Small audio transformers for isolation and impedance matching

[Guitarist]

I'm looking at some small, affordable transformers. 600Ω:2.4K others are 600:600. The latter having DC resistance of 145Ω. I can see a few reasons to use these and one is in parallel effects loops where you get out of phase returning. Another is with two amps and/or when you prefer out of phase (which I discovered with acoustic guitar in to an SWR Strawberry Blonde). Or breaking ground loops etc.
I am wondering how to mitigate signal loss if your signal is only 2.4K above ground? What am I missing? Do they make a 100K:10K, a 10:1ratio?

[Helmholtz]

600 Ohm transformers are not suited for high impedance circuits.
They expect to see the rated load (say 600 Ohm) at the secondary, otherwise bass response will greatly suffer.
While the loading can be achieved with a resistor, this will result in significant insertion loss.

Quality high impedance audio transformer tend to be expensive.

Reversing the phase of an amp can be achieved by swapping the speaker wires (e.g. at the speaker plug).

[Leo_Gnardo]

What Helmholtz said ^^^. It's a bit inconvenient in some amps to swap speaker polarity especially if you're cramped for time. There are higher impedance miniature audio transformers that are friendlier to high impedance circuits. To flip phase with one of these you might try inserting in in your SWR effects loop, then there's no loading effect on your guitar and the amp I'm sure can drive it without a problem. Worth a try anyway. Mouser has fingertip size Xicon tran's very cheap. I'd try their part # 42TM018-RC for starters only $5 cheap. FWIW I've successfully used others in this series in DI boxes. Though the frequency response may seem limited on the spec sheets, that's for maximum signal level. Despite a prediction of serious bass rolloff, I've found them to work fine as long as they're dealing with signal levels that don't challenge their limits.

[Helmholtz]

It's a bit inconvenient in some amps to swap speaker polarity especially if you're cramped for time.

Doesn't have to be, at least when there's a jack plug for the speaker cable.

I built a small box with a heavy duty phase reversal switch and 2 jacks - e viola.

The bass rolloff of a transformer can be calculated if circuit impedances and primary inductance are known.
Depending on how and where the transformer is used, magnetic shielding may be required.
Professional audio transformers often have a mumetal shell.

[loudthud]

Geofex.com shows an A/B/Y splitter dated 12/8/04. Uses a Mouser 42TM018 for a transformer. Further down is something I've never seen before. Dated 6/07/00 the circuit uses opamp(s) to isolate the ground noise on two or more amps.

[Guitarist]

Thank you for the insights. I have another question now On my new RME interface, it has balanced Line In impedance of 13kΩ. If I wire a guitar's single humbucker which is 7.3KΩ, so it is balanced, will it work well? I know it's good to have 10x Ωs or more, at the input Z. I play with lots of distortion and it would be GREAT to get a quieter pickup sending and not deal with the jumpy, gate noise.

[pdf64]

A pickup with 7k3 DC resistance may an impedance at its resonant peak of over 10x that. Hence the use of controls with much higher resistance, eg 470k volume and tone pots, with amp input impedance of 1M.
Your guitar would sound very quiet and muffled if feeding a 13k input.
Rearranging guitar wiring for a balanced signal isn’t a trivial task.

[bbsailor]

Thank you for the insights. I have another question now On my new RME interface, it has balanced Line In impedance of 13kΩ. If I wire a guitar's single humbucker which is 7.3KΩ, so it is balanced, will it work well? I know it's good to have 10x Ωs or more, at the input Z. I play with lots of distortion and it would be GREAT to get a quieter pickup sending and not deal with the jumpy, gate noise.

Go to this web site:https://www.till.com/articles/PreampCable/ and look at the FET preamp cable. This device puts a high input impedance FET in the guitar plug end of the guitar cable to isolate any resistive or capacitance loading from being seen by the pickup and affecting the high frequency tones of the pickup.

The output impedance of FET preamp cable is low enough to drive your stated 13K ohm input impedance.
I built one of these and it really works!

I hope this helps.

Joseph J. Rogowski

[Leo_Gnardo]

Go to this web site:https://www.till.com/articles/PreampCable/ and look at the FET preamp cable. This device puts a high input impedance FET in the guitar plug end of the guitar cable to isolate any resistive or capacitance loading from being seen by the pickup and affecting the high frequency tones of the pickup.

The output impedance of FET preamp cable is low enough to drive your stated 13K ohm input impedance.
I built one of these and it really works!

Seems a lot like the Alembic Stratoblaster from way back when. I installed them in guitars, their owners were delighted. Of course same circuit could be built into a floor pedal or belt pak. Simple circuitry yields clarity of tone, can't go wrong. Long battery life too, Stratoblaster ran on a single 9V for weeks even months, as long as battery disconnected when circuit out of use.

[Guitarist]

I have made that fet thing too!
Perhaps, as I have two hiZ inputs I might spit the cable then. Then I get feed each side and hit the phase button. But then I have two tracks to deal with...hmm.
Maybe I can pull the transformer from a passive DI box I have here.
I have an old guitar wired straight-wire to a balanced 1/4" out. With a "stereo" cable, I tried it last night and though it is a poor guitar, it worked and was quieter than my PAF pickups.

[Guitarist]

I fixed my EH 12ay7 preamp by replacing the tiny, melted B+ toroid with a beefier one. Looking at the schematic, can I increase the input impedance on these 2N3958 transistors? This would accommodate, with varying "drag", different sources and suit my high gain purpose.
I also have different mics including an Octava tube mic which might favor a lower value here, idk.
Thanks for your considerations.

[Helmholtz]

It shouldn't be a problem to vary the gate ground reference resistors.

But are the 2 cascodes meant as one differential amplifier or 2 separate/independent (left-right) input stages?
I don't think you can leave the grids floating.
Please show complete schematic.

[Guitarist]

That good news, thanks,
The only schematic I have is one that is altered but the color-indicated mods, I don't think help things IMHO.

[Helmholtz]

Ok, grounding the grid (additional pink line) works.

The 2 cascodes at the input are wired as differential amplifier.
Differential or balanced input impedance is 440k.

How do you connect an unbalanced source like a guitar?

[Guitarist]

It is weird that the designer tied the grids together eh?
For this guitar pickup, I simply not ground one end of the single coil and run both balanced + shield. I have a basic, single-coil equipped guitar here (given to me from my neighbor and friend Del Dettmar of Hawkwind). It was a piece of cake to wire to a TRS jack.
I am finding that with the amount of gain I often use, that noise is too much, even with a noise gate, the taping down dynamics are lost. Some of my guitars are even star grounded. I also have the suspicion that I am losing complexity of tone, being unbalanced while passive.

So, do I have this right that if 'ratioing' down the impedance via a transformer—and there is no grounded negative happening—that with says 10:1 transformer I can drop the impedance to say 1K and be fine into a 10K'ish input (which my RME has)? Thanks for the Mouser part suggestion. I could see this being a great fix for piezo pickups too to lower their gnarly-high impedance passively.