Salas DCG3 preamp (line & headphone)

[Tea-Bag]

Why was the AD823 chosen in the first place ?

We used OPA2604 in early tests, but the AD823 was changed by Salas, better operating values or sound. He would have to fill that out.

 

[vgeorge]

I think Salas listened to many ICs and decided based on sound. But it is also very subjective. You could try more types and dcide what you like more.

 

[Salas]

Why was the AD823 chosen in the first place ?

Among many I tested which could do the servo job alright this one I thought I could prefer a little more subjectively

 

[RCruz]

Thank you gentlemen

Will find the AD823

 

[Dimdim]

The right one must be fake. Mine have the same lettering and orientation edge dent like the one on the left.

So does mine. I got it from Mouser.

 

[nashbap]

Salas, does the harmonic picture change as the gain/feedback is changed? I think the pics of FFT you posted on the first page are with the standard gain of 10. Will there be any change to the sound if I changed the gain to 6?

Thanks. nash

 

[marcus1]

Probably being overly cautious but for mounting the uPA68HA (J1-J2) on the DCG3 board - does the thick white line on the board correspond to the letter N on the left side of the uPA68HA as shown in the photo?

thanks

 

[nashbap]

Probably being overly cautious but for mounting the uPA68HA (J1-J2) on the DCG3 board - does the thick white line on the board correspond to the letter N on the left side of the uPA68HA as shown in the photo?

thanks

Here's a pic from Tea- Bag's blog that will help.

 

[Hikari1]

Just some info for builders sourcing parts:

From mouser dn2540 came very well matched. All were between .01v of each other when measured across 6.8r resistor. Measured 1.08v for 160ma ish bias. Out of 5 devices they measured between 1.07 and 1.08v with 6.8r resistor. Nice.

Bc560 required about a dozen devices to get 4 with a hfe match within 5%.

I have not tested the bf256b. Will probably do that when it's in the circuit and adjust resistor values accordingly.

Here is some in progress pics. The case is too small, I know. Psu is lm317/337 followed by a cap multiplier that will sit on the sink opposite of the amp board.

Iam getting +17.6v and -17.8v out of my lm317/337 and cap multiplier when unloaded. Too high or just about perfect?

 

[ammel68]

Iam getting +17.6v and -17.8v out of my lm317/337 and cap multiplier when unloaded. Too high or just about perfect?

Just about perfect.
I say go for it dude!

If you're concerned about it being too high, why not adjust it down? You do have trimmers to adjust the voltages of the 317/337 don't you?

 

[Salas]

Salas, does the harmonic picture change as the gain/feedback is changed? I think the pics of FFT you posted on the first page are with the standard gain of 10. Will there be any change to the sound if I changed the gain to 6?

Thanks. nash

Those were for 10dB gain yes (3x). I have checked in the simulator the range 2x to 6x and it does not show significant FFT changes. I have also listened to a build that went from 3x to 4x that did not seem to detectably differentiate in tone.

 

[Salas]

Iam getting +17.6v and -17.8v out of my lm317/337 and cap multiplier when unloaded. Too high or just about perfect?

Prefer nearer to 17V final rail in the long term if you can so to stay away from the op-amp's max spec. You will see if that Cmult loses some on its own when loaded. If not, adjust it down to 17.3V

 

[Salas]

Probably being overly cautious but for mounting the uPA68HA (J1-J2) on the DCG3 board - does the thick white line on the board correspond to the letter N on the left side of the uPA68HA as shown in the photo?

thanks

Yes

 

[marcus1]

Thank-you.

Yes

 

[Salas]

Here's a pic from Tea- Bag's blog that will help.

Yep, left one uPA you can read its type, right one you see its back (when looking at them from the inputs/outputs PCB side)

 

[RCruz]

As for the relay, maybe I can use this one: G6H-2 12V

What do you think ?

 

[Salas]

Is that the one Omron we used in DCB1 after the NAIS (Panasonic) got difficult to get at a point? If yes its also equivalent to the NEC. All those three are top reputation Japanese brands

 

[Salas]

OK then, load your PSU with 100R 5W per side and check its outputs to prepare it off-line

 

[Hikari1]

Just about perfect.
I say go for it dude!

If you're concerned about it being too high, why not adjust it down? You do have trimmers to adjust the voltages of the 317/337 don't you?

Only got resistors. I'll check it with load.

 

[Hikari1]

OK then, load your PSU with 100R 5W per side and check its outputs to prepare it off-line

Thanks. Will check with load before. Or I can just bring down the psu voltage a little with a resistor swap.