OPENAMP1 - MM phono preamp open project

[PMA]

I have all the measurements you speak about, I can post them tomorrow. I also have measurements with vinyl groove etc. The preamp is really quiet enough and when properly wired it is absolutely hum free. The measurement with cartridge has 0dB ref 2Vrms. Dashed line is noise level over 20Hz - 20kHz ref 2Vrms output.

[SY]

Thanks, I should have read better.

[PMA]

Originally Posted by SY

It might be nice to post spectra with the cartridge compared to shorted input- that way, you demonstrate that the preamp is not the limitation (assuming that's not the case!).

OK, I have prepared several comparison measurement. They are measured by a system that is created by SB X-Fi HD USB sound card and Virtins technology Multi-Instrument 3.2 software. This SW is able to calculate properly SNR and noise level over defined frequency range. The noise level is measured over 20Hz - 20kHz frequency range, unweighted, and can be seen as dashed lines in attached images. The noise level is related to 0dBFS, which is 2Vrms in all these measurements.

1st image is a noise plot of the sound card (measurement limit) vs. noise of the phono preamp terminated with 47k at the input, i.e. in fact the input connector of the preamp is left open.

2nd image is a noise plot of the preamp with input terminated with 47k (open input) vs. input terminated with 50 ohm.

3rd image is a noise plot of the preamp with real turnatable and cartridge Shure M35X connected to the input.

The results are:

- noise level limit of the method is -99 dBFs
- noise level of the preamp terminated with 50 ohm at the input is -96 dBFS
- noise level of the preamp with input connected to the real turntable cartridge is -90 dBFS
- noise level of the preamp with 47k at the input (open input) is -84 dBFS

[PMA]

But, the main contributor of the noise is NOT the preamplifier, but the vinyl groove itself. I have a plenty of measurements of noises of plain vinyl surface, "empty" ("silent") grooves, grooves "without signal" from measuring records etc. The real record would worsen the noise about 30 dB compared to the measurement of the preamp connected to the cartridge with needle "in the air". Please see one of the measurements of the vinyl groove "without signal". Especially the low frequency noise of the vinyl is quite horrible.

[PMA]

As we can read in the manual, the recommended opamp for this preamplifier is OPA627 (bought at reliable supplier like Farnell, please avoid cheap offers, fakes for sure).

Let's see what happens with noise level when we replace the OPA627 with OPA134, TL071 and AD744. But, how much important is this considering the vinyl groove noise? It is up to the builder, but I still recommend the OPA627. For those who would like to use something cheaper I would admit to use the OPA134, but nothing worse. All these measurements with input teminated with 50 ohm.

[Benedetto]

Hello Pavel,
Mouser, for example, got various types of OPA627 in stock.
Could you give some hints witch one to pick ?
627

[PMA]

I have also simulated the noise of the phono preamp with cartridge impedance (R + jwL) at the input. The concordance with measurements of the noise of the preamp with cartridge is stunning.

[PMA]

Originally Posted by Benedetto

Hello Pavel,
Mouser, for example, got various types of OPA627 in stock.
Could you give some hints witch one to pick ?
627

Normally, OPA627AP/BP and APG4/BPG4 should be OK. Let's hope they are genuine. I have built my preamps from the old stock of Burr-Brown OPA627AP genuine components, but unfortunately I have no more pieces.

[Benedetto]

Do you think Mouser is in doubt.... ?

[PMA]

I hope not. But I am not 100% sure, I recall something like a warning of one of my colleagues regarding this part. But I am really not sure and do not want to start any panic. This should be the list of TI official distributors in Germany

Semiconductor Distributors: Germany - TI.com

and Mouser is in the official list, so it should be OK. You might also try to ask at your German analog-forum.de, maybe Holger or someone would know?