[Roger A. Modjeski]

Audio Circle is where I try out new ideas to see if I am on the right track with  the readers here. You might say it is where I test the best way to disclose my research that leads to new products. It allows me to tailor the product to the needs of the user.  As we get deeper into this discussion of OTL amps I wonder if I should approach it differently.

So far my purpose has been to educate the reader on the pros and cons of different OTL amps and how OTL amps in general differ from the more common amps we see made with tubes and transistors. The number of companies who have made OTL amps can be counted on two hands with a few fingers to spare. Many of them have played and lost. Atmasphere is indeed the longest standing. Because OTL amps can be made several different ways I set out to explain them. Of course this brings up a lot of controversy and many of the things that have been said about OTL amps are simply untrue. Some are so technical that even the most knowledgeable reader may not be able to sort them out. Only someone who has designed successful OTL amps knows their rather strange inner workings.

The alternative to the discussion I have launched is simply to write a white paper. For those who have been following this discussion as it unfolds here is the white paper for your comparison. Please vote as to which approach works better for you and which do you think would work better for the people who are not part of this circle.

Please vote, comment, suggest changes and proofread the White Paper below.

MUSIC REFERENCE OTL-1 White Paper

A good white paper should start with a story, If you don't care about the story skip down to “How it works”

Forty years ago I got my hands on a pair of Harvard Electronics OTL amplifier. They were not identical, which I found strange. However the difference was only in the output tubes employed.  I had plenty of experience with tube amplifiers at that time but I had never seen an OTL. The owner had a single page typewritten instruction on how to adjust the pots on the amplifier. It also went into great detail of what to do if adjusting the pots did not produce the specified results. One of the instructions I found a bit curious was about swapping tubes from one side to the other if things would not bias up properly. This takes a lot of time because you have to wait for the tubes to re-stabilize every time and one has to be careful not to burn oneself on the very hot tubes. Adjusting this amplifier was not a simple matter. Without considerable practice I doubt many owners could adjust them, hence they came to me. There was nothing wrong with the amplifiers and nothing had to be replaced. I observed they were well built.  Upon their return the customer was very pleased that he found someone who could sort out the rather odd instructions.

Thirty years ago I designed the RM-6 amplifier which I licensed to Counterpoint Electronics. I often get compliments that it is the best amp this person has ever heard. It's sound seems to linger in listeners ears long after hearing it. To my knowledge is was the first OTL to be direct coupled and employ a servo to automatically balance the DC output to zero. Other direct coupled OTLs require manual adjustment periodically.

Two years ago I started thinking about designing another OTL. Actually I never stop thinking about OTLs and I have made several breadboards in the interim, but none of them satisfied me. I did experiment with 6AS7 and 6C33 triodes in both totem-pole and bridge configurations. I experimented with pentodes and triode connected pentodes in SEPP. I made some 5,000 Volt SEPP OTL amps for Acoustat electrostats (those are very successful and we are currently making them). However SEPP circuits are not useful at speaker levels and impedance.

HOW IT WORKS:

The Music Reference OTL-1 is a low powered, ultra short path amplifier with minimal parts, no output cap, DC coupled internally from input to output, self balancing via a Servo, single bias adjustment, single output pair amplifier. Via its direct output it can source 1 amp of current. It can also supply 100 volts into a high impedance load. If one slows the servo, the amplifier is capable of DC response.

The amplifier uses NOS tubes designed and made at the height of the tube era by the best factories of the 1970's when tube production was at its highest level due to the popularity of Color TV. It is the Japanese invention of the transistorized TV that closed tube factories all over the world. The factories that stayed open the longest were in the countries where tube TVs were kept in service the longest (China, Russia and in Eastern Europe). I tell you this little known bit of tube history so you can appreciate that we are using the best NOS tubes made that are far superior to the current sweep tubes as the EL509 and PL509. Those tubes were great while needed for Color TV construction and replacement as they had to be rugged and at Sylvania the goal was to have them last 10 years.

To obtain low distortion, low noise and good damping a circuit was developed by Roger A. Modjeski that would  have the shortest and fastest path possible. It is desirable in an amplifier to have DC coupling to produce the best bass and wide open-loop bandwidth so that feedback can be applied and stability into a variety of difficult loads is guaranteed. The resulting amplifier has a bandwidth from below one Hz to above half a megahertz.

This amplifier is unique in the field of past and present OTL amplifiers. Distortion is lower than most conventional amplifiers at 0.5% or less. The damping is higher at 20. The frequency and power response are far wider as stated above. The tubes deliver their power directly to the speaker with no intervening coupling cap or relay.

Unique to this amplifier is the inclusion of an Impedance Converter (IC) that allows the amplifier to output 10 watts into a variety of speakers from 1 ohm to 32 ohms. The impedance converter can be bypassed, however the amplifier power is then reduced to 3.5 watts for 8 ohm loads and doubles as the impedance doubles up to about 32 ohms. The IC along with a fuse for each output tube provides excellent protection from harmful DC present on the speaker. The amplifier will not oscillate into any load. The circuit description has more details of how this was accomplished.

Thanks to the advent of more sensitive speakers it is possible to achieve enough power with only one pair of output tubes. This reduces the power consumption of the amplifier to 140 watts, a fraction of what most OTL amps draw. Tube replacement is easy and matched tubes are not required as the servo adjusts for unmatched grid bias However for best performance matched tubes are recommended and are available from us at a fraction of the cost of modern output tubes such as the KT-88, KT-120, EL-34 etc.

CIRCUIT DESCRIPTION

The driver consists of a single tube for each channel. The pentode section of this tube provides the entire voltage gain of the amplifier which is about 500x or 56 dB. The triode section is connected as a bootstrapped split load phase inverter which (contrary to some misinformed individuals) provides equal drive to the triode connected output tubes. These tubes provide the current gain and can source one ampere from the positive or negative supply.

Feedback is a controversial subject among audio enthusiasts. It is considered more bad than good and many amplifiers tout the fact that they have no feedback. In recent issues of Stereophile Magazine  there have been several reviews of amplifier which have feedback switches so that the listener can determine for  himself the effects of feedback. In the examples I have read the feedback is 2 dB which is virtually nothing. In each case the reviewer reported that this small about of feedback did not please them and they preferred the sound without feedback. I cannot imagine properly applied feedback of 2 dB would worsen the sound of the amplifier as reported unless it was very poorly applied. It would take careful listening to hear it at all being vanishingly small.

The application of feedback is a tricky thing. Amplifier circuits do not have a place marked “put the feedback  here”. One has to create a place to inject the feedback. I imagine the amplifiers previously mentioned might have created this injection point with the switch thus modifying the circuit as a whole when the feedback is switched in. The designer of the $75,000 amplifier admits putting the feedback switch on the amp to show people how bad feedback is. Well, one can certainly install a switch to make an amplifier sound bad.

In the OTL-1 the input tube literally rides on a the output terminal. So as the input rises the output follows it perfectly in phase. Rather than being an injection point for feedback this point exists in the fundamental circuit. This allows the amplifier to react immediately. In conventional amplifiers the feedback comes through the output transformer (with considerable phase shift that is load dependent). The feedback then needs an injection point which is often the cathode of the input tube or grid of a differential amplifier. Internal delays in the loop (phase shifts at low or high frequencies) can cause the negative feedback to become positive and make the amp oscillate at low or high frequencies or both. Many amps are unstable without a load and many have low frequency instability that causes the woofer cone to wander about its rest position.

The power supply consists of a 300 VA toroid bridge rectified and connected to two 2700 uF filter capacitors for exceptional energy storage. There are separate supplies for the driver, bias and servo.

The servo is a simple integrator that keeps the DC at the output below 20 mV at all times It does this very slowly and much slower than the lowest bass note from any instrument. Bias is set on the negative going tube and, due to DC coupling the positive tube adjusts itself to equal idle current.

A high inductance, low loss tapped choke provides protection from DC should a tube fail. Fuses prevent damage of other components if that should occur. Music Reference amplifiers are know for their excellent application of fuses. With only a pair of output tubes per channel tube failures are reduced and locating a bad tube is simplified. The choke is in parallel with the output of the amplifier to ground. It has three taps which allow for more power and lower distortion that would otherwise be available without it. It does not harm the sound of the amplifier in any way. For those who want to confirm that claim there is a direct output also.

A great deal of consideration to the user has been employed in this design. We hope that this amplifier will begin a new chapter in the history of OTL amplifiers.

[Kcin]

Grammatically:

"Music Reference amplifiers are know for their excellent application of fuses."  change "know" to known... otherwise nice read .

I personally do not like the idea of a "impedance converter". It is counter intuitive to what I would want to accomplish with a OTL. I understand the marketing and appeal to most people with a more universal product in a very narrow sector. Speaker systems in the past 30yrs do not necessarily appeal to OTL.

I've run OTLs and electrostatics for 30yrs... I have that original Futterman amp you speak of in one of your other posts. Yes, it is a bear to set up but with the right speaker it is really exceptional for 50yr old design.

[Ericus Rex]

In the last paragraph of your back story, the second sentence should read "Actually I never stopPED thinking about OTLs..."

I like your white paper.  I do wish it stated which tubes you're employing (did I miss that?).  The audio public, in general, thinks OTLs are incredibly unreliable.  I would put more language in the paper about what made past designs unreliable and what you've done to alleviate these issues.  You do mention your tricks for reliability in the paper, but OTLs have such a pervasive reputation for unreliability that you need, IMO, more direct language to counter this.  I think this should be the only mention of your competition, and names need not be named.  Just talk about the flaws of the designs.

Hope this helps!

[paul79]

"How It Sounds"

Also, maybe highlight its strength's vs. other amps topologies as well. I know it is in there, but a summary of sonic performance would be good I think....

[FullRangeMan]

Too long, maybe a short version would be useful.
An inevitable question would be: this circuit is similar to the ZOTL Berning?
Cheers

[Roger A. Modjeski]

Too long, maybe a short version would be useful.
An inevitable question would be: this circuit is similar to the ZOTL Berning?
Cheers

Thanks to all for your suggestions. I will do some editing soon and let you know when the new version is up.

At to the Berning ZOTL, It is so unlike what we think of as an OTL that I wish he had called it something else, at least he did put a Z in front.  What we think of as an OTL does the job of sending current from the power supply to the speaker via the output tubes. Berning's amp turns the DC into high frequency current and send that to the speaker via what appears to be a HF transformer and filtering. In my understanding it is more like a switching amp than what we think of as an OTL. I think David is a very smart guy and comes up with unusual designs, how they perform I do not know.

Here is the patent. Look at the diagrams. There sure are a lot of transformers in there doing a lot of driving MOSFETS. There is a tube in there but it is surrounded by an army of solid state devices and transformers driving them at radio frequencies (RF). http://davidberning.com/technology/patent5612646

I extracted the following from the paper. http://davidberning.com/technology/zh  This is the essence of what he is doing and this is not going on in any traditional OTL amps. A very complex way to make an amplifier that would not interest me. I don't know who would be able to fix one of these other than the factory. I will continue to make things that are serviceable  by anyone with basic electrical knowledge. I am willing to work on most anything but I would pass on this.

I found this picture on Audiogon where someone was selling a ZOTL. The four tubes are to the right. The RF transformers are the thing in the center with many wires coming out. There are six small tubes top right.



The Berning ZH amplifier performs the impedance matching function of the output transformer by applying special switching power supply technology. This technology (US patent 5,612,646) uses radio-frequency to change the tube transfer characteristics from their normal impedance plane to one suitable for driving a speaker. Because the audio impedance conversion is done with an RF carrier, it is not subject to the distortions imposed by the audio output transformer as shown in fig. 3.

[FullRangeMan]

Thanks Roger for your expert and detailed explanation on this amp.
This ZOTL had nothing to do with an audio amp, Iam surprised it work as a radio transmiter, I was complete unaware it as everyone else.

[Roger A. Modjeski]

Thanks Roger for your expert and detailed explanation on this amp.
This ZOTL had nothing to do with an audio amp, Iam surprised it work as a radio transmiter, I was complete unaware it as everyone else.

It puts out audio so let's call it an audio amplifier and a very unusual one at that. Its not an actual radio transmitter but does work at those frequencies.

[BobRex]

What's the input sensitivity?  Is it low enough to work with passive volume controls?

I like what I'm reading so far, is the 10 watts sufficient to drive your ESLs?

[Roger A. Modjeski]

In low power applications it is best to state the gain of the amplifier which is 12-24 dB. It is varied by selection of the feedback tap. I prefer the lower gain as it provides more damping. However even at the high gain the damping is at least 4. Much depends on your speaker and listening level.

I am driving my ESLs with it to a level of 80-85 dB average. I see about 20 volts peak on the scope from 100 Hz up. I have a sub for the bottom end.