101 lines
4.5 KiB
Markdown
101 lines
4.5 KiB
Markdown
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date = "2018-09-12"
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title = "Design: Phono X, a RIAA Phono Preamplifier"
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tags = ["design", "audio", "preamp", "phono"]
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categories = ["Audio Design"]
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summary = "Building a quality phono preamp is still one of the joys of the audio electronics enthusiast. This is my first take on a RIAA phono premap."
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aliases = [
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"/2018/09/12/phono-x-riaa-phono-preamp/"
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]
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+++
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When I started out designing this preamp I only had a few requirements:
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* Perform RIAA equalisation and pre amplification for a Moving Magnet (MM) cartridge
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* Connect my [Technics SL-3310](https://www.vinylengine.com/library/technics/sl-3310.shtml)
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to my [Denon AVR-2113](https://www.denon.co.uk/uk/product/homecinema/avreceiver/avr2113).
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* Sound decent (e.g. no 50 Hz hum, etc.)
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* Directly powered from 230VAC
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Searching the internet yielded quite a few pre-designed phono preamp stages. The main
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issue I had with most of them is the sheer amount of customization put into each design.
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Take the [Muffsy](https://www.muffsy.com/schematics.html) for example. The Muffsy tries to
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accomodate many different cartridges by making input impedance and gain configurable. This
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is great, but I just have _one_ turn table. I might get another one in the future, but I'm
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pretty sure it's going to be mainstream moving magnet again.
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I also found [Elliot Sound Products' Project 06](http://sound.whsites.net/project06.htm
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), a Hi-Fi Phono Preamp with RIAA equalisation and good reviews.
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With all the different designs in the back of my mind, I started out designing Phono X.
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## The power supply
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The power supply is normally the last thing I design. When everything else is finished, I have
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a clear picture of power supply needs. Because the main components are a pair of [OPA2134] opamps,
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I want to have a clean ±15VDC power supply. The OPA2134 can be supplied with up to ±18VDC, but
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±15VDC seems more than adequate for the job. ±15VDC is also what Elliot recommends in his design.
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So, to get a clean supply voltage, I opted for an almost classic linear power supply, using a
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20VA 2x 15VAC toroidal transfomer.
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`AC1` and `AC2` are connected to the transformer. The `D1-D4` diode bridge rectifies the AC
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into DC which gets filtered by the capacitors `C1-C4`. Next is a +/- pair of an LM317 and LM337
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to regulate the voltage to ±15VDC, including protection diodes.
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## Pre-amp circuit
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Below is left channel preamp circuit, the right channel is identical. Most notable detail is
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the OPA2134, which is a dual channel opamp. One unit is used for L+R RIAA equalisation and
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another unit is used for amplification.
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This is almost an exact copy of Elliot Sound Products' Project 06 design with a few small
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tweaks.
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* I omitted the `CLL` and `CRR` cartridge loading capactiors.
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* For `C19` and `R22` I used 100nF and 750Ω instead of the specified 82nF and 820Ω, as per ESP's instructions.
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* The 10uF and 100nF by pass capacitors for the opamps are omitted in favour of `C15-C17` in
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the power supply.
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## PCB Design
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This is the final PCB design. In retrospect I should have better divided the available
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board space between different functions. Power supply and preamp are a bit mixed up. Also,
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the signal traces for the right channel are tad longer than left. I'm not sure this results
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in an audible difference, but it still doesn't feel right.
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The width of the board was chosen to fit in the pcb brackets of the enclosure.
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## Retrospective
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So, what did I learn doing this build?
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1. There's (presumably) a considerable inrush current into the big `C1-C4` capacitors. When
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the amp is already powered up, switching on the Phono X will cause a not-so-nice popping sound. I should
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probably add some soft
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1. When powering off, the capacitors keep the amp working for quite a while. It might be nicer to
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really shut down the output when turning off.
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1. Transformers affect signals. Keep them away from signal traces or maybe use some sort
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of shielding.
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1. Add the recommended [Project 99 Subsonic Filter](http://sound.whsites.net/project99.htm) to prevent sub 20Hz noise coming through.
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1. Get the right equipment to measure performance of my build myself ;-)
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1. Use a more symetrical design for the PCB, and maybe split the power supply and signal boards into two separate boards.
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## Datasheets
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* [OPA2134] High Performance Audio Operational Amplifier
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[OPA2134]: http://www.ti.com/lit/ds/symlink/opa2134.pdf
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