Bass pre-amp DI output

diybass

Member
2012-06-04 8:30 am
I am looking to add a balanced DI output to my DIY bass preamp. It consists of a copy of the Carl Martin Compressor and a single channel copy of an Alembic F2B preamp, connected in an enclosure. The Alembic F2B output is line level. I would like to be able to switch between pre comp and post comp+preamp, but failing that I could easily live with just post output.
After the preamp, I have added a master volume control, so I don't know how that would work with a transformer? I guess I would have to add an active DI out circuit? I want a DI output that isn't affected by the master volume control. A volume control for the DI would be cool, but not essential.

Any suggestions? I'm still really new to electronics, so any help is really appreciated! Cheers.
 

tinitus

Ex-Moderator R.I.P.
2005-11-24 1:47 am
After the preamp, I have added a master volume control......

I want a DI output that isn't affected by the master volume control....

A volume control for the DI would be cool, but not essential.

I guess you could 'just' connect DI out SE/balanced line trafo before your volume control

but like you say, failures from wrong impedance matching scares me too
 

diybass

Member
2012-06-04 8:30 am
Here's the schematic, I think it's pretty accurate.
 

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250k pot; a touch high for most DI transformers. Is the volume pot linear or log? If it's lin you could just about get away with loading it with a decent passive DI transformer, which will probably load it with about 100kΩ. But do you really want the DI After the master volume, or will you want DI level to remain constant while you adjust your local listening level?

Anyway, with an impedance transformer like that, output will be mic level; if you want a balanced line level out, you really would like a buffer; a cathode follower would be a good solution (oh, sure we can do a solid state buffer; but that's your recording output, wouldn't it be nice to tube all the way? And the power supply's there (I assume there's some reserve power?), whereas, with transistors… I suppose we could run something off 300V, but it's a challenge.

With a buffer, no problem about putting it pre/post compressor (if you plan it to be switched, obviously. If you take the grid reference from the previous stage rather than doing a complete internal bias, sure switching is complex, so don't. It's only a few extra components).

Trouble is, it's another tube socket, more heater wiring, quite a lot more mechanics.

With a 10k output drive, for a bass, you need a big output capacitor, and your output transformer's going to be massive so as not to saturate the core with the low frequencies, and don't forget the ground lift switch.

Is that preamp really only giving line level off a bass? I''ve had a volt and a half off a fender on snap; not that that circuit would have any difficulty handling it, I just wouldn't be surprised at much higher peaks.

Where do you insert the compressor? Before the master?
 
JENSEN TRANSFORMERS, INC. - LINE OUTPUT TRANSFORMERS
http://www.jensen-transformers.com/datashts/10k611m.pdf

check out the sample tube driver circuit for the DI...

We always favored the BM series for just 600:600 because they were SO hefty you didn't have to adjust the levels so often, they just sounded good if you got in the ballpark http://www.jensen-transformers.com/datashts/11bmcf.pdf but that's not direct from a tube. Main output from a mixing console...

The Trace Elliot V8 I'm starting to build a clone of uses only op amps for the DI, no transformer; it is considered the worst part of the amp.
 
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diybass

Member
2012-06-04 8:30 am
The compressor is instrument level in and out, so it is before the preamp and has effects loop sockets on the back of the enclosure. Yeah, the preamp output is line level, but it is quite a hot output, that is one of the reasons I added the master volume.

I want the DI output to be mic level with a ground switch, mainly for gigging. I want the the DI level to be constant, so the master volume only controls my power amp and speaker cab.

I would like to keep it as few components as possible, Failing that I could go tube, I just don't have alot of money. That jensen sample tube driver circuit looks good, I don't know where I could get a 12BH7 from in NZ though, could be pricey. I don't mind spending a bit on a good transformer though. I do want to keep away from op-amps, the compressor is the only thing with op-amps in this unit.
 

diybass

Member
2012-06-04 8:30 am
So, could I use just a transformer? How would I wire it? Or is the master volume control creating problems? The master pot is 250K audio taper. I don't mind changing the output pot value to make it work.

It would work with two transformers wouldn't it? One for DI out, one for line out with volume control after tranny? But that is out of the question because of the price!
 
Not just a transformer, but a DI transformer and a big coupling capacitor should be OK.

The DI transformer doesn't want to see any DC from a single-ended tube. The 1/2 Alembic is single-ended class A so it has DC that gets blocked by a cap in its output or the next stage input at the power amp. Single-ended output transformers are special things with air gaps and large cores to cope with the DC. So Jensen really wants you to provide a push-pull driver ideally, otherwise I guess you drive the transformer thru a large isolation capacitor so you don't lose bass. No need for another transformer for your normal high-impedance output. I would recommend a volume for each output, with a pull-switch range pad on the DI volume pot. A pull-switch on the main high impedance output to mute to a high-impedance dummy load might be convenient if you want to hear just the DI. The DI should have a ground lift, and if you plan to use it for monitors you want a DPDT polarity-reverse switch on the DI output too. Neutrik makes nice jacks that take an XLR, TRS balanced 1/4, or TS unbalanced 1/4, and the transformer will work balanced or unbalanced, so that's a nice jack to use and you can shove in just about anything. The load impedances should be much higher than the source impedances, so you shouldn't have trouble with the 2 volumes interacting as long as neither load is ridiculously capacitive or inductive, and they shouldn't ever be.

But get the opinion of a real EE on-list, not just me.
 

diybass

Member
2012-06-04 8:30 am
So the big coupling capacitor would connect to pin 1 of the 12ax7, then to the tranny, then other end of tranny primary to ground? What does the circuit for a volume control and pad look like for the DI out? Nice idea to have a phase reversal switch!

And what should I use for the dummy load for the high z output? Is a dummy load necessary for a preamp? I thought it was just for tube power amps?

What exact tranny anyway?

Sorry if some of my questions sound stupid, but I'm still fairly new to this. Having a heap of fun learning though!