Hammond 307A Transformer in audio splitter application

[Jayz]

I have a 1970's Hammond 307A I'd like to use to split a guitar signal to two amps simultaneously. The only info I can find is that it is a 1:1 txfr. Does anyone have an old spec sheet showing the color coding of the leads?

Thanks!

[Enzo]

A 1:1 means the two windings are the same. Use your ohm meter to determine which wires are paired. Pick one to be primary and the other becomes secondary. Phase shouldn't much matter, but if you detect a phase problem, reverse one pair of wires.

[g1]

Not sure if this is of any help to anyone.


Appears that primary can be 125 or 500 ohms (4 connections) and secondary is center tapped (3 connections).
I imagine the phasing issue would depend on where the two signals are going. To 2 amps with some separation it probably wouldn't matter, to a stereo setup it may. As Enzo said, if there is a phase problem just reverse one side wiring.

[Enzo]

If that catalog sheet is accurate, there are no wire colors, just terminal numbers.

[Jayz]

Thanks guys! The catalog sheet must apply to a different era of the 307A. The 307A I have has green-black-yellow on one side and blue-red-brown-red/white wires on the other. Not much help I guess but I appreciate your advice.

[J M Fahey]

Besides the wiring or colors, I'm more worried about *how* do you intend to use it.
You will connect the Guitar to one winding and take signal from separate windings for different amps?
Please confirm.
Thanks.

EDIT: with that cleared, I can suggest you how to measure it and sort wiring, (whatever I'd do if I had it on my own bench) but not otherwise.

[Jayz]

Thanks Juan! I intend to take the guitar output signal straight to one amp and also through the 307A (to isolate one amp from another) to the other amp. I used the 307A in a DirectBox circuit years ago that provided a balanced output signal. I since lost the wiring diagram and memory has failed me.

[Steve Conner]

To be any use in a passive guitar splitter, the transformer would need an extremely high impedance, more like 100k than 500 ohms. It is actually a pretty tall order for a transformer designer to make a 100k transformer with a decent bandwidth. I think there is one expensive Jensen model that does it.

If you add an op-amp buffer, you can get by with much cheaper transformers. RG Keen published a nice circuit for this.

[J M Fahey]

Agree, that's what worried me.
You can't hook it straiht from a passive guitar.

Anyway, although ground problems are n'rt nothing to be sneered at, they aren't either such a big problem for your application.

I bet plugging both amps and the pedal PSU to the same power strip (so to same ground) will avoid most of your grounding problems, at least those caused by amp to amp interaction.

[Jayz]

Thanks! As you may have gathered, I only know enough to be dangerous. I should have said that in the directbox circuit, the input to the 307A was from a speaker level signal. I guess that means it won't be any good with an instrument level signal.

[J M Fahey]

I should have said

You bet
It's *VERY* common here (and in other Forums) that threads go on and on, many pages, dozens of answers, many cracking their brains open, and then the OP says "oh, I forgot, such and such thing is what really happens" ... which turns everything upside down.
Oh well.

There's a *slight* difference between 50mV 200K impedance guitar signal and a 25000mV 8 ohms one found at a guitar amp's output.

[Jayz]

My apologies - sorry to lead you down a rabbit hole!
I'll try rigging it up again as a direct box capable of a speaker level input based on a Jensen schematic. Any suggestions as to how to determine what wires go where?

[Steve Conner]

First step is to figure out which wires belong to the primary winding and which belong to the secondary. Usually all the primary wires come out one side of the bobbin, and all the secondary wires come out of the other side. But you can also check with an ohmmeter: there should be no continuity between the two sides.

Next, measure the resistance between the various wires of each winding. The two wires with the highest resistance measurement between them are the ends of the winding, and the others are just taps that you probably won't want to use in this application.

From the Hammond data sheet it looks like you have two primaries, and you'll probably want to connect them in series to get the 1:1 overall ratio. Use a continuity check to figure out which of the 4 wires belong to the two primaries, and then use trial and error to figure out the phasing. If you get it wrong, you'll get no signal at all.

[Jayz]

Thanks Steve. Is the Hammond data sheet you are referring to the one in this thread with terminals or have you seen a data sheet showing one with wire leads?

[Steve Conner]

I'm referring to the same datasheet posted in this thread. If I had the datasheet you were looking for, I would just have posted it instead of going to the trouble of typing out an explanation on how to identify the wires.