Author Topic: 6N1 preamplifier with phono  (Read 3160 times)

Offline Mars

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Re: 6N1 preamplifier with phono
« Reply #15 on: August 13, 2016, 07:38:45 PM »
Thanks all to the useful comments. Much appreciated :thumbs:

The CCDA did not want to balance out with equal amounts of current running through the two triodes. I went back to the more simple option, as shown in the circuit diagram above, of parallel triodes in grounded cathode formation. I think the gain is a bit high; the phono preamp hiss is quite audible. Voltages have jumped up a bit, since the CCDA run quite a bit more current.

Effectively the power supply is a C-R-C design, as simple as you get. I do not like solid state voltage regulation, preferring good capacitors and resistors for a more musical experience. That is the reason for the MKP caps in the phono preamp section.

I was worried I would get hum on the preamp output. I am listening to the preamp now, and it sounds clear and lovely. Preamps are really good for improving the listening experience of youtube videos! My Dell tower internal sound card needs all the help it can get.

The phono is playing both channels. We'll test with turn table tomorrow.
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Offline Ampdog

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Re: 6N1 preamplifier with phono
« Reply #16 on: August 14, 2016, 06:21:04 PM »
Ah! good for you, Mike!  :point:

Keeps me alert. Perhaps I should have said: Maximum performance/efficiency attainable with used components .......

(How about saying yourself: What, whence - according to whose hearing!   :Ooooooh: But perhaps to stop somewhere, before someone is triggered into another  :banned: )

Hope you are well!
Creativity requires the courage to let go of certainties.  (Erich Fromm)

Offline handsome

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Re: 6N1 preamplifier with phono
« Reply #17 on: August 15, 2016, 06:58:13 AM »
Are you sure the CCDA did not work? Remember it is the signal currents that are balanced NOT the DC currents.....

Offline Mars

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Re: 6N1 preamplifier with phono
« Reply #18 on: August 15, 2016, 10:03:38 AM »
You might have a point there :-[ The current through the cathode resistor of the cathode follower was more than double the gain stage current. It did sound good, though. Maybe I only need 5 watt cathode resistors :thinking:
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Offline handsome

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Re: 6N1 preamplifier with phono
« Reply #19 on: August 15, 2016, 11:28:11 AM »
CCDA attempts to pull a constant current from the PSU. Constant current is a DC current by definition. The two triodes can draw unequal DC currents as that is a constant draw according to the PSU (it doesnt know how the current is split between the triodes). By using matching load resistors the signal voltage will create equal but opposite AC currents in the respective load resistors (the cathode follower has a notional gain of 1 so it is the same magnitude of signal voltage that will appear across both load resistors). The currents therefore cancel and the total current draw is constant.

Resistor wattage you will have to calculate according to the DC current

Offline ScottulusMaximus

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Re: 6N1 preamplifier with phono
« Reply #20 on: August 17, 2016, 10:52:42 AM »
Karel still hard at work, shame I think this build may be the end of him!
Please PM me for anything you need imported. See here.

Offline Ampdog

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Re: 6N1 preamplifier with phono
« Reply #21 on: August 17, 2016, 11:24:31 PM »
Nah

He's still young compared

And there is always some 'Cape Juice' close by to - er - assist muscle-and-mind.
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Creativity requires the courage to let go of certainties.  (Erich Fromm)

Offline Mars

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Re: 6N1 preamplifier with phono
« Reply #22 on: August 18, 2016, 11:19:46 AM »
Building something like this, is a bit of a sweat, but what worthwhile is't?

I think I have the CCDA under wraps. Changed the cathode resistor to 10K 5watt. Dissipation is 3.3watt, so the resistor heats up, but no discolouration or smoking! This preamp is a non-smoking zone! :2thumbs:

I measure 180V across the cathode resistor (of the cathode follower), so that means 18mA through the triode. This is just inside the max rating of 20mA as printed on the data sheet.

I also found a wiring error in the phono stage. No hiss any more. Time to get the LP's out!
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Offline ScottulusMaximus

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Re: 6N1 preamplifier with phono
« Reply #23 on: August 18, 2016, 11:55:53 AM »
Woohoo!!!!
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Offline handsome

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Re: 6N1 preamplifier with phono
« Reply #24 on: August 18, 2016, 12:14:54 PM »
If you don't mind me commenting on your build sir, and if you do please ignore the following (or just tell me to STFU)

18mA is way too high for a 12AU7 you saved the resistor but you are going to cook the poor tube  :flame: if your cathode resistor has 180V across it then the tube has only 120V across it and with 18mA of current that means it is biased positive and running extremely hot.....

I assume your Ht is ~300V so try this: choose a resistor value for the first tube's anode and the second tube's cathode - make it between 22k to 39k (good values for a 12AU7). Then adjust the value of the first tube's cathode resistor until you get the same current through each tube (between 5 and 10mA) OR aim to get approximately a third of the HT voltage across the second tube's cathode.

The CCDA is just a special case Cathode Follower direct coupled to a Common Cathode stage - special because we aim to balance the AC currents. In any direct coupled layout the anode voltage of the first tube sets the bias conditions for the second tube. So by choosing a reasonable load resistor value you can then adjust the first tube's cathode resistor to then set the conditions for both tubes.

You will have to ascertain the first tube's anode voltage from the datasheet curves as trying to measure the voltage on the cathode follower's grid is not possible as the multi meter's impedance will create an error.


Offline Mars

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Re: 6N1 preamplifier with phono
« Reply #25 on: August 21, 2016, 12:41:13 PM »
Thanks handsome; it's all coming back to me now :teach: I used the same tweak to set up the dc coupled phase splitters :facepalm:

OK, so I did that. I pulled out my decade resistance tool, and tried to set up the 10k resistors for equal current flow (same DC voltage drop across the 10k resistors). Not even 0 ohms could balance the CCDA. Next I moved to 33k resistors. I could bias the CCDA nicely with a 470 ohm resistor, but the current flow was about 4mA through the triodes. I would like to get closer to the 10mA handsome suggested. So next is 22k. I had some 5watt resistors and it all balanced nicely with a 100 ohm cathode resistor on the first triode. The voltage across the 100 ohm is 1.2V, and the voltage drop across the 22k's is about 145V on average. That is about 7mA per triode. Closer to the 10mA target number. I could try something lower, but I do not want to bias the input triode any lower than 1.2V, so I'm running with the setup a bit, to check if the sound is good.

I left the preamp with emtturntable to check out the phono stage. There should be some comment by Monday, I hope.
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Offline GECO

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Re: 6N1 preamplifier with phono
« Reply #26 on: August 21, 2016, 02:14:41 PM »
Looking very good Karel.

Interresting point you make about solid state vs conventional valve power supply. I really think it comes down to design an obviously personal choice.
If you look at Allen Wright he used a super regular on his own design and recommended nothing else. I found it also true when experimenting with various option. Conventional SS regulation just didn't cut it. But with a little more effort you can create something really special and extremely musical.

I will make you a spare board of my quitus power supply with the next run and send it down to you. In essence it is a vbe multiplier with buffered output

Offline ScottulusMaximus

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Re: 6N1 preamplifier with phono
« Reply #27 on: August 22, 2016, 09:10:38 AM »
I would like to pretend I know what you're all talking about... But it seems to be under control, so I'll relegate myself to cheerleader!

Go Karel go!!!
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Offline Mars

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Re: 6N1 preamplifier with phono
« Reply #28 on: August 22, 2016, 12:43:11 PM »
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Offline handsome

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Re: 6N1 preamplifier with phono
« Reply #29 on: August 22, 2016, 06:35:15 PM »
Hi Mars, 1.2V over 100R is 12mA not 7mA?

You cannot measure the anode voltage of the first triode as it is DC coupled to the second's grid. This is a very high impedance point and any multimeter will give you an incorrect reading. You will have to estimate the first triode's anode voltage by measuring the current and voltage across the cathode resistor. Then use this current to calculate the voltage drop across the anode resistor and then subtract that voltage from the HT.

Digital has a nice fixed output maximum of 2V RMS. Vinyl unfortunately does not. Plus with records you have scratches and dust which can cause very high transients way above program material. Because of these factors you should try and bias the first triode to -2V or better to ensure no clipping. All tubes experience grid current as they approach zero grid volts so i always try to bias (if possible of course depending on the tube) a 0.5 - 1V margin - in your case -3V.