An Idea for saving Laser Life - amplifying the current?

Gentlemen - no answers?
The circuit above could bear an universal solution for first to second gen. CD-Players.
Everyone used the Sharp LT022 Laser diode, Sony, Philips, Olympus.
Therefore, I assume that the photodiodes had similar values and
that one can transfer an eventual solution of photodiode amplification
to other players relatively easily.
So:
-I assume, the capacitors C301A-D correct DC-offset?
- Only current is amplified?

All the best,
Salar
 
A friend explained :
"This is an AC (RF) voltage amplifier, inverting, and the I/V happens in the 470R at the input.
The output voltage is relatively low impedance and could easily be transmitted a few cm.
Then you need an inverting V/I with inverse transfer factor, and which behaves as if it were the reverse current of a photodiode (the Nakamichi schematic is wrong in this respect, the diodes are drawn with the wrong forward direction). This would somehow work with a PNP transistor current source, AC coupled at the input.But it needs a proper DC bias because of that, and that's where the problem will lie. In other words, if the average DC output current of this equivalent circuit is not the same (in magnitude) as that of the photodiode, the whole thing probably won't work even if you get the AC current modulation replicated fairly accurately.
Maybe you can get something like that with another circuit, best fully DC-coupled (discrete or with fast OpAmps)."
Mooly also hinted to the same problem in post #32

Any ideas how this circuit could be achieved?
Thanks, Salar
 
I'm said friend in above post, and now have checked back with an expert on transimpedance photodiode amps, laser diodes and also well-versed in CD-player repair.

Bottom line: You can try, but it won't solve your problems and he also said you diagnosis is likely not catching the issue (it there is any, that is).

Now for the circuit, it is a DC coupled I/V opamp stage, a standard inverting transimpedance opamp stage and it is assumed you can dimension it to be stable and properly select the feedback R (R_f) for best signal (large, but enough headroom to avoid clipping). Bipolar supply is assumed and reference at GND, thus output will be negative. Transfer that over your cable (with RF impedance termination if deemed necessary, which introduces a factor of 0.5x) and amplify it with opamp inverter stage of gain 1 (or 2 in case of terminated line). Output voltage is now positive and can be converted into the original current by using the same R_f to inject that current into the CX20109, assuming that the operating point of the CX20109 inputs is also close to 0V. With some added elements you may be able to have some amplification factor on the AC part of the signal while keeping the DC operation point, that is you can increase the gain for AC only and or inject an offset current into the summing node of the inverter. Same goes for any operating point issues (if the operating point is not near 0V). If you are able to design, build and debug that circuit (with all additional fine print not mentioned, power-up behavior and such) from above description then go ahead, otherwise don't try to wast your time.

The main thing he said (as for diagnosis), have you looked at the test points for RF amp output (before and after the comparator) and also Focus and Tracking Error outputs? If the RF-output test point (TP102) is clean and if you don't see glitches after the comparator (EFM output, Pin20) for good CD's, in a dual trace display, you will not improve stuff significantly. With bad CDs you will still have the same error patterns because those error patterns do not mainly come from the pickup signals being noisy, but rather from the signal source being systematically corrupted. Making the corrupted signals "louder" does not help.

As for general idea of saving laser diode life, the diode is already biased to optimum power level by the circuit designers (and laser diodes always have a feedback loop with a photodiode to afford the constant laser power). Lowering laser power will quickly increase noise (of course also in any gained-up receiver circuit) and below a certain level the diode simply stops lasering alltogether. Higher power will just increase wear.
 
Hi KSTR,
many thanks for the reply! Some things to put into perspective:
The player IS in ballpark - for years. I did even replace the coarse original trimmers
with fine trimmers to align the unit more precisely, this happened maybe 10 years ago.
But it is an average tracker. The best trackers, according scratches are some Philips CDM-1
based units from 1983 and a Sony from 1996. Though tracking principles are differnt,
both have their opamps close or on the laser unit.
Also a CDP-101 of mine has a very clean exepattern. RF-Am on the laser.

The playerI like to mod just has the design flaw 80% of all players on the market have-
the RF-Amp is not on the laser, the weak current travels about 15cm through flexible cable.
I once tried to shield the cabling- atlhough the shielding was not tight,
it already damped the current too much, the player did not start.

I assume a lot of unnecessary noise is inserted by the cabling- and in older players
like the one I want to mod, the E-F signal geoes direct into the servo.
You are correct, making the E-F signal "louder" does not channge a thing.
But making it louder allows to run the diode with less power.
See post 65 in this thread:

https://www.diyaudio.com/community/...fe-amplifying-the-current.329551/post-5604526
Not much noise and the lasing power could be reduced.

So it is NOT about repairing, just getting more out of it.
That it can track bad CDs better than already out of the box.

But my knowledge is not sufficient in designing this pre-amp.
There is also no time, as I have to research topics
besides my regular work for later pitches.

Can't photocouplers be used after preamplifucation for damping and keeping
DC operation point before CX20109 - are they too noisy?

Many thanks, take care,
Salar
 
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As lasers age, the dark defects increase, so the threshold current will go up. I am not aware of any process that could damage the diode by running it more gently
We have easy to use high speed and cheap SMD opamps that would make very good photodiode amplifiers compared with what was available back in the day