Record player modification

A few weeks ago I bought a record player for a few quid at the local flea market. Even though we already have a nice vintage Marconi record player that’s in great shape and sounds fine, it has two disadvantages: 1) There are no external connectors, so all the sound comes from the built-in speaker. That’s fine, but it prevents you from either listening to your records with headphones, or to record them somehow (unless you use a mic at the speaker, of course). 2) The player is a mono player only. Again, that’s fine for playing both mono and stereo records, but of course you won’t hear any left-right differences when listening to stereo records. This second record player was supposed to fix both of these problems. It did, however need a few modifications.

The record player I bought was a Sanyo brand model. What I hadn’t realized when I bought it was that it was originally intended to be used with a specific home audio system, probably one of these all-in-one units with radio, tape decks and CD player. Because of that, instead of having standard connectors and a power cord, this record player had two cables coming out. One was terminated with a normal 3.5mm stereo plug, the other a three-stranded wire with a 0.1″ header at the end. The former was presumably for the audio signal, the other for power; opening up the player confirmed this. The stereo cable connected directly to the wires of the phono cartridge, and the power cable turned out to really only be 2 separate wires, as two of the three strands connected to the same point. Tracing these wires it was easy to identify which one was the positive and which one the negative lead, and luckily the printing on the back of the record player stated that it needed 12V DC at 100mA. Powering it up with my bench supply, it seemed to be working.

I could have just found a external 12 DC power source and left it at that. However, that would have still left one problem. Even though the audio signal came out of a standard audio connector, the signal itself is not well-suited for typical audio applications. For instance, trying to connect it to a computer and recording the music would require significant boosting of the incoming signal (after all it’s the raw voltages from the pickup coils in the cartridge). And even boosted, the music would sound “off”. The reason for this can be found in the way vinyl records are made. Before cutting the grooves into the disc, low frequencies are significantly attenuated, whereas high frequencies are amplified. To get the original signal back, this needs to be reversed, a job typically reserved for a “phono preamplifier” called “de-emphasis”. I therefore decided to build such a pre-amplifier into the record player.

There are many different phono preamp circuit designs available online (e.g. here and here), many of which appear to be based on the NE5532 (dual) op-amp. Since my local electronics shop had this opamp available, I decided to build the design from the second link above using some stripboard. Beside the opamp, only a handful of resistors and capacitors were needed. However, I also needed a positive and negative voltage supply. I found a transformer-based, isolated 12VAC 250mA power supply in my parts bin, which after rectification (and using one of the inputs as virtual ground) gave me around +/-16V DC. These would serve well as the positive and negative supply rails for the opamp. I also used the positive rail to power the motor using a standard 12V linear voltage regulator. In addition I added a power switch and power indicator LED, and managed to cram everything into the empty spaces of the record player. After some quick troubleshooting (I forgot to cut some of the traces of the stripboard as required), everything worked perfectly fine. In fact, I’m a bit surprised at how well it actually sounds – certainly good enough for me, and I’m sure better than in its originally intended configuration.

turntable_mod

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