A Power Transformer Less Amplifier
Julius Futterman, in a 1954 publication in Audio Engineering Society Journal, presented a 10-watt OTL amp without a power transformer. Keep in mind that this was the age of two prong power cords. Grounded cords and their outlets came many years later and are a benefit in safety but often a devil in getting all hum out of a system. I have long contended and had positive results by grounding just the power amp and floating all other equipment in my system.
Futterman briefly mentioned that his amp (which he was encouraging others to build from his published schematic) must be plugged in with the neutral and hot properly connected. He even mentioned how one could know which way was correct by using a neon pilot lamp brought near the chassis to find if it was hot or safe.
Two years later he published the same amp and added a power transformer but made it clear that it must have very low resistance windings to preserve the amplifier's ability to produce high currents in the load. He also noted that a great many people had built his non-isolated amp with very satisfactory results though he had received some criticism for not isolating it. By the way, Futterman did have a small power transformer in both versions to supply preamp power and some boost for the driver tube.
The advantages of direct connection to the power line are many:
No noisy power transformer
Better supply regulation
Taking 15 pounds off the amps weight making it safer and easier to ship
Lower cost
I have made such an amplifier both with and without a power transformer and the results very clearly show that the power transformer less (PTL) amplifier is clearly superior. Each amp is mono and weighs about 15 pounds vs. 40 pounds for an RM-200.
With grounded power it is very easy to build in a neon lamp to show that the outlet the amp is connected to is of the right polarity and properly grounded. If there are hum problems (very unlikely because the gain of the amplifier is a rather low figure thus requiring a preamp) other components can have their grounds lifted and let the power amp carry the fault current should there be one from these other components.
It would also be possible to build in a GFI. My concern here is product liability. However, we deal with appliances every day (microwave, toaster ovens, etc.) that could just as well have a hot chassis. I believe that GFI outlets are already in most building codes for bathrooms and kitchens. I do recall my parents having a deep fryer that would give me a shock if I had one hand on the metal counter strip and tapped the cooker with a spoon. A GFI would likely trip if one were there. The house I grew up in was built in 1956 and I do not recall it having three prong outlets and I am still here. In fact, only my distortion analyzer and one power supply are currently grounded on my bench and those grounds are easily defeated on the front panel.
The 30-watt RMS mono-block amplifier that currently sits on my bench has six output tubes with ceramic plate caps and two driver tubes. The dynamic power is about 100 watts. All OTL amps have low RMS power ratings because the IHF preconditioning requirement (which makes little sense for listening to music) would burn up the tubes. I know why the test was added, however completely disagree with it as we are not listening to sine waves at full output. Music Power or Dynamic Power is a very good way to rate an amplifier and it can be measured with tone bursts which is a better measurement as music peaks are typically ten times average levels. If they are any higher clipping is almost certainly assured. In other words, you really cannot play a 100-watt amplifier much over 10 watts average. With modern speakers that gives us 100 dB average and 110 dB peak levels. So many of us are using so little average power that we just need a good 10-watt amp with 100 watts peak. Unfortunately, the IHF test precludes this.
This amplifier has balanced inputs that allow those who have a balanced preamp to keep the preamp and other balanced components grounded. Keep in mind that any component in your system can go hot if the power transformer windings short to the frame. It works equally well, as does my RM-200, with unbalanced preamps though sometimes attention to grounding of other components must be lifted.
[Source: circa 2011]