Odd but True Output Transformer Wiring

A few popular output transformers out there have one (nearly) overwhelming quirk. They cannot be easily wired for multiple output impedance selection (such as 4, 8 and 16 ohm). At least, that's the complain I hear often from do-it-yourself builders and hobbyists. I have actually come across this issue with the original (and re-issue or clone) Marshall JTM45 outputs and with most of the Hammond brand transformers. Well it is time to relax. DST's VP of engineering has done the work for you. Yes you can use common switches and get the flexibility of multiple output impedance selection. Check out the schematics below.

The Hammond transformer is shown first. This is such a common, inexpensive and easily available transformer, it's likely that many first time builders will pick one for their project. It is simpler than you think at first. I have built and tested this circuit and can say it work's very well. The switch is a three position rotary type that I purchased from the Mouser electronics catalog. However, there are also good ones available from many distributors from manufacturers such as Greyhill, Oslo, Lorlin, etc. Just make sure you pick a switch suitable for the currents your amp will put out.

Next up is a quick and easy wiring scheme for the infamous JTM45. This one is simple and to the point and allows 8 ohm and 16 ohm selection (the two most common values used). It uses a standard issue double pole double throw (DPDT) toggle switch.

Have a question or want more details? Contact Bob Dettorre, VP Engineering, DST Engineering
bob@dst-engineering.com

DST Engineering Dual Bias adjustment - Copyright (c) 2002