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Category: RIAA Page 1 of 2

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

Thorens Power Supply Upgrade TD-150, TD-160

pcbThis is a very neat and nice PCB upgrade for Thorens turntables. It’s possible to use it with almost every other turntable that includes 16 to 24 pole synchronous motor.

The idea was to have a small PCB which includes both protection and power-off function. The PCB itself is a one-sided board with black solder mask on both top and bottom layers. I’ve decided to include optional drills for other capacitor sizes. This will make it possible to “adjust” the board for other Thorens models of even other turntable makes like Linn or Ariston.

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  <<<

Multisim Inverse RIAA Function

For those of you who want to simulate the accuracy of the calculated (and corrected) RIAA equalization network. Multisim has a transfer function that can be used to simulate the signal coming from the turntable. The picture shows the exact settings and connections for the function.

ECC85 RIAA Preamplifier

This idea came to me when a friend from the forum asked for a simple schematic based on the ECC85 tube.
He had plenty of these around and wanted to try them out. So I sat and and made some calculations. Since we had a 300V B+ supply I had to make calculations for this supply voltage. The preamp itself is a pure classic. Two stages separated by the RIAA EQ network. The RIAA parts were calculated using this tool:

http://www.kabusa.com/riaa.htm

I have used it many times. It’s always spot on. Of course when calculating RIAA equalization some parts of the schematic must be taken into account. Since the output impedance of the common cathode stage is pretty high it can not be neglected. The output impedance is effectively in series with R1(from the link), altering it’s value. This of course leads to an incorrect RIAA equalization. The other part altering R1 value is the next stage input resistor. This must be taken into account too.

Of course before calculating the RIAA equalization network I needed to set the operating conditions of the two gain stages. Those stages are identical. Both sections are biased at 175V anode voltage, -2V grid, 6mA anode current. This setup gives about 28 of voltage gain using a 20K anode resistor. Here is the bias point:

The total gain of the preamplifier is about 37.9dB. This will give you around 400mV output from a cartridge giving around 5mV of output.

The complete schematic is available for download HERE.

Here is the predicted nonlinearity of the RIAA equalization for this project (+/-0.3dB):

update:

Here are some pictures from a complete project. This one was built by a fellow mate Stoyan Tsonev.

 
THANK YOU FOR READING THIS ARTICLE

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