tube mic preamp

[jcm-jmp]

hey guys

i have been asked to design and build a tube mic preamp.

now rather than useing big iron i think i can just use line voltage (120vac) and a bridge rectifier for my B+ plate system. i will have to buy a small transformer for the heater circuit.

but...... im used to dealing with big huge high gain monsters! this low gain clean recording preamp unit is new territory for me .

you guys got any suggestions as to gain parameters and tricks in preamp circuits for thick clean signal reproduction?

thanx

[jhadhar65]

>...i think i can just use line voltage (120vac) and a bridge rectifier for my B+ plate system.

Dangerous - no isolation there.  You can get 1:1 toroidal transformers with line isolation, which is a safer, more logical choice.  I'm sure you're looking for "small and neat", but a PT is unavoidable.  You'll already be less an OT and the toroids are real space-savers.

[bnwitt]

Well if you have to have tube (and believe it or not there are great preamps without tubes)  a kit may be the way to go:

http://www.hamptone.com/

If on the other hand you don't need tubes, there are some other kits that are fantastic too:

http://www.seventhcirclestudios.com/SCA/SCA.htm

and for little dough there's always Craig Anderton's stuff:

http://www.paia.com/tubestuff.asp

[bluesbear]

I have a wonderful old DuKane 3 channel tube preamp I bought at Goodwill for $20. I'd look around for something cheap that was built by a company that actually knew how before I'd build something or pay an insane price for a new one.
Dave

[TubeGeek]

I built the Hamptone Jfet and tube pre's and like them alot.  Matched with a nice AD converter into Logic Pro....nice sound.

I am going to build a bunch of seventh circle audio Neve 1073 clones in October.  

The PAIA kits are very affordable. I might have to try one of them.  Has anyone used one or heard one?

It'd be nice if we could design something also.

I'd recommend building a kit first.

TG

[amp_mangler]

Very NICE idea :) valves and microphones work superbly !!!
its like valve guitar amps versus sand ( silicon ) both work, one sounds superb, the other is just sound :D
now isnt THAT going to make some discussion ? I have made quite a few small mixers and mic pre,s over the years, and have found they cant be beaten for sheer prescence, the vocals being NOT in yer face but surrounding you.

I have a whole bunch of circuits that ill scan later today and post here, er and ALL of em use a power transformer for isolation apart from providing the various voltages required[size=16][/size]amps without isolation should be canned untill they are safe !!![size=11][/size]


watch this space

Joe

[Megachunk]

Thanks for those links, bnwitt!

Those Hamptones look great, and a buddy of mine is considering buying one. Theres a company that makes LA2A clone kits, but the name escapes me.

[amp_mangler]

as promised circuit one
origionally designed for high quality tape recording
obviously the PU inputs arent needed (maybe ) but can be utilised as line level, as they need about 1 volt p-p
the concentric pots can be separated so ya have 2 mono channels
as many of these as ya like can be tacked together to make a mixer as large as ya like
tone controls can be added as well, requiring another 12AX7 or similar
I usethis circuit as a building block, adding what is needed as I go
its VERY low noise, and I have changed the EF86 to a 6BR7 as the noise is even lower
the power supply needs VERY careful design and Ive found that a separate supply box is best
BOTH valves should have screening cans

more to come

regards Joe

[TubeGeek]

Cayocasta has plans for a LA2A on the net but I have not seen a kit.  The LA2A is also on my project wish list.  I am thinking of trying to build one this winter.

I am ramping up my studio again.  I just purchased an RME fireface 800 to run in my analog preamps to Logic.

Get this...I received a call from someone today that had 17 Celestion Vintage 30's and 2 Eminence (unsure of the model) and four 4x12 cabinets for sale.  I offered $500 for everything and she took it.  No they are not stolen items.  

[OldHouseScott]

Yet another plan

NewYorkDave one-bottle mic pre

Saves on tubes, but requires an input transformer.

[PRR]

You will NOT build a hot-chassis (direct line powered) preamp.

Aside from killing your firends, you will find that you want more B+ than 150V. For the same reason we do in guitar preamps: The signals before any system volume control are ALL over the place, soft and LOUD. You need supply voltage to cover the loud sounds without excess damage.

>> a tube mic preamp.

> requires an input transformer.

This is now fairly true. Nearly all studio mikes are low impedance balanced. Many of them can not be run unbalanced. Many of them demand Phantom Power, which demands a specific input configuration (and a 48V 10mA supply which is awkward in tube work).

Low-impedance dynamic mikes have a self-noise voltage far below any tube. To get full low-noise dynamic range, you "must" use a step-up transformer. Happily this also allows a proper balanced input to an inherently unbalanced tube.

Low-Z high-output condenser mikes are a glut on the market and very popular. Put these near most pop-music sources and they deliver huge voltages, as much as a volt. Put any sort of gain after them, and you need high supply voltage.

The input impedance of most studio Line Inputs is annoyingly low for a tube. ADC inputs are often 10K. If a studio runs long lines from mike preamps to next box, 600 ohms may be desired to preserve highs and reject crosstalk.

GOOD tube mike preamp design ain't easy. (OTOH, sometimes it is astonishing how often a "not-good" design can work very well for a specific purpose.)

Follow the link Scott gave. That specific plan is well considered and simple. Trawl that forum a bit and you find other ideas with more or less merit.

IMHO: corrupting a signal before it reaches tape is just dumb. "Capture" and "Color-blend" are two different things. Use a good squeaky-clean SS preamp, capture verbatim. Then in mix-down, route out through a line-level color-box to taste. But I know this is not the fashion.

[jcm-jmp]

IMHO: corrupting a signal before it reaches tape is just dumb. "Capture" and "Color-blend" are two different things. Use a good squeaky-clean SS preamp, capture verbatim. Then in mix-down, route out through a line-level color-box to taste. But I know this is not the fashion.

i agree.

it is more work when tracking, but by adding "color" to a very low db master track produces a very large and full final mix. with the added benefit of an un-tainted master track to reference in the future or during final mix.

[Megachunk]

Hey Tube Geek!

Want to sell any of those V30's?

[TubeGeek]

Yes I do want to sell some of these speakers...

Is $65 +s/h each fair?

[thermion]

to keep the package small don't bother putting xlr inputs on your preamp as you already have this feature on your mixer. for mono, run a paralleled 12a_7 or such tube as your preamp at line level between your mixer and power amp/ recorder/ whatever you're doing this for. for stereo, just split l/r into each side of the tube. use a 0.15uf coupling cap for full range, or take a cue from your favorite hifi schematic.  we recorded to analog all day sunday with this setup and the sounds were as corpulent as they were dumb, possibly moreso.

Is $65 +s/h each fair?

maybe if you hadn't mentioned how cheap you got them for...

[TubeGeek]

$65 is still better than $120

[thermion]

you will find that you want more B+ than 150V

i forgot to mention mine's 265vdc on a 6sl7 type, pretty close to max. since it just sits next to the mixer on the table i splurged and got hammond 269ex bolted to the chassis.