An Idea for saving Laser Life - amplifying the current?

Thanks David! But according to the datasheet, the proposed amplifier circuit
(Fig. 70) seems rather complicated.
I only need to double the current from the photodiodes
as the emitting laser diode early reaches lower threshold
and therefor changes beam characteristics...

I have many old players: Found this "Head Amp" TA7331P in a second generation Toshiba XR-Z70.
I own five, one is not working.
The XR-Z70 was rebadged for many other brands:
Alpine, NAD, Kenwood, Luxman, Uher.

But I have no clue about the amplification ratio...
 

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O.k., found a description in a Kenwood DP-1100B.
Two outputs have a feedback resistor for adjustable gain.
The B and C inputs coming from the photodiodes have to be reversed.
Still, as a layman I do not know: The TA7731P does an I/V conversion
and outputs a voltage, correct?
But the CX20109 input is fed by a current. So will I get an amplification in current?

Next week I will try the mod proposed earlier by changing the feedback resistors around CX20109. If this does not work, I will try a mod using the headamp TA7731P put before the CX20109.
All the best,
Salar
 

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Simple answer is no, not directly. The I/V is really like an opamp configured as an inverting stage and with the current (from the photo diode) fed directly into the inverting input.

You could add a series resistor to make it a conventional inverting voltage stage but then you are into deteriorating the signal with noise... whether that would matter I don't know.
 
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Its not that simple...

If the photo diode delivers say 100uA and that is converted to 1 volt you can not then apply that voltage directly to another current input stage. You would need to convert that voltage to a current again which can be done by adding a series resistor in front of the next I/V stage. A 5k resistor would allow 200uA to flow into the next input stage (1V/5000) making that stage think it was seeing a 200uA photodiode current.

The big problems are noise (you are impairing the SNR) and also the real issue of DC level changes. The original circuit will not just need the wanted voltages/current, they also have to sit at the right static DC level.
 
It works.
At least for the feedback resistor of the internal RF-Amp
of CX20109. The resistor is between the pins 3&4.
Below the eyepatterns of an aged laser diode (LT022MC) in a
Sony BU-1C. Right before the mod, left after the mod.

Before the modification the amplitude of the eyepattern it was about 0.7V p-p
almost half of the the original value, due to aging.
The feedback resistor was 18k.

I increased the feedback resistor to 27k.
Back to Ballpark - 1.2V!
Probably the first photo ever published of an amplified eyepattern,
that was raised in level by not touching the APC circuit :D

So, this modification might help anyone who wants to extend the life
of a laser diodes life in a classical Sony CD-player using the CX20109 RF-Amp:
By lowering the current of the diode and raising the amplification of the CX20109.

The feedback resistors for the E-F opamps (RF104 / RF105) and focus error amp (R108 / R109) will be changed next.
Please note that the values of R108 / R 109 in post #18 I marked red are wrong.
They must be higher, not lower.
 

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Thanks. But I have to figure the cutoff frequencies and by this the values of
the paralelled resisitors. I don not know if raising the value of the capacitors
by the same amount as the resistors -i.e one third for R104: From 390k/18pF to 560k/27pF - is the right math.

Here is the corrected schematic. Only R102 has been changed and tested for now.
 

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I suspect those caps are just for stability rather than to give some pre determined response. Normally you would decrease a cap value as the feedback resistor was raised.

The E and F diodes feed directly into the chip as expected and so tbh I'm not certain what those resistors actually do. They seem far to high in value to be feedback resistors around an I/V stage and so I wonder if they are just to supply some static low level bias of some kind. Not sure on that. You may well have enough extra gain available with the tracking and focus gain presets.
 
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The RF amp isn't in the feedback loop for the laser. The laser current is determined solely by an onboard photodiode mounted on the same die as the laser. Its a self contained circuit and nothing in the player has any input over it apart from its supply voltage and laser on/off command.