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Improving S/N ratio of Photodiodes in CD Players

Herbert

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Nov 26, 2018
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Dear Friends,
in many first gen. CD Players - but also a few later models - the RF-Amp for the photodiodes receiving the laser signal
was mounted directly on the laser head. This resulted in a very clean Eyepattern as the weak current of the photodiodes
did only travel a short path before being amplified. But in 80% of all players, the RF-Amp sits on the main PCB.
The current travels at least 3 inches, the Eyepattern is always noisier.
No problem, as long as a disc is within specs.
But it becomes a problem if the disc is a (Re)-Writable or heavily scratched,
making the Eyepattern even more noisier...
And especially in an old player, this noisy signal is the base for the analog(!) servo-circuitry.

I have a third gen. player with Sony´s CX2019 RF-Amp, a standard IC in many players
from the early to late eighties. The RF-Amp is on the main PCB.
I have maintained it well but the Eyepattern is noisy and I can crosscheck between several new laser heads.


I have the mechanical skills putting the RF-Amp on the laser assembly but it is too complicated,
as the RF-Amp-section includes trimmers. So Instead I would like to amplify the photodiode
signals before they reach the RF-Amp.

My question:
The very first thing an RF-Amp does is an I/V conversion - see attachment, a CX2019 RF-Amp from Sony.
The Inputs 5 and 6 receive the current of the four main diodes A-D (they are summed in pairs) and
Inputs 8 and 11 for the E-F Photo-Diodes tracking signals, typical for 3-beam lasers.

I assume theoretically I could amplify the diode signals that leave the laser using Opamps,
(let´s say by a factor of ten) then send them over the cables (with a high S/N ratio) to the main board,
then dampen the signals by the same ratio before they enter the RF-Amp.
But can this be done with current, not voltage?
And will I get a better S/N?

If so, there could be another side effect: With some light amplification, the laserpower could be reduced - the laserdiode lasts longer.

Ideas and help appreciated!
I attached the circuit of the CX20109 from two sources, Nakamichi (where the values of the internal feedback resistors are given)
and Sony, with a typical implementation.
All the best, Herbert
CX20109-RF-Amp-Internes Schaltbild-Nakamichi.png
CX20109-RF-Amp-Internes Schaltbild-Sony.png
 
Last edited:
OP
H

Herbert

Addicted to Fun and Learning
Joined
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Messages
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433
Hm - no one? In fact I do want to do the same as in the attached schematic,
taken from a first gen. Nakamichi OMS-5 / 7:
Q301 amplifies the diode current before ot is sent to U 101 and U 102 on the main pcb.
But in my case, instead of U 101 / 102 it will be a CX20109, doing an I/V conversion first.
RF-Preamp.JPG
 
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