Planning a jfet input phono preamp stage

[Hari Iyer]

My current phono pre-amp uses op-amp for the RIAA stage and is a very old-fashioned designed one which compress lows and mid-bass to much and has too screechy highs.

I recently used a cambridge audio azhur 651A phono stage and it was sounding quite different and good. But again its very very flat and does not have much life as per me. I would say its very neutral.

This is tempting me to built one jfet input stage + op-amp phono stage which hopefully would be able to remove the issues in my current phono-stage. The phono-stage is a very important part of the vinyl audio and any mess-up in that will ruin the entire sound stage of a turntable.

Keeping my fingers cross as of now.

Cheers,

 

[omishra]

Use good opamp like AD823 or two LM49990 which has inbuilt JFETs at input stage.

 

[jls001]

@Hari: planning MM or MC phono preamp?

 

[Hari Iyer]

@ omishra - its a full discrete design and no Op-amp will be used.

@jsl
MM & MC both is configurable with internal 4pole dip switch.

If you need more pcb's let me know and i can get it built for you too.

 

[jls001]

I'm interested

Let's build it. One can't have too many phono stages

 

[Hari Iyer]

I will ask the pcb fabricator to make a total of 10 boards for the time being. Will update you after that.
Currently am travelling and should take another 10 days to get it in my hand.

Cheers,

 

[Hari Iyer]

Done some mods in my Sony opamp based RIAA ckt - replaced by pass caps from electrolytic to MKP and found significant improvement in coloured highs with smoother highs.

Now this week end will be adding my jfet input stage which has a constant current source.

Will post a ckt if that works ok.

 

[Hari Iyer]

The phono pre-amp has a DC blocking capacitor 6.8uF MKP type. I am contemplating of by-passing this. The best coupling is to have no coupling capacitors. Will have to measure for DC at the output before i connect them to my amp.

 

[IndianEars]

A circuit diagram would be nice to study

 

[Hari Iyer]

A circuit diagram would be nice to study

Am uploading. But there are some modifications to this too, but in principle its the same. Its borrowed from LSK489 datasheet application notes.

I am not using the 100uf + 0.1uF bypass capacitor anymore. A single 22nf capacitor on both the +ve and -ve rail is a replacement. No input capacitors, no DC blocking capacitors either. Output resistors reduced to 100 ohms and some by-pass added to RIAA filter cap 100uF, NP.

 

[IndianEars]

Am uploading. But there are some modifications to this too, but in principle its the same. Its borrowed from LSK489 datasheet application notes.

I am not using the 100uf + 0.1uF bypass capacitor anymore. A single 22nf capacitor on both the +ve and -ve rail is a replacement. No input capacitors, no DC blocking capacitors either. Output resistors reduced to 100 ohms and some by-pass added to RIAA filter cap 100uF, NP.

Thanks for sharing the Circuit diagram, Hari. My 2 cents :

Appears to be an "old school" approach with TONS & TONS of feedback, which often takes away the "Jump Factor" from the music.

A Single (The second) op amp does all the work, from Providing the gain, to RIAA Eq (which includes another 20 dB of LF gain required by RIAA) and the output is throttled via a 100 Ohm series resistor, to look after the Op Amp.....

The circuit seems to be a complete anti-thesis of your Great efforts on the (Valve) Power Amp side.

Would be interesting if you constructed a Valve Phono preamp which by its very intrinsic nature will be a low feedback design

 

[Hari Iyer]

Thanks for sharing the Circuit diagram, Hari. My 2 cents :

Appears to be an "old school" approach with TONS & TONS of feedback, which often takes away the "Jump Factor" from the music.

A Single (The second) op amp does all the work, from Providing the gain, to RIAA Eq (which includes another 20 dB of LF gain required by RIAA) and the output is throttled via a 100 Ohm series resistor, to look after the Op Amp.....

The circuit seems to be a complete anti-thesis of your Great efforts on the (Valve) Power Amp side.

Would be interesting if you constructed a Valve Phono preamp which by its very intrinsic nature will be a low feedback design

Ok. Don't have space on my rack for a tube phono preamplifier.