Concept
The circuit is mainly inspired by the riaa-amplifier "Supra 2.0" published from the electronic magazine Elector in edition June 2016. Here, the circuit and parts are very exclusively (four op-amps per channel from Linear Technology where each part costs 10 Euros) or hardly to find (Styroflex capacitors).
I redesigned the circuit with target lower costs: I used one (double) op-amp as preamplifier (set two op-amps in parallel can theoretically reduce the noise level by factor square of two) and one (double) op-amp for equalisation. I replaced the Styroflex capacitors (for filtering) by polypropylene types.
Hardware
Normally I'm using always NE5532A from Texas Instruments in audio circuits but always the same chip is also boring... Here I experimented with a NJM4580 from New Japan Radio Co.,Ltd.. According to the datasheets a NJM4580 has a little bit worse performance than a NE5532A (or it isn't so detailed specified). But the chip sounds good (very smoothly - perhaps a question of the produced distortions?) - so I will keep this part...
The power supply of the op-amp's has an additional filtering. With RV1 you compensate the input current (not the output offset), on pin 3 and 5 should be 0V DC-voltage after adjustment. The whole KiCad-project (including Gerber-files) can be found here. Remark: it was made under Linux with KiCad-version 4.0.7 using this additional libaries. The bill of material (bom) as semicolon separated text file contains some comments about part dimensioning / quality.
I measured the quality of the equalisation with different methods: the first channel with gain factor 1, the second channel with the normal gain factor 10. The gain factor has no influence on the equalisation but I can measure signals better if the gain isn't so high (or the input signals not so small). I would say, the circuit works quite well :o)
After placement the circuit into the metal shield housing I heard a lot of hum. I checked many things (grounding, errors in the circuit) but I didn't find a solution. My good old Pioneer A-117 amplifier doesn't produce any hum on the phono channel (and it was a very cheap device 20 years ago...). So I opened it and saw: Pioneer placed a 47nF capacitor directly from the cinch ground to the housing. I copied it (sorry) and the problem was solved...
License
Please keep in mind: all content on this page is licensed under the terms of the GPLv3.
falk.richter*at*yandex.com
Last update: December 20, 2019