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Old 20th April 2009, 01:30 AM   #1
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Default 12AU7 Preamp build

My second tube project, and first to not be from a kit. The schematic is based on the 12AU7 Tube Preamplifier Project from DIYaudioprojects.com, which I have done extremely minor changes to. The chassis is made from extruded aluminum with custom endplates and a custom mounting system for it. The bottom piece of the chassis slides off, so work will be easy. The transformer is a Hammond 269AX with a 120V primary, and a 250V CT secondary and a 6.3V filament secondary.

The chassis is nearly complete, so here are some pictures.

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I'm sure I'll use this thread when I run into electrical problems as I work on this over the summer. I want to do a majorly professional job on this one, no more rats-nest wiring jobs like everything I've ever built in the past.

Comments and thoughts are appreciated, anything I can do better I will. Thanks for reading.
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Old 20th April 2009, 01:43 AM   #2
gni is offline gni  United States
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Looks like a good start. . . .
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Old 20th April 2009, 02:20 AM   #3
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Some pics of the tubes and Transformer in place. The Volume pot I bought from tubesandmore.com didn't come with any hardware, so I've got to figure something out for that at the hardware store.

The other holes are for LEDs.

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Old 20th April 2009, 05:40 AM   #4
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Quote:
which I have done extremely minor changes to
Hi Matt,

Is this a guitarpreamp? Because that is probably the only thing you could use it for. It has a -3dB point of 20Hz and high gain of 45dB which means you have a sensitivity of 6mV for 1V out when loaded by 100k. If you want to use it for hifi you will need to make MAJOR changes to it. Or preferably use another design alltogether.

The buildup and box looks good so far !
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Old 20th April 2009, 12:36 PM   #5
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Thought of the best way to use a pair of ECC82 and this could be a possible solution. This is, if it is intended for hifi use. R5 is to set the overall gain.
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Old 20th April 2009, 02:17 PM   #6
Giaime is offline Giaime  Italy
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I would just use a SRPP stage. Fewer components, that's good for starters. Just Google "ecc82 srpp" or "12au7 srpp" will bring you many schematics.
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Old 20th April 2009, 03:06 PM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by Giaime
I would just use a SRPP stage. Fewer components, that's good for starters. Just Google "ecc82 srpp" or "12au7 srpp" will bring you many schematics.
I built such a pre-amplifier in the late 1990s commercially, and it sounded surprisingly good.

Very simple, just 8 -10 resistors, two coupling caps, and the tubes - excluding the supply which was ~ 250V..

Have attached screenshot (from Linux) showing the basic circuit. Need good clean plate supply and recommend dc filaments particularly in cramped space.
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Old 20th April 2009, 05:52 PM   #8
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Quote:
Originally posted by revintage


Hi Matt,

Is this a guitarpreamp? Because that is probably the only thing you could use it for. It has a -3dB point of 20Hz and high gain of 45dB which means you have a sensitivity of 6mV for 1V out when loaded by 100k. If you want to use it for hifi you will need to make MAJOR changes to it. Or preferably use another design alltogether.

The buildup and box looks good so far !

I was hoping I could use it for my turntable as well as my guitar. Right now I can't use either on my K16LS kit tube amp. I was also hoping to use it as a preamp to solid state stereos on occasion.

Is the issue that the schematic I was basing off of doesn't have much gain? How did you determine the -3dB point and high gain point?

Since I bought all the parts already for that other amp, I need to find a schematic that can use the Hammond 269AX and the 12AU7 tubes, the rest of the parts were relatively cheap and not a big issue if they go unused.


Quote:
Originally posted by kevinkr


I built such a pre-amplifier in the late 1990s commercially, and it sounded surprisingly good.

Very simple, just 8 -10 resistors, two coupling caps, and the tubes - excluding the supply which was ~ 250V..

Have attached screenshot (from Linux) showing the basic circuit. Need good clean plate supply and recommend dc filaments particularly in cramped space.
Thanks for the schematic. What is the resistance value of the volume pot? Or would a 100k pot work?
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Old 20th April 2009, 06:52 PM   #9
Giaime is offline Giaime  Italy
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Quote:
Originally posted by mattthegamer463
I was hoping I could use it for my turntable as well as my guitar.
You can't. High gain is good for guitar if you want it: but for turntables, you need high gain AND RIAA equalization, which is a sort of low-pass filter for de-emphasis. And I tried running my Strat through a phono preamplifier, it sounded

I suggest going for kevinkr schematic, maybe with a 50k potentiometer or, better, a stepped attenuator (you can have them cheap on ebay).
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Old 20th April 2009, 07:00 PM   #10
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Quote:
Originally posted by Giaime


You can't. High gain is good for guitar if you want it: but for turntables, you need high gain AND RIAA equalization, which is a sort of low-pass filter for de-emphasis. And I tried running my Strat through a phono preamplifier, it sounded

I suggest going for kevinkr schematic, maybe with a 50k potentiometer or, better, a stepped attenuator (you can have them cheap on ebay).
So basically, I can't have a general purpose preamp? Not being able to use my guitar is fine since I have a op-amp preamp with distortion that I built that works great for that, but can I run my MP3 player or PC through a phono preamp? Would I need some tone-control if I wanted to do that?
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