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What's in the Creek CD-60 i/v section?
What's in the Creek CD-60 i/v section?
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Old 16th April 2018, 10:08 PM   #1
hollowman is offline hollowman  United States
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Default What's in the Creek CD-60 i/v section?

The unit in question is a Creek CD-60 compact disc player from the early 1990s (designed in 1991). It uses the classic Philips chipset (SAA7220/TDA1541).

In the following schematic excerpt from the CD-60's I/V section, what's Creek doing with that LM78L05 tied into the OPA2604 inverting input pin (pin 6)?

Click the image to open in full size.

Methinks maybe some sort of biasing (as often the case with discrete I/V's) ... but never seen an opamp biased this way?
Anyone know?
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Old 17th April 2018, 02:14 AM   #2
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First, remember that the 1541 goes from 0 to -4mA, current sink.

That 7805 regulator and associated resistors (and LPF) injects a fairly clean 2mA current to null the TDA1541 midscale current (-2mA). 5V / 2.4K = 2.08mA

This way the opamp operates in a better condition, that is, with 0V output when the dac is at midscale.
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Old 17th April 2018, 04:23 AM   #3
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Default Hmm....

Quote:
Originally Posted by Alexandre View Post
First, remember that the 1541 goes from 0 to -4mA, current sink.

That 7805 regulator and associated resistors (and LPF) injects a fairly clean 2mA current to null the TDA1541 midscale current (-2mA). 5V / 2.4K = 2.08mA

This way the opamp operates in a better condition, that is, with 0V output when the dac is at midscale.
How much (if at all) should one tweak that 7805 injection current if the stock opamp is replaced with another model.
In other words, did Creek design an ad hoc current injector specifically for the OPA2604?
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Old 17th April 2018, 06:15 AM   #4
Alexandre is offline Alexandre
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Quote:
Originally Posted by hollowman View Post
In other words, did Creek design an ad hoc current injector specifically for the OPA2604?
Nope. That should do the job for any opamp. You could change the first resistor to a trimpot, say 1K5. Then you can trim for 0V at the output of the opamp.

You could also make sure the caps are good quality and effective at high frequencies. The regulator has poor HF line rejection.
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Old 17th April 2018, 06:32 AM   #5
Alexandre is offline Alexandre
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Quote:
Originally Posted by Alexandre View Post
That should do the job for any opamp.
Well, sorta...

It depends on the input impedance of the opamp in inverting mode. It should be very low, but more importantly, it should remain low above audio frequencies. It would be desirable to have it flat into the MHz. That will likely be the case with modern high speed opamps such as the THS4031.

Last edited by Alexandre; 17th April 2018 at 06:35 AM.
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Old 17th April 2018, 07:22 AM   #6
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Hard to understand why they created a specially regulated 5V supply for this purpose. They wanted to inject a current, so it must follow that the higher the source impedance, the better. There will already be regulated rails for the opamps (probably 15V) so why not run direct from them? From 15V that would make a 3X higher source impedance source.
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Old 17th April 2018, 03:19 PM   #7
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Setting aside the opamp approach to I/V, I'm curious ... how many of the discrete I/V projects for the TDA1541 (posted here on DIYA) use the "current injection" scheme to "balance out" the analog output stage?
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Old 17th April 2018, 03:24 PM   #8
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From an old Stereophile review of the CD60, I read that its PCB has footprints for using large HQ film caps at the output. But Creek ended up using electros.
Not sure that decision was related to their current-injection strategy?
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Old 18th April 2018, 12:34 AM   #9
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Originally Posted by abraxalito View Post
Hard to understand why they created a specially regulated 5V supply for this purpose. They wanted to inject a current, so it must follow that the higher the source impedance, the better. There will already be regulated rails for the opamps (probably 15V) so why not run direct from them? From 15V that would make a 3X higher source impedance source.
May I ask why higher source impedance is better here? And, why a 15V rail will provide 3X hiehr source impedance? Thanks

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Old 18th April 2018, 12:40 AM   #10
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The ideal current source has infinite output impedance. This means the current does not vary with output voltage. Just as an ideal voltage source has zero output impedance, meaning no variation in voltage with output current.

If the output voltage doesn't vary then this won't really matter, but here the output voltage will change due to the extreme rise/fall times coming out of the DAC. The opamp's virtual ground cannot keep up so while the opamp is slewing there will be a non-zero voltage at its -ve input. Hence with a varying output voltage there won't be a constant current through the biassing R (R17 here). A higher source impedance means there's less variation of current with output voltage.

The R here is 2k4 but if we ran from 15V we could use (say) 7k5 to get the same (2mA) current. That's around 3X higher source impedance.
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