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Old 11th March 2011, 02:35 PM   #1
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Default 12ax7 preamplifier with modified power supply!!! please help!

Hi guys!
I would like to build a tube preamplifier! I'm simulating various diy preamplifier projects from internet with LTspice IV. I'm not an expert expecially in tubes! I tried 3 different projects from www.shine7.com
and 2 other projects from www.diyaudioprojects.com.
Well I find out that all this projects have very different gain levels.
I need a line preamp and I think I need something with a resonable gain (2 to 4 I think! am I wrong?), while the projects of Shine7 Audio DIY Page have a high gain levels (gain of 51 for the 12ax7 project!!).
Is this suitable for any amplifier? Will I burn my power amplifier with a so high gain?
Finally simulating the 12ax7 project from DIY AUDIO PROJECTS - Do-It-Yourself Hi-Fi for Audiophiles I find out that this should work for me! It has a gain between 3 and 4!
The only problem is that this project is incredibly cheep and the power supply is not really for a hi-fi or hi-end application. So I've tried to build a better one! this is the preamp with the two power supplies (the first for the tubes plates and the second for the heaters:
Click the image to open in full size.May be this schematics can solve also some other thread in this forum! I find people with my same problem!
I think using a power mosfet and an LM317 voltage regulator the power supply should be a lot better than the original PWM PSU. It seems to has very low ripple too!
I'm a newbie with tubes. So... may be somebody can help me with this project!
My questions are:
1) what gain a standard preamp should have?
2) Is this power supply suitable for this preamp?
3) If I use two power transformers 55-0-55V (dual mono configuration) how many VA I need? are two 50VA or 80VA enough?
4) Has this preamp the right input and output impedance?

Please help! Thanks in advance!
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Old 11th March 2011, 04:29 PM   #2
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IMHO, the 12AX7 is about the most inappropriate tube one could possibly select for use in a low-gain circuit, much less TWO of them! This two stage circuit was obviously designed to produce a very high open-loop gain so that the circuit can employ a great deal of negative-feedback to get the closed-loop gain (and distortion) down. It looks more like something you might see in a solid-state amplifier than an tube amp.

I suggest that you seriously consider building a pre-engineered and proven kit unless you are quite sure what you are doing. The available number of such kits are numerous.
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Last edited by Ken Newton; 11th March 2011 at 04:41 PM.
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Old 11th March 2011, 10:00 PM   #3
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The stock 12AX7 preamp is the PAS2/PAS3/PAS3x. See schematic here 200 OK The difference in the revisions is whether the tone controls have center detent off switches or not (PAS2 not). The gain of the magnetic phono/tape head stage is about 50. The following line level stage I don't know, run it through your sim if you know how. These preamps ran great into high gain tube amps like the ST70, and the first generation of dynaco transistor amps (ST120) which had a gain 150 transistor in the first stage. Modern power amps need more gain from the preamp. My CS800s is not as loud as either ST70 or ST120 with the CS800s gain all the way up. But it won't distort at full volume, so it has more headroom than the ST70 or St120 that I can run into clipping with the pas2. The modern CS800s has about 280 W/channel into 8 ohm, which is the kind of speakers I have. It is only 29db gain into 4 ohm, however, with DDT knob all the way up.
The advantage of the 12AX7 is it is still in production and probably will be for a long time. The disadvantage is 250000 ohm output impedence, just about enough to drive a 6' RCA coax cable. For actual PAS3 clone PCB's see the recent thread about that on analog source forum. Classicvalve.ca is one source in Canada.
When I completly over drove my ST120 with about 8 V PP due to forgetting I had a 10X scope probe, the protection circuit cycled on and off with a period of about 2 seconds. An incandescent lamp on the same circuit dimmed at the same period. Other amps may not have this much protection. I had two 8 ohm 225 watt resistors as loads, so I don't know what it sounded like and my speakers were not stressed or damaged. The transformer was going "whang, whang" without a speaker, so it was pretty funny. No fuses blew.
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Last edited by indianajo; 11th March 2011 at 10:08 PM.
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Old 12th March 2011, 12:05 AM   #4
kevinkr is offline kevinkr  United States
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Take a look at designs using the 12B4 this will give the approximate gain you are looking for and there are a number of designs featured here. (Use the search function)

Multiple stages of tube anything with massive amounts of global feedback to reduce the gain are inelegant solutions to simple problem IMHO..

Your proposed design has a number of flaws including the most critical one which is the extremely low values of the feedback resistors you used, followed by the plate load resistor in the second stage - the 12AX7 has linearity problems driving anything including feedback networks of under several hundred K ohms. It will work, but will not perform anywhere near optimum. The rp of a 12AX7A typically ranges from 66K - 90K depending on operating point and for decent linearity the load it drives (AC and DC combined) needs to be at least 3 - 4X greater than this.
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Last edited by kevinkr; 12th March 2011 at 12:11 AM.
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Old 12th March 2011, 12:58 PM   #5
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Thank you Indianajo!
I really like Dynaco amplifiers! I'm impressed about the design of the power supply! this is so simple! Next days I will simulate the whole circuit to better understand it!
However the preamp schematics I'm looking for are pretty far from this design! I don't need a preamp with all this functions...
While the 12B4 design I think will match exactly what I need! I will read the whole thread (Oh god! that's a long thread! ).... This 12B4 preamp really looks interesting to me! Thanks so much Kevinkr! I perfectly understood now the linearity problem you told me with 12AX7! I think this will make me change not just the design, but the tube selection too!
Hey guys I found another interesting design! A generic preamp schematics to play with! I think it has to be fun! Can I know you opinion about it?
http://www.users.globalnet.co.uk/~valveamp/LinePreProject.htm
May be playing with it I can find something that will match my needs and my taste!
However from what I read I will definitively try the 12B4 design! I have more confidence with it! (especially for a newbie like me!)
Thanks so much!
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Old 12th March 2011, 06:18 PM   #6
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Why not do the Shine7.com 6922 preamp....gain and sonics much more in line with what you are trying to do. Nice simple circuit as well. Heck, if you've already built the Shine7 12AX7 preamp all you would need to do is change out a few of the resistor values and the pinout on the tube and you're done.....
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Old 13th March 2011, 02:48 PM   #7
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Default PSU for the shine7 project?

Can somebody suggest me a good Power Supply for the Shine7 6922 preamplifier project?
The measurements of THD and noise looks very good! I think I will love a preamplifier like this!
Thanks!
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Old 15th April 2011, 03:55 PM   #8
radio is offline radio  Macedonia
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Hello All!

To be clearer?
This preamplifier with ECC83 (12AX7) great play, .. but delivers the real valve sound!?
Because of this preamplifier higher third harmonic of the second and so forth,..
Best Regards,...

Last edited by radio; 15th April 2011 at 03:58 PM.
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