12AU7 Preamp build

[mattthegamer463]

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.

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.

 

[gni]

Looks like a good start. . . .

 

[mattthegamer463]

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.

 

[revintage]

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 !

 

[revintage]

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.

 

[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.

 

[kevinkr]

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.

 

[mattthegamer463]

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.

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?

 

[Giaime]

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).

 

[mattthegamer463]

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?

 

[kuroguy]

an MP3 player or PC would not need the gain or RIAA equalization of a phono preamp. I believe MP3 and PC have somewhere between 1 and 2 volts peak to peak output, so you shouldn't need anything other than your power amp to drive speakers. I assume the phono preamp would take the cartridge output and produce 1 volt peak to peak (with RIAA equalization) to send directly to the power amp.

 

[kevinkr]

Hi Matt,
You are confusing the functionality of a line stage with the specialized purpose of a phono stage. The line stage will pair perfectly with your kit amplifier and allow you to connect tuners, cd players, computer sound cards/outboard dacs to the amplifier - anything really that is line level.

For a turntable you need a dedicated pre-amplifier to provide both the required gain and the RIAA playback equalization. This would be used for just the turntable and would be plugged into the line stage you are currently building.

You can plug the guitar into your line stage, but I would not recommend it, probably not the best sound quality, but also playing guitar through a hifi amplifier and speaker is a recipe for blown speaker drivers and possibly depending on the ruggedness of the output transformer that as well.. I have more than one friend who found that out the hard way.

Yes a 100K pot, 50K or even 20K pot (solid stage sources only) will work just fine. Try to find an inexpensive stepped logarithmic attenuator on eBay - there is a seller who from time to time who sells nice ones quite inexpensively. Quality here pays off handsomely in the long term.

 

[mattthegamer463]

Hi Matt,
You are confusing the functionality of a line stage with the specialized purpose of a phono stage. The line stage will pair perfectly with your kit amplifier and allow you to connect tuners, cd players, computer sound cards/outboard dacs to the amplifier - anything really that is line level.

For a turntable you need a dedicated pre-amplifier to provide both the required gain and the RIAA playback equalization. This would be used for just the turntable and would be plugged into the line stage you are currently building.

You can plug the guitar into your line stage, but I would not recommend it, probably not the best sound quality, but also playing guitar through a hifi amplifier and speaker is a recipe for blown speaker drivers and possibly depending on the ruggedness of the output transformer that as well.. I have more than one friend who found that out the hard way.

Yes a 100K pot, 50K or even 20K pot (solid stage sources only) will work just fine. Try to find an inexpensive stepped logarithmic attenuator on eBay - there is a seller who from time to time who sells nice ones quite inexpensively. Quality here pays off handsomely in the long term.

In that case, this project will be a line stage preamp. Which schematic would you recommend? Would the one I was originally making be sufficient? If you think the one used in the article and the one that it was derived from are decent I'd like to build one of those since I have all the parts for both.

 

[revintage]

Would the one I was originally making be sufficient?

Hi Matt,
As I said in my first post it is totally unsuitable! In other words a total disaster whatever you want to use it for. WORTHLESS!

If you go SRPP you should use max 20k-50k as volume pot, as with higher values frequency response is worse at 50%(middle of the pot). The one I suggested is independant so you can use 100k if you want and it also has adjustable gain by adjusting R5 in the NFB.

The SRPP suggested by Kevinkr and the one above are about the same with respect to Zout and should not be loaded by less than ca 100k.

But go for the SRPP as this will be easier to build (even if SRPPs IMHO does not sound good enough for hi-end use). The SRPP will also have a little higher THD but this does not matter either, as it is only second order harmonics.

Your PT has OK voltage when used with a bridge rectifier w/o the CT connected. You should stay well above 250V B+.

EDIT: SEcond thougths, the one below will sim just a tad worse THD than the SRPP. But it might sound even better......

Why not try them all !

 

[Alex Kitic]

PREAMP WITH ECC82

Just a few comments to all that was written so far:

1) The preamp you are building is not worth the time;

2) Do not switch to SRPP, as the stage is not the best option for a line stage preamp (working into unknown and possibly varied load);

3) Try the CCDGC instead - for reference see my website, basically it's the line stage of the RPA. You can build it easily using the existing chassis and rest... once you feel ready to upgrade, add a decent power supply and full selector if you need it

Regards,
Alex

 

[mattthegamer463]

@ revintage

Thank you, that second schematic you supplied looks quite do-able for me. It looks as if the triodes are in parallel though, how does that work? I just don't think I've ever seen tubes connected like that instead of being lined up basically in series.

Also, could I put a pot at the front of it, maybe putting my 100k pot in parallel with one of those 50k resistors on the input? I'm just trying to figure out the function of those resistors.

 

[revintage]

The two 50k IS a 100k volume used, when I simmed the circuit . But you should not go above 50k. So just substitute the two 50ks with a 50k pot.

This is a simple solution that you can begin with and rebuild later.
Zout is halved and Gm is doubled when parallelling.

 

[mattthegamer463]

The two 50k IS a 100k volume used, when I simmed the circuit . But you should not go above 50k. So just substitute the two 50ks with a 50k pot.

This is a simple solution that you can begin with and rebuild later.
Zout is halved and Gm is doubled when parallelling.

What sim program did you use? My college course uses Multisim 9 but I tried doing a tube circuit in it and it didn't work, so I'd like to try another one.

 

[revintage]

LTSpice from www.linear.com is the "diy-industry-standard".
Lots of tube models on the net. Check into Sim City at the www.intactaudio.com forum.

 

[mattthegamer463]

I've been fiddling with LTSpice now for a few hours, but I'm having a real hard time loading tube models into it. I got a library of triodes and its applied to the schematic but when I run it says "Too few parameters for subcircuit type '12au7' " and I'm baffled. I've read a few guides but I can't figure out whats wrong.