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Old 1st May 2012, 03:49 PM   #1
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Default Looking for project suggestions - 12au7 pre-amp?

Hi all,
I am looking for an amp and pre-amp projects and in this thread I am looking for a good 12AU7 pre-amp project that is very well documented and easy to follow.

Me looking for an amp can be found here.

I am sure you are familiar with the Shine7 12AU7 pre-amp and I am looking for something similar or if I can actually build the same at a reasonable price. The only problem I have with Shine7's project is the 300V power supply for the tubes. I also have some JJ 12AU7 tubes that I am itching to use so i am also looking for an easy to follow headphone amp too maybe. He also uses some really expensive atteniator/pot and input switch.

I am looking for more of a build rather than design to get a baseline going for future design.

criteria:
1. Well documented, schematics, hook up diagrams and BOM
2. PCB availability (if there is a PCB I can buy it would be great, but I can do my own layout if need be or if i can bread board it will be great too)
3. Power supply schematics and BOM and hookup diagrams
4. Very good price to performance ratio
5. low budget, toatal around $100 in Parts and PCBs (flexible)
6. 10-20db gain? Or whatever is most appropriate.


Below are some parts/samples that I have acquired.

I am baselining the sound quality from my Onkyo TX-SR508 Reciever..... which sounds really good, but I want to build somethign that sounds better if possible

Power amp chips/tubes:
LM4780TA
LME49811TB
LM3875TF
LM4780TA
JJ 12AU7
JJ 12AX7

regulators for power supply:
LM317AEMP
LM317T
LT1086CT-12#PBF
LT1086CT-3.3#PBF
LT1086CT-5#PBF
LT3015ET-12#PBF
LT3015ET-5#PBF

I also have a bunch of very popular audio op-amps from Texas Instruments/Burr Brown and Analog Devices (except for OPA627 :/)


I would appreciate your suggestions!!!!

Thank you
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Last edited by zxgravediggerxz; 1st May 2012 at 03:52 PM. Reason: amp thread added
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Old 2nd May 2012, 06:10 AM   #2
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Can't help you much but if you are looking for an easy to build and well documented headphone amp using 12AU7, Alex Cavalli's SOHA II is one, but it is a hybrid. Kits are available too.

However it is a low voltage design that runs tube at lower operating point than what is probably typical/optimal. Mine seems to sound good, I said "seems" because I haven't done casing it up and do some serious listening with it. Btw I have mine modded to run hotter (still cold by most's standard).

The CTH is also quite similar.
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Old 3rd May 2012, 03:50 AM   #3
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Quote:
Originally Posted by Navyblue View Post
Can't help you much but if you are looking for an easy to build and well documented headphone amp using 12AU7, Alex Cavalli's SOHA II is .........

The CTH is also quite similar.
Thank for the reply! I will look into them!

I have also found other projects that I can use. When I find a bit of time this week I will post my findings/links and which one I will tackle.

Hope it can help others too when I post it.
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Old 3rd May 2012, 09:04 AM   #4
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Since OTL headphone amps can be used as pre amps too (but not the other way round), if you are into headphones and budget is a concern it's probably is a good idea to kill 2 birds with one stone. I always like to put a pair (or more) RCA out on the headphone amps that I built.

Broskie has many line stage amplifier too, and a few headphone amps, I am sure at least some can be made to use the 12AU7. They are all available as kits, so worth a look too.
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Old 3rd May 2012, 06:20 PM   #5
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1. Are you absolutely sure you need a preamp? Modern sources have a very high output level (1.5 - 2v rms) and most amps are quite sensitive, reaching full power at less than 1v input.
So why do you need preamp gain?

2. You didn't choose a very good tube for your project. I have auditioned lots of 12AU7 tubes, including the JJ ECC82, and none sounded great. They are notoriously less linear than other equivalents.
Use a 12BH7, for example, instead. It will sound better. If the rest of the system is high quality, that is. In a crappy amp you won't hear a difference, probably...


3. I can't recommend the SOHA II. It grossly misuses the tube by biasing it at its most non-linear point. The 12AU7 and its equivalents were meant to be used with *at least* 5 mA of current going through them, preferably more. And that differential input stage is just a waste of a good triode IMO, but the amp looks better with two tubes instead of one... lol.
I built a SOHA II. Made my own PCB for it. It sounded OK, but I ran the tubes at 120v and 7 mA, as their Maker intended!
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Old 4th May 2012, 01:20 AM   #6
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I am not sure I would call it "grossly misuse". But it appears that the designer's objective is to make the tube work just well enough and keep the voltage as low as possible, which is not anywhere near optimal. May be the designer wants to appeal to those who are averse to high voltage, I don't know.

Btw, what do you have to replace to run the tube at 120V 7mA? I was thinking if I have to replace the whole HV supply I might as well pick a different tube while I am at it.
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Old 4th May 2012, 06:12 AM   #7
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I used another HV supply indeed, and you can use almost any low to medium mu tube in this amp. I liked the 6211, the 12BH7, the 6N23P-EV...
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