3 stage Tube Mic/line amp build completed.

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lassoharp

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Joined
Jan 3, 2009
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2,100
Location
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Here's some pics of a tube preamp I recently finished.  Two channels, fixed gain is adjustable from 60 to 72db, 3 triode stages with interstage volume control and input pads that go from mic to line range.  Design could be loosely considered as an RCA BA-2 with an extra gain stage.  Plate loading on low level stages is a bit lower than BA2, output stage is exact same with same OTs.  Iron compliments of emrr.  Sounds great and represents the best I could think of for a SE direct coupled output tube mic amp in terms of gain, flexibility, and fidelity.  Took the better part of a month to complete including bench test time and research.

front:

http://i409.photobucket.com/albums/pp179/lassoharp/DYI%20preamp%20%20Nov%202011/frontweb.jpg


3/4 side R:

http://i409.photobucket.com/albums/pp179/lassoharp/DYI%20preamp%20%20Nov%202011/P1030039.jpg

3/4 side L:

http://i409.photobucket.com/albums/pp179/lassoharp/DYI%20preamp%20%20Nov%202011/P1030038.jpg

gut:

http://i409.photobucket.com/albums/pp179/lassoharp/DYI%20preamp%20%20Nov%202011/P1030045.jpg


rear:

http://i409.photobucket.com/albums/pp179/lassoharp/DYI%20preamp%20%20Nov%202011/DIYpreampFBrearBW.jpg
 
Very lovely indeed. Wish it were mine!

Nice work ;)

Just love that style of build - old style : point to point, large front shallow rack.

 
Thanks for the enthusiasm guys!  It's always nice to know there's other folks out there who get as excited over the old style stuff as I do!


I'll post the schematic later this evening.
 
schematic:



http://i409.photobucket.com/albums/pp179/lassoharp/DYI%20preamp%20%20Nov%202011/DIYpreampschematicweb.jpg
 
That's a straight ahead 3 stage SE circuit for sure.
i'd love to hear it on a di'd guitar.

Have you had a chance to look at the noise spectrum?

60-72dB is one whole bucketload of gain!

Really interested in how quiet the old tubes are.

Cheers

 
In my experience the old tubes are quieter than the smaller bottles, and far quieter than early SS.  Quieter than a lot of pro SS into the mid-70's. 
 
I haven't been able to do any detailed noise measurements yet.  What I generally do on the bench is put a suitable resistance load on the input and run output into digital meters and see where noise starts to register.  At all but the highest of gain settings it stays somewhere less than -65 to 70.

With triodes it's usually not a concern.  The one amp of comparable gain (~62db) I have with a pentode and no NFB is objectionable and it will be rebuilt as a triode circuit soon.

Tracking vocals is where I really love these amps - other instruments do fine too but I like not having to patch in compression or limiting as much during mixdown.

 
Great build.  Sure would like to find some of those Transformers.  I have the other parts ready to go.
 
letterbeacon said:
Wow!  This looks amazing!

Have you had a chance to take any frequency response measurements at all?



Thanks.  No official measurements yet but it sounds right on the mark on the few tracks I've recorded so far.
 
How does it stack against your BA-2 build?  Same with more gain, or was it steered in another direction by unforeseen factors?  Here you have RCA inputs and outputs, but slightly different outputs and later era inputs. 
 
I haven't had a chance to do a side by side A/B using same mic direct into DAW yet, probably this week.  Right off the bat my impression was that it was a little ahead of everything in - . . . some clarity, hi-fidelity? aspect. 

I'm debating best method to do a sound clip test.  You can sometimes get so many different tones through the gain range.  I'm looking at setting up 3 to 4 gain settings using test tone to match output voltage into a given load, and then using DAW to equalize any volume differences.

 
bruno2000 said:
Man, that is great!  Are you working on a 500 version? ;o)
Best,
Bruno2000
hahaha!
very nice build!
mabe an output attenuator could be useful to be able to generate distortion (as an option?)
i never tried this myself but its been a while i'm thinking about building a similar tube preamp with lots of gain, and implement an output attenuator and see how it sound (on electric stuff not classical music of course) mabe it sounds like crap !
 
mabe an output attenuator could be useful to be able to generate distortion (as an option?)
i never tried this myself but its been a while i'm thinking about building a similar tube preamp with lots of gain, and implement an output attenuator and see how it sound (on electric stuff not classical music of course) mabe it sounds like crap !


That's where the NYDave mixer comes in handy - I just run into it before the recorder if I need to push something hard like DI guitar or bass.  Works great for that. 
 

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