[Roger A. Modjeski]

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 your system. I have long contended and had many positive results by grounding just the power amp and floating all other equipment, including a ground isolator for the TV cable is one is present in the 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. BTW, he did have a small power transformer in both versions to supply preamp power and some boost for the driver tube. I may do that also though my present amp does not have one.

The advantages of direct connection to the power line are many.

1. No noisy power transformer
2. Better supply regulation
3. Taking 15 pounds off the amps weight making it safer and easier to ship
4. Lower cost

I have made such an amplifier both with and without a power transformer and the results very clearly show that the power transformerless (PTL) amplifier is a clear winner. 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 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 and various appliances) 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 were there one there. The house I grew up in was built in 1956 and I don't 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 of 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 and amplifier, it can be measured with tone bursts and 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 cant 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 input which will 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-200k with unbalanced preamps though sometimes attention to grounding of other components must be lifted.

If one wants to be extremely cautious he can purchase a 300-500 watt isolation transformer for under $100 that would bring added safety and would help users eliminate hum with unbalanced sources.

Personally I would use the amp direct connected and grounded. Because the amp won't power up without seeing a ground I have no fears of safety.

Please cast your vote and ask brief questions. Feel free to point out typos or things that don't make sense.

[FullRangeMan]

I like 6C33 tube, OTLs and so PTL, Iam less fan of the 6AS7 or 509/519.
I would like this amp since it:
1) It is a SE parallel, not a Futterman or PushPull.
2) It had great sound, even bass performance.
3) It use 2 or 4 6C33 in SE parallel, more than 4 tubes is excess.
4) Output power=20W already very good, to sensitivity speakers.
5) Realistic price.
Afew years ago I see somewhere on web, a artificial ground device, maybe it could help this amp.
Congratulations for work on a exciting amp as this PTL design.

[steve f]

Intriguing to say the least.  Two things concern me about OTL amps.  First, lots of expensive tubes. Second, some tube types used are incapable of really sharp transients. I absolutely agree that a whole lot of power isn't needed. These days speakers tend to be more efficient.  I checked my power usage, and often it's just around two watts.

[poseidonsvoice]

Okay, but this is assuming that other manufacturers are making tube amps that do 10 watts with 100 watt peak ratings. I don't see that at all (If Roger can design something like that, then all the power to him). I want my 100 watt peaks for those small transient times when you need that output even with 95dB speakers as I have had them hit 110 dB from 10-12 feet away. That translates into a good 120dB at 1meter from the speaker. That takes power. But not 10 watts of power. And recordings are not made at the same output level. There are many variables here including but not limited to:

Listener preference of music/music genre
Sensitivity of loudspeaker
Impedance swings of loudspeaker
Dispersion of loudspeaker
Point Source/Line Source nature of loudspeaker
Seated distance

Anand.