[Duke]

Let me reword too You are conflating "maximum power output vs load impedance but independent of input voltage" with  "power output vs load impedance given a constant input voltage (and while in a linear operating region)".

Now I follow you, and you're right.

To put it another way, "constant current" and "current limited" are not the same thing. You can have a current-limited voltage source.

Agreed. 

[JohnR]

 

[Roger A. Modjeski]

Just to clarify: A good OTL (Futterman and mine) will have low output impedance (high damping), Atmasphere amps do not.  A constant current amplifier has high output impedance and poor damping. All amplifiers have voltage and current limits though we think more of clipping as caused by voltage, it can as well be caused by current.  OTL amps tend to have lots of voltage, however current is limited by the number and type of tubes. Current in OTLs is more difficult to obtain than voltage. Hence, OTLs like higher impedance loads. If the world was 32-64 ohms OTLs would rule.  We can, however, use a wide-band auto-transformer to convert our low impedance speakers to anything we want. These transformers are far more ideal than those we can make in a push-pull traditional amplifier.

[bdp24]

Roger---If you're asking for wants and/or preferences for the OTL's you're working on, mine would be mono-block form with balanced inputs. I like to have my amps right by the speakers, with balanced lines to them from the balanced-output pre-amp.

[steve f]

Any updates?

[Roger A. Modjeski]

Any updates?

I brought my small 2 output tube per channel amp to Berkeley and its doing a great job on the QUAD 63's. Hope to make some soon or a kit. They have amplifier building classes here at the Randall Museum.

[Clio09]

Hi Roger,

Jack Elliano told me about the time he and a number of other designers spoke at one of the Randall Museum classes quite a few tears back. It pretty much put his 300B DRD and Ultrapath preamp on the map. Jack mentioned quite a few of those DRDs were built by DIYers as a result of that class and are still in operation to this day.

Would love to see the OTL as a kit or made part of a future amp class project.

[Roger A. Modjeski]

Hi Roger,

Jack Elliano told me about the time he and a number of other designers spoke at one of the Randall Museum classes quite a few tears back. It pretty much put his 300B DRD and Ultrapath preamp on the map. Jack mentioned quite a few of those DRDs were built by DIYers as a result of that class and are still in operation to this day.

Would love to see the OTL as a kit or made part of a future amp class project.

I will be taking over the Randall amplifier class in January. Not sure what we will start with.

The four output tube stereo OTL is finished and we are making a pilot run on RM-200 chassis. If anyone wants one please write [email protected] for details. It is capable of 3.5 watts into 8 ohms direct. With the included inductive impedance converter it can deliver over 10 watts into any load from 100 to 2 ohms. It is direct coupled with a servo to control bias and zero the DC on the output.

[Roger A. Modjeski]

There are are three points I want to make concerning how this low power amp is different from other low power amps. As speakers are becoming more efficient and people are realizing that playing a 100-1000 watt amplifier at 1 watt is not doing them any good the interest in low power is growing. We have 3 pre-sold at this point and will deliver them in early July.  We are starting with the deluxe version $6,500 built on an RM-200 chassis. It looks just like an RM-200 with plate caps on the tubes (picture soon).

I am making these points so that we don't lump this OTL in the same camp with other low power amps. To my knowledge this is the first of its kind to be a low powered OTL with built in impedance conversion for low impedance speakers. The impedance converter also allows exceptional speaker protection.

1. Something that impressed the speaker maker I visited was that the amp was very "clean at high levels" or lets just say low distortion compared to his single ended 300B. That is because single ended amplifiers have distortion that rises with output. When you play them softly at say 1/10 of their rated power they sound great. As you get louder and especially is there is a lot of bass information they get muddy. This is simply because their distortion is typically 0.5% at low power but several percent at high power. John Atkinson usually uses 1% distortion as his cutoff for power and thus some amplifiers get rated (by him) at much lower power than the manufacturer's spec. JA kindly raises his distortion cutoff to 3% for SE amps and sometimes agrees with the manufacturers rating. Most SE amp makers rate distortion at 1 watt and dont tell you what it is at full power.  Generally the distortion doubles every 6 dB above one watt and rises rapidly at clipping.

My OTL has distortion less than 1% right up to clipping, JA will like that.  Typical distortion at 1 watt is 0.4% direct which reduces to 0.1% via the first tap on the impedance converter. As the impedance converter (IC) converts the 8 ohm load to 32 ohms we would expect this 4/1 reduction in distortion. Along with this also comes a 2/1 reduction in current supplied by the output tubes and an 2/1 reduction in plate dissipation which will extend the life of the tubes considerably. The IC also allows 4 ohm speakers even more advantage. By going down another tap the 4 ohm speaker is also converted to 32 ohms. There is one further tap to go even lower. As you know typical OTL's have a real problem with 4 ohm speakers and anything less might as well be a short. The problem is not just output power and distortion but high dissipation shortening tube life. I am doing everything I can to make an OTL that will enjoy the same long tube life that Music Reference amps are know for.

2. One other thing that comes up in tube amps with taps is the "preferred tap". It is somewhat true that the tap from which the feedback is taken is the best tap on the amplifier and this is true to a varying extent, mostly dependent on the quality of the output transformer. Most output transformers are 3 dB down around 40-60 KHz and the phase varies on the different taps making the feedback rather tap dependent. The IC is down 3 dB at 200 KHz on all taps. Because of this, and some other things, I can now move the feedback with the tap. On this amp there is a feedback link right by the speaker terminals terminated with a spade lug that can be moved to the selected tap. The feedback link can be put to any tap to affect both damping (if you want more or less) or gain (if you want more or less). I find this very useful and I hope I am not going to confuse the user with too many choices. One thing that confuses me is the listener who is reluctant to experiment. Although he may spend days playing with speaker cables that make little difference, when I give him something that makes a real difference, that doesn't cost any $$ to try, that is easy to try, he calls me up and says. "Hey, which one should I use?" The answer is "which ever one sounds the best!" Then they usually ask "which one do you use?" What can I say at that point? I use the one that sounds best in my system to my ears which are different from your system and your ears.

3. The third point is the effect of damping on the speaker. Most good tube amps, including mine, do not have damping in excess of 10. The ones I have measured that are greater than 10 (I have not found one more than 20) are unstable into certain loads. SE amps without feedback have typical damping factors of 5 or less. The PrimaLuna Dialogue has damping as low as 0.5 in some modes and never more than 2 in any mode. I find this unforgivably poor design work. This amplifier is a thorn in the side of every speaker manufacturer.

I will say this with certainty, If you want to hear your speaker the way it was voiced by the designer then you always want high damping because if he had any sense at all he designed your speaker with a high damping amplifier.  Stereophile readers can simply look at JA's measurements and see the frequency response shift with his simulated speaker load. I agree his load is typical of most box speakers and quite representative of what is going to happen in your listening room. Unless you pull out a generator and voltmeter you won't know exactly how much you have modified the response of your speaker but you sure will hear it. With damping around 5 there is typically a +/- 2 dB shift in response. When a builder brought his OTL and hooked it up to my QUAD 63's I immediately heard more bass than ever and not good bass. When you hear a big change like that something is wrong. I measured a peak of 8 dB at 50 Hz. That is not what Peter Walker intended.  There will always be a peak at the woofer resonance where many speakers rise to 50 ohms and some dips at the crossover where a speaker may fall to half its rated impedance. These are large audible differences. The peaks are always more noticeable than the dips and there will be several. In a later post I will give some links to speaker and amplifier tests so we can see these effects.

I sincerely hope readers are getting something from this discussion. There are many points to be covered and questions to be answered.  I encourage your comments.

[atmasphere]

Just to clarify: A good OTL (Futterman and mine) will have low output impedance (high damping), Atmasphere amps do not.  A constant current amplifier has high output impedance and poor damping. All amplifiers have voltage and current limits though we think more of clipping as caused by voltage, it can as well be caused by current.  OTL amps tend to have lots of voltage, however current is limited by the number and type of tubes. Current in OTLs is more difficult to obtain than voltage. Hence, OTLs like higher impedance loads. If the world was 32-64 ohms OTLs would rule.  We can, however, use a wide-band auto-transformer to convert our low impedance speakers to anything we want. These transformers are far more ideal than those we can make in a push-pull traditional amplifier.

Further clarification:

The Voltage Paradigm (use of a voltage source) and Power Paradigm (use of a power source) to drive a loudspeaker are common means. The current method (use of a current source amplifier) never developed much of a following over the years and decades.

To create a voltage source out of nearly any amplifier usually you apply negative voltage feedback. To create a current source instead of voltage feedback you use current feedback. Current source amplifiers have very high impedances that are multiples of the load they are driving, even if they are solid state.

To create a power source, the amplifier has to have zero feedback or can also be done by having voltage and current feedback in equal (canceling) amounts- there were a number of amplifiers with damping controls made in the 1950s that employed this latter technique. The resulting output impedance will be at or near the impedance of the speaker down to some fraction of the load impedance. There is a very long and healthy tradition of loudspeakers that are designed with power source amplifiers in mind; most of the horns made in the 1950s expect such response and the world's first acoustic suspension loudspeaker, the Acoustic Research AR-1 was designed for such amplifiers.

The Power Paradigm fell out of favor in the 1960s as getting a good match between the amp and speaker did not always yield predictable frequency response. The Voltage Paradigm was meant to solve this but it was not 100% successful- equipment matching issues remain as all audiophiles know.  During the 1990s the Power Paradigm began to gain favor among many amplifier designers- the SET phenomena is part of that.

[Carl V]

in the latest Stereophile magazine July mentions favorably
a Japanese Miyajima 2010 OTL amp

[Roger A. Modjeski]

in the latest Stereophile magazine July mentions favorably
a Japanese Miyajima 2010 OTL amp

I hope John Atkinson will get to measure it. So far all we know is Art Dudley liked it on his rather dated Altec 16 ohm speakers. Unlike modern speakers these have high sensitivity and I imagine fairly flat impedance curve typical of those days.

The circuit is unknown but it does use the 6080 which his the industrial version of the 6AS7. It has lots 16 pots to adjust, such fun.

[bdp24]

Just to clarify: A good OTL (Futterman and mine) will have low output impedance (high damping), Atmasphere amps do not.  A constant current amplifier has high output impedance and poor damping. All amplifiers have voltage and current limits though we think more of clipping as caused by voltage, it can as well be caused by current.  OTL amps tend to have lots of voltage, however current is limited by the number and type of tubes. Current in OTLs is more difficult to obtain than voltage. Hence, OTLs like higher impedance loads. If the world was 32-64 ohms OTLs would rule.  We can, however, use a wide-band auto-transformer to convert our low impedance speakers to anything we want. These transformers are far more ideal than those we can make in a push-pull traditional amplifier.

Roger, are you making and selling an Auto-Transformer?

[Roger A. Modjeski]

Roger, are you making and selling an Auto-Transformer?

Yes I am making the ones for the OTL and can make others. Autoformers for audio are wonderful things. The ones I make are better than the torroids that others make in both the high end and low end. Is there something you would like?

[bdp24]

For one thing, if I wanted one or two of them I would like to buy them from you, Roger. I have nothing against Mr. Speltz or his products (in fact, I think quite highly of both, and am planning on getting a pair of his speaker cables), that's just the way I feel (does this ass-kissing get me a discount? ). Plus, MR being within driving distance (by the way, Santa Barbara is amongst the handful of best places in the world to live, Los Angeles amongst the worst. Not counting third world countries and/or ones the U.S.A. has invaded in the last, let's say, sixty-five years. Remember "My Country, right or wrong"? Ah, memories.), I might be able to hear one with a speaker we both posess (original Quad). For another, it seems like a good idea in general with the old Quad, no matter the amp. True? As it happens, in my power amp stable there reside a pair of A-S M60's. At an almost purely resistive 12 ohms, would the Eminent Tech LFT-8b Magnetic Planar panel (without it's woofer) benefit from one when used with an RM200, rather than from one of the taps on the amp? Seems like it would be the opposite. By the way, anyone interested in, or already an owner of, the Magneplanar line of speakers, the ET is a double-ended/push-pull (opposing magnets on either side of the Mylar) design, with far less distortion than Maggies. And with a far higher nominal impedance (though about the same sensitivity), a better candidate for tube amp use. As the kids these days put it, just sayin'. Speaking of Music Reference and Eminent Technology, one of Brooks Berdan's longest-term customers and a reviewer at Positive Feedback, Dave Glackin (who passed away last year, and was younger than me. As was Brooks. Yegads! Brooks was a drummer in High School, but his life-long disease prevented him continuing to play in adulthood. Sheila Berdan asked me to put a band together for Brooks' 50th birthday party, and the happiest I ever saw him was while he was playing "Wipeout" on my drumset ) had a reference system based around an RM9 and a pair of LFT-8b's (the pairing in his system for over ten years, surely a record for a review.....uh, make that even an audiophile. Except for owners of Quad and/or McIntosh, if you consider THEM audiophiles. Harrumph.). I admit to having been tempted to call his Widow, to ask if she had plans for the amp. Hey, I didn't, okay?

[bdp24]

Yes I am making the ones for the OTL and can make others. Autoformers for audio are wonderful things. The ones I make are better than the torroids that others make in both the high end and low end. Is there something you would like?

Here is a separate question: Regarding your new line of OTL amps, will there be one that you consider an optimal match (perhaps with the aid of your Autoformer) for the original Quad Loudspeaker? Enough power to enable the speaker to give it's maximum output without the amp going above 1% in distortion, but not enough more to damage the panels. This assumes the Quad's use from around 180Hz and up. Would the same model be appropriate for stacked Quads, also crossed-over around 180Hz? Is the Autoformer also usable with a stacked pair of "57's"? I know the Direct-Drive ESL Amp is preferrable, but I'm a little shy of the $ for that amp, damnit.

[Roger A. Modjeski]

There is this whole mythopogy that the OTL and electrostatic speaker are a match made in heaven. That original match was the Futterman OTL and the KLH model 9 ESL and that is the only pairing that makes any sense. All the other ESL dip so low in inpedance that any OTL has trouble with the current needed. I have in my files a copy of the impedance of the KLH 9 at many frequencies taken by Julius Futterman many years ago. He was certainly interested in having his amplifier drive that speaker. The KLH 9 had a minimum impedance of 16 ohms. The KLH 9 also had very well insulated panels and, I believe, was pretty difficult to damage by excess voltage. The QUADs, especially the 57 have no such insulation and above 1500 volts the midrange/tweeter panel will arc. There are three ways to prevent this. Have an amplifier that puts out no more than 35 volts peak (the RM-10) or get the clamp boards installed or get my direct drive amp.

For stacked 57's you need a lot of current if you are going to run them in parallel. The OTL with the clamp would be good for that if you put them in series.