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Old 25th August 2012, 08:25 PM   #261
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Quote:
Originally Posted by zigzagflux View Post

As far as the oscillator, I built my own 35V non-regulated outboard power supply, which was actually putting out closer to 37V at 35mA. This caused the oscillatorís local power supply to run a little too warm for my comfort; you could really see drift of circuit performance as the entire pcb warmed up. The TL431ís were running around 55-60C absolute. So a 100 ohm resistor in series was sufficient to bring voltage down to 34.7V, where the shunt power supply was much more happy and cooler.


Your thoughts?
So, Victor's oscillator is sensitive to supply voltage? I was hoping to run it off of 4X 9V batteries (36V). Well, I suppose that's not a good idea because 9V batteries are anything but... Maybe 3X 12V lead acid batteries? sigh... probably not a good idea either.
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Old 25th August 2012, 08:26 PM   #262
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Those numbers are in the same range as I was getting. There are resistors (carbon composition) that are worse and many other passives are much worse.

I would propose replacing the resistors with a current source for the power supply. I have use this with good success. Use the High voltage version of the LM317. It also provides short circuit protection and contant performance as the supply fluctuates. Just insert in series with the supply and bypass or reduce the values of the 100 Ohm resistors.
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Old 25th August 2012, 08:33 PM   #263
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The performance will change as the FET warms up. Takes a few minutes to stabilize. This is typical of all of these systems. With the 100 Ohm resistors the shunt regulators will go from little dissipation to a lot with a small variation in supply voltage. That's why the current source (really a current regulator) is preferable. You also get really good supply isolation.

The LM317 + 4 9V batteries would be about ideal. No ground loops or AC leakage to deal with. The 9V battery should give a continuous 24 hours of operation. Remember to turn it off and you could get months of operation.
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Old 25th August 2012, 09:13 PM   #264
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Fine suggestions. I am fairly certain with a shunt setup like this the quality of the outboard supply is less critical than the overall dissipation resultant. I can set up a CCS easy enough, but at this point the oscillator in its current state is the highest hanging fruit.
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Old 25th August 2012, 09:59 PM   #265
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Quote:
Originally Posted by 1audio View Post
Those numbers are in the same range as I was getting. There are resistors (carbon composition) that are worse and many other passives are much worse.

I would propose replacing the resistors with a current source for the power supply. I have use this with good success. Use the High voltage version of the LM317. It also provides short circuit protection and contant performance as the supply fluctuates. Just insert in series with the supply and bypass or reduce the values of the 100 Ohm resistors.
Just to be clear, you're talking about the two 100 ohm resistors R32/R33?

What current do you have your LM317 circuit set to? I can get constant current diodes in SMD that could possibly be soldered in place of at least one of the resistors, but they come in specific current ratings. I can't remember how much current this oscillator draws so they may not make one large enough. Is it 35mA?

The LM317 needs a minimum voltage drop in order to function. I think it's about 3V, in the same range as a CRD. So, doesn't the battery supply need to be 35+3V at least? If I remove the resistors entirely then there's 3.5V right there.
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Old 25th August 2012, 10:04 PM   #266
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[QUOTESo by the numbers, my low-Q notch filter performs as follows:
2nd: +30.9 dB
3rd: +34.9
4th: +36.7
5th: +37.7
6th: +38.3
7th: +38.7
8th: +39.0
9th: +39.2[/QUOTE]

If I have understood your notch filter design as being passive without feedback into the bottom of the leg, the low Q will cause the notch to attenuate the second H by about 9db and the third H by about 5db. So you will need to adjust your measurements to have accurate results.
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Old 25th August 2012, 10:11 PM   #267
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Those numbers are adjusted.

+40 dB fixed gain - 9.1 dB loss = +30.9 dB gain total for 2nd harmonic.

My calculations of distortion took the notch characteristic into account at each frequency.
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Old 25th August 2012, 10:29 PM   #268
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Originally Posted by dirkwright View Post
Just to be clear, you're talking about the two 100 ohm resistors R32/R33?

What current do you have your LM317 circuit set to? I can get constant current diodes in SMD that could possibly be soldered in place of at least one of the resistors, but they come in specific current ratings. I can't remember how much current this oscillator draws so they may not make one large enough. Is it 35mA?

The LM317 needs a minimum voltage drop in order to function. I think it's about 3V, in the same range as a CRD. So, doesn't the battery supply need to be 35+3V at least?
I calculated for 25 mA. That is what the circuit draws, including the shunts. The 317 will work down to close to 2V, maybe less. You have 1.25V for the adj that is the real limiter on minimum voltage. I know the LM 317 is stable used this way. There is 5V drop across the two 100 Ohm resistors so you can shunt or replace them with something like 10 Ohm resistors and have plenty of compliance with 4 9V batteries.

25 mA is too large for a current diode and the heat output would be a problem. Don't put two in series or one will be useless since the current is defined by the smallest current limiter. any others will collapse to a minimum voltage. You could use them in parallel. That gets pricey pretty quickly.
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Old 25th August 2012, 11:37 PM   #269
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OOOPs sorry, missed a post.... Nice work -- did you add in the harmonic attenuation of the passive notch? That would raise the 2nd by 9.5dB and the 3rd by about 5.5dB, with lesser amounts for the rest.
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Old 26th August 2012, 01:52 AM   #270
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Originally Posted by 1audio View Post
I calculated for 25 mA. That is what the circuit draws, including the shunts. The 317 will work down to close to 2V, maybe less. You have 1.25V for the adj that is the real limiter on minimum voltage. I know the LM 317 is stable used this way. There is 5V drop across the two 100 Ohm resistors so you can shunt or replace them with something like 10 Ohm resistors and have plenty of compliance with 4 9V batteries.

25 mA is too large for a current diode and the heat output would be a problem. Don't put two in series or one will be useless since the current is defined by the smallest current limiter. any others will collapse to a minimum voltage. You could use them in parallel. That gets pricey pretty quickly.
They make a 20mA diode, but the minimum drop is 4.5V ($1 each). It's surface mount. I could parallel a 15 and a 10 (about $2.50 each). Maximum voltage across the diodes is not a problem. But, maybe the LM317 will work just fine. Thanks for the tips.
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