I bought a turntable with little or no regard to how I would eventually hook it up to the rest of my system. High end systems being what they are tend not to include many features so an external Phono stage was necessary.
There are many different ways this could have been built but it needed to fit a few simple criteria:
- High fidelity (and thus accurate reproduction of the RIAA EQ curve)
- Cheap preferably built mostly from spare parts
- Simple, an on / off button
- Designed for high-output cartridges, loaded with 47kΩ and with a gain of ~45dB
- Built in rumble filter to prevent subwoofer damage.
With these goals in mind I eventually found the original El-Cheapo Project. This project was then modified by a member of diyaudio to simplify its design and add a Class-A bias, and it was this design which I then took and modified again! The final circuit looked similar to the circuit below except with minor bolt-ons such as a resistor on the output so the 1uf cap acts as a high pass filter to tune out rumble, and removal of R1.
Theory of Operation
On the input C14 and R3 (R1 has been jumpered) are to load the turntable cartridge and should be adjusted to suit the cartridge used.
The amplifier itself, a high bandwidth, low noise, FET input type is configured to have a gain of approximately 45dB @ 1khz: 20 × log10( 1 + R5/R2 ). R4 || C15 and C16 || C5 and R6 then form an eq curve to boost bass by 20dB and drop treble by the same amount in accordance with the RIAA EQ curve. The result is within ±0.1db over the audio range, and tracks the middle point between the standard and enhanced RIAA curves.
The effect of using 5% tolerance components can vary the response additionally by approximately ±0.1dB. These deviations are not perceptible and thus can be safely ignored.
R7 and the two NChannel JFETs form a constant current source which bias the the phono stage into class A when driving high impedance loads. Effectively these load the output forcing the opamp to compensate to sink the current and achieve it’s stable 0V output.
Finally C17 blocks DC offset induced by the large gain of the amplifer, and with the addition of a parallel resistor of approx 10-15kΩ to ground forms a low-pass filter to remove speaker damaging sub-10hz rumble.
The powersupply is a bog standard LM317/337 and the last of my designs to use these regulators for reasons which are explained under the DAC project. It also has a simple flipflop triggering a relay to allow the PSU to be switched on and off via a pushbutton.
All in all the design sounds much better than the phono stage and tape loops of my cheap receiver (R.I.P) and so it should. I unfortunately have not yet had the chance to compare it to anything substantial, but at a total cost of under $20 (mostly the casing) thanks to having most of the parts in my spare parts bin how much can really be expected. That said on the whole my vinyl system surpassed my digital system at the time of writing. Time to finish that DAC.
This project was completed in October 2006 and only recently posted to wordpress.