DIY Low Gain Preamplifier

[Eric.DeYoung]

First post - Got directed here from the Parts Express forum, so here it goes!

I'm in the middle of a school semester, and I'm in desperate need of a fun project. I'm looking to build a tube preamplifier - but I would like a low gain (0db to 12db or so) design so that I can insert the preamplifier between a line out from my sound card, CD player, etc. and my receiver. I've always wanted to try tube gear, and I think that this would be a great way to get started without having to spend a huge amount of money on huge power transformers and output transformers. I've been looking around online quite a bit, and found a couple designs such as http://www.shine7.com/audio/6922_pre.htm
that I really like - however the gain is much to high for my liking, and it seems that some people aren't too fond of the cathode follower arrangement that acts to lower the output impedance. Do any of you have some links laying around that might be what I'm looking for?  I just want something to go between two line stages, so I'm assuming that a low gain is necessary - correct me if I am wrong!

Thanks!
Eric

 

[Doc B]

Our Quickie kit fits the description:

http://www.bottlehead.com/store.php?crn=220&rn=438&action=show_detail

 

[Eric.DeYoung]

That kit looks alright, but I would really like to start from a schematic, design my own PCB, and purchase parts myself to build it - I am a 4th year electrical engineering student and I'm pretty comfortable with solid state, but I've had very very little experience with tubes - and starting from a schematic and building the entire project would be a great learning experience for me!  Also, if a jump in price means a jump in performance, I am willing to spend a bit more - I would like this to be a great addition to my 2 channel music system and have a positive impression of what this tube stuff is all about!  

Thanks Again!
Eric DeYoung

 

[Paul Birkeland]

Well, there are a number of arguments of using PC boards in tube audio gear...

I like the idea of keeping signal currents off of PC boards whenever possible (I didn't come up with this, by the way), and I really like keeping tube sockets off of the PC boards also.  Tubes generate a lot of heat (except the Quickie tubes), and a lot of that gets transferred to the PCB material, introducing significant wear. 

If you bought an Extended Foreplay III kit, it would come with all the parts and the schematics for both versions.  All you would have to do is buy some 9 pin PC mount tube sockets and your own cabinet.  You'd probably not have a hard time doing a PCB that is switchable between 12AU7's and 6CG7's. 

If you wanted to use the schematic you posted, consider adding some tone controls between stages to eat some of the gain (it would seem to be designed for that since it is a circuit that could easily be directly coupled but isn't). 

 

[Paul Joppa]

Quickie is 7dB; Foreplay III has 10dB gain.

The circuit posted is topologically very similar to the original Foreplay, except for C7 which is unnecessary and the use of the 6922 (Foreplay uses 12AU7). I'll forgo the temptation to otherwise critique it; I haven't even heard it.

One reason Foreplay has lower gain is the use of a shunt mode level control, which provides 10dB of attenuation (optionally more) before the signal reaches the tube's grid. You can use that with any preamp of course.

Do look at my white paper on signal levels and noise, linked from the Community page. I usually apologize for the level of detail, but as you're an EE I probably don't have to ...  :^)

If you are going for good tube sound, try to avoid circuits with negative feedback. Cathode followers are of course negative feedback circuits; that's why when we use them we go to some lengths to minimize their sonic impact. A more expensive way to get good sound at low gain is to use an output transformer, which also provides a low output impedance but without any feedback.

However, I strongly recommend that you spend less than half your budget on your first try - you are likely to learn something, and want to do it better the second time! I learned this philosophy from a really excellent teacher in a class on design of experiments...

 

[Eric.DeYoung]

Thanks for all of the replies!  I think before I start building, I obviously need to start getting a better handle on the basics of tube amplifiers.  I understand the very basics about tube operation, but that is about it.  Time to do some research!

Eric DeYoung

 

[Paully]

Another place you can research archives and ask questions is:

http://www.audioasylum.com/forums/tubediy/bbs.html

Just type in preamp schematic and see what you get.  Also, is low gain absolutely necessary?  Why not build a regular preamp and pad down the outputs if the gain is too high?

There are some books available on designing and building tube amps, might be some material in there on the pres as well.  I think Rozenblitz (sp?) is one and there are some others.  If I recall, some books were actually at parts express.

 

[Paul Birkeland]

Thanks for all of the replies!  I think before I start building, I obviously need to start getting a better handle on the basics of tube amplifiers.  I understand the very basics about tube operation, but that is about it.  Time to do some research!

Eric DeYoung

Start a new post in Tech Discussion with questions you come across.  Their answers will help build the knowledge base in the forum quite well. 

I kind of like PJ's idea of doing a practice run, then really going at it.