LCR Phono: design notes (Part III)

My previous design wasn’t good for two reasons:
  1. Input capacitance was too high due to Miller effect.
  2. Overall gain wasn’t enough: 55dB was marginal as 60dB would be ideal for an MC stage. Obviously this doesn’t apply to an MM cartridge where 40dB should be more than ok.

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RIAA Preamp Power Supply

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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JFET RIAA Preamp – battery supply

I built a new pack of 12V+6V lead-acid batteries to provide +/- 18V for my JFET RIAA MC phono stage. Despite the bad reputation of these being noisy, Geoff tried them with great success. The Haze brand are the recommended and the low capacity ones (i.e. 1.3Ah) are very quiet.

I decided to build and test it. As the proof is in the pudding!

Here is a simple test on my workbench of the 18V battery set with a 20mA load to simulate the RIAA stage consumption. The LC is formed by a 33mH choke in differential mode + 100uF/20V OSCON capacitor.

No twisted cable pair, just banana alligator clips. The noise level is really low and is obvious that 3.3μV 50Hz hum is picked from the workbench. I also listened to it for a while and can hear the difference. I monitor the FFT with no average and lower FFT size and also couldn’t see any spikes due to chemical reactions. They may happen in future though 🙂

18V lead-acid battery noise test

The performance of this battery pack is outstanding, so far so good!