DESIGNING A PHONO PREAMPLIFIER
Part II: Wish List
Wish List is a set of targets that a product needs to meet or exceed. As far as electronic equipment goes, phono preamplifiers are among simpler products and we can concentrate only on few most critical specifications:
1. Very low distortion: <0.01% (target 0.005% or better). These days it's possible to go really low with this number (think 0.0001%). However, my lab is not equipped to accurately measure such a low number. There are tricks to measure much lower distortion but, having not tried it yet, I will stick with the number I can comfortably measure.
2. High accuracy: RIAA tracking within 0.2dB (target: 0.1dB). This is a very ambitious target, component tolerances and standard value ranges will work against us, but qualifying the problem will be a huge step in the right direction.
NOTE: Not only is this interesting as an engineering challenge, transparency is important for component matching. Consider a setup consisting of a tube preamplifier and single ended amplifier. Adding a phono preamp with strong character will be a hit or miss situation, with a significant risk of too-much-of-a-good-thing. Or, transferring LPs to a hard disc: the most linear preamp is desired if a software equalization is to be used.
3. Amplifier gain: >40dB. There are several constraints limiting the phono preamp gain, one of the most critical ones being noise and dynamic overhead. These two constraints tend to pull a design in opposite directions and it's up to the designer to find a compromise that will reflect his view of what the unit should be able to do.
4. Signal/noise ratio: >70dB (target >80dB). The goal here is to be 10-20dB above typical SNR of a phono cartridge. That will ensure that the preamp's noise is well below other major noise contributors'. It's likely that the number will be much better, but going with a more ambitious number right from the start will significantly impact price without real contribution to the sound quality.
5. Small size: 4.5x1.5x2.5". The dimensions come from readily available nice little steel boxes that I'd like to use.
6. External power supply: This will actually be an external AC supply, or a simple transformer. Removing transformer and it's magnetic field from the box hosting the preamplifier circuit is a simple, straightforward choice.
With the Wish List in place, it's time to grab a pen and paper and jot down some circuitry that can meet the requirements....
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