Simple question looking for a simple answer for my simple mind.. 
I am looking for a follower circuit to reduce impedance (such as a cathode follower) to follow a basic starved ecc88 anode followed design, but don't want any additional gain from the circuit than already there from the tube. Also (sorry if this is technically a solid state question..) I don't want to run a cathode follower as I don't want a second tube.
For example, many modern little tube headphone amps (Like the Bravo V2) run an ecc88/12au7 followed by a IRFxxx/LM317 combo. This IRFxxx/LM317 setup does the job, but adds a ton of gain that I don't want as this is a preamp, not HP amp.
What is a simple circuit to replace the IRF/LM. Passive would be fine as current draw is at a premium in this setup (hence not wanting a second tube for this duty..)
Thanks!
I am looking for a follower circuit to reduce impedance (such as a cathode follower) to follow a basic starved ecc88 anode followed design, but don't want any additional gain from the circuit than already there from the tube. Also (sorry if this is technically a solid state question..) I don't want to run a cathode follower as I don't want a second tube.
For example, many modern little tube headphone amps (Like the Bravo V2) run an ecc88/12au7 followed by a IRFxxx/LM317 combo. This IRFxxx/LM317 setup does the job, but adds a ton of gain that I don't want as this is a preamp, not HP amp.
What is a simple circuit to replace the IRF/LM. Passive would be fine as current draw is at a premium in this setup (hence not wanting a second tube for this duty..)
Thanks!
The IRFxxx/LM317 combo in the Bravo actually reduces gain slightly. The gain comes from the tube.
I finally found the Bravo web site.
The tube gives all of the voltage gain. They are running a FET with a current source in the source leg making it a source follower.
If you want less gain, you need to add a volume control going into it, or redesign the starved circuit for less gain.
If you don't want to add a cathode follower (additional tube), I would suggest a Source Follower. What is your B+ voltage.
The tube gives all of the voltage gain. They are running a FET with a current source in the source leg making it a source follower.
If you want less gain, you need to add a volume control going into it, or redesign the starved circuit for less gain.
If you don't want to add a cathode follower (additional tube), I would suggest a Source Follower. What is your B+ voltage.
I am using a 12au7 (in a starved plate setup) with the output tied to the plate (like a normal tube setup). I now want to reduce the impedance of that output signal to a usable level to drive a solid state amp, such as a cathode follower would do. I however want as little additional gain from this circuit as possible and don't want to use a cathode follower as I don't want two tubes.
input--12au7--output off of anode--(circuit I'm looking for to reduce impedance with no or little additional gain)--output to solid state amp
So basically I want the exact function of a "cathode follower" circuit in regards to effect on output impedance, but not a tube.
input--12au7--output off of anode--(circuit I'm looking for to reduce impedance with no or little additional gain)--output to solid state amp
So basically I want the exact function of a "cathode follower" circuit in regards to effect on output impedance, but not a tube.
Sorry, you guys got a reply out while I was still working on mine
I didn't realize all the gain was from the tube, that's kind of cool actually..
My V+ is 24vdc.
So with this 220, pin one ties to the anode output, pin 2 to +24, and pin 3 is my output? Or is this wrong? Any support parts?
How does this affect SQ?
My fear in just running a volume pot (which I plan to do anyway) is not enough attenution.. Right now the line out (not hp out) of my Ipod Classic runs into a Bravo V2, then into a NAD2400 amp. That was too much input gain, So i added a 100k log pot between the Ipod and V2.
Even with that, the 100k log is at about 9 oclock, and the 10k lin on the V2 is still only at just off zero for where I want to listen.. too much gain for me.. but sounds nice..
I didn't realize all the gain was from the tube, that's kind of cool actually..
My V+ is 24vdc.
So with this 220, pin one ties to the anode output, pin 2 to +24, and pin 3 is my output? Or is this wrong? Any support parts?
How does this affect SQ?
My fear in just running a volume pot (which I plan to do anyway) is not enough attenution.. Right now the line out (not hp out) of my Ipod Classic runs into a Bravo V2, then into a NAD2400 amp. That was too much input gain, So i added a 100k log pot between the Ipod and V2.
Even with that, the 100k log is at about 9 oclock, and the 10k lin on the V2 is still only at just off zero for where I want to listen.. too much gain for me.. but sounds nice..
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Well, in tossing around the idea of having a switched headphone output onboard anyway, I guess also I could just stick with the LM/IRF combo (now that I know it is not adding gain) and just put a hefty fixed Lpad r network on the preamp output side of the headphone out/preout switch prior to the stereo volume pot.
A 2N2222A (or your favorite npn small signal transistor with a diode from base to ground to get about 0.74V (I used four schottky diodes), a 22K resistor from base to V+, a 330R resistor from emitter to ground will make a nice current source.
Adjust the emitter resistor to change the current.
Connect the collector of the current source to the source pin of a 2N7000, connect the drain to V+, and connect the output at your tube anode to the gate.
Ta-da, a current source loaded small signal source follower.
Adjust the emitter resistor to change the current.
Connect the collector of the current source to the source pin of a 2N7000, connect the drain to V+, and connect the output at your tube anode to the gate.
Ta-da, a current source loaded small signal source follower.
Even though I have no idea how or why those parts do what they do in that situation, your explanation was very easy to follow, Thanks!
So varying my 330R in your setup will adjust my output level, correct? Larger value, lower output I assume?
Any guesses on what vicinity I will be in for output impedance? How does this circuit affect sound quality, tone, and frequency response? (I know some transistors roll off top end by up to 6db, etc..)
Thanks!
So varying my 330R in your setup will adjust my output level, correct? Larger value, lower output I assume?
Any guesses on what vicinity I will be in for output impedance? How does this circuit affect sound quality, tone, and frequency response? (I know some transistors roll off top end by up to 6db, etc..)
Thanks!
No, the 330 ohm resistor is a sense resistor for the current level.
The diode from base to ground sets the input of the transistor at the base to 0.74 (in my case) volts. The transistor will pass current up to the point where the emitter voltage is sufficient to reduce the base current and reach a stable operating point.
The voltage is approximately the base voltage minus the base to emitter voltage. At low current, the BE voltage is small, so an approximation is the current will be the base voltage divided by the resistor value:
Ve~= Vb = 0.74V
Ve/Re=Ie The current is equal to the emitter voltage divided by emitter resistance.
Ve/Re = 0.74/330 = 0.002242424.....
Emitter current is approximately 2.2mA
If you want more current you decrease the emitter resistance.
If you want 5mA to flow, .005 = Ve / Re => Re = Ve/0.005 = 0.74/0.005 =148 Ohms.
The current does not determine gain. There is less than one (~=0.9998) for an source follower.
The current does in part determine drive capability and thus distortion.
The diode from base to ground sets the input of the transistor at the base to 0.74 (in my case) volts. The transistor will pass current up to the point where the emitter voltage is sufficient to reduce the base current and reach a stable operating point.
The voltage is approximately the base voltage minus the base to emitter voltage. At low current, the BE voltage is small, so an approximation is the current will be the base voltage divided by the resistor value:
Ve~= Vb = 0.74V
Ve/Re=Ie The current is equal to the emitter voltage divided by emitter resistance.
Ve/Re = 0.74/330 = 0.002242424.....
Emitter current is approximately 2.2mA
If you want more current you decrease the emitter resistance.
If you want 5mA to flow, .005 = Ve / Re => Re = Ve/0.005 = 0.74/0.005 =148 Ohms.
The current does not determine gain. There is less than one (~=0.9998) for an source follower.
The current does in part determine drive capability and thus distortion.
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