Hey guys, one of my next projects will be a new buffer line stage. The preamp I built a while back using 12AU7s sounds good enough, and for some things it is ideal. But it has too much gain. I am finding that many modern sources (like my DAC or an audio interface) have an output more than sufficient (by a fair bit) to drive an amplifier. With my current preamp (and it is by no means a high-gain design), I have the volume set at about 10% to achieve a normal listening level.
While it would seem that having a simple pot-in-a-box (passive) line stage would be the best, I do have some amplifiers (some of my big solid state amps, for example) have a pretty low input impedance, and while a tube line stage will add distortion, I find that even a low distortion design like my current preamp can take the slightly harsh edge off digital sources (all subjective and subtle). Beyond that, a passive preamp is rather dull and unexciting.
While I will use whatever transformers I have on hand, I think it would be good to design this to be cost effective using off-the-shelf parts in order to make it more beginner friendly. When I was looking for my first project (about nine months ago... I'm not a "seasoned veteran" by any means), I was continually frustrated by the number of designs using rare or difficult to obtain parts.
Enough rambling. I am thinking of doing a very simple cathode follower with a regulated power supply and possibly a constant current source. I've modeled some simple cathode followers in LTspice (just learning how to use it... whole new world of opportunities), and the 6DJ8, 12BH7, 6SN7 and 12AY7 seemed to perform fairly well. I have not modeled these with a CCS. The 12AU7 did not seem to perform quite as well.
As a side note, I think I have a pair of Jensen JT-11-FLCF transformers to use for a balanced output. These would be easy to omit, however, if someone wanted to save the $80 a pair of these would cost, especially if they didn't regularly use it with balanced-input power amplifiers.
My thinking, based off of inexpensive and obtainable transformers let's look at a B+ of around 260-300 volts.
So here's my real question for you all. What tubes do you all like for a cathode follower?
Ideas and suggestions are appreciated.
Thanks and sorry for the long post,
H713
While it would seem that having a simple pot-in-a-box (passive) line stage would be the best, I do have some amplifiers (some of my big solid state amps, for example) have a pretty low input impedance, and while a tube line stage will add distortion, I find that even a low distortion design like my current preamp can take the slightly harsh edge off digital sources (all subjective and subtle). Beyond that, a passive preamp is rather dull and unexciting.
While I will use whatever transformers I have on hand, I think it would be good to design this to be cost effective using off-the-shelf parts in order to make it more beginner friendly. When I was looking for my first project (about nine months ago... I'm not a "seasoned veteran" by any means), I was continually frustrated by the number of designs using rare or difficult to obtain parts.
Enough rambling. I am thinking of doing a very simple cathode follower with a regulated power supply and possibly a constant current source. I've modeled some simple cathode followers in LTspice (just learning how to use it... whole new world of opportunities), and the 6DJ8, 12BH7, 6SN7 and 12AY7 seemed to perform fairly well. I have not modeled these with a CCS. The 12AU7 did not seem to perform quite as well.
As a side note, I think I have a pair of Jensen JT-11-FLCF transformers to use for a balanced output. These would be easy to omit, however, if someone wanted to save the $80 a pair of these would cost, especially if they didn't regularly use it with balanced-input power amplifiers.
My thinking, based off of inexpensive and obtainable transformers let's look at a B+ of around 260-300 volts.
So here's my real question for you all. What tubes do you all like for a cathode follower?
Ideas and suggestions are appreciated.
Thanks and sorry for the long post,
H713
I read this long ago and learned a lot. Recently I got hold of some input transformers and playing with them, I am now asking myself if, instead of biasing the tube with the 10M resistor and using the required 10nF cap, one couldn't put the servos output to the now grounded side of the input transformer and get rid of the 10nF cap.
other "capless" buffer I like is this
Passive line stages and the TCJ stepped attenuator & NO-Gain/No-Pain Tube Line Stage
but the question was "which tube". For line levels a lot of tubes will work: if a tube is wanted I would take something with a gain > 30 and some 10mA/V transconductance - more transconductance gives lower output impedance, but also prone to oscillations (and super low output impedance is generally not needed).
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I like this article, well written and well based on real world knowledge.
Now it's getting complicated.
My question is : Will this be more audio correct then a resistor ?
The Valve Wizard -Cathode Follower
My question is : Will this be more audio correct then a resistor ?
The Valve Wizard -Cathode Follower
The bottom tube essentially creates a constant current source. It should be more linear with a lower Zout than using a resistor, and the power is dissipated above the chassis instead of inside it.
If you calculate the current and voltage relationships, you can replace the bottom triode with a resistor. The beauty lies in the fact that most tubes will bias up fine in this circuit, whereas using a resistor, you're limited to the tube it was designed for (10k for 6DJ8, 22K for 6CG7 etc).
If you calculate the current and voltage relationships, you can replace the bottom triode with a resistor. The beauty lies in the fact that most tubes will bias up fine in this circuit, whereas using a resistor, you're limited to the tube it was designed for (10k for 6DJ8, 22K for 6CG7 etc).
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Here's my contribution to the simple tube buffer pile (12AU7):
Muchedumbre Buffer Preamp – wauwatosa tube factory
No fancy parts or tricks; just a cathode follower. Buffers make good preamps in my experience. Like you, I've found that most tubes have too much gain as grounded cathode amplifiers.
Muchedumbre Buffer Preamp – wauwatosa tube factory
No fancy parts or tricks; just a cathode follower. Buffers make good preamps in my experience. Like you, I've found that most tubes have too much gain as grounded cathode amplifiers.
I agree that most power amps have more gain than they really should- Both tube and solid state. It's a power amp, not an integrated amp. But that said, modifying the power amps for lower gain isn't always an option. And not all power amps have volume control.
Seriously though, my phone can output enough to drive some amps into distortion. And all of this gain, no matter how well you lay out and design things, will result in noise.
For what it's worth, I am leaning towards a 6SN7 or 6J5 with a CCS.
What do you all think about a tube CCS vs say, a 10M45?
“Passive preamps have recently become a popular option to serve these ends. A passive preamp is just a simple switch to change between sources and a volume pot (or stepped attenuator). That’s cool and minimalist and all, but if you don’t respect impedance, it will take a hot dump all over your sound quality. “
Well said.
And fantastic site. I’ve bookmarked it.
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Nice project!
In the article:
"I’ve received several questions about the 1M resistors across the input. I added these as a precaution in case the pot craps out or becomes intermittent (scratchy). The great John Broskie has also claimed these reduce cross-talk. They are an optional part, so feel free to omit."
Shouldn't the 1M resistor be at the wiper to avoid scratchy noise when it loses contact intermittently?
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