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Old 23rd September 2010, 01:05 AM   #1
Rafael L is offline Rafael L  Brazil
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Default Output DAC

Hi

I have the player Philips CD610 and want to replace OPAMPS and put in common GND with the output of the transformer, but I know little about DAC. I wonder what the function of trasistors the output of opamp?

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Old 23rd September 2010, 06:10 PM   #2
guido is offline guido  Netherlands
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mute function
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Old 24th September 2010, 07:19 PM   #3
Mooly is offline Mooly  United Kingdom
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Output DAC
Don't understand the "transformer gnd" bit. What do you mean.

As Guido says, four transistors at the right are muting. Middle two are deemphasis switching.

Swapping the opamps would be worthwhile... and being very boring I'll still suggest OPA2604 although you may want to try the LM4562.
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Old 24th September 2010, 10:54 PM   #4
Rafael L is offline Rafael L  Brazil
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mute function
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As Guido says, four transistors at the right are muting. Middle two are deemphasis switching.
The circuit is confusing to me, the transistor on the left I think the are two filters, one is active by switching transistor.

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Middle two are deemphasis switching
Not understand

The 22k resistor (3675,3676) before the two transistors mute funcion, I seem to make a voltage divider, more leaves to very high output impedance.

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suggest OPA2604 although you may want to try the LM4562.
I was thinking of using the OPA2134, The opamp OPA2604 would be a better option?

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Don't understand the "transformer gnd" bit. What do you mean.
Is to put the preamp and CD player in even GND to avoid a ground loop, see figure.
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Old 25th September 2010, 07:38 AM   #5
Mooly is offline Mooly  United Kingdom
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Output DAC
The 22k resistors reference the output electroylitic and mute transistors to ground. Common practice to do that. The resistor is high enough to have no effect on the output at all as it's fed from the opamp output.

The mute transistors, two are used per channel to get a "deep" enough level of muting in conjunction with the 470 ohm resistors. These two 470 ohm in series define the output impedance of the player.

The deemphasis transistors are the two in front of the opamp that change the hf filter characteristics.

All the transistors and diodes at the top are used to provide the logic levels required to operate the mute and deemphasis circuits.

The OPA2604 is just a personal favourite I find works well
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