[Andrea]

Hi,

I'm puzzled by this doubt -- might the AD744Ks that I put in my Shellbrook Ascent suffer due to the class-A biasing (which is implemented just like in the Pimeta, with FETs), being their output current so low (20 mA short circuit) ?


Thanks to whom clears my doubt

 

[Andrea]

In other words, is this alright?

 

[steinchen]

I'd recommend to use the bypassed output of the AD744 which works already in classA and deactivate the classA biasing of the amp

just use a dip-8 socket, shorten pin-5 (compensation) and pin-6 (output) between the pins, bend up pin-6 of the AD744 (must NOT contact the socket) and plug the AD744 into this adapter. The class-A biasing stage of the amp has to be deactivated !

 

[Andrea]

Quote:

Originally Posted by steinchen I'd recommend to use the bypassed output of the AD744 which works already in classA and deactivate the classA biasing of the amp just use a dip-8 socket, shorten pin-5 (compensation) and pin-6 (output) between the pins, bend up pin-6 of the AD744 (must NOT contact the socket) and plug the AD744 into this adapter. The class-A biasing stage of the amp has to be deactivated !

Thanks, that's definitely an option! Of course I'd have to cut the resistors in series with the FETs to deactivate class-A biasing, which requires some pondering first.


But why use a DIP8 socket? I could just cut pin 6 on the op-amps so that it doesn't plug in the amp's sockets, and then jumper it to pin 5 with a little careful soldering. Is it correct?


Anyhow, my doubt remains -- would it be alright to leave things as they are, i.e., will the AD744's normal output (pin 6) support the amp's class-A biasing system correctly? Thanks in advance.

 

[steinchen]

Quote:

Originally Posted by Andrea Of course I'd have to cut the resistors in series with the FETs to deactivate class-A biasing, which requires some pondering first.

I'd socket the resistor or the jfet with pins from a dip-breakoff-strip, this way you can easily activate/deactivate the biasing

Quote:

Originally Posted by Andrea But why use a DIP8 socket? I could just cut pin 6 on the op-amps so that it doesn't plug in the amp's sockets, and then jumper it to pin 5 with a little careful soldering. Is it correct?

the adapter method doesn't touch the opamp, you can easily take out the adapter-socket, bend down the pin of the opamp and got the original setup

Quote:

Originally Posted by Andrea Anyhow, my doubt remains -- would it be alright to leave things as they are, i.e., will the AD744's normal output (pin 6) support the amp's class-A biasing system correctly? Thanks in advance.

Depends on the biasing current. To how many mA is the biasing set ? Is there a way to adjust the biasing ? As long as the biasing is set to a few mA (aprox 2mA) everything should work fine, at high biasing levels the biasing current stresses the opamp too much and affects sound quality

 

[thedoctor]

Quote:

Originally Posted by steinchen I'd recommend to use the bypassed output of the AD744 which works already in classA and deactivate the classA biasing of the amp just use a dip-8 socket, shorten pin-5 (compensation) and pin-6 (output) between the pins, bend up pin-6 of the AD744 (must NOT contact the socket) and plug the AD744 into this adapter. The class-A biasing stage of the amp has to be deactivated !

might not be my place to say this, but i don't think you should deactivate the Class A bias. if i'm seeing thing correctly, the amp is buffered. a few miliamps of current taken for Class A bias should still be under the amp safe limit if it's buffered.

 

[steinchen]

when using the bypassed output of AD744 (which is already biased with 2mA) you have to deactivate the amp's classA biasing, otherwise you'll get a very high DC offset at the output of the amp

 

[ryssen]

"I'd socket the resistor or the jfet with pins from a dip-breakoff-strip, this way you can easily activate/deactivate the biasing"


You could use a switch to.

 

[Andrea]

Quote:

Originally Posted by steinchen I'd socket the resistor or the jfet with pins from a dip-breakoff-strip, this way you can easily activate/deactivate the biasing

I don't wanna risk doing any least damage, so I think I'll leave it as it is and just see how I like the sound. If not enough, AD744's will go out and AD843's in.


Quote:

Depends on the biasing current. To how many mA is the biasing set ? Is there a way to adjust the biasing ? As long as the biasing is set to a few mA (aprox 2mA) everything should work fine, at high biasing levels the biasing current stresses the opamp too much and affects sound quality

That's comforting and exactly what I wanted to hear. It seems reasonable to me that the biasing level would be the same for this amp as for the Pimeta, so I guess about 2 mA. I think I'll just put the case together (when I receive the Cardas RCA jacks I'm waiting for) and see how I like the sound...