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Post by spendorspain on Sept 12, 2006 16:22:04 GMT
Hi
Is the AD797 op-amp fully interchangeable with others like AD843 and AD744, in order to directly compare them? It seems a very good audio op-amp (the one preferred by Rick, I believe), but is it as easy as unplug one op-amp from the 8 DIL socket and fit the AD797 in its place? I'm going to use it in a WNA headphone amplifier which has neither input nor output capacitors, so I'm concerned about the AD797 possible DC offset (as is the case with LM6171 which needs a capacitor at amplifier input and a trimpot to null DC offset).
Thanks in advance Regards Jose
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rickcr42
Fully Modded
Rest in peace my good friend.
Posts: 4,514
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Post by rickcr42 on Sept 13, 2006 1:44:47 GMT
I have had good luck using the AD797 with low impedance inputs (microphone) and very high gain without a hint of DC offset so on that score yes but this puppy does place special needs on both power supply bypassing and possibly feedback loop stability compensation (see data sheet for details) making "drop in" situation dependant more than a universal "yes" though you can try it and see.If for some reason you DO have DC issues see fig.54 on the data sheet for a DC balance trim.Use a 10 turn trim pot here if you want a stable adjustment. For original designs meant from the start to use this part there are certain "rules" you should follow for best results : 1-if you need to or want to use the opamp with high impedance input sources you will need to add your own discrete jfet input buffers.By "high impedance" I mean anything over 600 ohms drive 2-Power supply bypassing : look at fig 35 from the data sheet : www.analog.com/UploadedFiles/Data_Sheets/336938187AD797_e.pdfsee the left figure ? I use a 0.1uF stacked film in parallel with 10uF film and foil and a 2.2 ohm carbon composition damping resistor.AVOID the recommended ceramic/tantalum data sheet recommendation.The power terminal of the opamp should be directly connected to the caps with the ground end connected right at the output jack ground terminal (load) 3-For unity gain applications read the entire section on feedback loop bypassing and adjust your loop caps accordingly (resistor dependant). NEVER run the feedback loop as a "wire from output to inverting input" as in most unity gain apps but use the resistors as shown then bypass them with a compensation cap,again as shown Fig.37-39 and Table 4/5 4-I am also in the habit of using decoupling on the output if direct coupled to the next stage/headphones (jensen OLT or a DIY version of) and rfi bypass caps across the input load setting resistor but that with ALL op-amps out of habit.Better to avoid a problem than to have to track one down and fix later 5-The 797 can be used at anything from unity gain to X1K gain but really is most comfortable in the X10-X100 range and even though it boasts a 50mA current output delivery this part really is more comfortable driving a high impedance load such as a discrete "current pump" output stage or as a compound amp see fig.49 ) when driving headphones and a 1:1 600 ohm transformer either with or without a buffer as a long line low noise line driver. a couple of "actual" circuits I personally use the 797 for are in my distribution amp based on this design www.jensen-transformers.com/as/as041.pdfand in my balanced line drivers (work well as combination line drivers/headphone amps with the headphone output before the output trafos ) www.jensen-transformers.com/as/as033.pdfwww.jensen-transformers.com/as/as023.pdfThis is a very high quality part and has a nice sonic profile but for best results does have a set of requirements psecific to the part even though mostly usable as a pin compatible "drop in" BTW-You may find this chart interesting www.jensen-transformers.com/as/as050.pdf
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rickcr42
Fully Modded
Rest in peace my good friend.
Posts: 4,514
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Post by rickcr42 on Sept 15, 2006 18:30:54 GMT
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