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Old 21st May 2010, 10:04 PM   #21
sonidos is offline sonidos  United States
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jcx - much obliged! I'll scan Walt's site as well to get further insight on how this is done.
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Old 23rd May 2010, 09:33 PM   #22
klewis is offline klewis  United States
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Quote:
Originally Posted by sonidos View Post
klewis,

I started playing with CFB op amps, too. Any luck on your servo after a year? I contacted Nat Semi and they sent me right back over here. ha!
I was able to get one to work, then blew it up some how... I've been running the CFB preamp for over a year without offset compensation and not particular trouble. Someday I will put the servo back together. I'll try to post the schematic of the amp tonight.

Ken
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Old 24th May 2010, 01:53 AM   #23
klewis is offline klewis  United States
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schematic of pre amp and servo attached. The gain of the preamp is high with the values shown. I get 10v p to p as shown. As I recall, the servo is a Douglas Self design, straight out of the book... but, it works (I think).

Post your result here if it works for you. I would be interested in any improvements.

Search NS site for current feedback design literature. I believe that the AN are under the Comlinear or similar name - apparently purchased by national some time ago. Lots of helpful hints - for example to keep the feedback loop short, I put the feetback resistor on the bottom of the board, almost a direct connection between the two pins.

Good luck.

Ken
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File Type: pdf preamp schematic.pdf (20.5 KB, 125 views)
File Type: pdf pre amp servo.pdf (15.5 KB, 127 views)
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Old 24th May 2010, 02:45 PM   #24
sonidos is offline sonidos  United States
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Ken,

Thanks to you as well. Audioman54 told me this CFB op amp is one of his favorites and urged me to try it. I built it on a breadboard and I am very happy with it. I've been busy but I'm getting back to trying different resistors, caps, and rail voltages with it.
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Old 24th May 2010, 06:59 PM   #25
klewis is offline klewis  United States
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Audioman54 influenced me as well. He's the one who suggested the inverting configuration and the metal can parts.
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Old 24th May 2010, 07:07 PM   #26
sonidos is offline sonidos  United States
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Ken,

Did you try non-inverting vs. inverting? I took some time over the weekend to read up on CFOAs and the remarks across manufacturers was that the inverting is much lower in impedance than non-invertiing. I tried a rudimentary form of going into the inverting input and thought something was lacking.

If you had success going the inverting route, I'll go back and try it and play with my resistor values. I noticed you increased your Rfb values from the recommended 1k2 value.
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Old 24th May 2010, 07:29 PM   #27
klewis is offline klewis  United States
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I tried both, but, Mark (audioman54) strongly recomended inverting, so, I went that way. I thought it would make the servo harder to implement, but, in the end the servo worked.

Regarding value of Rf, I've attached the application notes. Increasing Rf reduces the bandwidth slightly, I don't need 100Mhz... the notes talk about overshoot in the time domain. I took this the mean overshoot of a square wave, (maybe traditional square overshoot is in the voltage domain )which in simulation, was a problem for CF op amps. In testing my amp, I don't have overshoot.

Ken
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File Type: png feedback_resistor.png (17.3 KB, 164 views)
File Type: png Gain_vs_Frequency.png (14.2 KB, 163 views)
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Old 28th May 2010, 06:33 PM   #28
sonidos is offline sonidos  United States
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I had the same thought re. the Rf value. I am definitely going to play with increasing it to see what audible gains I get through my headphones.

So I looked at the schematic and I see you went DC coupled w/o any cap in the signal chain. So the 49713 must be pretty stable for DC offset.

I have a heavily modified Aunt Corey's with BUF03's that have to be trimmed on an annual basis. I'll definitely do a bit of burn in time and see what happens to offsite under temp changes and varying V+/V- values.
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Old 30th May 2010, 01:42 PM   #29
klewis is offline klewis  United States
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Quote:
Originally Posted by sonidos View Post
I had the same thought re. the Rf value. I am definitely going to play with increasing it to see what audible gains I get through my headphones.

So I looked at the schematic and I see you went DC coupled w/o any cap in the signal chain. So the 49713 must be pretty stable for DC offset.
I think it relatively temp stable vs DC offset. The trouble is that the offset varies with Rin. So, as you turn the volume control, the offset changes. I think I'm just lucky that my volume setting doesn't produce much DC.

As I remember it, the servo was working. I can remember how I cooked it, something dumb on my part. I would recommend including the servo in your design. I made the servo as a daughter board that plugs in directly above the output section of the preamp. It keeps the traces quite short.

Ken
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Old 1st June 2010, 06:46 PM   #30
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ok, interesting. I'll look for the same results to see what happens to DC offset with Rin.

It was busy around our home with the 3 day weekend, but I've got some parts to order and try it out with the servo. I'm putting away the 49713 cans for now for this testing because they are pricey. I ordered plenty of the soic version for experimentation just in case I do something dumb, which of course happens from time to time.

I built my proto on a breadboard with some 9 volt batteries. I put on Stereophile's recording of Rhapsody in Blue for my wife. She is not into any of this and she is just as happy with a boombox. The hairs of her arms stood on end when she heard the music coming through my HD580's. Mark and company did a really nice job on this opamp and hopefully I can squeeze the max out of it. I'll be back later to report on the servo performance.
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