Designing a Phono Preamp - Part V: Schematics

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Designing a Phono Preamp - Part V: Schematics
« on: 2 May 2009, 11:53 pm »
Part V: Schematics

Putting all the pieces together, we come to a sketch of the schematics.

Little crooked, but nevertheless useful to connect major sections outlined in previous sections.

Time to fire up Schematics Capture software and make first draft:

Very similar to the hand drawn sketch, this schematics has some more detail - we see input capacitor and resistor for matching to most commonly used MM cartridges. Also there are a couple of jumpers for selecting different options. First jumper to the left can be used to select MM or MC cartridges by changing input resistance and gain of the first stage. The second jumper, when disengaged adds a low frequency roll-off in order to comply with European version of RIAA curve. When jumper is inserted, we have standard, US version of RIAA.

Most component values are missing in this stage - those that are in are not final, they are just an indication of the approximate value that will need to be calculated before we finalize the schematic.

At this stage I would typically walk away from the design and let it settle for a few days. When I come back, I take another look and revisit the concept and execution. It is interesting how many times I find some obvious errors or a better execution of a specific idea after the brain has had a chance to relax and work on other things. I cannot recommend highly enough this practice to any designer or DIY-er.


Re: Designing a Phono Preamp - Part V: Schematics
« Reply #1 on: 9 May 2009, 10:41 pm »
After another revision or two, the final schematic came up and about:

Now we have connectors, power supplies and component values. Also, the schematic suffered some changes - jumpers at the input got thrown out and we have new jumpers bypassing the passive RIAA. This bypass can be used for software based RIAA compensation. Conceivably, a software will provide even more accurate compensation and options for selecting different compensation curves. In that case, we just need a high gain, high accuracy amplifier to bring the signal to the computer input.

Jumpers at the input added complexity to the circuit and prevent us from optimizing layout where it's most critical. The preamplifier still can be easily optimized by replacing resistors/capacitors at the input without detrimental effects on the extremely small signals coming out of phono cartridges.

Even though the schematic is pretty complete at this stage, it is safe to assume that some modifications will take place during bench testing and listening of the built prototypes. (NOTE: In fact, right after posting this I saw that R71/72 and R81/82 will have to change to much lower values and/or R91/92 will need to go to a higher value, otherwise in FLAT mode, RIAA will run in parallel to the bypass resistors.  :duh:   Ironing wrinkles in a design is normal part of the process, so I decided to leave the original SCH with this note, rather then correct it).

Before we move to printed circuit board (PCB) layout we need to select components (packages), create footprints for every part and associate it with the component's schematic symbol. Once that is done, we will export the so called Netlist, which is a file describing how are components interconnected, along with each component's reference designator (R1, C15, U3....) and footprint name. PCB layout software will then import the netlist, comb its libraries for available footprints (based on the name from the netlist) and pull everything together on the screen for layout to start.

In the next post I will have some interesting links with more reading materials. While we're busy preparing the schematic for export to layout software, you may read some interesting stuff and find more details about the circuitry described in this and previous sections.


Re: Designing a Phono Preamp - Part V: Schematics
« Reply #2 on: 10 May 2009, 01:51 pm »
I was reluctant to add jumper in line with op-amp input, but that is by far the most elegant solution. Now the first op-amp sees an identical load with and without the bypass, second op-amp has the same input signal (at 1kHz) in both cases, and layout is not affected at all, as the jumper fits fine between Gain 1 and Gain 2 stages. After redoing RIAA bypass, the schematic comes out to:

Now we can move on....