I have just purchased the latest Sound Lab M1

Hi Winston, There are several good amp recommendations here but if you can get a demo with the new Berning Quad Zs, I have no doubt you would be left with a big smile. When the MBL 9011 amps left me cold, I talked/begged David Berning into working his magic with amps that would power the most difficult speakers made, read M1s, Revel Salons and MBLs. The units offer large current delivery, COOL running tubes, slam and finesse. No other high output amp offers this combination that I know of.

Hi Allan, there is a product called the ZERO (see zeroimpedance.com). It allows our smaller amplifiers to drive loads down to 2 ohms. Its an outboard output transformer, but one with a very low turns ratio.

Without it, our smaller amps can't drive really low impedances like 2-3 ohms. You made my point for me: a transformer is in use in the Berning amplifier, which allows it to drive low impedances, i.e. its not an OTL; failing the definition of an OTL in that it has an output transformer (walks like a transformer, quacks like a transformer, its a transformer). So call it it a ZOTL (although I've got no idea what the acronym stands for, Harvey never told me), or make up a more accurate acronym, if it happens to be that you even *need* an acronym.

I don't object to the technology, never have and I've always admired its ingenuity and said so any time I've mentioned it in the past (you can't get better advertising can you?). This is about something else: for example I am sure I would get castigated if I started calling our amplifier a transistor amp when it has no transistors.

Ya know, one of the biggest marketing issues any OTL manufacturer has to overcome is the legacy of the Futterman amplifier. That legacy is that the amp is unreliable, finicky and can't handle difficult loads. I really feel that we changed that over the years, but what puzzles me is that David must *know* this as if he chooses to call his amp an OTL, people will challenge him on these issues as they have Atma-Sphere. What I don't get is why David would take that on when the amp isn't an OTL in the first place.

About 9 or 10 years ago, in a letter to Positive Feedback, David mentioned that he found it difficult to explain how his amp worked to the public, so he told the public it was an OTL, because it was easier. The admission that the amp was not really an OTL was published that long ago.

Hi Winston,

I don't have any amp recommendation for you (I'm in the high efficiency/low power camp), but I thought it was an interesting coincidence that we both have an Epifania, Vector arm and Basis table (I have a Debut Vacuum and a Lyra Titan cartridge). My Epifania has Teflon caps and is equiped with remote control of volume, source selection, balance and mute.

I like the Soundlab M-1s. Looks like you have impeccable taste in gear.

Some would conclude that my description above in many ways describes the characteristics of an audio output transformer, except for the dc-coupling attributes. In fact, the ZOTL can accurately be described as an ideal output transformer emulation circuit. When I claim that it is OTL, I am saying that the ZOTL lacks the physical item known in the audio industry as an audio output transformer. The physical object known as an audio output transformer is made using an iron-based core that is wound with perhaps a kilometer of magnet wire to obtain the required inductance to couple the full range of audio frequencies from a primary winding (tube side) to a secondary winding to which the speaker is normally connected.

The audio output transformer is a good way to couple tubes to speakers, and makes a lot of practical sense. It is the most economical way to make a tube amp, although I think the ZOTL could compete if made on a high-volume scale. For this reason, almost all tube amps ever made use such a transformer.

But let’s face it, the audio output transformer, with its hysteresic iron core and perhaps a kilometer of wire is plagued by parasitics that adversely affect sound reproduction. The first major OTL pioneer, Futterman, and later persons like Ralph of Atmasphere realized the sonic limitations of output transformers and went to great lengths to find a way to use tubes but eliminate the output transformer. (Apparently Futterman developed his earliest amplifiers with the goal of reducing cost, not achieving better sound). It should be noted that not everyone likes the sound of the OTL amp and some prefer the output transformer coupled sound.

For many years I made transformer-coupled amps, and I am very familiar with the sonic issues of these amplifiers. Almost any amplifier will sound good on reasonably well-recorded music of the pop and jazz variety. But I worked very hard to make some of my difficult recordings sound right. These were still well recorded, but the music itself was difficult to reproduce convincingly. One of my test records was Shostakovich String Quartets on L’oiseau-Lyre DSL011. I struggled with this record for years. I could tweek one resistor this way and make one instrument sound right, but another instrument would then sound wrong. The first ZOTL I built was soon put to the test with this record, and suddenly all of the instruments were reproduced correctly. My output transformer based amplifiers were suddenly obsolete in my mind.

The point that I am making with all this is that the audio output transformer is a sonic filter that while some prefers having it, there is a reduction of reproduced transparency particularly at the frequency extremes. Both traditional OTL designers like Ralph and designers like myself who come up with a new way to beat the colorations of the output transformer have the common goal of integrity of the sound reproduction. In my view, an amplifier that eliminates the audio output transformer, both physically and in sprit, falls into a class that is simply known as the OTL amplifier. In other words, either the amplifier has an audio output transformer characterized as described above, or it doesn’t. The sonic attributes of the ZOTL and the traditional OTL amplifiers are far more similar than different, although differences can be expected based on speaker matching.

Yo Ralph.
Dave did say something like that but what was meant was that the least this amp is, is an OTL, the least it is! So that people can get a little grasp, because it's not output transformer coupled, so then what is it???

Having a transformer that does not operate at audio frequency or anywhere near it, is not a audio output transformer, as Dave has pointed out above it would compare to your power transformer, then if that's the case neither of our amps are OTL's !

After all, this technology is to over come all of the OTL's troubles, as noted in your posting, with the Futterman as well as with many other OTL's but with keeping all of the positive sonic attributes of an OTL.

You have to remember Futterman came out with the OTL in 1954 and his primary reason was for cost savings, he eliminated the audio output transformer because they were costly and he had access to tubes so he could load up the thing with a zillion hot tubes, now Dave's invention does away with both the audio output transformer and the Zillion smoking hot power hungry tubes, just because this is so radical, does not make it an output transformer coupled amp, or anything else other than an invention of tube amp technology with responsibility suitable for modern times.