Push-pull microphone preamp with UTC LS and Tamura transformers

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MaxDM

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Indeed, I mentioned this already in post #8:
"Moreover I like the idea of using opposite triodes correcting each other when wired in a long tail circuit. Idem dito for the 'crossed feedback' connecting the opposite sides."

In my opinion this is one of the qualities of the design...

Interesting paper!
Conclusion of the paper: " In a class A amplifier the use of a bypass capacitor generally reduces the intermodulation distortion, although it may either increase or decrease the harmonic distortion.
The decision as to whether or not to use such a capacitor depends mainly on the magnitude of the distortion. If it is very small, then it may be safely left off, with no possible audible difference."


The 2 charts (5kHz and 10kHz) of fig.9 show that when the output is low, - 6V with a charge of 15 ohm - the intermodulation distortion without bypass capacitor is lower in both cases. A low output of 2.4 Watt has lower intermodulation distortion without cap. Most of the music information played in a normal living room with reasonable high rendement speakers is within the first couple of watts.

In the microphone preamp the maximum output level used to record is low. Average level very low.

Not only the unbypassed common cathode resistor is playing a correcting role. The interplay with the cross-coupled NFB is important. This NFB is also influencing the behavior of the double j-fet. The voltage on the drain on both sides is constantly changing with the signal.

Whatever, the output connected to the RME AD convertor is measuring flat on scope. I'm pleased with the result when recording ...

And... of course it is easy to bypass the 16k5 resistor or to connect 2 separate 33k resistors on the cathodes and bypass both.

Paul

Yes, the IMD issue is probably not such a great deal at these voltages and currents.

I would still be concerned in driving an output transformer with a cathode resistor that large. The maximum current available is limited by it, in addition to the plate resistance summed.

Better would be to use a combination of a large cathode bypass, and adding whatever negative feedback from the anodes to the grids in whatever way you prefer, which could be cross-coupled as well, IMO.
 

aazaa

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Apr 11, 2018
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Yes, the IMD issue is probably not such a great deal at these voltages and currents.

I would still be concerned in driving an output transformer with a cathode resistor that large. The maximum current available is limited by it, in addition to the plate resistance summed.

Better would be to use a combination of a large cathode bypass, and adding whatever negative feedback from the anodes to the grids in whatever way you prefer, which could be cross-coupled as well, IMO.
The paper is rather convincing not to bypass the cathode resistor. The better values without the capacitor for intermodulation distortion below 2.4 Watt are not in the charts. Harmonic distortion is uncertain at the low frequencies and: "At the higher frequencies the addition of the capacitor quite consistentely increased the distortion."

I don't know the load you want to use. We don't have to drive a speaker.

Maximum usable current with the 16.5k resistor is 150/16.5=9mA, with or without the capacitor.
If more current is wanted a lower resistor can be used.

About the voltage needed: my RME convertor is happy with 2V to have full output.
With a load of 600 ohms at the low impedance output I measure maximum 18V 20Hz-43kHz. Same limit with or without the bypass capacitor. At higher frequencies there is a lot more output.

Limitation is in the capacity for the lows of the output transformer. Raising the current is giving only little improvement. With the capacitor there is a possibility to go slightly higher at 46Hz, not at 20Hz.

With an output of 3V at the fixed output (no bypass capacitor, 600 ohm load): 8Hz-43kHz.

I don't use the fixed low impedance output to connect the variable output.
Input impedance of the RME is 10k. More logical and possible to use the other one. There is a better ratio, a lot more headroom.
Transformer ratio: input 4/4, output 5/1

Thanks for the reaction, MaxDM!


Paul

(And: If you want to draw maximum current, it is a bad idea to use 2 separate 33k resistors.)
 
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MaxDM

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Well, I would look at square wave response with the 16.5K resistor and pay particular attention to the sharpness of the corners of the square wave, and possible ringing.

The maximum current flow is due to the sum of the cathode resistor, and the resistance of the plates as well.

But if you like the way it sounds, that's what's important.
 

aazaa

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Apr 11, 2018
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Well, I would look at square wave response with the 16.5K resistor and pay particular attention to the sharpness of the corners of the square wave, and possible ringing.

The maximum current flow is due to the sum of the cathode resistor, and the resistance of the plates as well.

But if you like the way it sounds, that's what's important.

No visible difference at square wave response with or without cap...

And yes, it's all about the way it sounds!
 
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aazaa

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The maximum current flow is due to the sum of the cathode resistor, and the resistance of the plates as well.
Agreed MaxDM. When choosing the tubes the internal resistance has to be low enough and the maximum dissipation high enough.

In this configuration, when capable tubes are choosen, it is not the sum of the cathode resistor and the resistance of the plates which is decisive for the maximum current flow.

In the preamp the common cathode resistor is functioning as a current source.
The choice of this resistor is decisive for the current trough the double triode.
In the preamp 150V/16.5=9mA.
If a larger 22k resistor was choosen: 150V/22=max 6.8mA, with 33k only 4.5mA, with 47k a low 3.2mA...

Same result with different tubes having different internal resistances.

Paul
 
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aazaa

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Apr 11, 2018
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In a new experiment, the LSK389-circuit was built at the second channel with the UTC LS12 (1:14 ratio). To reduce partially the extra gain of the transformer, 390k resistors were added between gates and cathodes on both sides of the LSK389/ECC83. (Of course, to avoid the additional NFB the values of the 2k7 and 22k resistors can be changed.)

I suppose the lowest possible noise is reached now. There is 5dB more noise when a 180Ω resistor is connected at the 600Ω input than with an open input. (Visible on computer with the RME app)

No more 40kHz. A roll off starts at 16.5kHz. Was expected, the LSK389 has 25pF at the input. When a 1:10(Ls10) or a 1:14(LS12) transformer is used, the LSK489 (4pF) would be a better choice. The tiny bit more noise of the LSK489 is largely compensated by the gain of the transformer.

A 1:4 transformer at the input and the LSK489 as monolithic matched dual J-fet is probably the best choice.
No additional NFB or changing of the resistors necessary + a better possible performance of the transformer/preamp.

There is also the possibility to keep the UTC LS30 (1:1 ratio) and add a parallel dual J-fet for lower noise.

Paul
 
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