Simplistic NJFET RIAA

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Confused with the different sonic results from bypassing riaa caps in my three riaa, I decided to buy a decent LCR.

I got a TENMA 72-8150 that states an average precision of 0.5% in capacitance reading.

After that I desoldered the 47n caps in the 48dB unit and measured 47.6 R 47.1L.

No wonder the sound lost magic when I added 680p //

In this design we should not overshoot 47n :)

Congratulations for the new LCR, could you post a picture? How much cost it?Now I will ask you to measure mine caps:)

Thks for advice not overshoot RIAA caps.
 
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Today I dismantled the 57dB machine, desoldered one leg of the 47n cap, measured 46.5 and replaced the // 680p by a //560p.

This happened in both chanels..... The difference is amazing.... Now it really sings (and it uses only one shunt) ... maybe a second shunt is redundant but I will try it anyway :)
 
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Now I completely removed hum !!!

It is incredible.... using an inboard r core and beeing carefull about GND I got even lower noise with the 57dB than on the 48dB I built before with external double psu´s.

After studying the schematic for a while, I decided to lift the psu GND and now there is no hum.

Maybe I will need to rethink this when I add the second shunt but for now I have a "floating GND" psu.

:cool:
 

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  • 57dB GND setup 1 shunt floating GND.pdf
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Two shunts are definitively worth.
Even with only one tx and one rectifying bridge followed by two smoothers the difference for 1 shunt is producing much better separation between channels... The image grows laterally and the instruments are better focused.

Care must be taken with GND. To avoid hum at maximum, star earth linked to mains earth and case is connected to both shunts (-).

Do not connect the rectifier bridge directly to GND as this intruduces more hum.

Ricardo
 
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I'd recommend a shunt for each stage... and for grounding, use star ground to the negative rail of each shunt.

Hi João

You are correct... I had trouble earthing the (-) of the rectifier bridge and opted to use floating earth... (it works in this case) but after some studying I decided to ground the (-) of both shunts and that produced the best results.

Did you build one of these ?

Ricardo
 
Hi João

You are correct... I had trouble earthing the (-) of the rectifier bridge and opted to use floating earth... (it works in this case) but after some studying I decided to ground the (-) of both shunts and that produced the best results.

Did you build one of these ?

Ricardo

No I didn`t build one yet. My expericence is from my tube amps and as well what my old professors - one of them was a designer of high frequency amps & tuners - told me from the experience of her works.

I've got my transistors and will nuild a version from Salas phono amp with a 600 ohm LCR-RIAA. It will be interesting because I'm building up my new tube phono stage with LCR-RIAA as well!