iso transformer for 1/4" unbalanced lines

[mike_mccue]

Hi,
Today I was tracking a guitar amp and a dry signal coming off a split. I had a ground loop causing hum on the dry signal so I bought a cheap iso transformer; a Rolls HE18 Buzz Off.
It seems to have gotten rid of the hum but there's a signifigant reduction in signal and the frequency response seems to be less than flat... there's a peak that's associated with a portion of the fretboard positions.



I know I've got some issues because I have 1/4" TS unbalanced lines going into and out of a device set up for 1/4" TRS lines. I'm having a hard time finding a unit just for 1/4" unbalanced. At least the Buzz Off has 10kΩ input and output impedance.
I searched tonight and can't find any premade 1/4" TS unbalaced iso or hum eliminators.
I would like to avoid going from unbalanced to balanced ala direct box because I want to use the hi-z input on the API preamp for my dry guitar tone.

I took a quick look at the Jensen site and am thinking about a D.I.Y. project built around one of their trannys.

Does anyone have a low signal loss, flat and wide bandwidth solution for this? Especially something that maintains similar impedance bridging as would occur with a direct connection to the API preamp.

thanks,
mike

[dai h.]

if you are a pro and you have the money, I think it would be worth it to look into a balanced power isolation transformer (avail. from equi=tech, furman, and maybe others). These let your equipment run off of balanced power which is like balanced signal lines and eliminates the fundamental problem which causes ground loops (i.e. current flow in the ground line--this the reason for instance why you have to pay attention to the specific way you ground various points in a tube amp, for example if you want to avoid hum and buzz and instability). If you can borrow one from a friend, acquaintance or a store I would try running your signal chain without the isolation transformer. If you get no buzz from a ground loop and it sounds good, then it would be working as it should. Other than that I understand that there exist high quality A/B switchers and signal splitters meant for gtr. use with transformer isolation so you might look into those as well (Lehle comes to mind for A/B, I forget for the splitter). As far as the iso transformer you are using it sounds like the impedance chosen could be incorrect (sounds too low) and impedance mismatching can cause power loss, and also I think you need to terminate one side with a resistor in the correct range otherwise you get peaks in the response. I'm no expert but these are some suggestions from what I know.

[Steve Conner]

Have you tried lifting the ground on your guitar amp? It's unsafe and against electrical code, but quick and easy, and would probably work. The amp chassis will still be grounded through the guitar cords and the API preamp, so your death shouldn't be too quick or grisly. Just for goodness' sake put it back as soon as you're done and tell your next of kin not to sue me!

Barring that: As far as I know, it's nearly impossible to make a transformer that will work accurately over the whole audio band with the high-impedance signals from guitar pickups. Probably even Jensen would struggle to do it, although I'd take a $100 Jensen transformer over a $10 Behringer part in this application any time... I've seen a 1:1 isolating transformer with 10k impedance from Palmer, but even 10k is probably a little too low.

All solutions I know of use a buffer amplifier ahead of the transformer, to help the guitar drive it. Maybe you could use a buffer pedal ahead of your Rolls Buzz-Off box, or one of those guitar cords with a JFET buffer built into the jack plug.

[dai h.]

another DIY possibility (transformer isolated hum free splitter from R.G. Keen's GEO site) :

http://www.geofex.com/FX_images/humfree2.gif

(the Mouser transformers in the schematic shouldn't be very expensive)

also, re: ground loops, using a small network of an R, cap, and diodes can work and there is a product which apparently contains this in a small plastic package which goes between the AC plug and outlet, and while it should help break ground loops, from what I gather they've kept raising the price so to me (although the initial outlay is more expensive) the balanced iso tranformer looks like a better value, esp. if you have one with capacity to have a bunch of equip. hooked up to it (then you wouldn't have to worry as much about dealing with ground loops).

and the thing Steve mentions with the buffer before the transf. to lower the impedance--that would be easy to try also

[TD_Madden]

Ebtech Hum-Eliminator (wants a pedal in front of it though, for a signal boost)or Ebtech Hum-X (I use this one).

[dai h.]

the Ebtech is what I'm referring to above. Lists for $90, street price around $60? The balanced power thing gives more value for the $ IMO.

[TD_Madden]

if you're handy, R.G.'s plan works great.

I like the Ebtech Hum-X, bacause I can just through it in the case, no problem

[mike_mccue]

Thank you for all the good suggestions. I have some buffer amps I can try.

One thing that's confusing me is the sugestion that the impedance bridge may be inappropriate.

I thought the output of my guitar pickup was apprx 6.5kΩ and the input of my guitar amp (1970 Fender Champ) was 68kΩ or even higher.

So I'm really confused about how a 10kΩ load would drag down the level. I can understand that the frequency response could skew... and I'm familar with a generalization that perhaps the treble would attenuate and the bass might swell.

Do I need a very high input impedance to bottleneck the current so that the voltage can maintain a high level?

I found this schematic at Jensen transformers which seems like I could easily build:

http://www.jensen-transformers.com/as/as013.pdf

but it only has a 10kΩ input as well.

best regards,
mike

[Steve Conner]

Guitar pickups might have a DC resistance of 6.5k Ohms, but because they are coils with a lot of inductance, their impedance at signal frequencies is much higher. So the impedance of your guitar is mainly determined by the shunting effect of the volume and tone controls, putting it in the ballpark of 100k.

Therefore, the impedance level in your circuit- a 100k guitar in parallel with a 68k amp input- will be of the order of 40k, and anything else you plug in will have to have an impedance several times higher than 40k if it's not to affect the amp tone.

The Jensen JT11-P1 transformer is specced with a damping network that lowers its impedance to 13k at high frequencies, which you would probably notice.