Part I: Concept Part II: Wish ListPart III - Conceptual Design
The way I see it, logical, deliberate and careful design should get me to 90% of the work done. Another 5% will be lab work, tweaking the circuit parameters to validate design and meet the specs.
Upon validation, 95% of the design work is done, unit performs safe, reliably and within specifications.
Then....last 5% ....you guessed it - listening tests... This is where the product gets its personality (or multiple personalities - we may like more than one option). Friends and I listen to different components, different gain distributions, grounding schemes....
It's all about right compromises.... and I rely on the Concept and Wish List to guide me through....First thing to decide - solid state or tubes?
Considering requirements for high accuracy, small size and reasonable price, solid state wins. Furthermore, for a while now I have been tempted to try some of the popular operational amplifiers ("op-amps") and some of the new ones, designed specifically for high end audio Only a few years ago the category of "high-end audio op-amps" did not really exist, we had to make do with instrumentation or video amplifiers and end result was pretty much hit-or-miss proposition. Next - active or passive RIAA equalization?
One quick look back, at the concept and requirement for high accuracy and flexibility makes passive equalization a clear winner. Separating gain stages from filtering required to meet RIAA specification allows to treat accuracy separately from gain related issues (gain, THD, dynamic overhead).
Well, let's then see what will the block schematics look like:
Going from left to right we see the first gain stage, which also acts as a low impedance source for RIAA filter and second gain stage which also provides high impedance load for the filter. With low impedance input and high impedance at the output, filter's job of modifying signals as predicted (calculated) is pretty easy.
Then, there is the Offset Compensation circuit. This circuit's job is to cancel the DC offset at the output, so we do not have to use output capacitors. Some capacitors sound bad, some may even sound good (euphonic second harmonic distortion of Aluminum Electrolytic capacitors is not uncommon), but all of them add distortion. Less capacitors, less distortion. Keepin' it simple.