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Category: Schematics Page 1 of 2

6SN7 Preamp With Parallel Sections + CCS

Here is my design of a 6SN7 (6N8S) based preamp with parallel sections of the tubes. I plan to use just two tubes, one for each channel. As we all know 6SN7 is a double triode and for my design I will tie the two sections in parallel. The main design parameters fall into three points – low-distortion, low-gain and low output impedance without using a output buffer/cathode follower. In order to achieve some of these goals I will be using an active load – constant current source(CCS).

Setting the bias point

First of all lets begin with the obvious – setting the tubes into the desired operation point. Lets consider a fairly typical power supply voltage of around 300V and a 30kohm plate resistor which results in a plate current of 10mA. From these three values it’s easy to draw the load line, so here’s what it looks like:

6SN7 Load line and biasing
6SN7 Load line and biasing

The LPRO-101 DIY Rubidium Clock Generator

The DIY LPRO-101 ruibidioum clock generator. This box together with some additional electronics on a separate PCB will transform this LPRO unit into stand alone equipment. I’ve always wanted a good and affordable frequency standard for my home lab. Searching the eBay one might find quite a lot of solutions. The good thing is that nowadays we have plenty of options.

Low-noise Power Supply

Here is a quick article describing a low-noise power supply for low current applications. It’s simple, and easy to build. All parts are widely available and cheap. I’m using this power supply to power-up my RIAA preamplifier. It’s more that capable of delivering excellent results. Build one and try it for yourself.

First of all lets see the basic functional diagram of the regulator.

Positive voltage regulator:

Error Amplifier Low-Noise


Here is my circuit adopting the famous LME49811 amplifier front-end and ThermalTrakTM

transistors. This one is a rather conventional topology but implementing some tricks to improve the performance.

RIAA Considerations

The following link is a great source of info about RIAA designs. A great collection of schematic topologies and measurements. A must read material. The site includes variety of industrial schematics. Some of them are pretty old but could be easily adapted and brought back to date using the modern component base.

Now this one is a real gem:

It’s a design by Linsley-Hood. This one could be easily optimized for even lower noise. What comes to mind is the Toshiba’s 2SK170 JFET. This will drop the input bias circuit and the need of input capacitor. Plus – more JFET’s can be paralleled to reduce the noise floor even further at no real penalty. The only problem is the hfe matching between the JFET’s.

Be sure to read the article here – >>>  LINK  <<<

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