LCR Phono: design notes (Part II)

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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

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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)

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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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RIAA Preamp Power Supply

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Introduction

I ran my JFET folded-cascode RIAA preamp for more than a year with batteries. Charging the batteries has been painful enough for me to decide to look for alternative supplies

A DC supply for an MC stage is not an easy task to accomplish. My battery pack ran out to a point that frequent recharging periods became a real nuisance, so decided to build a DC supply.

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The new power supply (far left), the Lenco turntable (right) and the RIAA JFET stage (middle)

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Situbes digital panel meter review

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I’m a heavy user of fixed-bias output stages. Yes, I do prefer them despite the additional complexity. However, I’m not looking to open a can of worms around this subject. On the contrary, I wanted to report a fantastic product developed by Situbes.

SiDPM Digital “Panel” Meter

Here is the brief description from the website. I suggest you take a look at the datasheet as well:

The SiTubes DPM is a digital “panel” meter packaged in a standard octal tube envelope.

It measures a DC input voltage from 0 to 2V full-scale.  Several selectable legends (V, mA, A, etc.) can be selected by an external programming resistor.  It is particularly useful to measure supply voltages (such as plate voltage) and tube bias levels (normally plate current) in tube amplifiers.

The DPM can be powered from an AC or DC voltage.  The power input is isolated from the measurement input, so floating measurements can be made up to 1500V above or below the power input.  This allows high-side current measurements – for example, sensing plate current directly at the tube plate –  or the ability to be powered from a supply that is not referred to ground.

 

My review

Looking at their construction you will realise instantly the high-quality of this product. Impressive finish and presentation. The tube is made of glass and fits very tightly to the 8-pin plastic base.

I did a simple and basic test on my work bench to test this device and its accuracy. In 5 minutes I wired it on my curve tracer to access the pins easily without soldering a test rig. With a 1Ω 1% resistor I configured the device to current mode and placed my 5½ digit bench meter in series for reference. The refresh cycle is very good, more than what you’d need in normal operation. Accuracy with the reference resistor was great. It’s calibrated as provided by the seller and error was below 1mA up about 200mA which is the planned use case for me. The OLED display has the right brightness for day operation. It’s just great.

You can use them easily to measure anode/cathode current, grid bias or supply voltages in multiple configurations.

They are pricey, but worth every penny. A top quality product which I’m keen to use shortly in one of my next builds.