installing balanced transformers into a guitar?

i would like opinions and advice on transformers (before i buy).

long story short, i have in mind to do a mod on my acoustic guitar.

this guitar has a dual-source pick up system (one piezo out and one battery powered mic out).

the pick up system is the Fishman Prefix Premium Blend assembly (common technology).

each pick up's lead (out) is unbalanced.

in the past (of course), i would run each output cable into a high end DI box (for balanced signal).

this is of course how everyone connects an acoustic guitar (on stage or studio).

but, recently, i thought up the idea to install high-end DI transformers INSIDE the guitar.

then, i would wire each pick up out cable with a balanced XLR jack (allowed for dual balanced outputs).

it seems like it would be a simple/lightweight mod (eliminating heavy DI boxes and unbalanced line).

some people think i am crazy for this. but, taylor guitars does this!

have any of you ever heard of a mod like this?

would the transformer for the piezo be different than the one for the mini condenser mic?

should i buy Jensens, Cinemags or ???

It should work.

I've thought quite a bit about this in the past, and it really still seems strange to me that we run guitars unbalanced, with all the associated noise and interference etc.

I know it goes back to tube amps having inherently high impedance inputs, but if you are going to go straight into some kind of mixer or preamp then running balanced seems sensible.

Any decent make of tranny would be OK. Something like 10K to 600 ohms would work for a normal wound pickup. I'm afraid I know little about the Fishman pickups' output impedance.

Zebra, 10k:600 ohms (roughly 4:1) on a magnetic guitar pickup is going to sound dull. Even 10:1, which is what most passive DI boxes use, is pretty low. An average mic input will be around 2k, so the reflected impedance the PU sees with a 4:1 will only be 32k. That may be more than the pickup's DC resistance, but its true output impedance rises to several hundred K near its resonant peak. The pickup's resonant peak will disappear completely, in fact you'll get a falling response beyond 2-3 kHz. For good reason any decent guitar input is 470k to 1Meg, sometimes more.  So even a 10:1 is on the low side at about 200k reflected impedance. 15:1 would be better, but of course you lose even more level.

To come back to the original question: You really need to know the output impedance of your pickup system. Perhaps you can find it in the spec sheet. Or maybe they specify the recommended load impedance or at least the type of input it is designed for i.e. guitar amp, line input, mic input? Transformers are variables. How good or bad they perform depends on the input and output impedance. As a rule, an electronic output (like a piezo preamp) is happy driving a load impedance that is 5 to 10 times its output impedance. So if its output impedance is 1k, it should be happy driving a 10k line input, but for a 1 to 2k mic input, you'd better use a 4:1 (10k:600 ohms) transformer, which also does the balancing.

Perhaps another thing to consider: If you buy another guitar as a replacement or addition, you're back where you started. So maybe consider an external solution. Also, as you've seen, a transformer is only a good solution for one type of input. You won't get optimal performance plugging into a guitar amp and plugging into a mic input from the same transformer.

great to finally be among techs that see my dream.

take note, the guitar is an ACOUSTIC (so no magnetic pickups...just a piezo and a mini condenser mic (both powered by battery, inside the guitar). so, some of your noted concerns and warnings would thankfully not apply. but, i will indeed find out the output specs of the fishman system (which is here if you all are interested: http://www.fishman.com/products/view/prefix-premium-blend-1).

i think i am going to go through with this (and order the transformers today). i am excited and will keep you all briefed!

In my experience, piezo's seem to perform best in to a 1meg load.  A transformer may not be a good solution.  An active circuit with a 1meg input and a balanced output may be a better solution for you.

usekgb said:

In my experience, piezo's seem to perform best in to a 1meg load.  A transformer may not be a good solution.  An active circuit with a 1meg input and a balanced output may be a better solution for you.

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yeah, i think i was doing that before when i actually ran 2 countryman DI's (active). you might be right. i might need to go active. still researching this and have tech support calls into Fishman (pick up) and preamp (D-tar).


hmmmm....

To me it looks like that the Fishman Blender amplify both signals (pickup, mic) and puts out a line level(ish) signal.(mono or stereo)

Electrical Specifications:
Nominal Input Level: -20dBV
Input Impedance: 20MOhm
Output Impedance: Less than 3.5kOhm

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Why not use a buffer and electronic balancing, avoids possible interactions between x-former and stray fields. X-formers adds to weight also....

if there is a battery in there then i bet the piezo has a fet amp attached to it,

so you could use a transformer on the output side of the piezo amp to the guitar amp,

maybe a 2.4K:600 , 2:1 turns ratio would work,

if the condenser mic also has a preamp, you could do the same with it,

maybe get a dua; pri xfmr and wire each pri to a pickup output so you could use 1 xfmr to mix and output the signals, i believe there is a balance control so you could adjust input levels to the xfmr to get a good balance,

great to have all these helpful replies.

i also have an email into tech support from both Jensen and Cinemag.

they asked for my Fishman specs so that they could brain-storm and suggest ideal transformers.

i will keep you all briefed!

http://www.fishman.com/files/prefix_premium_blend_user_guide.pdf

Nominal Input Level: -20dBV
Input Impedance: 20MOhm
Output Impedance: Less than 3.5kOhm
Nominal Output Level: -12dBV
THD: Less than .04 %, -20dBV input
Noise Floor: -97dB
Dynamic Range: 102dB
Battery Life: 100hrs. (tuner off)
Notch Filter Range: 30Hz – 300Hz (-15dB)
Bass Control Frequency: 60Hz
Treble Control Frequency: 10kHz
Contour Control Frequency: 250Hz – 10kHz; Q = 0.5
All specifi cations subject to change without notice

Rossi - you are completely right! I was of course matching the pickup winding impedance rather than the load impedance.
This is what happens when I try to think before coffee.. 

i heard back from Cinemag tech support. after they carefully studied my pick up specs, they suggested 1.1 ratio and to order two CMLI-15/15B transformers (passive). here was his comment:

"The output signal level is -20dBV, which is a perfectly decent level for most microphone preamplifiers.  If the signal level is good your your applications, you need a transformer with a 1:1 turns ratio.  The CMLI-15/15B ($52.78) would work for the output of the Fishman preamp.  If you need more than one output, which is what I understood, the question becomes whether the signal level is the same.  If so, just use 2 of these transformers, hooking up the primaries in parallel".

these Cinemags seem to have a more impressive history then Jensens or Countrymans. I just might go for it.

zebra50 said:

Rossi - you are completely right! I was of course matching the pickup winding impedance rather than the load impedance.
This is what happens when I try to think before coffee.. 

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Actually, as I know you also wind your own pickups, have you experimented with low-Z pickups? I recently got interested in the Les Paul Recording model again. As you may know, it uses low-Z stacked humbuckers and a step-up output transformer for use with regular guitar inputs. It wasn't a very popular model, but it seems like a cool concept. I wonder how much hum reduction you get from low-Z concept alone, even without humbucker winding. In any case, the low-z concept allows to alter the resonant frequency in a wide range with parallel C.

richard_2277 said:

i heard back from Cinemag tech support. after they carefully studied my pick up specs, they suggested 1.1 ratio and to order two CMLI-15/15B transformers (passive). here was his comment:

"The output signal level is -20dBV, which is a perfectly decent level for most microphone preamplifiers.  If the signal level is good your your applications, you need a transformer with a 1:1 turns ratio.  The CMLI-15/15B ($52.78) would work for the output of the Fishman preamp.  If you need more than one output, which is what I understood, the question becomes whether the signal level is the same.  If so, just use 2 of these transformers, hooking up the primaries in parallel".

these Cinemags seem to have a more impressive history then Jensens or Countrymans. I just might go for it.

Click to expand...

It really depends on what kind of input you're using. 1:1 may be fine for a line input (typically 10k), but for a mic input (typically 1 to 2k), a step-down (at least 2:1 or better 4:1) seems like a more adequate solution for your 3.5k source. It's not just about level, it's about impedances. Your guitar electronics may not sound good driving a low-Z input such as a mic input. A 4:1 tranny will lower reduce your output level by 12 dB, but a mic input is likely to have enough gain to make up for that.

You could try a Neutrik NTE4 wired backwards. This is a very inexpensive (15 $) microphone level 1:4 step up transformer. But you can use it backwards as a 4:1 step-down. Of course you can find better and bigger transformers. But it may actually be adequate for your uses, and since it is so small it should be easy to install inside your guitar.

A little off topic
http://www.lacemusic.com/Alumitone_Humbucker.php

Gus said:

A little off topic
http://www.lacemusic.com/Alumitone_Humbucker.php

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woooh...those are kool..

not my territory as i am only an acoustic player but nice to see like minds making stuff.

all these mods might not be for everyone but, Balanced is a nice upgrade, especially when you gotta run through a mixer and compete with other musicians in the mix.