Dansette Major HT?

[paulsherwin]

Absolutely, the TL071 has an FET input so is capable of being configured to present a very high impedance. Quite why the designers have gone for only 10k is a mystery, even for use with modern transistor kit. A higher input impedance would work just as well and be more versatile.

[OldTechFan96]

I added a 1M resistor in between the cartridge and the preamp and it made things quieter.

Back to the drawing board. I'll see what I can do to get more gain out of the preamp.

[paulsherwin]

A 1M resistor into a 10k input impedance is going to cause massive attenuation. You need to modify the resistors setting the TL071 voltages to raise the input impedance. There will be lots of circuits on the net or at the end of the TL071 datasheet.

Have you tried the cartridge without any preamp?

[OldTechFan96]

Find attached a photograph of the preamp board.

Would I be right to suggest that changing R5 from 10K to 1M would set the input impedance of the circuit? I get the impression that R6 (1k) is used to set the gain of the op-amp.

I have not tried the cartridge on its own without the preamp.

This is one of those situations when a test tone record would be handy to give objective output numbers!

[Herald1360]

An actual circuit for the preamp would help a lot! The pictures are a bit too blurry to make out resistor values. A mirrored track side shot alongside a clear shot of the component side would enable some remote reverse engineering.

[OldTechFan96]

Thanks for the help so far.

I was doing some reading about opamps and came to the conclusion that R6 was used to set the gain of the circuit.

With an input impedance of 1M, R6 was changed to 10M for a gain of 10.

There is now bags of volume with the gain adjust potentiometer set half way. The sound is loud and without distortion.

I'll probably put the original Siemens rectifier back in and retest things. I'm happy to conclude that this is a circuit that runs off a low HT level.

[Michael Maurice]

I've been rather lazy of late, I was thinking of designing a replacement module using either an ECL82 or ECL86 using the original O/P and mains transformer but using an 1N4007 or 1N5408 rectifier on the new board.

I really should try and make it using Kicad. What would be best the ECL82 or the ECL86?

[John10b]

I have modified a number of amps over the years and always use ECL82.
John

[Edward Huggins]

The ECL86 should be a better match to the existing EL84 OPTX primary.

[John10b]

ECL82 is readily available at a reasonable price, I have on a number of occasions replaced the O/P Transformer.
John