Selecting the right transformer for the job?

[brad]

In the seemingly endless quest to isolate the hum from my magnetic double bass pickup, I ditched the use of a transformer thinking it would cost too much -the Jenson transformer needed is US$75+.  However, upon further research, I noticed a cheap 1:1 transformer could do the same job.

Most of them appear to have crappy telephone frequency responses that only start at 300Hz, but I found this 10K:10K transformer with a response of 20Hz-30kHz for the princely sum of AU$9.95:

http://www.altronics.com.au/index.asp?area=item&id=M0710

The idea is to wire it up like this to leave the output hum-free: Diagram

Would the 10K:10K transformer fit the bill?  Perhaps I should just employ someone to keep their finger touching the output socket.  A Hum Roadie!

[brad]

Maybe what's inside this Car Stereo Line Isolation Transformer would work instead?

Car Stereo Line Isolation Transformer

The telephone transformers would be fine if their lower limit went below 300Hz.  I can imagine the loss of those low frequencies wouldn't matter on guitar, but would probably be noticable on a double bass!

[R.G.]

Dang, that's some transformer if you can believe the datasheet - and if what the datasheet doesn't say is truly unimportant.

If I were you, I'd nab one and try it. Chances are it's going to be good for your project.

The cynicism I was expressing at first was a relic of my being burned on datasheets back when I was an engineerling. Engineers have their own saying about datasheets - "There are lies, damned lies, and then datasheets." The community mores on datasheets - at least back in the 70's in the USA - were such that a manufacturer would guarantee that his devices would work to the specified standard but nothing else, and usually under the most restrictive interpretation. So things like specifying transistor gain as a max of 100, typical of 90 and no specified minimum would make you think that most of them were up in the 80s and 90s.

Not so, grasshopper. What the manufacturer REALLY told you was that there would be no transistors over 100, nothing else. You *might* get a transistor or two or thousand with a gain of zero, and they would still be within spec.

So that kind of thing is always in my mind. I can't help it. I got it in the war and I can't get rid of it.    

On the transformers, they specify them at a response of 20Hz to 30kHz. Well and good. But they don't specify what loss they might have at those extremes. 20 Hz response might be -24db and still fit their wording.

I agree, probably not, but it's one of the things that I have this twitch about. Probably it's -3db at the ends. That would be more common.

Whatever the response, you can make it better by driving the primary with a low-impedance buffer like I did on the hum free isolators and A/B/Y boxes at GEO. That will extend the low end a fair amount.

But do try it. See what it does without active electronics, then try the actives if you need them. Non-active loading will cut treble on you, but you may be OK on that.

[brad]

Thanks for the informative post R.G.  

I might give both the 10K:10K and the telephone transformer a go and see what difference they make.  To be honest, I wasn't even sure my wiring diagram was correct or if a cheap transformer would even work because this is the first time I've dealt with transformers!  It didn't even occur to me that a range of 20Hz to 30kHz was out of the ordinary lol

[Rob Strand]

From your ol dpost it sounds like you have a buzz problem rather than a humm problem.

You say:

he socket isn't physically touching the aluminium on the bracket b

That is a *very bad*  configuration - that is asking for a buzz problem.

When you get buzz problems like this on electric guitars and basses invariably the solution is to shield the pickups and the control cavity.  You should be trying to mimic this configuration by puting some sort of shield around the pickup and connecting it to the cable's ground, and in addition connecting your plate to that ground also.  If you haven't tried this you are possibly solving the wrong problem.

After doing that, if you still have buzz then consider the transformer.  Note the transformer you have chosen is *unshielded* this means it can pickup buzz, and it can possibly  pick-up humm if it's not designed well.  An alternative to the transformer is to build a a differential input preamp (a high input impedance one) this performs the same function as a good transformer..  The Jenson site argues the transformer is better, well it's debateable anyway, but if the transformer isn't of high quality like a Jenson then the electronic solution is actually better.

Something to think about.