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kkurosawa 24th November 2012 12:28 PM

Expensive transformer for bass DI?
Just a bass player here, deer-in-the-headlights type, one each.

I've read that the secret to a great sounding bass preamp/DI is an expensive transformer. Seems to me I ought to be able to locate such a transformer as surplus/used instead of bleeding money.

Can anyone point me to an understanding of what a transformer in a preamp/DI does, why it is critical, and what I'm looking for? I'll take the smallest, simplest clues, pointers to stuff others have tried, basic tutorials, anything. I could google, but I wouldn't be sure I'd be avoiding irrelevant or bad advice.

Thanks in advance!

pinkmouse 24th November 2012 12:47 PM

Basically, a DI transformer converts signal level and impedance from the format that your pre/bass puts out to what a mic input on a desk expects. This is a standard item, so searching manufacturer's websites such as Jensen or Sowter will give you plenty of options. However, in your situation, I'd just buy an EMO single channel jobbie. They are an industry standard in the pro world and will do everything you need.

Shoog 24th November 2012 03:37 PM

OEP make quite reasonable PA type transformers at a fraction of the price of Jensen and Sowter;

OEP manufacture wound components and power sources to suit a vast array of applications within the OEM electronics manufacturing sector


DF96 24th November 2012 04:19 PM

A DI also provides ground isolation, sometimes switchable.

This thread should be in Instruments & Amps section.

SemperFi 24th November 2012 06:13 PM

Edcor have some that work great for DI. I think they call them 2.5watt impedance matchers or something. I use the 10k:600 driven by a 6922.

kkurosawa 24th November 2012 11:22 PM

I didn't want to get into the gory details, but this is about the latest studio good luck charm, the $745 Reddi DI that has a 6N1P and is very heavy. There's a huge following for this box. Now, I'm somewhat of a tube freak, if an ignorant one. I rolled hundreds of tubes and found a winner, and to me, the "tube sound" is all about astounding accuracy: .So I was much taken aback that this box, which to my ears sounds like it's all about mushy bloom (listen to samples on )is all the rage. Well, it's probably true. Everyone says just get one, engineers are used to working with it, it takes no tweaking so they're delighted to see one, but when I suggested that one of its major proponents try a better Russian milspec tube, he said he did, but, it made a hump in the EQ and he blew it off. What I'm guessing happened is it reproduced his signal accurately. And since he admitted it was otherwise an improvement, I'm guessing the stock tube sucks. Now I want to find out what all this is about, this big transformer and all. I don't know what else is in the Reddi, but I figure if folks here can tell me how to build the best single-tube preamp/DI I can build, then I'll have something worth wrapping around one of the two magic tubes that turned up for me, and I'll maybe be able to demonstrate what I suspect, that folks are confusing "tube sound" with their favorite EQ flavoring, which is ugly to my ears, muffled and muddy and masking what a good bass really sounds like, which, on reflection, may be unappealing to them, but the electric bass is what it is, and it's good enough for me, magical, even.

kevinkr 25th November 2012 10:36 PM

:cop: Moved to the proper forum for this discussion.

cyclecamper 26th November 2012 06:28 PM

We would probably need to know more about how you want to use it; live or studio, direct to pwoer amps or crossovers or mixing board, consistent setup or continuously changing.

There's cheap direct boxes and expensive ones. Places like Guitar Center sometimes have really cheap direct boxes on sale as loss leaders selling for less than the parts cost. The input for a board or even power amp input is sensitive enough that you really only need a little transformer. BUT live sound can have a dynamic range far exceeding the norms for any recorded material, and you also need to accomodate the mismatches and volume changes that occur during a live gig. Low-frequency extension increases the amount of iron required, but IMHO the advantage of a bigger tranny is being able to drive any load, even if the guy at the board has a big pad on your channel.

T A Weber sells a kit for building a balanced low-impedance output into your amp. It uses a teensie transformer, and works as long as you drive it with the correct voltage, so it includes resistors and a potentiometer. I'm sure you can source the parts even cheaper. It's intended to connect to the amp output, not preamp output, but can probably be used for either. But it has no meter, so I imagine you have to adjust it every time you change the volume at guitar or preamp, and work with the guy at the mixing board to adjust his pad and level using his VU meter, as you need to keep your drive low and the board input sensitive so that you don't overdrive the transformer.

For many years the Jensen "BM" series balanced output transformer has been nicknamed the "Big Mac" (for "BM" series), and it is big and handles incredible dynamic peaks at low frequencies while delivering considerable voltage into low-impedance loads. They're commonly built into high-end PA mixing board outputs, active stage box outputs, high-end tube mic preamps, etc. It sure doesn't sound flabby. That's probably the high-end. It will drive very long lines, very low impedances, at very high levels. It delivers a balanced lien level, much more than just balanced mic level. Sounds GREAT IMHO.

Little transformers can work fine when adjusted perfectly, but the big ones are more forgiving.

The transformer isn't just to make the match, it's also to keep the output balanced so it doesn't pick up hum or RF. Of course, if you're not driving long cables you can use higher impedances, unbalanced, with much higher drive levels and not even use a transformer.

Whether to spend the money depends on how often you plug the preamp directly into different boards or other equipment with different input impedances or need quick setup for live shows.

I have been very happy with Edcor for very good value with other transformers. I have not tried their matching tranformers for a balanced low-impedance output.

The bottom line is that you need to test for yourself.

Passinwind 26th November 2012 10:03 PM


Originally Posted by kkurosawa (
Just a bass player here, deer-in-the-headlights type, one each.

I've read that the secret to a great sounding bass preamp/DI is an expensive transformer. Seems to me I ought to be able to locate such a transformer as surplus/used instead of bleeding money.

What's your budget, and is the DI side or the preamp one most important to you? Personally, I don't consider Jensens to be all that expensive when put into the context of my design and build time. A friend of mine on Talkbass swears by a cheap reversed mic transformer-in-a-can though.

cyclecamper 27th November 2012 01:13 AM

I guess it depends on whether you want to achieve balanced line levels or balanced mic levels. Sorry I don't have time to look up the details, I used to just call and order the "BM" line-level output transformer. I've always been very pleased w/ Jensen (not to be confused with the speaker company), good input transformers too, good shielding etc.

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