If all that is needed is a tube line stage (preamp) that doesn't have to drive headphones, then here's something that is both very simple and will have a little bit of 'tube character' to it.
The balanced current idea has been recently popularized by John Broskie of Tube CAD Journal fame.
Gain should be about 9X (about 19dB).
The output impedance should be about 600 ohms, which will be low enough to successfully drive a 5k ohm load. The higher the load impedance, the better the performance. It can sink 6mA into the load, which should drive a reasonable amount of cable capacitance.
This preamp will not drive headphones, though.
This circuit will need a +300V DC supply, which is a relatively high voltage. This can kill you. Voltages that high must be respected, but they are not so high that they are ridiculous hazards (like the +1000V DC supplies in some single-ended power amps).
The largest expense will be the power transformer. A 40mA rated part would be plenty. A 500VCT 40mA power transformer shouldn't be hard to find. Allied 6K88VG would work.
Hammond Manufacturing - Transformers - 6K88VG - LEADS 6.3V @ 2.0A 500VCT @40DCMA 50/60HZ PRI:110-120V TRANSFORMER - Allied Electronics
That also has a 6.3V AC winding for the heaters.
The Hammond 270BX would also work, but is twice as expensive.
--
For better performance you could use a 6SN7 or Russian 6N8S instead of the 12AU7. All parts values could remain the same. The 6SN7 is a bigger tube, so would also look more impressive.
--
The balanced current idea has been recently popularized by John Broskie of Tube CAD Journal fame.
Gain should be about 9X (about 19dB).
The output impedance should be about 600 ohms, which will be low enough to successfully drive a 5k ohm load. The higher the load impedance, the better the performance. It can sink 6mA into the load, which should drive a reasonable amount of cable capacitance.
This preamp will not drive headphones, though.
This circuit will need a +300V DC supply, which is a relatively high voltage. This can kill you. Voltages that high must be respected, but they are not so high that they are ridiculous hazards (like the +1000V DC supplies in some single-ended power amps).
The largest expense will be the power transformer. A 40mA rated part would be plenty. A 500VCT 40mA power transformer shouldn't be hard to find. Allied 6K88VG would work.
Hammond Manufacturing - Transformers - 6K88VG - LEADS 6.3V @ 2.0A 500VCT @40DCMA 50/60HZ PRI:110-120V TRANSFORMER - Allied Electronics
That also has a 6.3V AC winding for the heaters.
The Hammond 270BX would also work, but is twice as expensive.
--
For better performance you could use a 6SN7 or Russian 6N8S instead of the 12AU7. All parts values could remain the same. The 6SN7 is a bigger tube, so would also look more impressive.
--
Attachments
Last edited:
Does this preamp have to be able to drive headphones?
Simple yes, but not high performance at all. The volume control is in the wrong place, for one thing. If you put the volume control on the output, it will present a varying load to the preamp tube. Also, if you think about it, the tube will be running at full gain all the time, with highest distortion. Turning down the volume control (if placed at the output) will not change this. You'll just be burning off the excess gain you're getting from the tube (again, at its highest distortion). If the volume control is at the input, you reduce the level of the input to the tube's grid. Distortion in a triode is proportional to level, so the lower level at the input means lower distortion at the output.
And since that circuit is based on tubes with relatively high plate resistance (12AU7 rp will be about 10k), the preamp won't do well at all driving the low input resistance of a typical solid state amp (10k to 47k ohms). A 12AU7 driving a 10k load will sound rolled off and sleepy. But that might be the 'tube sound' you're after. It is kind of enjoyable.
A proper preamp could be done if you take that 12AU7 and use a cathode follower after it. Some will say the 12AU7 is a bad choice because it is a poor performer for audio, but I'll bet it would be good enough if done right. You could even use this old circuit from ye olde RCA Tube Manual:
I wouldn't implement it exactly that way. I'd increase the plate current and change component values to suit. But that's the general idea.
Incidentally, this RCA circuit is basically the same idea as used in the Bravo headphone amp, except the Bravo uses a MOSFET source follower instead of a tube cathode follower.
None of the above will drive headphones, by the way.
--[/QUOT.No I had intended to use it to be a preamp for the SS coming off of my pots into the brovo and from there into the SS amp boards sound inputs.... But as far as sound quality I'm looking for an accurate reproduction of kick drum to bass guitar, voices to cimble on a drum . I have some very good quality custom built Line Arrays that deliver awesome sound coming out of a cheap Techni-pro Hybrid amp but I will admit the amp has allot of help from a mixer and 32 dual band EQ and a Behringer Super X Pro cx3400 rack mount crossover that has a tone of tuning on it . I sometimes think I'm wasting my time even to consider a tube preamp . I may already have the best quality of sound I can get from any other source ....https://www.facebook.com/photo.php?...413.1073741831.100002107078171&type=3&theater
Last edited:
Does this preamp have to be able to drive headphones?
Simple yes, but not high performance at all. The volume control is in the wrong place, for one thing. If you put the volume control on the output, it will present a varying load to the preamp tube. Also, if you think about it, the tube will be running at full gain all the time, with highest distortion. Turning down the volume control (if placed at the output) will not change this. You'll just be burning off the excess gain you're getting from the tube (again, at its highest distortion). If the volume control is at the input, you reduce the level of the input to the tube's grid. Distortion in a triode is proportional to level, so the lower level at the input means lower distortion at the output.
And since that circuit is based on tubes with relatively high plate resistance (12AU7 rp will be about 10k), the preamp won't do well at all driving the low input resistance of a typical solid state amp (10k to 47k ohms). A 12AU7 driving a 10k load will sound rolled off and sleepy. But that might be the 'tube sound' you're after. It is kind of enjoyable.
A proper preamp could be done if you take that 12AU7 and use a cathode follower after it. Some will say the 12AU7 is a bad choice because it is a poor performer for audio, but I'll bet it would be good enough if done right. You could even use this old circuit from ye olde RCA Tube Manual:
I wouldn't implement it exactly that way. I'd increase the plate current and change component values to suit. But that's the general idea.
Incidentally, this RCA circuit is basically the same idea as used in the Bravo headphone amp, except the Bravo uses a MOSFET source follower instead of a tube cathode follower.
None of the above will drive headphones, by the way.
--
I also agree with what you have said. But it is simple and does sound very good. I changed the plate resistor to 50K to better optimize the 12AU7 tube the only one I am using. I also know about changing the pot from the output to the input but I have not done so. I have built as you say some better preamps since building this one but it is easy for a beginner and and it does sound very good if you like a lot of 2nd harmonic thrown in.
I like the schematic you posted. Something definitely worth trying. I like simple if it gets the job done. I've printed it out to savor over and look at some more.
Last edited:
- Status
- Not open for further replies.