whole shebang is, of course, already available from the turntable supply. The ±15V, 10VA DC supply was in fact a bit of overkill, being capable of much more than demanded by the preamp. But then, we always had this little project in the back of our minds.

About the only change we'd make from the project published in March would be the use of larger U-shaped heatsinks to cope with increased current (there's tonnes of room on the PC board for them) and the use of PC-board mounting screw-in terminal blocks on at least the power supply board (where there is plenty of room) and possibly the RIAA board - where space might be a tad tight.

Of course, there's nothing to stop you using this project as a general-purpose headphone amplifier, as long as it is fed with "line level" signals - such as the output of a CD/DVD player, tuner, etc.

Performance of Prototype Output level 90mΩ (max) into 8W headphones Frequency response 0.5dB down at 30Hz and 20kHz (see Fig.2) Input sensitivity 0.83V RMS for full power Harmonic distortion see Fig.3 & Fig.4 Signal-to-noise ratio -95dB unweighted (20Hz to 20kHz) with respect to 500mV input signal Separation between channels -50dB between 20Hz and 10kHz

The amplifier circuit

This design is a tried-and-true circuit we have used before. It's rather similar (or at least one channel of it is!) to the one John Clarke used in the mono headphone monitor amplifier in the 8-channel Mixer (described in November and December 1996). We've made just a few changes which we will go into shortly.

All components except a stereo (dual ganged) volume control pot mount on a single PC board measuring 65 x 65mm.

The biggest change from the earlier circuit was to duplicate it for stereo. Rather than use two TL071 op amps (one for each channel) we used a TL072, which is effectively two TL071s in the one 8-pin DIL package. The other changes involve the components in the input and feedback circuits - these have been adjusted to make them optimum for the RIAA preamp output. That's not to say it won't work with other audio sources - it should be pretty-well "universal."

A DC blocking capacitor has also been included in series with both inputs. This is not to prevent any DC present in the preamplifier output from reaching the op amp inputs; rather it is to prevent any DC offset voltage at the op amp inputs (albeit tiny) from being impressed back on the potentiometer.

DC on pots is a no-no: most "noisy pots" that you come across are not the result of mechanical wear and tear but the result of DC flowing in them and to the contact point of the wiper.

How it works

Signal from the RIAA preamp (or other "line level" audio source) is applied to the ganged pots, VR1a and VR1b.

The pot simply forms a voltage divider across the preamp output, presenting the op amp input with a level of signal dependent on the pot setting. At minimum setting, very little (if any) signal gets through; at maximum most gets through. Presto: a level or volume control.

As both stereo channels are identical we'll just look at one (the left) channel.

After passing through a 56kΩ resistor (which basically sets the input impedance), the signal is applied to the inverting input of one of the TL072s twin op amps.

Normally, the feedback loop for the op amp would go from the output back to the input - but in this circuit, transistors Q1 and Q2 are included in the loop. This helps minimise harmonic distortion.

The transistors themselves boost the output current capability of the op amp. To minimise crossover distortion, the transistors are slightly forward-biased by D1 and D2 between their bases.

The 33Ω emitter resistors on each transistor maintain the bias stability. They, along with the 68Ω resistor between their junction and the headphones, limit the output current to less than 150mA under catastrophic failure conditions, protecting the headphones from damage.

The feedback loop previously mentioned consists of a 5pF capacitor in parallel with a 330kΩresistor. This limits the response to about 50kHz and sets the overall gain of the amplifier (op amp plus output pair) to about six (330Ω/56kΩ).

Construction

Start by checking your PC board for any defects, then mount and solder components in the usual way: lowest profile (resistors) first, the link (using an offcut from a resistor), then the four small capacitors, the electrolytic (watch the polarity) and finally the semiconductors: diodes, transistors and the IC. Again, watch both the placement and the polarity of the semiconductors.

The headphone socket mounts on the board - provision is made for sockets with pins either side. The power supply is connected via an on-board screw terminal block while signal is connected via four PC stakes.

Checking it out

Check your component placement, polarity and soldering before proceeding. If you're happy with it after a thorough examination, connect the +15V, 0V and -15V from the RIAA preamp supply to this board. Note that the order of the terminals are not the same on both boards: you have been warned!

With no headphones nor any inputs connected, turn the power on. Absolutely nothing should happen (at least as far as visual observation is concerned!).

Check that you have +15V and -15V on the board and (respectively) on pin 8 and pin 4 of IC1 and (again respectively) on the collectors of Q1 and Q2. All of these voltages, by the way, are measured with respect to 0V.

Check that you have 0V (or within a couple of millivolts of 0V) on pins 2, 3, 5 and 6 of the IC. You should also have 0V (or very close to it) on pins 1 and 7 (the outputs). Now check the voltages across the diodes and between base and emitter of the four transistors. In every case it should be around 0.6V, give or take. Obviously, in half the cases the polarity should be reversed compared to the other half.

Finally, check the voltage between the outputs and 0V - again, it should be pretty close to zero.

If all this passes muster, it's time to plug in a pair of headphones. This done, apply the "blurt" test: touch your finger on the input pins (not the earth pins!). You should hear a healthy "blurt" out of each of the headphones in turn.

If you do, you can reasonably assume the system is working (what you're doing is simply connecting a big source of hum and noise - you!) If you don't get any sound, check that (a) the headphones are turned on or up, if they have any controls; (b) they are plugged into the socket; and (c) you still have power to the board.

There's not much that can be wrong given the tests you have already undertaken but if you have sound in one channel only, it's nice to know there is a "reference" right alongside with which to compare voltages.

The pot

Wire the 50kΩdual-ganged potent-iometer as shown. It's important that the inputs to the amplifier go to the wipers of the pots and that both the earths (preamp and amplifier) connect together at the bottom end.

If when using your amplifier later you find that a lot of hum is generated when you touch the volume control, it might pay you to solder a wire from the pot body (you'll probably need to scrape off some of the passivation to bare metal) to the earth braids.

The pot should be a "logarithmic" type to match the sensitivity characteristics of the human ear. But if all you have (or can get) is a linear pot, you could use it - it won't do any harm but there won't be much control: you'll find that minimum to maximum is squeezed into a tiny section of the pot's travel.

If you are fitting the amplifier board underneath your turntable as with the power supply and preamp, you're going to have to mount the board so the headphone socket emerges from an appropriately-drilled hole in the plinth.

And the volume control will also need a mounting hole drilled - make sure that neither the board nor the pot foul any under-deck mechanical operations.

Just remember to keep the input side of the PC board as far away as possible from the mains transformer and the motor.

Being a line level input, it's not as critical as the RIAA preamp input - but it's better to play safe.

Parts List Headphone Amp 1 PC board, 65 x 65mm, coded 01105021 1 PC mount stereo headphone socket 1 PC mount 3-way terminal block 4 PC stakes 4 mounting screws (as required) Suitable lengths red, black and white hookup wire Semiconductors 1 TL072 dual op amp (IC1) 2 BC338 NPN transistors (Q1, Q3) 2 BC328 PNP transistors (Q2, Q4) 4 1N914 or similar silicon signal diodes (D1-D4) Capacitors 1 10μF 35VW PC electrolytic 2 0.27μF MKT polyester (code 274 or 270n) 2 5pF ceramic (code 5 or 5p) Resistors (0.25W, 5%) 2 330kΩ 2 56kΩ 2 22kΩ 4 10kΩ 2 68Ω 4 33Ω 1 50kΩ dual-gang logarithmic pot (VR1)