Discrete Opamp Open Design

[fireworks]

Originally Posted by jan.didden

... flying off the handle ?

Re: thinking: I once had a manager, who saw me siting at my desk, hands folded behind my head, ask what I was doing. "Thinking".
Said he: "I 'm not paying you to think, I am paying you to work". And I swear, he was dead serious.

jan

This is an excellent joke ! I need to remember it so that I can tell it to other people.

P.S.: I'm not saying it didn't happen, just that the story is very funny.

[grhughes]

"......I noticed the 357 Ohm resistors could be a tad larger the ratio to the position held by the 243 Ohm one should be closer to 5/3, this only adds 400uA in that branch which does not do much and comes out in the trim." S. Wurcer

Is 383 ohms too much? I'm hard pressed to find those Panasonic DMG/DMC2040020R at either Mouser or Digi-key. Thanks, Ray

[linuxguru]

Originally Posted by grhughes

... I'm hard pressed to find those Panasonic DMG/DMC2040020R at either Mouser or Digi-key. Thanks, Ray

NXP has BC8xx in dual NPN/NPN, NPN/PNP and PNP/PNP, both matched and unmatched, in SOT363. If you want a larger package with higher power dissipation, the Rohm IMX- (NPN/NPN), IMT- (PNP/PNP) and IMZ- (NPN/PNP) series in 6-pin SC74/SMT6 packages fit the bill nicely. Be very careful with the pinouts on the Rohm packages, though - some smaller packages have the standard 123456 pinout, while some of the SMT6 packages have 321654 (reversed) pinout.

[scott wurcer]

Originally Posted by grhughes

"......I noticed the 357 Ohm resistors could be a tad larger the ratio to the position held by the 243 Ohm one should be closer to 5/3, this only adds 400uA in that branch which does not do much and comes out in the trim." S. Wurcer

Is 383 ohms too much? I'm hard pressed to find those Panasonic DMG/DMC2040020R at either Mouser or Digi-key. Thanks, Ray

383 is fine, and all those general purpose duals should be fine, I've used the NXP devices before.

I just bought all the Panasonic ones from Digi-Key last week?? Try a search for Panasonic and the package you want. The numbers get long maybe I missed a digit.

[PHEONIX]

Hello Scott,

I like the take on the hardware.
What are you using to measure the distortion and is it possible to measure the whole stage including the input stage for distortion.

[grhughes]

Originally Posted by linuxguru

NXP has BC8xx in dual NPN/NPN, NPN/PNP and PNP/PNP, both matched and unmatched, in SOT363. If you want a larger package with higher power dissipation, the Rohm IMX- (NPN/NPN), IMT- (PNP/PNP) and IMZ- (NPN/PNP) series in 6-pin SC74/SMT6 packages fit the bill nicely. Be very careful with the pinouts on the Rohm packages, though - some smaller packages have the standard 123456 pinout, while some of the SMT6 packages have 321654 (reversed) pinout.

Thanks guys! Is it going to be necessary to add a choke trap output filter (back door filter) to the output of the preamp driving a power amp with about 20 feet of cable? I'd like to avoid the back door trap filter if possible. I guess I'll just have to try it. Ray

[RNMarsh]

SW--- is this InterFET part any good? --> jFET P/N IF3601/02 (.3nV). -RNM

[RNMarsh]

Any time you use an impedance in series with the output (build-out R or inductor) it raises the Z of the whole cable interconnect which then makes the cable more susecptable to emi/rfi pickup. place such at the load end, if possible. -Thx RNMarsh

[kgrlee]

Originally Posted by RNMarsh

Any time you use an impedance in series with the output (build-out R or inductor) it raises the Z of the whole cable interconnect which then makes the cable more susecptable to emi/rfi pickup. place such at the load end, if possible.

While Mr. Marsh is correct about the 'input' end being the more important, EMI/RFI can be injected into outputs too.

Clip on ferrites for computer cables are an inexpensive aid if you have problems. They need to be as close to the end of the cable as possible but outside the box.

Best with ceramics AT the input/output sockets too.
________

Mr. Hughes, I'll leave Scott to answer the question about acceptable capacitive load but you might want to look at AD797 datasheet for how this might be presented. It will be affected by the compensation capacitor too.

Belden 9451 has 35p/ft conductor/conductor and 67p/ft conductor to shield. Nominal Z is 45R but 'best transient' response over a long length is with about 90R.

Yes the extra resistance will degrade EMI/RFI but only if you don't have the ceramics at the sockets.

[fas42]

Originally Posted by kgrlee

While Mr. Marsh is correct about the 'input' end being the more important, EMI/RFI can be injected into outputs too.

There's a lovely snippet in the 'bible', H & H: opamp closed loop impedance rises with frequency, say 10 to 100 ohms at 1Mhz. Nearby, digital circuitry is slewing at 0.5V/ns. We want an error coupling of less than 0.1mV to appear in the analogue.

What does the capacitance between those two signals have to be below to achieve that? 0.02pF, something everyone here I'm sure will know how to maintain ...

Frank