AD826- Ready to download: EARphone PURE AUDIOamp AD826

[ygg-it]

Here you may find the project, photos, circuit, measurement, curves, schematic, full documentation of my headphone audio amplifier based on AD826 opamp

Free for non-commercial purpose only. Please read the disclaimer



http://ygg-it.tripod.com/



Please leave here any your comment that will be very appreciated!

[Dxvideo]

Congrats... I am sure it sounds perfect.

But some questions;
- The input buffer is labelled "Class A". How can you provide that? I couldnt see any bias system for that opamp....
- At the second and output buffer sections, the DC offset is determined 0mv... Also how can you provide that? There is no DC servo and/or DC decoupling on NFB line..
- I cannot see any frequency compansation circuit in the amp.. In this case you can listen a lot of Russian (or Arabic) radio station...

????????

[ygg-it]

- The input buffer is labelled "Class A". How can you provide that? I couldnt see any bias system for that opamp....

Already answer in FAQ section

- At the second and output buffer sections, the DC offset is determined 0mv... Also how can you provide that? There is no DC servo and/or DC decoupling on NFB line..

RE: for ideal op-amp in order to minimize DC (goal is zero but 10-20mV at final output is even tolerated) the input (+) and input (-) should see the same DC impedance.
To do this you need that Rin(+)=Rf||Rin(-). And this was done with 1% precision resistor for both the stages. This usually is enough to get a very low DC offset. In real world you have also DC voltage output due to the AD826 input offset voltage, input bias current and input offset current.
These values can range from microvolt to millivolt, can be either polarity and vary among devices along a Gaussian curve: with an accurate selection of Op Amp devices and the right cascade connection you can furtherly minimise the Dc offset. I don't like too much the Dc servo solution since it would feed the opamp with power supply noise.


- I cannot see any frequency compensation circuit in the amp.. In this case you can listen a lot of Russian (or Arabic) radio station...

RE: you are completely right, but fortunately this is not my case... It was very risky and in fact my first project had a small 100pF capacitor in parallel to the input. If you are lucky you can take it away since any additional capacitor is additional distortion...But you should have a very good layout, star grounding circuit and short shielded wiring inside.
In my circuit you may also find three ferrite beads that wrap wires and stop RF from going in (one onto the input cable, the second onto the pot cable and the third onto the main earth wire.
Also remember that the big EPCOS 33uF capacitor has a self resonance of about 320KHz (see Metallized Polyester Film Capacitors (MKT) EPCOS datasheet- B32524 - pag.20 ) due to its intrinsic inductive property. So from this frequency it starts to act like an inductor and this may help too.

[ygg-it]

The project is finished and the web site completed !

[peranders]

Is it only on vero board? You haven't made any pcb? The circuit looks OK but I have no idea how good the 230 VAC arrangment is.

Why have you excluding the cap in the offset trimming circuit? Isn't it good to block incomming supply noise directly into the audio signal?

You write 21 kHz bandwidth? How do you manage this without any filters and using 50 MHz opamps?

[analog_sa]

The "Hi Definition Dual Power Supply" is particularly clever

[peranders]

Marketing you know You are free to call it whatever you'll like. My stuff is "high performance", "ultra high performance" or even "extremely high performance" but then I can't figure out the next level What do you call stuff that is more than extreme? über-extreme maybe?

[ygg-it]

"Hi Definition Dual Power Supply" : check this site to really understand what I mean. It seems that a 100nF cap is even too much!!

lm317 article


"Hi definition" name is used since it is the best compromise to get the best performance for image precision and the less AC noise. There is no marketing intention on it... I tried to go down with less than 100nF but the amplifier started to oscillate. By the way the 100nF cap is the only one which bypass the power supply after the voltage regulator.

[peranders]

Softness,
Tone realism,
Dynamic,
Definition,
Bass level,
Bass depth,
Space clarity,
Voice position

Can I read...

If you read carefully the datasheet it says something about power supply bypassing and personally I would be very carefull when I have a 350 V/us with minimal decoupling and a not too fast voltage regulator. The opamp is much faster than the regulator.

If a LM317 is high definition what might a super regulator be which is 20-50 times faster?

[analog_sa]

Originally posted by ygg-it
"Hi Definition Dual Power Supply" : check this site to really understand what I mean.

From the site: "ceramic or plastic for savage sound". Now i get it.