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DESIGNING A PHONO PREAMPLIFIER

In the following series of articles I will go through the process of designing a phono preamplifier (but you probably already guessed that). I will outline typical steps that most of our products go through, concentrating on some specifics of this type of audio electronics. This is an open ended design (I have just started and results are yet to be obtained), and I expect to learn some interesting stuff during the process and hope that myr readers and customers will gain a better understanding of how I work (other designers probably follow a different process).

Part I: Concept

High quality and low price can pretty much summarize what I want to do here. Unfortunately, that doesn't tell me what exactly am I to do, as quality and price are so relative. So let's drill deeper - what I want (or don't) want to do:

1. I want small physical size. Small size is inherently less expensive to make (less material), but there are other benefits as well. I will need to place components in close proximity, minimizing traces and trace loops that can pickup environmental noise and negatively affect sound quality. A small unit can be placed in close proximity of the turntable and reduce cable length, again helping reduce noise in the system. I will work with my friends from AudioLimits to make a short phono cable for this specific application.

2. I want high accuracy. There are two components to a phono preamp accuracy: Distortion (as measured by THD+N) and RIAA tracking accuracy. Circuit configuration, component and topology selection will be mostly responsible for low distortion. RIAA tracking is a bit trickier. Optimum values for RIAA compensation are easy to come by - basic circuit theory works here and there are many online calculators that simplify the task. Problem with the calculations is that they give us exact values, such as 311.586nF... well, good luck finding that one at your friendly electronics distributor. If you are a DIY-er with healthy budget you may want to buy many capacitors with close nominal value (say 330nF in this example), and try to measure each one (by using a decent RLC meter, not your handheld DMM) to find the closest match. This does not really work for production, so I will try to analyze RIAA tracking error for standard vs. theoretical component values. Moreover, every component comes with a tolerance number, typical for resistors is 1% these days, and 5 or 10% for capacitors. I will look at how component tolerances affect the accuracy. Monte Carlo and Worst Case analysis are handy tools to use.

3. I will not do a revolutionary design. A "revolutionary" design will take a lot of time (you will understand why as we go through all the steps in coming weeks), will carry significant risk to the planned schedule and will go against  the "low cost" concept. I am appreciative and thankful for hard work and generosity of Walt Jung, Joe Curcio and many other notable engineers and companies that supported them and who put their work in public domain through various application notes. Readers are encouraged to explore vast libraries of Analog Devices and National Semiconductor where these and many other fine designers published their work. However, I will try to make a step forward in attaining the goals. In every design I try to address a specific problem, here that will be a RIAA tracking accuracy.

4. Flexibility is the name of the game. Even though small size requirement works against flexibility, I still want to have options to try different parts in the circuit. The circuit needs to be able to accommodate MM or MC cartridges with simple component changes (ideally only one component per channel).

Here we are, with a simple set of goals (which, by themselves, are not so simple). There will be more as I proceed to develop specification which, at this point, I will call a "wish list".

NEXT: Part II: Wish List

 

 

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