1:1 isolation transformer?

[888guitars]

I bought a couple of 15k 1:1 audio isolation transformers for guitar. I was wondering if someone had some ideas on how to fix the signal loss? High end loss. Buffer the input?

[mhuss]

What are you driving it with? 15k is a pretty hard load for guitar pickups. Unless it's already in an amp or something, yes, a buffer in front will help.

--mark

[888guitars]

It was either 15k or 600ohms. I will try the buffer. I wanted to have this as a stage box/tool for guitar players.

[R.G.]

Transformers do not have impedances. Transformers have RATIOs.

What makes a transformer 15K:15K instead of 600:600 is the design of the core and windings so the primary inductance is much greater than the specified impedance at the lowest frequency specified. So if your transformers are specified for a frequency range of 100Hz to 15kHz, the primary inductances are much greater than 15K and 600 ohms respectively at 100Hz. If the spec is 20Hz to 20kHz, the primary inductance is much more than 15K or 600 ohms respectively at 20Hz.

But the load the signal source on the primary sees is the RATIO times the secondary load, in parallel with the primary inductance. The transformer ratio is usually specified for the impedances that make the transformer look best on the data sheet. So the 15K:15K ... and... the 600:600 are likely to give their best responses when loaded with a 15K and 600 ohm resistor, respectively. Other loads will reflect in the same ratio to the input, and the frequency response will change some.

Leaving the secondary essentially open by loading it with, say, 1M, reflects 1M to the primary. But that 1M is in parallel with the primary inductance, which has not changed.

For guitar, you want loading over about 1M at 82Hz. Of the two transformers, the 15K:15K is clearly the "better"; but it's still something like 150K - 500K loading for guitar. This does go up as frequency increases, so it's not terrible, but it's quirky and may have odd resonances.

So yes - if you're seeing signal loss, especially treble loss, buffer it. I did a design of an isolator using a cheap ($2.50) 10K:10K transformer with a 300Hz-3kHz spec and got 60Hz-22kHz response after buffering. More to the point, it had the 1M+ impedance of the buffer presented to the guitar. Works well.

It is quite difficult to design a good isolation transformer for a guitar or bass signal without using a buffer for the guitar. Probably not impossible, but very difficult. Much beyond buying transformers from ebay.

[888guitars]

Thanks RG, I hope to get this built up and check out the response. Do you prefer some fets over others for buffer?

[R.G.]

Yes, I do. I prefer no FETs - use an opamp unless you just can't, for power or other reasons. The drive is better.

A FET might possibly be found that would do a decent job with a 15K reflected load, but it's a much trickier design. And the steps to correct this all lead in the direction of opamps.

Not just any opamp, of course. The LM833 is a good starting point.

If you want some soft limiting, do it in front of the opamp with a single N-channel JFET. Then let the opamp do the driving.

[Leo_Gnardo]

Couple of weeks ago got to repair an ABY box "Bigshot". Used an inexpensive 15K:15K transformer to isolate the second line and even flip its polarity. No buffer. Didn't notice any loss of response but then I was only listening for a short time thru "ordinary" guitar amps.

For cheapness & simplicity, just the transformer will do. If you notice a degradation of fidelity or need some volume boost, add the buffer of your choice. I've done it with TL071 series op amps (TL072, TL074) in powered circuits, and TL061 in battery-operated.

FWIW the problem with the ABY was the transformer got knocked off the circuit board, probably by being dropped. If you have one, open it up and put a dab of glue where it'll do the most good. Stick that transformer to the circus board lest it become unstuck by accident.

[888guitars]

This is what I am building. Can I trade the tl072 for the LM833? Looks to have same pin out. I have made a few of these on stripboard and didn't know about the 833. Looks like people do like the 833 better? I did a few searches but did not see a schem/layout for a buffer using a 833.

[Tom Phillips]

This is what I am building...

Did you intend to include an attachment? There is no "This."

[888guitars]

Uh ya. .

Can I sub lm833? Or know of a layout for a LM833?

[Leo_Gnardo]

Can I sub lm833?

Why not build your buffer with an 8 pin DIP socket, then you can swap IC's to your heart's content.

Not familiar with the 833 so can't comment one way or t'other.

Swap til ya drop, & if you hear anything that sounds terrific, report it here!

[888guitars]

I usually do build with sockets. Especially when using fets. ( static charge ) but, I'm not sure the surrounding components are suitable to a opamp like the 833 ?

[R.G.]

It's fine. The LM833 works in the same socket, and has a lot of drive. You might run out of voltage. That's why I ran the transformer driver over at geofex on a battery plus a charge pump inverter to get more available headroom. the final 560 ohm is a bit too big. I'd cut that back to 100, maybe 47 or 10 ohms. No sense wasting the power supply voltage headroom and limiting the drive current you worked to get.

[888guitars]

Great! I will chuck one in the socket and see what gives!