Author Topic: 6N1 preamplifier with phono  (Read 5421 times)

Offline Mars

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6N1 preamplifier with phono
« on: July 29, 2016, 09:51:05 AM »
Hi All

As you know from Scott's thread, I am building a integrated line stage/phono preamp for him using 6N1P tubes. I am following the audio section of this schematic:



I am using a R-core voltage doubling power supply and I want to see if I can get away with AC on the filaments and CRC regulation.



All is well, except the plate load resistors for the triodes need to be recalculated.
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Offline Mars

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Re: 6N1 preamplifier with phono
« Reply #1 on: July 29, 2016, 10:10:04 AM »
Looking at the first triode of the phono stage: If I enter the stated values for a 6DJ8 triode, the output impedance is:



The calculated output impedance is: 2.54k ohm.

I am using the svetlana 6N1P data  http://www.svetlana.com/pdf/svetlana/6n1p-svetlana.pdf

This is what I glean from the pdf:



If I use the same TUBECAD software to calculate a plate resistor that delivers the same output impedance, I get this:



I need to use a 4400 ohm plate load resistor for the 6N1P? Am I correct? :thinking:
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Offline gavinbirss

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Re: 6N1 preamplifier with phono
« Reply #2 on: July 29, 2016, 10:11:14 AM »
Almost though by reading heading you were talking about the Chinese 6N1 tubes.

You have any preference on brand of 6N1P tubes?
My 6N1P's (Svetlana) always seem to produce a slight amount of hum (in Schiit Lyr headphone amplifier).
Heaters are AC also.

Offline ScottulusMaximus

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Re: 6N1 preamplifier with phono
« Reply #3 on: July 29, 2016, 01:41:12 PM »
Oh Karel, now you're just teasing me...

I actually nearly know what you're talking about too, that should cause great concern to all!
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Offline handsome

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Re: 6N1 preamplifier with phono
« Reply #4 on: July 29, 2016, 05:15:44 PM »
you could adjust the 130k resistor to compensate for the different tube characteristics .....

Offline Mars

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Re: 6N1 preamplifier with phono
« Reply #5 on: August 09, 2016, 02:49:53 PM »
Hi All

I have been cautiously powering up the preamp over the last two days, and the circuit is starting to take shape.

While contemplating the power supply design and the practical aspects of the parallelled 6N1P gain stage after the volume control, it occured to me that the design on the schematic above relies a lot on the voltage regulation in the power supply to give a noise free output. Having two triodes available for a single output stage, could be used more efficiently and have a better power supply rejection. The favourite with me at the moment is the so called CCDA that is blogged about on the Tube Cad Journal.

The other advantage is the possibility to affect the gain of the preamp by using 12AU7 is stead of 6N1P for a lower gain output. We might need this in matching the pre amplifier with the power amplifier/speakers/room. This would be a plug-in-and-play option. Ideal for the non technical user.

Here is a photo (a bit blurry) of the filament wiring in place:



Next I soldered the earth rail to the turn table earth point. This is also the star earth of the whole pre amplifier.



Here you can see the two line inputs and the output RCA's



Here is a close up view of the CCDA and the two 100ohm resistors that creates the virtual centre tap for the filament reference. To clarify: the heaters are powered with 6.3V AC, but the filament circuit has to be referenced to earth somehow. If you do not do that, the sound suffers.



Here is another close up of the filament junction. I use a polypropylene capacitor to AC earth the virtual centre tap. This stops a lot of nasty noise getting into the circuit. I recommend the capacitor on all pre amp circuits with AC heaters. The blurry cap in the fore ground is soldered onto the 100ohm junction and the earth rail.



Usually I reference the filaments 10 or 20V above ground for regular circuit, but in this case the cathode follower of the CCDA is 100V above ground. A reference closer to 50V would put the filaments reference half way between the voltages reflected on the two cathodes..

I used a 220k and a 47k resistor for the job:



A last detail. I put a common mode choke between the voltage doubler and the next filter capacitor. This is to filter the high frequency noise from the diodes and mains power.

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Offline ScottulusMaximus

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Re: 6N1 preamplifier with phono
« Reply #6 on: August 09, 2016, 05:53:58 PM »
Drooooooooooooooooooooooooooooool
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Offline Mars

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Re: 6N1 preamplifier with phono
« Reply #7 on: August 10, 2016, 06:57:31 PM »
Completed the phono stage today. All voltages exactly right. Resistors in the CCDA run a bit hot. Might have to change plate load and cathode resistors for higher value.  I stuck with the RJM circuit (since I am familiar with it and I have all the caps in stock). Excuse the blurred photos; it is a bit late :P




Next I need to clean up wiring and test the phono stage audio.
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Offline ScottulusMaximus

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Re: 6N1 preamplifier with phono
« Reply #8 on: August 10, 2016, 07:24:51 PM »
Beautiful job! You're going to have to explain it all to me once you're recovered ;)
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Offline Mars

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Re: 6N1 preamplifier with phono
« Reply #9 on: August 12, 2016, 09:55:55 AM »
Are you shure :Ooooooh:  There are plenty of basics to cover, since this preamp is quite basic , in principle. It is based on the grounded cathode triode with a cathode follower right on the end.

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Offline ScottulusMaximus

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Re: 6N1 preamplifier with phono
« Reply #10 on: August 12, 2016, 12:27:35 PM »
Haha don't worry Karel, you don't have to teach me the entire circuit ;) busy learning from basics myself.

I'm just interested in why such a simple circuit has turned into a fairly complex build that's turning you even more grey ;)
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Offline entropyagent

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Re: 6N1 preamplifier with phono
« Reply #11 on: August 12, 2016, 11:35:18 PM »
Perhaps the challenge lies in making it as simple as possible, but no simpler.

Offline DeonC

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Re: 6N1 preamplifier with phono
« Reply #12 on: August 13, 2016, 12:58:30 AM »
Haha don't worry Karel, you don't have to teach me the entire circuit ;) busy learning from basics myself.

I'm just interested in why such a simple circuit has turned into a fairly complex build that's turning you even more grey ;)

(Speaking as a non-EE observer) I think the idea is to keep the circuit simple, but at the same time to make sure the supporting circuitry (PSU, etc.) is very good, which can make for some complexity, as well as trying to maximise the layout and component choices in the circuit itself (simple circuits are a lot more revealing of small changes). Am I right in these observations?
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Offline Ampdog

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Re: 6N1 preamplifier with phono
« Reply #13 on: August 13, 2016, 04:07:06 PM »
Design ways need to be simple, but at the expense of performance. (In general, not particularly referring to the present topic.)

Eg.: NFB: Adds extra components and every component must degrade the performance because no component is perfect. Logical in an 'over simplified' ( ;D ;D ;D) sense but untrue. Because a few extra resistors and perhaps one cap can cause a factor 10 decrease in distortion and other improvements but whose 'degrading' effect is totally out-weighed, if present at all.

Which clumsy example meaning: One should rather look at ultimate performance, including all available knowledge, than over simplifying. "Simple' is after all a relative concept: Compared to what? Certainly not just absolutely necessary components.  (And then "necessary"  - compared to what?  :EGrin:  ;D ;D ;D )
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Offline fdlsys

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Re: 6N1 preamplifier with phono
« Reply #14 on: August 13, 2016, 04:49:44 PM »
Well, since you're asking... "Ultimate performance" - according to who and compared to what?
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