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lm3886 + cathode follower PS problem
lm3886 + cathode follower PS problem
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Old 18th March 2010, 11:28 PM   #1
nightanole is offline nightanole  United States
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Default lm3886 + cathode follower PS problem

I got a lm3886 kit from chipamp.com

Ive been looking for a simple way to make a tube buffer from it when i came across the lampizator cathode follower using the 6N6P.
Now he uses 2 circuitslinks at bottom)
CD DAC MAXI-Lampucera lampizator

In the hand drawn one:
grid R = 200k
cathode R = 330
then an output cap with no resistor to ground after it

In the hard to use office format:
It has an series input resistor of 1 ohm ??? going strait to grid
cathode R = 330
Then an output cap with a resistor(47k) to ground

Im assuming the hand drawn one is correct. Im also assuming (ms format) the input resistor is really the dac load output, and the resistor after the cap is a volume control pot.

Now heres were i have a problem, the heater and B+. My main transformer for the amp is a 22v + 22v (gonna be a 4ohm load). If i used the secondaries as a voltage source, what would be the best way to get 6.3vac(750ma) and 135-150vdc (i cant read his hand writing for current requirements, it looks like 011A or 019A for 2 tubes?

Since the B+ current requirements are so small i was thinking of tapping the rails for 44 and using a small 120/240 euro converter transformer.

Or should i just get a 6.3vac transformer and a 120/120 isolation transformer for the B+? I assume i cant get the B+ strait from the wall...
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Old 19th March 2010, 04:46 PM   #2
Rodeodave is offline Rodeodave  Austria
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lm3886 + cathode follower PS problem
Your assumption is right, DON'T EVER take any voltage "straight from the wall". This would be very deadly. Always use a transformer with two isolated windings (not a autotransformer) and a fuse.

Concerning your questions:

Not that I encourage you to do so, but you could wind a couple of turns of isolated wire onto the transformer's core (assuming it's a toroid) to get a 6.3V secondary. I'm only mentioning this because you may find this advice somewhere else and might even find some projects where people actually do that, but I strongly advise you not to!

Simply use a higher-voltage primary a regulator, 7806 or better LM317, and use DC for heating. The regulators can drop 20-30V and usually require a heatsink.
You should be able to determine the total current draw from the maximum dissipation of the tubes by looking at their datasheets.
To get the HT B+ from a LT supply can be done with a switchmode PSU, but these are usually noisy, complicated and less reliable. Use a designated transformer for the HT and heater, and make a nice star-ground.
Or even better, use a LT design for the tube buffer, like these:

Tube-buffered Gainclone
Building a valve-buffered Gainclone chip amp.
Tube Gainclone for DIY Buffs
Tubes & The Gainclones
valve buffer gainclone

Hope this helps.
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Old 19th March 2010, 05:33 PM   #3
Rodeodave is offline Rodeodave  Austria
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lm3886 + cathode follower PS problem
Exclamation Severe Typo

Quote:
Originally Posted by Rodeodave View Post
...
Simply use a higher-voltage primary a regulator, 7806 or better LM317, and use DC for heating. The regulators can drop 20-30V and usually require a heatsink.
...
Wow, I completely messed up this sentence. What I want to say is that you could use a (higher than required) 22V secondary(!) to get a regulated 6.3VDC.
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Old 19th March 2010, 08:19 PM   #4
nightanole is offline nightanole  United States
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Hmm. All these are completely different from the chipamp's non inverted setup. However i do like the idea of using a low voltage 6dj8/6922. I will have a bit cooler rails at 30vdc since im shooting for 4ohms. I dont know how to do the math to make a standard cathode follower using this tube.

Back to the lampi version with the 6N6P. I was going to use 2 of these:
http://www.mouser.com/catalog/specsheets/XC-600060.pdf

So i convert the wall to 6.3vac for the heater, then i connect the 2nd one backwards and convert the 6.3vac back to wall voltage. This should give me both voltages i need for about 12 bucks. Ive seen this done in several flea layouts.

Last edited by nightanole; 19th March 2010 at 08:30 PM.
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Old 19th March 2010, 08:34 PM   #5
woody is offline woody
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Why not just go for a real low voltage tube. Here is a line stage that runs
off 24v. Or have you thaught about just using Nelson's B1 jfet buffer?
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Old 19th March 2010, 09:12 PM   #6
nightanole is offline nightanole  United States
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So this uses 2 E188cc for stereo, 24vdc rails, and 12v heaters?

Hmm. If i went with this, could i use the 30vdc rails for B+? And with the 12v heaters i could just pop in a 12v regulator from the rails too, no extra transformers necessary.


EDIT: Holly crap these are $100 a pair or more...

Last edited by nightanole; 19th March 2010 at 09:16 PM.
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Old 24th March 2010, 07:20 AM   #7
Rodeodave is offline Rodeodave  Austria
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lm3886 + cathode follower PS problem
Here are two examples of not-so-standard PSUs:

DIY 12AU7 Tube Preamplifier Project with back2back transformers, and
12AX7 Tube (valve) Preamplifier Kit with a DIY SMPS.
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