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