Guys,
I got a pair of icepower 500asp's.
These have a input sensitivity of 1.87v and 8k input impedance.
I would like to use a tube input buffer to raise the input impedance. Only want to use balanced (xlr) input.
I have seen a commercial product using a standard 12AX7/ECC83 tube.
It raises the input impedence to 100k
For power it uses the +/-12v output of the icepower.
Do you guys have any ideas how ik can build it myself? Need 2 balanced mono buffers . (Just to raise input impedance, i don't need a preamp.)
I got a pair of icepower 500asp's.
These have a input sensitivity of 1.87v and 8k input impedance.
I would like to use a tube input buffer to raise the input impedance. Only want to use balanced (xlr) input.
I have seen a commercial product using a standard 12AX7/ECC83 tube.
It raises the input impedence to 100k
For power it uses the +/-12v output of the icepower.
Do you guys have any ideas how ik can build it myself? Need 2 balanced mono buffers . (Just to raise input impedance, i don't need a preamp.)
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Off the top of my head..
A line transformer might be the easiest and most effective method. Absent that, if this were my deal to do, I'd try what I call the 'simple SN7' line stage, which can be implemented with a pair of either 6SN7 / 12SN7, 7N7 (Loctal) or 12AU7. Might also be done with 12BH7, or even 6BL7 / 6BX7. This is a well-known circuit with the first section of each tube direct-coupled to the second section, and a cathode-follower output from the second. For the 12AU7 version values go something like this:
1st section: Rp = 100K / Rk = 4K7. 2nd section: Rp = Direct to B+ / Rk = 100K. Plate of sec 1 -> Grid of sec 2. Output from K of sec. 2 via 3-10uF cap. B+ should be 300V or so.
The circuit has a little bit of gain, not so much.. but it does drive a line well.
To drive 8K you're going to want a fairly large opt. coupling cap, in the 3-10uF range, I'd think. But that is quite do-able with Solen, Aeon or even Illinois Cap parts (you know IC makes 3-10uF film parts in the 250-400V range, eh?).
A line transformer might be the easiest and most effective method. Absent that, if this were my deal to do, I'd try what I call the 'simple SN7' line stage, which can be implemented with a pair of either 6SN7 / 12SN7, 7N7 (Loctal) or 12AU7. Might also be done with 12BH7, or even 6BL7 / 6BX7. This is a well-known circuit with the first section of each tube direct-coupled to the second section, and a cathode-follower output from the second. For the 12AU7 version values go something like this:
1st section: Rp = 100K / Rk = 4K7. 2nd section: Rp = Direct to B+ / Rk = 100K. Plate of sec 1 -> Grid of sec 2. Output from K of sec. 2 via 3-10uF cap. B+ should be 300V or so.
The circuit has a little bit of gain, not so much.. but it does drive a line well.
To drive 8K you're going to want a fairly large opt. coupling cap, in the 3-10uF range, I'd think. But that is quite do-able with Solen, Aeon or even Illinois Cap parts (you know IC makes 3-10uF film parts in the 250-400V range, eh?).
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For an 8K input impedance, you'll want to look for tubes that have an Rp down around 2k I think. In a cathode follower, higher Mu means more feedback and lower THD. Higher Gm means lower output impedance.
Are you intending to run the tube solely off of the +-12V power for the ice amp? If there is no way for you to incorporate a small transformer to provide a 150V+ B+ supply, you might find that running a tube input at such low voltages will create more new problems than it solves (as far as sound quality goes).
If you can add a power supply for the tubes, John Broskie has some balanced board/kit solutions, I think.
Are you intending to run the tube solely off of the +-12V power for the ice amp? If there is no way for you to incorporate a small transformer to provide a 150V+ B+ supply, you might find that running a tube input at such low voltages will create more new problems than it solves (as far as sound quality goes).
If you can add a power supply for the tubes, John Broskie has some balanced board/kit solutions, I think.
Not wanting to steal your thunder, but if your only reason is to raise Zin, a couple of JFETs do the job without heaters, no high voltage required, can easily be fit inside the existing case, cheaper....
Jan
Build this- The heretical Preamp. You could do a bare-bones version with an edcor input transformer, and without bias servo, or output delay relay.
The Heretical Preamp
Use the 6DJ8 as designed, or the russian 6N1P, or 6CG7, or 6Sn7. All will work fine for your purpose if you use a high-value input resistor with a volume pot across it, and set the CCS current to suit the tube you use.
I've built one with the 6CG7, no input transformer, and no servo, and it truly is a high impedance piece of wire that will not negatively impact the sound at all. use a toroidal transformer from Antek, or a cheap isolation transformer and a cheap filament transformer, and it is a very affordable build.
The Heretical Preamp
Use the 6DJ8 as designed, or the russian 6N1P, or 6CG7, or 6Sn7. All will work fine for your purpose if you use a high-value input resistor with a volume pot across it, and set the CCS current to suit the tube you use.
I've built one with the 6CG7, no input transformer, and no servo, and it truly is a high impedance piece of wire that will not negatively impact the sound at all. use a toroidal transformer from Antek, or a cheap isolation transformer and a cheap filament transformer, and it is a very affordable build.
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