I have posted extensively about my prototype and build process on AudioKarma. I figured i would post it here because if you all are anything like me you like to look at other peoples work for inspiration. Hopefully this will encourage you to build whatever it is you dreamed up. Chances are it will work better than you ever thought.
I have been in need of a good tube preamp for a long time. I play with tubes, my rule. I had a Dynakit PAS3 which I had modified, It sounded good, and technically works right but doesn't impress me at all. It has too high of an output impedance, which resulted in me never being happy with the bass. I sank a lot of money into replacing nearly every part in it and then subsequently modded it into almost sounding acceptable. It works and I'm done messing with it.
I had previously built a SRPP line stage using 12AU7 tubes it worked really well, but i don't listen to CDs or tapes, or anything that doesn't require RIAA equalization if ya know what I mean... I need a tube preamp with a phonostage, and a line stage.
I had also gotten some experience with the phono-stage in my HH Scott LK72
In the process of rebuilding one I ended up totally blueprinting the entire stage including the RIAA equalization network within the feedback loop. I noticed that all the parts were common values and still available today and cheap. The stage sounded damn good in its original carbon composition implementation, and My mind immediately went to scratch-building a copy.
Sometimes the only way to have the really cool stuff that you want is to build it yourself, but then again you already know that. I did it the way i did it because it works well for me and I like it. I personally have an aversion to printed circuit boards. I have built kits from circuit boards, and they are not a challenge to me. Simply populating a circuit board and dropping it in a chassis can result in a fantastic amp, but I feel like my potential is wasted on such a thing. I personally find there to be no challenge in it. I didn't want to use any circuit boards in this build. Some of my other design criteria were to have a "conventional" case made entirely of metal (aluminum)
I didn't want the tubes sticking out the top like many DIY "plate" style construction amplifiers. Nothing against them just not what i was shooting for.
I wanted the tubes to be easily rollable without removing the top cover, and also to be shielded and isolated from vibration.
I wanted to have an extremely low noise floor without the added complexity of a separate chassis and special cord. I decided to house the power supply in a walled off section of the same chassis. It worked extremely well for a variety of reasons. The power transformer is potted (adding an additional shield) and sits right smack in the middle of the amp, which keeps weight centralized. I cant stand it when an amp is too light and slides around when you move the controls or the cables. The potting can as well as a shield around the actual transformer both act to contain magnetic fields. There is virtually no hum in the phono section and none at all when the line section is selected.
Even though I do all of my listening on vinyl, i thought it would be wise to go ahead and have some line level inputs for a cd player, radio, or to plug into a phone... I designed in a relay section which will give me 2 phono inputs and 3 line level inputs, switched on a grayhill 6 position switch. The 6th position will de energize all 6 relays and short all inputs to ground through resistors. unused inputs are (you guessed it) shorted to ground through resistors.
I mounted the tubes on a piece of aluminum 90 degree extrusion. I tapped all the holes so that the majority of the amp can be assembled without nuts. (no trying to align 2 parts with a screw and dropping the nut.) Its another thing which i had to have but which you might decide to do differently. The extrusion is mounted on little rubber grommets which give the tubes some isolation from vibration if the chassis gets to shaking with the music.
I think that it turned out pretty sweet, I built a prototype in a par metals chassis and decided I liked it enough to order the parts to build a few more. I will be offering them to members of our local audio club, or friends and family to recoup costs. After all who doesn't need a really sweet point to point tube preamp...
Anyway I'll let the photos do the Talking. I intend to bring this and some big ultralinear power amps to the Burning Amp festival in SF this year along with a few of the other members of the club.
Here is a photo of the chassis before I cut the divider plate. The sub-chassis for the audio section is in place with the shielded tubes going out the back.
Here is the unpopulated audio subchassis, with Elco shielded sockets
This is the back panel of the prototype i built.
here is a top down view of the high voltage power supply caps and the potted transformer
here is the power supply wiring on the PROTOTYPE. next one comes out much neater...
The proof of concept was complete and the amp works. The next one I build is now done, and is in a Black anodized Par metals chassis. I bought 4 of em and I used the plasma cutter and sheet metal brake to bend the power supply sub chassis. It turned out much cleaner and the bigger chassis allow me to be a bit cleaner with the layout. You can see that in the next photo...
it even fits in the chassis well .
My NEC relay section built on a PERF board.
The front pannel layout and controls. Each LED indicates a different input selection. All very analog. The flying pigtail will be attached to a Cinemag step up transformer once the hardware arrives to mount it.
this is the top down view of the whole amp. Im thinking about mounting the other audio caps on terminal strips in a row with the 2 big red film caps, so that anyone who wants to roll them in the future can easily access them.
And the Schematic I drew for myself with a parts list.
Thats all i have for you right now. Suffice to say the preamp sounds really wonderful. It grabs all of the info off of the disk and presents it accurately. When you lift the tonearm up it is extremely quiet at a very high volume level. Im really excited. I got what I want and now my mind can move on to building bigger and better things.
I have been in need of a good tube preamp for a long time. I play with tubes, my rule. I had a Dynakit PAS3 which I had modified, It sounded good, and technically works right but doesn't impress me at all. It has too high of an output impedance, which resulted in me never being happy with the bass. I sank a lot of money into replacing nearly every part in it and then subsequently modded it into almost sounding acceptable. It works and I'm done messing with it.
I had previously built a SRPP line stage using 12AU7 tubes it worked really well, but i don't listen to CDs or tapes, or anything that doesn't require RIAA equalization if ya know what I mean... I need a tube preamp with a phonostage, and a line stage.
I had also gotten some experience with the phono-stage in my HH Scott LK72
In the process of rebuilding one I ended up totally blueprinting the entire stage including the RIAA equalization network within the feedback loop. I noticed that all the parts were common values and still available today and cheap. The stage sounded damn good in its original carbon composition implementation, and My mind immediately went to scratch-building a copy.
Sometimes the only way to have the really cool stuff that you want is to build it yourself, but then again you already know that. I did it the way i did it because it works well for me and I like it. I personally have an aversion to printed circuit boards. I have built kits from circuit boards, and they are not a challenge to me. Simply populating a circuit board and dropping it in a chassis can result in a fantastic amp, but I feel like my potential is wasted on such a thing. I personally find there to be no challenge in it. I didn't want to use any circuit boards in this build. Some of my other design criteria were to have a "conventional" case made entirely of metal (aluminum)
I didn't want the tubes sticking out the top like many DIY "plate" style construction amplifiers. Nothing against them just not what i was shooting for.
I wanted the tubes to be easily rollable without removing the top cover, and also to be shielded and isolated from vibration.
I wanted to have an extremely low noise floor without the added complexity of a separate chassis and special cord. I decided to house the power supply in a walled off section of the same chassis. It worked extremely well for a variety of reasons. The power transformer is potted (adding an additional shield) and sits right smack in the middle of the amp, which keeps weight centralized. I cant stand it when an amp is too light and slides around when you move the controls or the cables. The potting can as well as a shield around the actual transformer both act to contain magnetic fields. There is virtually no hum in the phono section and none at all when the line section is selected.
Even though I do all of my listening on vinyl, i thought it would be wise to go ahead and have some line level inputs for a cd player, radio, or to plug into a phone... I designed in a relay section which will give me 2 phono inputs and 3 line level inputs, switched on a grayhill 6 position switch. The 6th position will de energize all 6 relays and short all inputs to ground through resistors. unused inputs are (you guessed it) shorted to ground through resistors.
I mounted the tubes on a piece of aluminum 90 degree extrusion. I tapped all the holes so that the majority of the amp can be assembled without nuts. (no trying to align 2 parts with a screw and dropping the nut.) Its another thing which i had to have but which you might decide to do differently. The extrusion is mounted on little rubber grommets which give the tubes some isolation from vibration if the chassis gets to shaking with the music.
I think that it turned out pretty sweet, I built a prototype in a par metals chassis and decided I liked it enough to order the parts to build a few more. I will be offering them to members of our local audio club, or friends and family to recoup costs. After all who doesn't need a really sweet point to point tube preamp...
Anyway I'll let the photos do the Talking. I intend to bring this and some big ultralinear power amps to the Burning Amp festival in SF this year along with a few of the other members of the club.
Here is a photo of the chassis before I cut the divider plate. The sub-chassis for the audio section is in place with the shielded tubes going out the back.
Here is the unpopulated audio subchassis, with Elco shielded sockets
This is the back panel of the prototype i built.
here is a top down view of the high voltage power supply caps and the potted transformer
here is the power supply wiring on the PROTOTYPE. next one comes out much neater...
The proof of concept was complete and the amp works. The next one I build is now done, and is in a Black anodized Par metals chassis. I bought 4 of em and I used the plasma cutter and sheet metal brake to bend the power supply sub chassis. It turned out much cleaner and the bigger chassis allow me to be a bit cleaner with the layout. You can see that in the next photo...
it even fits in the chassis well .
My NEC relay section built on a PERF board.
The front pannel layout and controls. Each LED indicates a different input selection. All very analog. The flying pigtail will be attached to a Cinemag step up transformer once the hardware arrives to mount it.
this is the top down view of the whole amp. Im thinking about mounting the other audio caps on terminal strips in a row with the 2 big red film caps, so that anyone who wants to roll them in the future can easily access them.
And the Schematic I drew for myself with a parts list.
Thats all i have for you right now. Suffice to say the preamp sounds really wonderful. It grabs all of the info off of the disk and presents it accurately. When you lift the tonearm up it is extremely quiet at a very high volume level. Im really excited. I got what I want and now my mind can move on to building bigger and better things.
Could you resize your images please. It's virtually impossible to read your post.
Ok, never mind.
jeff
Just a thought, there is no grid leak resistor shown from pin 7 of the 12AX7 to ground. Probably an oversight.
Looks good though.
Looks good though.
Yes it was a mistake on the schematic, the amp definitely has the resistor. something in the order of 2 megs if memory serves me correctly. My notes are always late getting updated...
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