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Old 23rd May 2011, 05:55 PM   #1
Ar4 is offline Ar4  Latvia
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Default Low gain preamp.

Hello everyone.

I am looking for schematic for low gain tube preamp. I need it for inverting chipamp. I've built cathode follower,running from 180v, and I dont want to change feedback/input resistors for chipamp, so I think I could use grounded cathode -> my cathode follower. Or any other design, that can be used to feed inverting amp. I have only two 6n1p's left, so I think I could just use one double triode for a channel. As I understand, I cant use only grounded cathode stage, because it is not capable to drive low impedance load.
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Old 23rd May 2011, 06:04 PM   #2
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Ar4 View Post
Hello everyone.

I am looking for schematic for low gain tube preamp. I need it for inverting chipamp. I've built cathode follower,running from 180v, and I dont want to change feedback/input resistors for chipamp, so I think I could use grounded cathode -> my cathode follower. Or any other design, that can be used to feed inverting amp. I have only two 6n1p's left, so I think I could just use one double triode for a channel. As I understand, I cant use only grounded cathode stage, because it is not capable to drive low impedance load.
Whether or not it can drive a low impedance load depends entirely on the tube chosen to do the job. 6DJ8/ECC88, ECC99, 5842, 5687, etc can do the job in anode follower configuration and will drive 10K loads if designed to do so.

If you are concerned about the phase inversion you can always swap the speaker connections to correct the inversion in the amplifier. Depending on the material being played the difference may be inaudible or not.. (And it's often arbitrary from recording to recording.) Something I would not worry about too much.
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Old 23rd May 2011, 06:14 PM   #3
Ar4 is offline Ar4  Latvia
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Well still I am not sure about the best way to set gain so low. Resistor dividers? Or maybe I could use some kind of "feedback resistor", from grid to output?

EDIT. I attached two schematics. 6n1p buffer which I am using at the moment, and second one I found I've made some time ago. As I remember, It had low gain, I am just unsure how appropriate it is.
Attached Images
File Type: jpg Tube_Buffer_6N1P.jpg (116.7 KB, 493 views)
File Type: jpg Tube.jpg (42.3 KB, 472 views)

Last edited by Ar4; 23rd May 2011 at 06:21 PM.
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Old 23rd May 2011, 07:24 PM   #4
Ar4 is offline Ar4  Latvia
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Ok, I found this one, with gain = 22 DIY 6922 / E88CC Tube preamp

As I have 180V power supply I have to recalculate anode and cathode resistor values, but to decrease gain I have to reduce plate load resistor, and to increase a cathode resistor? I am a bit confused with setting gain for tube preamps.
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Old 23rd May 2011, 07:29 PM   #5
SY is offline SY  United States
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What is the purpose of this stage? A chip amp will typically have a gain of 10-20 (20-26dB) so needs no voltage amplification before it. A chip amp is likely to have an input impedance of 10-20k, so doesn't need current amplification.

If you're trying to add distortion, then use a very low plate load resistor, which will also drop the voltage gain.
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Old 23rd May 2011, 07:41 PM   #6
Ar4 is offline Ar4  Latvia
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Well I wanted to reduce chipamp's distortion, so it haven't enough gain, and as this LM4780 is point-to-pointed, changing feedback resistors is not so easy now. So now I need a little voltage amplification, and I have heard that inverting gainclones sound better if fed with higher current signal. I could use simple opamp stage, but well, tubes look better, and they sound nice too.
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Old 23rd May 2011, 07:45 PM   #7
SY is offline SY  United States
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How will this circuit reduce distortion? Are you planning to place it within the chip amp's feedback loop to increase open loop gain and hence increase feedback?
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Old 23rd May 2011, 07:57 PM   #8
Ar4 is offline Ar4  Latvia
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Well I have nothing against "tube distortion" . Well lets just say I cant (dont want to) change amplifier gain settings, so I have to use preamp to amplifiy input signal. Also tubes introduce 2nd harmonics. I plan to use this just after music source. Just like common preamp. As I have 3 more LM4780 chips, with next one I plan to try http://www.tubecad.com/2011/02/06/Su...iode%20GKA.png
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Old 23rd May 2011, 07:58 PM   #9
Vix is offline Vix  Yugoslavia
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Ar4,

I don't want to rain on the parade, but if want to use inverting chipamp, you may just find it easier to use J-FET based buffer, like Pass (firstwatt) B1. If you really want make a hybrid amp, I see a tube being much better suited as a voltage gain element, and a solid state as a current buffer, (i.e. a follower, like Pass F4 amp).
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Old 23rd May 2011, 08:12 PM   #10
Ar4 is offline Ar4  Latvia
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Vix, I understand your point, but well, call it placebo effect or what, I preffer tubes. They are simple for p2p, they glow and my previous gc, made using JLTi's hybrid design used to sound great ( CF + inverting GC)
Anyway, thank you for suggestion.
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