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DOA - Discrete Op Amps
DOA - Discrete Op Amps
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Old 10th February 2019, 02:59 PM   #81
jfetter is offline jfetter  Costa Rica
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Mr Ken, For exact thd numbers please look at earlier post DOA - Discrete Op Amps


I'm only now running battery of tests on the grounded grid so for that check back later. I have removed the test cap green Muse and back to DC operation.
In my world, if the thd is below 0.01% then testing becomes more subjective with fft visualization more of interest.


I've used both high performance op-amps and general purpose protected types. The GG is more susceptible to oscillation with high performance parts and I'm still characterizing it. In GG the anode is shielded from cathode by the grounded grid.( No Miller capacitance)


The epiphany listening moments occurred when I repaired an old Zenith Transocean then tuning in by accident to a local AM station playing music. It was I suppose you could say life-changing. Then soon after I built hand wired discrete tube preamp(+-24V) connected to a $3.00 class D TPA3118 amp. More awaking there too. Later I listened to a Sphinx hybrid then was sure on right path. Products_sphinx.htm

I don't like inversion clipping op-amps. Its ugly so I came up with a simple thermal compressor. Yes just purchase newer inversion protected op-amps but a simple limiting compressor allows using legacy op-amps.
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Old 11th February 2019, 02:15 PM   #82
jfetter is offline jfetter  Costa Rica
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~20db using OPA1612 and 6922/6DJ8 , still clean and very low dist.
(pics)
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File Type: jpg 6922.JPG (31.3 KB, 232 views)
File Type: jpg WP_20190211_08_39_46_Pro.jpg (827.9 KB, 233 views)
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Old 11th February 2019, 03:04 PM   #83
Ken Newton is online now Ken Newton  United States
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Curious circuit. Because the first stage is a unity gain follower, it's not immediately apparent to me the exact effect that the triode's non-linearity has on the net output. Since the input stage is a follower and so doesn't feature voltage gain, and the total THD is low, the triode may be affecting the bandwidth and/or the distortion spectrum of the op-amp. So, my first thought is that, in conjunction with capacitance at the inverting input, maybe the triode is modulating the op-amp GBW as a function of the signal level? Whatever effects the triode is producing it would interesting to see the distortion spectrum of the output.
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Old 11th February 2019, 10:12 PM   #84
jfetter is offline jfetter  Costa Rica
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It's still unknown how the anode is powered and too if/how the electrons are avoiding the grid.
I reduced the heater to 4.35vdc and no sign of degradation. Below that no emission.
Different tube and different opamp can't be an anomaly.
Bandwidth is good for DC to 50-100khz in unity depending on opamp.
I looked at the opamps schematics no clue.
The first stage output is hi-z but always voltage correct.
Maybe some sort of charge pump effect but it's dc correct.
I'm laying out a general purpose board for easier testing.

Yep as soon as I an get my analyzer back will put up a waterfall.

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Old 13th February 2019, 01:09 PM   #85
Ken Newton is online now Ken Newton  United States
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This topology intrigues my curiosity. I may be forced to construct an version for evaluation purposes.

I do not doubt the net low THD you report. I suppose, that since the triode isn't functioning as a gain setting element, it's non-linear resistance wouldn't appear to induce net output distortion from the first stage. What parameter(s) is the triode effecting on the final output signal? I suspect that the distortion spectrum is being altered, although, since it's low in sum total anyhow, I wonder the subjective significance.

It would be interesting to compare the signal fed to the second stage versus that appearing immediately at the the output (pin-7) of the first stage op-amp. Which would be where the signal corrections for the presence of the triode's non-linearity is apparent.
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Old 13th February 2019, 03:40 PM   #86
jfetter is offline jfetter  Costa Rica
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I think I'm conceptualizing it incorrectly.

If the anode is held at a constant virtual ground, then modulating the cathode will cause the anode to follow. The overall graph shows anode voltage but for a follower it is aprox the same for a cathode adjusting for bias offset. The opamp takes care of that.

I measured grid current under normal operation and it was below what my meter could measure. During clipping it rises to about 200uA.
The available potential is 30V so looking at the 6DJ8 plot for zero volt grid, it begins to make sense.
The first plot is with grid at signal ground. It is very linear. The 330 ohm resistor helps the opamp remain in SOA.


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File Type: jpg 6dj8.JPG (235.0 KB, 148 views)
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Old 14th February 2019, 03:40 PM   #87
jfetter is offline jfetter  Costa Rica
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Why not test 6SN7 so here is. Same stellar performance.
Cleaned up the connections for better layout.



No time now for additional tests. Heading out to the Gulf of Mexico on new ship the VISTA , back in a few weeks.

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File Type: jpg 6sn7sch.JPG (34.0 KB, 118 views)
File Type: jpg 6sn7-04_Pro.jpg (753.7 KB, 119 views)
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Old 15th February 2019, 02:43 PM   #88
Ken Newton is online now Ken Newton  United States
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Quote:
Originally Posted by jfetter View Post
Why not test 6SN7 so here is. Same stellar performance.
Yes, I would expect relatively low THD via most triodes. In this follower configuration the triode doesn't determine the op-amp's gain, therefore, the triode's non-linearity does not non-linearly determine the op-amp's gain. However, I suspect the distortion spectrum is being altered, the extent of which is worth determining.

Quote:
No time now for additional tests. Heading out to the Gulf of Mexico on new ship the VISTA , back in a few weeks.
Have a pleasant journey.
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Old 15th February 2019, 02:53 PM   #89
Ken Newton is online now Ken Newton  United States
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Quote:
Originally Posted by jfetter View Post
Why not test 6SN7 so here is. Same stellar performance.
Yes, I would expect relatively low THD via most triodes. In this follower configuration the triode doesn't determine the op-amp's gain. However, I suspect the distortion spectrum is being altered, the extent of which is worth determining.

Quote:
No time now for additional tests. Heading out to the Gulf of Mexico on new ship the VISTA , back in a few weeks.
Have a pleasant journey.
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Old 12th March 2019, 07:09 PM   #90
jfetter is offline jfetter  Costa Rica
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I decided to do a simple pc board to evaluate this circuit before proceeding.
This particular 6SN7 design is beyond the scope of a DIP8 plug-in but it is reducable to very small form factor using the sub-minature triodes.

Note on measurements, my test interface has 0.003-4% THD as shown in loopback test. The proto board measures around same so I cannot measure the actual THD using this equipment.
These plots have a TL072 in one socket and OP275 in other. See filenames.

I'm still puzzled as to why the circuit even works. ( some opamps do not work at all)
An old vacuum tube text from 1947 mentions that some electrons will still get to the anode with open plate, even low energy slow electrons. Maybe that is the explanation. Or maybe it the op-amp input bias current, don't know.


plots attached.


Cheers
Attached Images
File Type: jpg OP275-PowerS.JPG (40.4 KB, 16 views)
File Type: jpg OP275-THD.JPG (79.0 KB, 19 views)
File Type: jpg OP275-WFALL.JPG (45.9 KB, 23 views)
File Type: jpg sch.JPG (42.7 KB, 40 views)
File Type: jpg TL072-RESP.JPG (68.3 KB, 40 views)
File Type: png UR22-MkII-loopback-baseline.png (6.9 KB, 22 views)
File Type: jpg TL072-THD.JPG (77.5 KB, 18 views)
File Type: jpg WP_20190312_12_45_20_Pro (2).jpg (729.7 KB, 34 views)
File Type: jpg WP_20190312_12_44_30_Pro.jpg (795.6 KB, 35 views)
File Type: jpg WP_20190312_12_47_44_Pro (2).jpg (883.5 KB, 36 views)
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