A DIY Tube Preamp/Line Driver by Passinwind

[Passinwind]

This is still a definite work in progress, but here goes anyway. When I built my DIY solid state preamp, I always intended to add a line driver stage eventually. I actually found that a balancing transformer was all I really needed, but still...I can't leave well enough alone I guess.

I wanted to keep it very simple, but also at the same time to try out some different approaches than using just the typical 12Axx tube gain stages and basic power supply. I found a nice looking circuit board for the line driver itself here: Aikido Line Driver, and went with a kit from Welborne Labs for the power supply. The turret board is also from Welborne; it contains a biasing network that I wired up for the DC filament supply. The rack case came from Par-Metal, the output transformer from Jensen. So far I have been going between two 6SN7s and one 6SL7 paired with one 6SN7. I don't really need the extra gain from the 6SL7, so I'll likely settle on two 6SN7s in the long run.

Initially I set the line driver up with a fairly high input impedance so I could use a passive bass directly into it, if desired. I've since decided that I prefer it with either my SS preamp or my active basses straight in, so I've optimized things for that now. I may implement a "loading" function if I find it useful in testing.

I'll post some clips and updated distortion and noise specs in a few days, after I dial in a few more things. As you can see, there's plenty of room for more features too. It already sounds pretty danged good IMHO though.

[Petebass]

Nice job!

[bongomania]

Very cool! I have an ignorant question though: I had thought a line driver was a booster stage one would insert in a long cable run to reduce signal loss over long distances. How would you use a tube line driver in your application? Is it essentially a powered DI with a hot output?

[Passinwind]

Originally Posted by bongomania

Very cool! I have an ignorant question though: I had thought a line driver was a booster stage one would insert in a long cable run to reduce signal loss over long distances. How would you use a tube line driver in your application?

My initial application was going to be driving my SpeakerPower amp directly from my active basses. Pretty similar to your description, although long lines aren't necessarily involved in my case. If you've read many Bergantino threads, you know this is something lots of people find desirable. But it's also turning out to complement my solid state DIY preamp really nicely.

Is it essentially a powered DI with a hot output?
There are a few different taps on the Jensen output transformer I used, so the output can just be balanced and at or below the original level of the bass feeding it if you want. Seems to work pretty well for a recording DI with a bit of tubiness.

[alexclaber]

Very nice!

Alex

[svtb15]

now thats a work of art

[OldogNewTrick]

Congratulations, really nice !
Looking forward to your sound clips.

How would your line driver compare to REDDI / The Brick / TD-100 in application and sound ?

***droool***

[A9X]

I wondered when someone was going to get around to using an Aikido in a bass pre as it's an extremely clever, well thought out and executed piece of design. It'll never catch on commercially as you can get the same amount of gain from 1/2 6SN7, but none of the other benefits of the topology this way.

Good choice of tubes too; the 6SN7 is about the best small signal tube ever made. I use 7N7/14N7 because I get better NOS tubes cheaper, but it's the same type in a different bottle. Glad you didn't go with the awful 12AX series.

[lneal]

I'm crazy about it. For clean, clear sound the 6SN7 can't be beat. Let's start a 6SN7/6SL7 revolution in MI amps!
Don't forget about the 6CG7 which is a 9 pin miniature with the same pinout as 12A_7 tubes except for the heater pinout, is modeled after the 6SN7, and with a heater voltage/pinout mod will plug right into a 12A_7 hole....................

[fdeck]

Purdy. Nice work.

[Passinwind]

Originally Posted by A9X

I wondered when someone was going to get around to using an Aikido in a bass pre as it's an extremely clever, well thought out and executed piece of design. It'll never catch on commercially as you can get the same amount of gain from 1/2 6SN7, but none of the other benefits of the topology this way.

Heh...I built my first Aikido-based bass preamp in December '05. That one has a fairly unusual tone stack which I'm still refining. A friend recommended tubecad.com as a source of scary mad scientist ideas, and I've been hooked ever since.

It was nice to just whip together a few pre-made circuit boards this time. This build really went fast once I had the parts on hand. I also built the matching stepped attenuator to go with the Aikido board, but I'm not sure yet whether I prefer it to a simple pot.

Thanks to all for the positive comments. Next time I'm going to work on a custom case with some trick graphics as well. I didn't feel like dropping a lot of extra dough on that this time, since there's currently only one (working) pot and one switch. I'm also getting my chops up on CAD for circuit board design at the moment. John Broskie at Tubecad has some nice hybrid ideas using power MOSFETs that have really caught my eye.

[seanm]

Very professional!

Did you use coax from the input jack to the amp?

Also you mentioned that you initially used a high input impedance. Is the input impedance configurable on the amp board, or did you add something in front?

[Passinwind]

Originally Posted by seanm

Did you use coax from the input jack to the amp?

Hi Sean,

Yeah, I did. On of the things I'll eventually try to dial in is input loading including capacitance, but I haven't stressed on this too much so far.

Also you mentioned that you initially used a high input impedance. Is the input impedance configurable on the amp board, or did you add something in front?

Input impedance is largely configurable off-board. The Tubecad site has a nice discussion of this relating to John Broskie's stepped attenuator. He specs nominal input impedance values from 600 ohms to 250K, but I'm not sure how practical that range is. I haven't actually gotten around to calculating (or measuring) where I'm currently at, but a 10K pot seems to be pretty nice for my purposes so far. As I said in post #1, these are still early days for this project.

[BartmanPDX]

Nice work, Charlie!

I've been impressed with the sound of everything you've done so far. This looks to be really nice!

[Passinwind]

Originally Posted by Passinwind

I'll post some clips and updated distortion and noise specs in a few days, after I dial in a few more things.

Heh...how about a year later?
A few guys have been PM'ing me lately about this design, figured I might as well put up a recording. That's my Crescent Moon fretless bass, loaded with Q-Tuner pickups.

1 minute clip in .WAV format. This is around 8MB.

Same clip in MP3 format. Around 2 MB.

[JimmyM]

Charlie, that sounds great! You are really something, dude. I wish I had half the electronics knowhow you have. Everything sounded so warm and toasty, even over computer speakers.

[seanm]

Very nice! Have you made any mods to it since you last posted?

[Passinwind]

Originally Posted by seanm

Very nice! Have you made any mods to it since you last posted?

I added a nice Jensen input transformer for balanced input and I also bought a new production EH 6SN7 to see how it compared to my stash of old ones. Pretty nice tube, actually.

[Passinwind]

Originally Posted by Passinwind

I added a nice Jensen input transformer for balanced input and I also bought a new production EH 6SN7 to see how it compared to my stash of old ones. Pretty nice tube, actually.

And now I am using a new Tung-Sol 6SN7 in the gain slot, and the EH tube in the second stage. As I was finishing up my newest DIY solid state bass preamp I found a few ideas that I thought might improve performance of this line driver as well. They worked very well indeed, and I am really happy with the sound now, especially using this in conjunction with the new preamp, which will get its own thread shortly. Here are the current measurements, done with Rightmark:




That constitutes a personal record for the best noise figure I've yet achieved in any DIY build. I'll record some new sound clips and post them in this thread ASAP as well.

[greenboy]

Great stuff, mang! Practically nobody is touching those figures!