Schematic for Pro-ject phono box

[Zombie]

schnulli, I made the mods for a friend. The sound in MC was more detailed etc with OPA604 but noisy. He put in the NJM and the sound was "softer" and less defined but no noise. The best result was with OPA2227 (singel ver - wrote 2228 earlier), good sound, no noise. It's even better in MM mode.
Cheers

[dimitri]

Stephan wrote > when investigating my Pro-Ject Phono Box,
it looks to me like C2 and C5 is pico-Farad instead of nano-Farad.

Dear Stephan,
I could not check right now, as I don't have the Phonobox at hand.
May be other readers could check?
But R2C2 will be 47*10e3*27*0e-12=1.27us that corresponds 125kHz, so the cartridge will not be loaded at all in audio band.

Stephan wrote > To me it looks that the purpose of the feedback servo loop is to create a "virtual" cartridge loading, hence the terminology "ACTI-DAMP circuit for automatic impedance adjusting" (badly tranclated from german product description found on the web).

If you have the original design by Sykora, mentioned by Upupa Epops could you share?
With R7=330k and C4=0.33uF it will be only integrator, otherwise C6 and R5 are in the wrong place.

Stephan wrote > Also, im am very intersted in Opinions about NJM4580.

Try to add 2k2 resistors between op-amp outputs and –15V rail

[schnullimaus]

Dimitri,

thanks for your reply.

Unfortunately, I do not have additional Information
nor the original design.

But what could be the purpose of having the
integrator in that place. Especially, it you reconnect
R9 as suggested, there will be no DC-Path to the next stage,
so DC offset removal is probably not the intention here.

DC-Removal would only make sense if R9 would be connected
to the output of NJM4580 in my opinion

What would we loose if we remove that TL061 and
all it's associated components at all ?

2K2-Resistors force the NJM4580 into Class-A, right?

Do you think a NE5532 would sound better ?

Thanks,
Stephan

[dimitri]

Stephan wrote > But what could be the purpose of having the integrator in that place. Especially, it you reconnect R9 as suggested, there will be no DC-Path to the next stage,
so DC offset removal is probably not the intention here.
I start to think, that the R6C5 network should be connected in series with C4, thus TL061 stage will have -1 gain at ac, so the input resistance will be R5/(1+R4/R3)=40kOhm.
Can you check R6C5 connection in your unit? In mine I checked it triple times.

Stephan wrote > DC-Removal would only make sense if R9 would be connected to the output of NJM4580 in my opinion
Why use R9 and C7 at all if R9 would be connected to the output?

Stephan wrote > What would we loose if we remove that TL061 and all it's associated components at all?
Nothing.

Stephan wrote > 2K2-Resistors force the NJM4580 into Class-A, right?
Yes

Stephan wrote > Do you think a NE5532 would sound better?
I wouldn't like to start a usual flame here but 4580 into class-A is more then enough

[Upupa Epops]

Guys, who will send me email, I will send him original schematic of Actidamp Mk II, not copy by Project. Questions ( if will be ) I can explain later.

[bobolix]

Hi guys,

the Phonobox / Actidamp II is quite old stuff. Nevertheless, there exists a new product, the CLEARAUDIO BASIC, which is made very similar way (from the same author), and this is assumed to be a flagship of the CLEARAUDIO phono preamp program.

Concerning the opamps: The very best is the AD745 for the MM, AD797 for the MC at the front end, but they are single ones in SOIC package - there is needed an adaptor. The absolute top among duals is Harris HA5222, but it is out of production yet. By measurement, the NE5532 has lowest distortion and very good noise, the JRC/NJM4580 is similar by noise and worse by distortion, but some say it sounds better. The OPA2227 is quite slow, but very good in respect of noise. The brand new AD8672 is very good, too, but it is produced only as SOIC, so again an adaptor is needed.

Concerning the ceramic caps - they introduce a lot of distortion, but some people like the resulting sound. Originally the polypropylene caps were used, the polycarbonates are even beter, but the good polyester work well, too, although they can be found some distorting pieces - probably due the weak winding of the foil.

And, finally, don't fear the bipolar elcaps - at the output voltage of the phono pickup they perform quite well.

By the way, you can follow the offer of Upupa Epops - take the original Actidamp II, although they are better things now, but not as an "open source"...

[schnullimaus]

Dimitry,

congratulations :

"I start to think, that the R6C5 network should be connected in series with C4" turned out to be true. Here is what I found in my
Phonobox:

R7 / 330 kohm sits directly between Output 4580, Pin 1
and negative Intput TL061 / Pin 2. C5||R6 is in Series with C4:

Pin 6 TL061 --C4 --(C5||R6)--Pin 1 TL061


Also, C5 and C2 are pico-Farad, not nanofarad. I unsoldered
and measured them to be sure.

What does that tell us ??

Regards,
Stephan

[Upupa Epops]

Little explanation : bobolix is Dr. Bohumil Sýkora, author of this preamp and certainly many others .

[zinsula]

Hey you Czech guys seem to be pretty up to the job in audio engineering! You know what you are telling. Keep up the good work.


Cheers
Tino

[dimitri]

Stephan wrote > What does that tell us ??

This is no dc servo, this is so called 'electronic cooling'.
From Marcel van de Gevel EW paper: "When the input stage and feedback network are properly designed, the thermal noise of the 47k resistor shunted across the input is usually the largest remaining noise contribution in the RIAA amplifier itself".
Such connection will lower noise current spectral density from 47k resistor 1+R4/R3 times, but overall s/n ratio will improved 1-3 dB