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cascode distortion Spectrum.
cascode distortion Spectrum.
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Old 18th January 2017, 03:10 AM   #1
mashaffer is offline mashaffer  United States
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Default cascode distortion Spectrum.

I have been looking at the triode cascode circuit as a phono preamp first stage as many have done. I see the advantages as high gain, low input capacitance,and relatively low noise.

To get a feel for the circuit I like to run spice simulations which I did in this case. I simulated for circuits using 12AU7, 6N1P, and 12AX7 using various biasing schemes.The difference in gain was about 9dB with the 12AX7 giving the highest gain (expected higher Gm tubes to have given the higher gain).

I also looked at the FFT knowing of course that the results would be of limited accuracy. Interestingly the distortion spectrum was amazingly consistent regardless of which tube was used or the bias points. HD2 was very low -60 to -80dB but HD3 was always between -36dB and -39dB. I kind of expected HD3 to dominate but was surprised that it was that high and that consistent.

So three questions for those who have designed cascodes.

1. Is the HD3 really this high (or the relative differences between HD2 and HD3).
2. Are there any design considerations that can be used to minimize HD3?
3. How objectionable is the HD3 in real life.

mike
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Old 18th January 2017, 04:46 AM   #2
artosalo is offline artosalo  Finland
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Quote:
...HD3 was always between -36dB and -39dB.
Seems too high distortion at signal levels present in phono input.
What signal level did you use in simulations ?
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Old 18th January 2017, 08:41 AM   #3
Rod Coleman is offline Rod Coleman  United Kingdom
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cascode distortion Spectrum.
Cascoded Triodes gives much better real-world performance than text-books suggest, and simulations are certainly not very reliable.

I started work on series cascodes over 12 years ago, and found that the important consideration is the cascode voltage. Using a triode at the top gives poor results compared to higher-gm devices, and bipolar transistors measure and sound the best.

Turning from Series to Shunt cascode improves performance again, and the gain can be increased greatly, without having to resort to high voltages.

It makes a superb RIAA EQ stage, because the output is a current, and so you only need a shunt-connected EQ network. this in turn means that the EQ performance does not get corrupted by the ra of the triode.


Here's an outline drawing of a 6Э5П shunt cascode, and a real measurement of spectrum at an output voltage of 1kHz, 75V peak-peak. No sign of 3rd harmonic problems at all!

The stage gain is about 170.
Attached Images
File Type: jpg Shunt-Cascode-OUTLINE.jpg (169.3 KB, 353 views)
File Type: png 25V2rms-74V4pp-102V-idle.png (49.1 KB, 342 views)
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Old 18th January 2017, 08:45 AM   #4
Rod Coleman is offline Rod Coleman  United Kingdom
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cascode distortion Spectrum.
The consistency of the Harmonic series in the output can be seen by looking at the 145V peak to peak output:


.
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File Type: png 49V8rms-145Vpp-109V-idle.png (49.1 KB, 332 views)
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Old 18th January 2017, 10:24 AM   #5
DF96 is online now DF96  England
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Quote:
Originally Posted by mashaffer
I also looked at the FFT knowing of course that the results would be of limited accuracy. Interestingly the distortion spectrum was amazingly consistent regardless of which tube was used or the bias points. HD2 was very low -60 to -80dB but HD3 was always between -36dB and -39dB. I kind of expected HD3 to dominate but was surprised that it was that high and that consistent.
Getting results which look wrong probably means they are wrong. Something in your circuit or analysis is not right. To a first approximation, a cascode gives you the distortion from a plain triode feeding into a lowish impedance so you mainly see the nonlinearity of transconductance. Second order should dominate.

Voltage gain is almost meaningless, as a cascode is a transconductance amplifier.
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Old 18th January 2017, 10:52 AM   #6
Rod Coleman is offline Rod Coleman  United Kingdom
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cascode distortion Spectrum.
Quote:
Originally Posted by DF96 View Post
.

Voltage gain is almost meaningless, as a cascode is a transconductance amplifier.
We are talking about the practical circuit: the load resistor is included. The outcome for an amplifier is a circuit delivering voltage gain.

Voltage gain is most relevant to the circuit performance, and the measured distortion also, at least somewhat.
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Old 18th January 2017, 01:00 PM   #7
DF96 is online now DF96  England
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With a low value anode resistor R, voltage gain is gm times R. The fact that the OP did not see this means either that he used a higher resistor value, or a different one for each circuit.

With a perfect CCS load voltage gain will be around mu^2. This will require a CCS with impedance much higher than mu x anode impedance - say CCS impedance of 20-50M for a 12AX7?

Typical biasing might mean an anode resistor roughly equal to the anode impedance. This will give a voltage gain around mu. I suspect that this is roughly what he saw, but modified by the exact resistor value.
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Old 18th January 2017, 01:48 PM   #8
mashaffer is offline mashaffer  United States
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Quote:
Originally Posted by artosalo View Post
Seems too high distortion at signal levels present in phono input.
What signal level did you use in simulations ?
5mV in output was around 300mV (Both peak).

mike
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Old 18th January 2017, 01:51 PM   #9
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by mashaffer View Post
I kind of expected HD3 to dominate
Why ever would you expect H3 to dominate in a single-ended triode circuit? Listen to DF96, he's never wrong.
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Old 18th January 2017, 01:55 PM   #10
mashaffer is offline mashaffer  United States
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The basic circuit...

Attachment 592967

I did diddle with the load resistor some with the other tubes (esp with the 12AX7 as the current was so much lower).

The above circuit gave this result.

Attachment 592968

Attachment 592969
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Last edited by jazbo8; 18th January 2017 at 03:14 PM. Reason: redundent images
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