In the workshop I have a couple of these Tascam M-1B line mixers. They’re a very useful box: they have eight inputs, each with a level control and pan pot, which are mixed together and fed into a master volume control. There’s also a pair of headphone outputs with their own volume control driven by a headphone amplifier with enough voltage swing to drive 600 ohm headphones properly. Into the mixer I typically feed the sound output of my PC, other audio sources such as a cassette player (yes!), and a spare cable lying around on the workbench ready to connect to whatever I’m working on, or my phone. The output is connected to the workshop speakers. With this setup I can hear all the sources at once without fiddling around, switching anything or unplugging anything.
The workshop I set up a couple of years ago turned out to have space for a new audio source: a turntable. Great! I can listen to vinyl (not ‘vinyls’, please). I had a spare turntable on the shelf, so I put it in place. The trouble is, the turntable has a magnetic cartridge which can’t be connected straight to the line inputs on my trusty mixer. It needed amplification and RIAA equalisation. I could have just gone and bought an off-the-shelf preamplifier, but where’s the fun in that? Looking at the back of the Tascam mixer, there’s a blanking plate which looks perfect for adding a preamplifier, neatly built in. No worries about trailing cables or yet another power supply to plug in. Now to find a suitable circuit.
A quick web search pulled up an application note from National Semiconductor for their LM833 audio op-amp which shows a simple preamplifier circuit. It then goes on to describes the circuit’s deficiencies and how a two-stage design can improve on it, but I decided to stick with the simple version.
I didn’t have any LM833 op-amps in the spares box, but did have some NE5532s, which are not only an excellent audio op-amp but live two to a package. Finding the passive components in the spares box also proved to be a challenge. Getting the right component values is one thing, but finding two identical ones of each proved to be impossible. However, all was not lost, so I thought. The RIAA equalisation curve needs three time constants, of 75µs, 318µs and 3180µs. I found pairs of resistors and capacitors that gave the same time constants as R1C1 and R2C2 even if their values weren’t what the original design specified. Here’s a circuit diagram with my substitutions marked in red.
I built up a small piece of matrix board and tried it out. It worked, but didn’t sound quite right: there was a noticeable lack of bass. I decided to check the frequency response. The application note shows a useful table of the RIAA response at various spot frequencies, so I had something to compare my results with. I used the ARTA STEPS program with a PC sound card to measure the frequency response of my circuit. It took a bit of fiddling with the levels to get good results, since the phono preamplifier has a lot of gain, but the result looked like this.
The curve looks convincingly like the RIAA curve, but it should be +17dB at 50Hz (relative to 1kHz). In fact it’s more like +13dB in my version, so I’ve lost 4dB of bass. At the other end of the spectrum it’s more accurate: at 20kHz it’s at almost exactly -20dB as it should be. (Note, when taking readings from the plot, that the level at 1kHz is about -3dB. It’s the relative levels that are important).
What I’d neglected in my component value calculations was that the relative values of the two RC networks are also important. The National Semiconductor application note actually goes on to explain more about this, but I hadn’t read any further than the circuit diagram. Rather than delving in to the maths and risking discovering that I needed more components I didn’t have, I started playing with the values I actually did have. Here’s a plot of the before-and-after empirical results.
The green curve is the modified version. The level at 50Hz is now +17dB relative to the level at 1kHz, as it should be. Below 50Hz there’s a bit of a rolloff, but that’s not something I’m worried about – my speakers don’t go down that low and it makes for a useful rumble filter. The modified circuit is shown here.
I had to change both RC networks to get a good result. The time constant of one of them (11k/6n8) is still 75µs, but the other (180k/22n) is more like 4000µs than 3180µs. I suspect it’s interacting with the time constant of R0C0 to give roughly the right result.
It fits neatly in to the slot in the back panel of the mixer. The toggle switch connects the output of the preamplifier to inputs 1 and 2. Switching it off leaves those inputs available for normal line-level use if desired. There’s also an earth terminal.
Time for some testing. What were the skies like when you were young?