PhonoClone

Good things need improvement

Part 1: Intro and Design

Last modified: 19-Jul-2003

Introduction

After finishing the GainPre project I immediately decided to work on a follow-on model. I'd like to think of it as an improved version, but for the moment follow-on is a better description and time will tell if I'm able to improve this one over the Gainpre.

The GainPre is an MC phono equaliser with a gain of 75dB and a fixed input impedance of 1 kOhm. But most people need a more flexible approach and connect their turntable for example with MM cartridge or optimise the imput impedance for a specific MC cartridge. And apart from that, I had the impression that battery power could be the solution for noise and humm problems while providing the best type of power for OpAmp circuits.

Design Criteria

These are the challenges I gave to myself for this project.

Suitable for both MM and MC cartridges (gain 75 for MC and 55? for MM)
User Selectable Input impedance
No exotic components (after all, the OpAmps are just chips, not?)
Use the same power supply as the GainPre
And be able to use Battery Power too (2* 9V block battery)
The AD797 OpAmp needs competition

Design

Based on my goals and design criteria I decided to modify the GainPre design as follows:

Support for MM cartridges can be easily made by changing the value of resistor R0 from 50 Ohms to about 5 kOhm. The amplifier consists of two stages and the first stage was designed for 31dB. For MM support the total gain needed to be 55 instead of 75dB and therefore the first stage needed 20 dB less gain. This means 10 times less amplification of the first opAmp and the easieast way to get this is by using a value of R0 which is ten times higher. However, I'm not sure this is the way to go since the OpAmp will be configured for a gain of 5, and it may not be stable with these gain settings.
MM cartridges need high impedance and therefore the Ri value needed to be replaced by a 47 kOhm one.
For the MC playback I wanted at least three resistor values for impedance setting (and by combining these three I would get even more). The values 100, 220 and 1000 Ohms are OK
A resistor of 1M parallel to C0 may be used to get a ground reference on pin 2 of the input.

I also wanted to change a few components:

In the input, the AD797 probably will be replaced by another OpAmp with less drift and that is easier to manage. In the output stage I will decide based on experience. As I used the OPA637 in the GainPre project already I plan to order a few LT1037 in order to test this OpAmp as well.
The design will be completely dual-mono, (including power rails etc.) and can be upgraded with separate power supplies per channel at a later stage.
All OpAmps have a 4.7uF and 0.1 uF Tantalum cap close to pin 4 and pin7, but also two 2200 uF caps per chip.
The audio cap between first and second stage of the amp is a Audyn cap.

Schematic

Starting point is the same schematic as used for the GainPre and improve from there. Therefore, the schematic found below will not be the final design for the phonoclone, but it's for the moment a placeholder and an object for further study.

I have reduced the value for Rx since I do not think that I need too much treble (2nd pole). Not shown in the figure are the V- of -15Volts on pin 4 and the V+ of +15V on pin 7. In order to have clean power close to the pins 4 and 7 I have put two tantalum caps parallel from these pins to ground with value 4.7uF and 0.1 uF (25V types will do just fine).

The value of C0 is such that the lowest frequency roll-off (-3dB) is around 5 Hz. Actually I used a larger value than necessary and about 100uF is enough for most applications.

Impedance

One of the most important enhancements in this design is the possibility to adjust the input impedance according to the specific cartridge impedance.

Most MM cartridges like an input load of around 47kOhms and 120pF capacitance. Most MC cartridges have a very low output impedance and need a load which is between 100-1000 Ohms in order to perform best.

Gain

By changing the value of R0 we are able to control the gain of the OpAmp. The gain is normally given by the following formula: 1+ (R1+R2)/R0. However, we will introduce a small error when changing R0 only because unlike passive designs we have C1 in the feedback loop and we'll have to lower the value of R2 as well if we change R0.

For the moment I do not worry too much about this error as it will filter the lower frequencies a little too hard (< 200 Hz) and this could be acceptable for MM cartridges if not of higher quality.

Of-course it is possible to use a passive filter design for the phono pre, but I'll need to work this one out in probably a separate project.

How to design the RIAA filter?

The design of the RIAA filter is too complicated to explain on this page. please refer to the RIAA background page for Opamps to read why I took certain design decisions for the RIAA filter on the this an dthe next page.

RIAA page 3: General background on active filtering and feedback, including formulas for computing your own resistor/capacitor values.
RIAA page 5: Specific background on PhonoClone RIAA filter design, including a step-by-step design process for your own Hybrid phono amp.

Please provide me with feedback regarding these pages so I can improve the background library (and myself)

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