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Old 2nd January 2013, 05:04 PM   #21
TheGimp is offline TheGimp  United States
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I'm testing in triode mode at 8Vp-p as that is the max into my sound card before clipping.

The gyrator represents a low DC impedance, and a high AC impedance to the tube, so gain is not reduced significantly over a CCS.

Yes, my sound-card is a 10K input (I expected 47K for some reason) and was the initial cause of the high distortion.

The LF353 buffer distortion is quite low at this level when driving the sound card (2nd below-120dB, 3rd below -110dB, 4th on below -130db), so it is acceptable as is. I have an OPA2134 but they didn't like the proto-board. They generated a lot of noise, probably due to poor power-supply bypassing. I'll eventually build a breadboard with ground plane for the OPA2134. For now I'll stick with the LF353 until it becomes the dominant distortion source.

I think testing in pentode mode will pose some problems. As you point out, Gain with either a CCS or Gyrator is going to be high.
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Old 2nd January 2013, 05:16 PM   #22
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The LF353 is an old design. Its open-loop output-impedance is not symmetrical,and so it may distort more when driving a capacitive load - either the follower's input capacitance, or the cable's.

How about trying a more capable op-amp - especially one design for capacitive loads, eg TI LM8261, or a LT "C-load" model - may well improve it.


The LM8261 can drive any capacitance, and has specified performance for a 2K load, so you won't need a follower on this one's output.
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Old 2nd January 2013, 05:38 PM   #23
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Part of the original question, I believe, was "before I measured worse performance with a similar circuit, now it's cleaned up. Why?"

Looking at the two FFT plots attached to Post #1, it's fairly clear that the one that shows poor performance contains a whole school of 60 Hz harmonics. The actual circuit performance is buried under all the 60 Hz garbage. With the new layout and ripple filter, most of the 60 Hz is gone, leaving a good noise/hum floor to make measurements.

I bet that's the cause of the better performance measured in the second plot with the new circuit and layout.

As for FET source followers driving tubes like the 300B. I've played with that quite a bit. I used an AOT1N60 NMOS for the source follower. It has very low gate-drain cap, hence, lends itself well to source follower applications. I found the THD of the complete amp to be better at low FET currents, actually. Not dramatically better, but measurably better. Same was true when using a cathode follower. I suspect this is due to distortion cancellation between the input tube (grounded cathode) and source/cathode follower. Morgan Jones writes about this in VA4. I measured and listened to both circuits and ended up preferring the cathode follower. YMMV.

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Old 2nd January 2013, 06:09 PM   #24
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Spectrum of tubes and FETs will be the same: in both cases are used non-linear devices with deep feedback. But distortions of FETs will be as the rule much lower due to higher transconductance. Of course, distortions caused by capacitance modulation will play higher role with FETs, so as usual the trick is to find optimal solution.
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Old 2nd January 2013, 06:14 PM   #25
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Rod, per your recommendation I've ordered a couple of LM8261s to try. However in light of the performance of the low distortion oscillator with the LM353 buffer, I don't expect to see a great deal of difference.

If the LM353 were the culprit, wouldn't there be a high level of 2nd order harmonic content when only driven by the low distortion oscillator?
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Old 2nd January 2013, 07:54 PM   #26
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Steven, the LM353 will probably work fine for many duties, especially where the capacitance of the load is minimal - but one of the biggest improvements in modern op-amps is the ability to drive difficult loads with comfort.

Knowing the actual value of load capacitance will help ensure the design is optimised.

The LF353 shows high open-loop output impedance (>30 ohms) which can be a recipe for trouble where more than a handful of pF are expected.

The problems include instability (which can be fixed with 100 ohms of resistance in the output), and added distortion.

The supply voltage also plays a role, and if you are not working with +/- 15V, the LM8261 will also improve performance. +5V single rail will suffice for many test environments.
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Old 2nd January 2013, 08:00 PM   #27
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I'm using +/-15V supplies and and inverting amp configuration since it looks to generate less distortion than the non-inverting configuration.

I'm using 6' of RG174-u at about 30pf/F for a total of 180pF. Say 200PF including connectors etc.
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Old 2nd January 2013, 08:10 PM   #28
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180pF is high enough to degrade a LF353, I suspect.

A good app note on the subject:

Analog Devices: Analog Dialogue: Ask The Applications Engineer - 25

And yes, inverting amplifier should be best, I agree.
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Old 4th January 2013, 04:50 PM   #29
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I built a buffer using the LM353 while I'm waiting for better op-amps on order.

I used a LM317 pre-regulator with two TL431 shunt regulators for +/-15Vsupplies.

The first FFT is of the low distortion oscillator only.

Second FFT is of the LDO through the buffer.

The buffer adds a slight degradation of S/N, the second harmonic is up about 5dB and the third harmonic is up about 3dB, and there is some garbage beyond 10KHz but otherwise looks pretty good.

Third FFT is of the output from the source follower (IRFI820) without the buffer.
Fourth FFT is of the output from the source follower with the buffer.

Here there is a significant improvement in S/N, and a lot of the EMI is eliminated. I suspect the lower impedance output of the buffer is reducing the EMI suscpetance of the wiring.

Again there is about a 5dB increase in 2nd Harmonic distortion, but 3rd, 4th 5th, and 6th are down significantly, or they were being masked by the EMI.

Possibly the lower load of the buffer compared to the Sound card input is resulting in greater signal levels of the harmonics, but there is only a 0.24 dB increase in the level of the fundamental compared to a 5dB increase in the second harmonic. Shouldn't all signals be effected proportionally?

Am I still seeing an erroneously high distortion level, or is this actually an accurate measure of the tube? (6J9P, NOS)
Attached Images
File Type: jpg LNO Only.JPG (173.8 KB, 187 views)
File Type: jpg Buffered-LNO.JPG (178.8 KB, 183 views)
File Type: jpg Tube-w-SF.JPG (180.9 KB, 179 views)
File Type: jpg Buffered-Tube-w-SF.JPG (174.3 KB, 177 views)
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Old 8th January 2013, 09:35 PM   #30
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To answer my own question, this is probably still not an accurate measure of tube performance.

The high levels of distortion are probably due to my driving the input to near 0dB ref, and although I'm not clipping I suspect that I am over-loading the sound card, in which case I would expect to get increased distortion.

I'm working on a buffered Twin"T" filter to reduce the fundamental and thus prevent me from overloading the input.
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