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Old 21st March 2014, 11:58 AM   #1
ishiru is offline ishiru  Indonesia
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Default 12AX7 preamp for DAC

hello there.

i need help to improve my DAC output stage.i have a CV4024 on my DAC and recently bought a CV4004 tube for an upgrade.

but now, i'm thinking about new circuits for higher improvement.


please don't suggest me to buy another tube since i'm running out of money right now.but i still can buy some parts like resistors and tube sockets since they're cheap enough.

my current circuit was attached.i forgot to add the volume pot at the output..oops
(anyway, which one is better?.volume pot before the tube, or after the tube?)

thankyou!

-add-

is this circuit from DIYAudioprojects good enough?..i'm thinking to use AT7 on the input and AX7 on the 2nd stage..
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File Type: jpg output.jpg (140.4 KB, 208 views)
File Type: png 12AU7-Cathode-Follower-Schematic.png (45.4 KB, 207 views)
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Last edited by ishiru; 21st March 2014 at 12:02 PM.
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Old 21st March 2014, 12:19 PM   #2
Ceglar is offline Ceglar  Australia
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It depends on the voltage gain that you need and the impedance of the following stage.

My vote goes for the first circuit (simple plate loaded triode), but as drawn with 12AX7 its completely unsatisfactory and 12AT7 would do better at 8x less plate impedance.

If the option is open to you to sell the CV4004, which (if you bought it - you would know) is an expensive tube. Then simply use something like triode connected E180F/6688. My guess is you'd get ten pairs of E180F to the single CV4004. Rp would be some 40 times less and voltage gain not too dissimilar.
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Old 21st March 2014, 12:36 PM   #3
ishiru is offline ishiru  Indonesia
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Quote:
Originally Posted by Ceglar View Post
It depends on the voltage gain that you need and the impedance of the following stage.

My vote goes for the first circuit (simple plate loaded triode), but as drawn with 12AX7 its completely unsatisfactory and 12AT7 would do better at 8x less plate impedance.

If the option is open to you to sell the CV4004, which (if you bought it - you would know) is an expensive tube. Then simply use something like triode connected E180F/6688. My guess is you'd get ten pairs of E180F to the single CV4004. Rp would be some 40 times less and voltage gain not too dissimilar.
thanks for the reply.

this DAC would be used in various amp so i couldn't give the exact impedance of the output.

yes.i bought the CV4004 half box plate but it's priced so low ..none of my friends are playing around tubes so i couldn't sell this (and i already know this tube is pretty high in price so i'll keep this )

i never found E180F or 6688 tube here.i listened to 6N1/2/3P but i liked none of them .

is there any way like SRPP or else?..but i'm thinking about the floating heater if SRPP used.my 12V supply was shared to DAC and the tube heater.could bridge supply help?..(i used this kind of supply right now)
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Old 21st March 2014, 01:11 PM   #4
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The 12AX7 is horrible as an output tube (awesome if you need lots of voltage gain). It will never have a low output impedance, so the suggested schematics/tubes are not a good option.

But what exactly are you trying to achieve as an improvement? More gain, lower output impedance, higher input impedance?
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Old 21st March 2014, 01:22 PM   #5
ishiru is offline ishiru  Indonesia
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since i currently use plate follower which is high impedance (am i right?), i want to go lower.i actually have 6DJ8 buffer circuit but i'm not sure if they could be used at high voltage.

actually i'm happy enough w/ the current circuit but if i can extract more juice, why not?
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Old 21st March 2014, 01:29 PM   #6
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Yes, a plate follower will always have a high output impedance. It's the plate resistance in parallel with the plate load. I don't know the figures of the 6DJ8 and your plate load, but is's probably much higher than preferable. The 6DJ8 is expected to run at lower anode voltages than e.g. the 12A*7 series.
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Old 21st March 2014, 01:38 PM   #7
ishiru is offline ishiru  Indonesia
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ah..i see..i'll stay using this CV40x4 then..

which one better?.SRPP or the one i use + buffer like the one i uploaded at the 1st post?
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Old 21st March 2014, 01:44 PM   #8
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Your second schematic uses a cathode follower. A CF will always have a lower output impedance than a (symmetrical) SRPP. SRPP's are good for higher current demands, like OTL stages, but still have a rather high output impedance. If a low output impedance is what you're after, a CF or white cathode follower (like the Aikido amps, nice little amplifiers), are more suited. But it all depends on what you're trying to achieve really.
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Old 21st March 2014, 01:46 PM   #9
ishiru is offline ishiru  Indonesia
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ah..i see..thank you very much!

does my heater supply okay for cathode follower (#3)?.i'm using my 12V trafo for heater and DAC.i don't want any sparks or electrocution
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Old 21st March 2014, 01:54 PM   #10
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It won't work. A LM317 by itself will never do anything. First of all, you need add a cap (or caps) in front of it to make rudimentary DC. The resulting ripple can't come below at least 1.5V of the output, or else the LM317 will drop out of regulation. Second, the 317 works by trying to maintain a constant voltage between the output and adjust pin of 1.25 volt. Check page 2 of the link below for the typical application:
http://www.ti.com.cn/cn/lit/ds/symlink/lm117.pdf
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