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#1
Posted to rec.audio.tubes
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Best op amp for wien bridge oscillator for highest BW.
This morning I well and truly fried a signal generator beyond
fixability. Solid state generators don't like +580Vdc poked down their outputs, but I managed it while testing a Dynaco MK6. There was a sickening sausage sizzle sound as a a bunch of parts converted quickly from plastic encased silicon to soot. So, I am re-building a 5 range wien bridge oscillator to go from 2Hz to 200kHz. So far, with NE5534 and with +/-6V rails I can just get the five full ranges but only 2.8Vrms output. Using higher rails above 9V up to 12V means the op amp struggles above 100kHz and the oscilations break up into a series of bursts rather than continuous nice oscillations. So what's the best opamp for wide bandwidth? Positive FB networks have caps ranging from 4.7uF to 470pF, and variable R from 1k5 to 16k. NFB network is about 200 ohms plus two series tiny lamps which when warmed are 100 ohms, or just right. With 11V rails I got 5.6V at below 50kHz OK with the NFB ohms totaling only 133 ohms with one lamp which means the opamp must move into class AB operation to do this because at idle the Idc = 11mA. I'd like 5 Vrms at 200kHz or higher F if I can get it. Any thoughts? Patrick Turner. |
#2
Posted to rec.audio.tubes
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Best op amp for wien bridge oscillator for highest BW.
"Patrick Turner" wrote in message
This morning I well and truly fried a signal generator beyond fixability. Solid state generators don't like +580Vdc poked down their outputs, but I managed it while testing a Dynaco MK6. That is nature's way of telling you that you should have put clamping diodes into the output circuit. There was a sickening sausage sizzle sound as a a bunch of parts converted quickly from plastic encased silicon to soot. I regret that I've made similar mistakes. Since I don't do much with tubes, microphone preamps with phantom power was the source of my most recent waterloo, a few years back. So, I am re-building a 5 range wien bridge oscillator to go from 2Hz to 200kHz. Been there, done that but I admit I stopped at 100 KHz. So far, with NE5534 and with +/-6V rails I can just get the five full ranges but only 2.8Vrms output. It's pretty well known that NE5532-4 chips are most happy up in the +/- 18 range, and will stay glued together up to +/- 22 volts. Using higher rails above 9V up to 12V means the op amp struggles above 100kHz Been there, done that as well. More properly stated the oscillator struggles. I'm not that sure that the op amp is the whole reason why. With VCC= +/- 22 I was able to get about 11 vrms @ 100 KHz. and the oscilations break up into a series of bursts rather than continuous nice oscillations. Many possible answers. Obviously an op map with more GBW and slew rate would give relevant evidence to the hypothesis (which may be true) that the 5534 was the problem. Counter-evidence is the fact that http://www.cordellaudio.com/instrume...d_analyzer.pdf shows a full THD test set (both oscillator and analyzer) that is alleged to work up to 200 KHz, and is based on 5534s. So what's the best opamp for wide bandwidth? The OPA 604 and OPA 2604 come to mind in the "classics" department The LM4562 is the latest-greatest from National. Positive FB networks have caps ranging from 4.7uF to 470pF, and variable R from 1k5 to 16k. NFB network is about 200 ohms plus two series tiny lamps which when warmed are 100 ohms, or just right. With 11V rails I got 5.6V at below 50kHz OK with the NFB ohms totaling only 133 ohms with one lamp which means the opamp must move into class AB operation to do this because at idle the Idc = 11mA. I'd like 5 Vrms at 200kHz or higher F if I can get it. Most of my experiments were based on the Wein bridge components of Heath's IG-18/5218. There are schematics on the web. |
#3
Posted to rec.audio.tubes
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Best op amp for wien bridge oscillator for highest BW.
Hi RATs!
Krone-Hite made a nice vacuum tube oscillator with three power supplies (two regulated) and 0.00001 CPS to 99,999 CPS range with output up to 10 volts, IIRC. Mine was an A-440, but there were others. It had a 5x10 array of buttons to select the freq Traded it for a bunch of SE OPTs. He checked it out with his AP at home. I believe his comment was: "We often dismiss the past achievements of engineers more clever than us." Or something along those lines. Worth keeping an eye out for. Happy Ears! Al |
#4
Posted to rec.audio.tubes
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Best op amp for wien bridge oscillator for highest BW.
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#5
Posted to rec.audio.tubes
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Best op amp for wien bridge oscillator for highest BW.
On 5 Feb, 15:17, Patrick Turner wrote:
This morning I well and truly fried a signal generator beyond fixability. Solid state generators don't like +580Vdc poked down their outputs, but I managed it while testing a Dynaco MK6. There was a sickening sausage sizzle sound as a a bunch of parts converted quickly from plastic encased silicon to soot. So, I am re-building a 5 range wien bridge oscillator to go from 2Hz to 200kHz. So far, with NE5534 and with +/-6V rails I can just get the five full ranges but only 2.8Vrms output. Using higher rails above 9V up to 12V means the op amp struggles above 100kHz and the oscilations break up into a series of bursts rather than continuous nice oscillations. So what's the best opamp for wide bandwidth? Positive FB networks have caps ranging from 4.7uF to 470pF, and variable R from 1k5 to 16k. NFB network is about 200 ohms plus two series tiny lamps which when warmed are 100 ohms, or just right. With 11V rails I got 5.6V at below 50kHz OK with the NFB ohms totaling only 133 ohms with one lamp which means the opamp must move into class AB operation to do this because at idle the Idc = 11mA. I'd like 5 Vrms at 200kHz or higher F if I can get it. Any thoughts? Patrick Turner. This could be interesting Patrick, I am also playing about and gathering parts for a nice Wein bridge oscilator. It seems that the NE5532/4 is are hard act to beat. Check out this page on Doug. Selfs site: http://www.dself.dsl.pipex.com/ampins/webbop/opamp.htm There are a lot of new opamps about now but I m not really up to speed on them.A couple I have been looking at are the LME49710 and 49720 and their cousin the 49600 as a buffer. They are expensive though and the NE5532/4 seems to be the best bang for your buck as they say. Matt. |
#6
Posted to rec.audio.tubes
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Best op amp for wien bridge oscillator for highest BW.
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#7
Posted to rec.audio.tubes
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Best op amp for wien bridge oscillator for highest BW.
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#8
Posted to rec.audio.tubes
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Best op amp for wien bridge oscillator for highest BW.
"Patrick Turner" wrote in message ... I've settled on the NE5534 for now, and have got protection clamping diodes in place and a 5uF cap on the output, 2 switched outputs for 0.0V to 0.32V and 0.32V to 2.5V with 800 ohm and 5k ohm log pots. At 2.5V maximum output voltage with +/- 6V rails I get the full 220kHz of bandwidth I wanted So Cordell was right - the 5534 was not an inhibiter to 200 KHz operation? , and I've never known a power amp needing more than 2.5Vrms to bring the output to clipping. Stabilization by what, lightbulb? THD at 1kHz measures 0.006%, the best I've ever had from an opamp oscillator, and range accuracy calibration on the dial is within 2%. There's very little bouncing or amplitude modulation. I'm happy. Yeah, getting rid of that amplitude modulation can be hairy. With lightbulbs distortion below 50 Hz can be a problem because the output stabilization circuit's time constant whether a lightbulb or more complex means. One solution is to just lower the gain of the stabilizer, but that can lead to AM on the HF ranges. For higher F maybe I will upgrade a tuned LC type of oscillator. At higher frequencies it seems to be pretty economical to make low distortion LC circuits. To make a square wave I can amplify the sine wave and clip it off. Schmidt trigger is best - nice discrete one in the IG 18/5218 schematic. I think it would work well with your 5v supply. Don't forget the clamping diodes for it! ;-) |
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