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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

ECC82 preamp for LM3886 opinons

siriusbg

Member
2011-12-16 1:53 pm
Hi, guys!
First of all sorry for my not so good english. :)
I'm writing here for advice about a circuit I made. I have LM3886 amplifier, but I want to try to add some tube flavor to it. The idea is to use the existing amp power supply and chassis. The tube is ECC82 in parallel with B+35V, Ia 5mA. I know that B+ is low, but I've made NP-100v12 project and I really like the sound. The tube stage gain is 3(i want to drive the LM to full power in 8ohms with 300vRms). The output inpedance is low enough to drive the inverted LM. I will be glad for some professional opinions about my design.

Best Regards to all!
 

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kodabmx

Member
Paid Member
2011-10-31 1:00 am
Toronto
Agreed. You'd be best to use a 6N27P/6GM8/ECC86. A 6DJ8 will also work better at that voltage than a 12AU7. Why not use the -35V rail too? Then you have 70V across the 12AU7 and since you already have input and output caps, it's almost as easy as a simple rewire.
 

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siriusbg

Member
2011-12-16 1:53 pm
So I've decided to use the two power rails for the tube stage. I'm providing schematic. The +-30v are after regulating them. My concerns are for the grounds. Because the tube stage ground is -30v below the LM3886 center tapped ground and they are from one transformer, so they are not isolated. Is this not a problem for a potential ground loop?

Thank again in advance!
 

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I'm not sure why you increased the voltage across the 12AU7, as surely the whole point of this exercise is to add a little distortion? You can do this with an inadequate supply rail voltage (as in the first design) or a very low anode resistor (as in the second design). The 12AU7 can provide mainly second order distortion, but you have added a little feedback to ensure some higher order re-entrant distortion too. Not to mention the thermal noise from the 22k input resistor, although this will be small at line level.
 
So I've decided to use the two power rails for the tube stage. I'm providing schematic. The +-30v are after regulating them. My concerns are for the grounds. Because the tube stage ground is -30v below the LM3886 center tapped ground and they are from one transformer, so they are not isolated. Is this not a problem for a potential ground loop?

Thank again in advance!

I think since you have -30VDC to the tube's input 'ground', you also need a DC blocking capacitor from the input signal ground to the -30VDC rail. Otherwise, you'll be injecting -30VDC onto the preceding output signal ground.

Why not use a 1:1 input transformer? More old school stuff in the signal path, and it will block DC in all directions.
--
 
If going for "tube flavor" a 12AU7 finally has a chance to shine as a gain stage, but running feedback may be counterproductive to the goal, even if it does reduce gain :)

As rongon suggests, a transformer would be a great solution for input. The cheaper 10k:10k stuff would do fine here, so long as it is flat enough across the audio band.

Personally, if you are using one bottle per channel and don't need much gain anyway, I would look to using a triode-connected pentode running across the power rails. Gain will be lower without feedback to begin with, and it will be less linear (giving more tube-ish sound) at lower voltage. It would be a great solution, however. Look at the 6V6 preamp Salas has here- 6V6 line preamp

You could do this with 6V6, EL84, or any of a number of common pentodes. It's a bit funky at low voltage, but will likely work better than 12AU7.

The russian 6P43P-E has been on my radar for a while for low voltage use-
el_43p_low.gif
 

siriusbg

Member
2011-12-16 1:53 pm
After your suggestions, I've come up with new clean design. No feedback, one rail power supply(30v) - no more ground problems. The tube is hot biased at -0.5v, 3.5mA per half(7mA whole tube). The anode voltage is at 20v DC witch is still far from clipping. I've also reduced the LM gain to 12 to compensate for the high tube gain. I will be happy for new criticism. :)

PS: Thanks for the suggestions for EL84 and isolation transformer, but I think both of the variants will have much high distortion.
 

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U130421

Disabled Account
2015-12-16 2:00 pm
After your suggestions, I've come up with new clean design. No feedback, one rail power supply(30v) - no more ground problems. The tube is hot biased at -0.5v, 3.5mA per half(7mA whole tube). The anode voltage is at 20v DC witch is still far from clipping. I've also reduced the LM gain to 12 to compensate for the high tube gain. I will be happy for new criticism. :)

PS: Thanks for the suggestions for EL84 and isolation transformer, but I think both of the variants will have much high distortion.
The lm3886 gives it's best with gain of less than 3 , but oscillate bellow 10 . By applying as shown bellow you can run it as at high frequencies C1(to adjust) brings the gain to 10 . The tubes than give necessary gain. At start up , to keep the 3886 muted until the tubes are ready , you can use the cathode voltage to unmute .
To remind you a phenomena about 3886 , if the ac impedance at the input exceed 20k , the IC generates a shower noise .
 

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