[alley-kat]

hello to all, i am new here and need some opinions. the ad 825 chip on another forum, has been suggested to bias from the negative rail by most but,some say positive rail. so, needless to say, i'm a little confused. any input would be appreaciated. alley-kat

 

[tangent]

Try both and decide for yourself. It's not hard.

 

[jeffreyj]

Quote:

Originally posted by alley-kat ... the ad 825 chip on another forum, has been suggested to bias from the negative rail by most but,some say positive rail...

See fig. 4 in the AD825 Datasheet for the simplified schematic of this op-amp.

One normally ties the class A biasing current source to V- because this forces the upper transistor in the output stage (NPN) to do all of the work; the lower transistor (PNP) then does nothing, unless the current demanded by a negative excursion exceeds the current source's setting. The reason the NPN is almost always used (and, therefore, the current source is tied to V-, not V+) is because NPNs are easier to fabricate on the silicon substrate and typically have superior specs to their PNP counterparts. The latest generation of fabrication processes, though, have narrowed the performance gap considerably, though, so soon it won't matter which rail you tie to.

The AD825 would probably best be served with the current source tied to V-, though.

 

[alley-kat]

hi folks, i have now tried both ways of biasing and with the positive, i seem to have more detail with the highs and actually cleaner sound but, less bass. the kick drum is a lot weaker, i guess this is just one of those places where you just can't have the best of both worlds. i think i will try the negative and try going a little higher and lower with the bias currents. thanks for the help, alley-kat

 

[tangent]

It depends on the application whether you can bias an op-amp into class A and get better sound. If your application means you need 20mA of bias to push it into full class A (as opposed to AB), you're likely activating the current limiting circuitry, and even if not you're straining the circuit. By contrast, amps like the META42 have buffered output stages so the input stage can easily be biased into class A with a miniscule amount of current, so the op-amp isn't loaded at all.

 

[alley-kat]

hi tangent, the negative bias seem to be sounding better all the time. i guess the 825s are begining to burn in. what i have them in is an audiosource home preamp, running at just over 2 milliamp. i don't think i would want to go over about 2.5 to be safe. thanks for your help, alley-kat