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PostPosted: 30 Oct 2013, 00:51 
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I've built a very basic stereo pre-amp, which is just a gain stage, followed by a cathode follower. Both stages use a 6SN7. It has weak bass, and although I haven't measured it with a spectrum analyzer, I'd swear there is a dip in the upper bass register. Frankly it just doesn't sound that great. Please take a look at the schematic and let me know if I've committed a design flaw that will just suck the life out of a pre-amp.

The gain stage is as basic as it gets (common cathode). It is biased so that it's passing 10 mA (which seems to be a popular point for the 6SN7). It is DC-coupled to a cathode follower. The potential divider between the 2 stages should only attenuate the AC signal, leaving the DC voltage the same as at the anode of the gain stage. The diode between the grid and cathode of the cathode follower is only there to protect the tube from arcing during power-up when the grid is at B+ and the cathode is at ground, and should effectively drop out of the circuit once the cathode starts conducting. This stage settles in when the grid is about 4V lower than the cathode and produces an idle current of ~10mA.

The potential divider is there because I don't really need any gain in my system, but used a gain stage on purpose to add some 2nd harmonic magic. Output voltage (AC) after the divider is ~1/4 of the input voltage.

As you can see, the power supply is fully passive CLC and CRC topology.

Here is my background with building pre-amps. About a year ago, I built an e-bay knock-off of the Jadis J200, using 12AX7s. It had beautiful velvety mid-range, crystal-clear highs, and tight bass, if not particularly strong. However I abandoned that design as it had horrible ground-loop noise that I and a pro amp building friend could never track down. We spent about a month trying to track the inject point.

For my next attempt, I decided to use an octal triode, and found a mu-follower design on the web for the 6SN7. I built it, point-to-point and it worked without a hitch (no ground-loop). It produced full, thunderous bass, so I know the 6SN7 is capable of producing low-end "umph." But the pre-amp had no magic. It was dry and clinical, and didn't get me emotionally involved in the music.

This final attempt was based on the assumption that a basic triode common-cathode gain-stage would be less linear than a mu-follower, and perhaps might give me some warm, inviting music. But it just sounds blah, and there is no low-end. Have I unintentionally created some kind of high-pass filter? Any thoughts would be appreciated.


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PostPosted: 30 Oct 2013, 07:39 
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Yup. That amp is going to sound like... well, you know.

It's your "cathode follower" that really isn't a cathode follower and the improper stage coupling. Then to top it off, the voltage divider is grounded through a cap. Overall, I'm not sure what you were going for, but this design has problems.

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PostPosted: 30 Oct 2013, 10:48 
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Hi, I agree, plus I would run the tubes at about 250 v and about 7 ma. It seems more like a sweet spot for low level audio use. Less noise as well. For a line stage there is no reason to go any higher. You have no need for 50 volts of output. I am a bit prejudiced, but a Forewatt will run circles around that one. As for the ground loop in the previous one...how about posting some pix and a schematic and perhaps we can find it for you.

Good listening
Bruce

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PostPosted: 30 Oct 2013, 20:22 
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Suncalc/Matt,
Please talk to me more about the improper coupling and the "not a cathode follower". There should be nothing theoretically wrong with the voltage divider. In fact, I pinched the idea from John Broskie's tube cad journal. All I want to attenuate is the AC signal, while preserving the DC voltage to bias the second stage. A 1uF cap should be enough to pass all reasonable audio (AC) frequencies to ground (while blocking DC), but if I have unintentionally created some secondary effect that is affecting either stage, I'd like to understand the mechanism to learn from my "journey of discovery"
Also, why is the 2nd stage really not a cathode follower?
Thanks in advance.
James.


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PostPosted: 31 Oct 2013, 07:18 
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m1: the schematic doesn't make any sense. Cut your loss and get a better design. The 6SN7 is a great tube and there lots of good schematics out there to build which are properly designed.

You will find on this forum a 6SN7 cathode follower schematic which will give great performance.

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PostPosted: 31 Oct 2013, 11:03 
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In order to get a bias voltage for the second tube, you need a voltage divider hooked up between B+ and GND, but the cap is blocking that path, which leaves the grid more or less floating. Besides you need a grid leak, but you have none. Or maybe I am partly wrong ...

Else check Blue Velvet:
http://www.faktiskt.se/modules.php?name=Forums&file=viewtopic&t=23232
http://www.blackdahlia.com/html/tip_44.html

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PostPosted: 31 Oct 2013, 22:06 
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People seem to be confusing the purpose of the voltage divider. :confused: It is only to attenuate AC, to reduce the amplitude of the first stage output to a level that allows me to use most of the range of my volume control. The DC bias on the CF grid is exactly the voltage on the first stage's anode (rock steady), because no DC current passes along that path, only DC voltage, which is what I used to calculate the 2nd stage's operating point (the measurements match my calculations). The grid is not floating, and I am not using the voltage divider to alter the DC voltage or set the grid bias point as a reference to B+. Only AC current passes through the voltage divider to ground and only AC voltages are being attenuated. The amount of DC current passing through the first stage's plate resister is exactly the same as the current passing through the cathode resistor. If my current measurements are equal (which they are), then, ipso facto, no DC current is passing through the voltage divider and no alteration to DC voltage occurs.


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PostPosted: 01 Nov 2013, 08:10 
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Plain and simple it will not work! That is why it sounds so bad! The diode will cause clipping the cap to ground will kill all low freq. This is severly flawed from the start.


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PostPosted: 01 Nov 2013, 23:33 
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Under what operating condition will the diode cause clipping? It is reverse-biased as soon as the cathode of V2 goes positive. Vgk is -4V once tube is warm.
Here is a good link: http://www.valvewizard.co.uk/dccf.html

Unless I am mistaken, the cap to ground would kill high frequency, not low frequency.


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PostPosted: 01 Nov 2013, 23:40 
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Here's an example of a commercial build I did that does about the same thing you want.

Cheers!


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