Tag: LCR

LCR Phono: design notes (Part II)

Introduction

This is a continuation of my previous blog post. I will try to share my experience through the design process of this RIAA stage through these individual posts with an attempt to spark some interest in others and in return to get some valuable input from the experience and knowledge of others. Hope this works!

First stage

LCR-phono-test5
Here is the initial design version for analysis. I’m working through this step by step and refining the circuit in every iteration. The initial circuit is very simple. The first stage is key. We want to achieve as much amplification as possible from this stage before we hit the LCR network. The choice of the 6S17K-V valve may appear as a surprise to the ones not familiarised with this valve.  Here are some notes from Wavebourn around this valve:

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LCR Phono: design notes (Part I)

Introduction

A phono stage is probably one of the most challenging circuits to build in audio. Clearly not for beginners, many make the mistake in adventuring in building one. There are several designs which are simple, albeit many are poor ones. In addition to the challenges related to high-gain and very low noise design, audio enthusiasts really overlook the fact that you need to be able to measure and adjust the RIAA curve for a successful phono stage build. This means that you need an IRIAA signal source and also an accurate LCR bridge to adjust the network. I personally built a great IRIAA box and procured several LCR meters including this one as part of my learning journey of phono stages.

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Refurbishing an LCR bridge

Recently I acquired a second-hand and untested TF2700 LCR bridge from Marconi. This is a fantastic piece of engineering and a superb LCR bridge that can only be beaten by an expensive impedance analyser, something I can clearly not afford.

Matching Russian NOS capacitors
Matching Russian NOS capacitors

 

This bridge is perfect for selecting accurately capacitors for a RIAA network with extreme accuracy. In fact, the 1% precision can be improved down to 0.25% by replacing the main bridge 0.5% resistors with 0.1% ones as suggested by Morgan Jones in his “Building Valve Amplifiers” (Second Edition) page 330 and 331.

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