Push-pull microphone preamp with UTC LS and Tamura transformers

Help Support GroupDIY:

aazaa

Well-known member
Joined
Apr 11, 2018
Messages
65
Optimised voltages and currents. The third harmonic distortion peak.

When the current of the ecc82 is increased slowly, you come to a point where the third harmonic distortion peak is very low. The dissipation of the tube easily reaches 2W/triode (max. is 2.5W). So the same experiment was done again with less voltage across the tube.

Surprise! With 150V across the test tube and a current of about 8mA/triode, the third harmonic distortion peak drops below -100dBu. Idem for the 12kHz and 15kHz IMD peaks. Once over that point, the distortion increases.
Dissipation is a very reasonable 1.2W/triode.

New B+ 325V. Stabilised B+ ! Fluctuation of B+ has to be kept as low as possible.

The high tension of the unregulated PSU fluctuated 15 à 20V depending of the hour of the day.

Regulation with a few zeners and 4 transistors: bc550c, bc560c, 2sb649ac and stp9n65m2. The zeners are fed by a very stable current source. No more chokes.
Current is limited to 100mA. Fluctuation is less than 0.7V.

Adapted anode and cathode resistors.
A 100k trimpot is adjusting the current drawn by the j-fets / the ecc83. The voltage across - and the current drawn by the ecc82 is set by the changed voltage at the anodes of the ecc83.

Nice to see that adjusting a trimmer at the very beginning of the chain reduces considerably the distortion of the tube at the end.

The adapted circuit:
uc


Below IMD and THD of the old Philips and Mullard ecc82 tubes. Incredible performance.
Tests with a french 12au7WAH Cifte tube of recent production: IMD 0.003% THD 0.004%.

uc


uc


uc


uc


Paul
 
Last edited:

aazaa

Well-known member
Joined
Apr 11, 2018
Messages
65
Playing with harmonic distortion.

The transparent sound of a tube preamp with almost no distortion is nice, but it can also be fun to add some extra colour. With the hybrid PP- preamp it is possible to increase the even harmonic distortion while keeping the odd harmonics very low.

A little experiment.

Overview of the adjustments:
2k trimpot: unbalance between the anodes of the ecc82: 0.00V
20k trimpot: unbalance between the cathodes of the ecc83: 0.00V
1M trimpot: second harmonic distortion turned up to -60 dBu
100k trimpot: odd harmonics lowered to below -110 dBu


THD is 0.13% almost pure even harmonic distortion.
And yes, fans of an uglier sound can do the inverse, low even and more odd harmonics.

A great control not only over the level, but also over the character of the distortion.

Paul
uc
 
Last edited:

Fdieck56

New Member
Joined
Dec 6, 2021
Messages
18
Location
Dallas
I am curious what your drain to source DC voltage is for the FET Pair input as I believe you are operating with less than the 10 volts on the data sheets for the given capacitance figures. The 2.7K resistors are gobling up more the Vds and the increased impedance seen by the drains is negating the cascode effect of the tube cathodes for the FET drains. 500 ohm gate resistors will really help reduce oscillations RFI susceptibility, and interaction with transformer leakage inductance. I would reference the grids of the ecc83 to get 12 to 15 volts Vds for the fets And get rid of the 2.7K resistor.


 
Last edited:

aazaa

Well-known member
Joined
Apr 11, 2018
Messages
65
I am curious what your drain to source DC voltage is for the FET Pair input as I believe you are operating with less than the 10 volts on the data sheets for the given capacitance figures.
The psu has been rebuilt very recently. The 3 Talema pot transformers were quiet but not dead quiet. The only downside of the preamp.
I decided to put the primaries of two transformers in series and run the third at a lower voltage. Voltage doublers both for heaters and B+. Maybe not ideal, but better than having to buy new transformers.

Now the psu is dead silent. Even when switching on, not the slightest vibration is audible. Only inconvenience: the high voltage has dropped to 288V. I will have to do a little revision to have the same ideal balance as before.

I can't give you the exact measurements for the posted preamp at B+ 325V.

The experiment with the increased second harmonic distortion is done at B+ 288.5V. Changing slightly the 18k cathode resistors resulted in the same low distortion figures as before.

measurements:
cathode ecc83: 23.35V
Vds LSK389: 14.15V


You can add gate resistors if you want, but in my build it really isn't necessary.
Functioning of the 2k7 and 68k resistors is explained in post #98.

Paul
 
Last edited:

Fdieck56

New Member
Joined
Dec 6, 2021
Messages
18
Location
Dallas
I really don’t understand how you get a DC cathode voltage of 23 volts out of a grounded grid ecc 83….. I mean I REALLY don’t understand it. With the stated drain voltages for the jfets a 2.7K drain resistor might near the “sweet spot” for distortion. I can understand not wanting to explain all the intricate details of the design of a nested feedback circuit, but please don’t take my comments as just putting my two cents in. I am a BSEE and have been designing with jfets since the mid 80s and interested in your design. Thank you for your reply.

Fred

PS I went back looked at the circuit again because I figured I MUST be missing something….. I see the grids are not grounded but lifted by DC through the plate connected feedback network. “The whole interaction of all the elements is too complicated to describe“ might be closer than I thought to summing things up. Thanks for stirring up the gray matter, I think following current events has turned my brains to mush.
 
Last edited:

aazaa

Well-known member
Joined
Apr 11, 2018
Messages
65
I really don’t understand how you get a DC cathode voltage of 23 volts out of a grounded grid ecc 83….. I mean I REALLY don’t understand it. With the stated drain voltages for the jfets a 2.7K drain resistor might near the “sweet spot” for distortion. I can understand not wanting to explain all the intricate details of the design of a nested feedback circuit, but please don’t take my comments as just putting my two cents in. I am a BSEE and have been designing with jfets since the mid 80s and interested in your design. Thank you for your reply.

Fred

PS I went back looked at the circuit again because I figured I MUST be missing something….. I see the grids are not grounded but lifted by DC through the plate connected feedback network. “The whole interaction of all the elements is too complicated to describe“ might be closer than I thought to summing things up. Thanks for stirring up the gray matter, I think following current events has turned my brains to mush.
Indeed Fred, you're right. The grids are not grounded.

Forgot to specify that the LSK389 used is the LSK389A

Maybe also this:
I always tune my preamps to the lowest distortion!

Paul
 

aazaa

Well-known member
Joined
Apr 11, 2018
Messages
65
Tune the ecc83 - or not, a different approach.

First this: there is no necessity to adjust the unbalance between the cathodes to 0.00V.
Without the 20k trimpot it is still possible to turn the distortion down to the same low levels with the 100k and 1M trimpots.

I knew that from the start. Nevertheless - driven by some obscure intuition(?) - I prefer to trim the cathode unbalance to 0.00V.

With my tubes - not selected ones - the 20k trimpot was turned almost all the way to one side. The anode resistances were near 180k and 200k.

A different approach: Why not adjust by means of a small correction of the 56k grid resistor on one side? Since the 20k trimmer was already installed, I combined both ways.

First channel: I adjusted the 20k trimpot to have exactly equal anode resistances. A 10M resistor in parallel with one of the 56k resistors reduced the unbalance to just a few millivolts. A small adjustment of the 20k trimpot was enough to set the unbalance to zero.
Correcting this way resulted in an exactly equal level of the mirrored signals at the anodes of the ecc83.

Second channel:
6.2M parallel with 56k (4.7+1.5M). Same result.

But - as mentioned - you can forget about it, omit the ecc83 unbalance correction and reach the same low THD level by trimming simply the 100k and 1M pots.
I like my approach more, but have no convincing argument for doing it my way.

Paul
 
Last edited:
Top