Low Noise Design Schematics

Posted on Feb 4, 2014

I liked the idea of distributed active RIAA correction and also the concept of using negative feedback to linearize the input stage. Now, the original Collin design has a few significant flaws (openly admitted by the author in the same article), one of them being the very poor PSRR in the input stage. To avoid hum and other noise artifacts to enter the signal chain, this design calls for

Low Noise Design Schematics
Click here to download the full size of the above Circuit.

a solid power supply, much better than the original. So I decided to add a Jung regulator on the same board. The plan was to feed HPS 1. 0 from a 24V wall wart, so some extra circuitry is required to split the power supply. I decided for a TLE2426 rail splitter. This little circuit works great and it`s ideal for balanced rails current; any imbalance flows straight through the virtual ground created by this circuit, so I would advise to be careful in your applications. The HPS 1. 0 currents are already imbalanced by the input stage JFETs, dragging current from the positive supply only. The TLE2426 gets a little hot, but nothing to worry about. However, further extending the same concept to more paralleled JFETs may create some serious power dissipation issues. There`s not much to tell about construction; just use good quality components (polypropylene caps, metal film resistors) and low noise opamps (AD797 or LT1115, LT1128, LME49710, OPA211 on a SMD adaptor) and the good results are guaranteed. The relays are DIP case 12V models. It is a good idea to mute the preamps before powering up/down or switching the gain to avoid a large thump into the speakers. For this build, the dynamic range (aka the headroom) is around 22dB at 1KHz, for the max gain of 64dB and +/-17V supplies. This is not bad, but has a certain audible as you`ll find later on this page. As you will find in the Measurement sections, the equivalent input noise...

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