Line stage preamp, based on SY's "Heretical Unity gain line stage" - Page 2 - diyAudio
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Old 18th June 2005, 02:16 PM   #11
Danko is offline Danko  Hungary
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Is it a big problem, if I put the B+ filtering RC-network on another board, and not on the tube's board? In this case (when thr RC net. is not on that board) I put 1uF - 1uF capacitors to the B+.

Is it a good idea, to create a biiiiiigbig plane on the component side, which is not connected to anywhere, except the GND, but the components aren't soldered to it? I'm thinking of somekind of shielding. Look ad the attached image!
The plane would be soldered to the central GND-star point with a wire.

There's another advantage this: I have ONLY 2-sided plain-PCB at home, and if I make single-sided boards, from one side of the board, the copper goes into trash.
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Old 18th June 2005, 02:32 PM   #12
SY is offline SY  United States
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You want that RC filtering as close as possible, especially the film bypass cap.
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Old 18th June 2005, 03:22 PM   #13
Danko is offline Danko  Hungary
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Is it close enough?

Can I connect the two circled foils? see attached file
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Old 18th June 2005, 03:24 PM   #14
Danko is offline Danko  Hungary
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damn, i have to scale down the image so much, it's very blurred
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Old 17th July 2005, 06:44 PM   #15
Danko is offline Danko  Hungary
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Talking Preamp /almost/ finished!!

Hi all!


First of all, I would like to say thank you for everyone, who helped me in this project, especially to SY, who designed the circuit!! :-)

And some technical stuff

I planned to use 100% switching mode powersupply for the tube, but the heating-supply wasn't good. I planned to use MAX8454, but it soooo small, I couldn't solder it
The plate supply was made with MAX1771 + IRF840. This boost-converter produces about 120VDC from 12V. After this, there come's a MOSFET stabilizer. This stabilizes the voltage to 108V. Of course, the mosfet doesn't just switch down instantly the plate voltage to the tube. The plate voltage slowly increases, in about 1 minute.
I miscalculated the summa voltage of the Zener-diodes, and since I soldered it, I haven't changed them... Is it a big problem, that the plate voltage is 108V instead of 90V ?

The heating is made with a 7805 + 2x1N4148. The diodes are connected series with the GND terminal of the 7805. When I first tested the heating supply, i have some doubt about how accuratly will be 6.3V the heating voltage, but I measured and it was 6.2X V, so I think it's OK.

This entire project is about a LINE STAGE amplifier, which is designed for a 10kOhm load impedance. But I tried to put my Sennheiser (HD202) headphones on this preamp. At my surprise, it sounded!! At maximum volume level the output voltage is about 600mV peak-to-peak. At this level the tube is distorting heavily.

Sy wrote, that the constant current generator must be set to 10mA. I put in series with the plates a 10 Ohm resistor, and I measured the current with this. Unfortunatelly I forget to buy 470 Ohm trim.pots, so I put instead of them 1kOhm trimpot. And /maybe becouse of this/ the minimum current, that I could set, was about 14mA. Isn't it too much?

Is it possible, to make some voltage amplification with this stuff? just a few deciBel -s, not 30dB

And once again, thank you for all, who helped me !!!
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Old 17th July 2005, 06:49 PM   #16
Danko is offline Danko  Hungary
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oh, by the way, i made a small A/B test with a relay.

I'm listening now with a Sennheiser HD202 headphone. And when I switch between the preamp, and between the PC (directly), I cannot hear so much difference.
Okay okay, i'm listening to mp3, but ....
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Old 17th July 2005, 07:31 PM   #17
SY is offline SY  United States
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With the adjustable CS, a higher resistance trimmer will result in lower current, not higher current. With a 1K pot turned to maximum, you should be able to get (1.7-0.7)/1K = 1mA of so. Are you sure you didn't slip a decimal place in your current measurement?

A 1K trimpot will still work, you'll just have to be working in the lowest part of its range.
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Old 17th July 2005, 08:18 PM   #18
Danko is offline Danko  Hungary
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Thanks, tomorrow I will measure it again.
Now I just want to listen to this lil' thingie :-))

BTW, how can I amplify a few dB with this?
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Old 17th July 2005, 08:42 PM   #19
SY is offline SY  United States
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You could use an input transformer with a small step-up ratio. That would be easiest.
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Old 17th July 2005, 09:45 PM   #20
Danko is offline Danko  Hungary
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huh, i would like leave transformers out of this project
If it's too difficult (it can't be solved be simply changing one resistor), then i will use in this state, without amplification).
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