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Reply with quote  #1 

I recently  changed my old NHT pro A 20 nearfield monitors from passive  (70V system, passive x-over) to active x-over with my old  SE-OTLs driving them. They sound almost as good (at 3-4 feet listening distance!) as Lowther BLH regarding response  and clarity but of course they don’t have the  natural sounding  bass foundation of a BLH  and the more realistic live sound in general. But very good for a  2-way system with typical woofer/tweeter.

I was so impressed that I thought I should  do the same with my second pair of 2- way speakers which I also use as monitors. They are based on Scan Speak Illuminator 18WU/4741T00 (87,2dB/4 ohm) and D3004/664000 (91,5dB, 4ohm). They sound good with a powerful solid state amp but not even close to an active system with OTLs…despite their “high-end” passive x-over.

And here is the problem: they are both 4 ohm and the midwoofer is only 87dB. In general one can get a lot more power out of the same  OTLs with active x-over and bi-amping as there are no resistors necessary for adjusting the speakers sensitivity and impedance. But still it can be problem at peak levels even when using them only as near field monitors.

So …I was wondering if it would be possible to create an OTL amp with extra peak power on demand that would also drive any difficult load easily? That’s obviously also a general problem with OTLs and people’s speakers as I could understand from the last TS Newsletter.

Is it possible to use an autoformer like this!/%E2%80%9CNaked%E2%80%9D-ZERO-Autoformers/p/14644819/category=3461167

but only when the OTL starts clipping? Is it possible to have it smoothly fading in and out or at least switch on/off of the autoformer connection to the amps/speakers so that we have pure OTL sound at moderate to high levels and easy loads and autoformer support only at peak levels or critical loads? Would the difference between pure OTL and autoformer be audible at all if we keep the output power of the OTL in reasonable range to the speakers’ sensitivity we use?

I don’t know how fast a circuit would have to react to DC in the signal (at first sign of clipping) that could for example charge a cap that would open a transisitor that would switch to autoformer (in/out) and back to pure OTL again? Fading in /out would be preferable but most likely a problem with the mixed impedance between OTL output and autoformer input, NFB etc? There are of course better indicators than clipping for the decision  when to switch , like  level of output current etc which would also extend tube life.

Any ideas or input?

Kelvin Tyler
Reply with quote  #2 
Hi Wolfgang. Thinking a little about switching between 'pure OTL' and 'autoformer' output, my initial personal opinion is that this would be very difficult to achieve without the sudden jump in power being quite noticeable. Like yourself, what interests me is how much OTL character loss occurs when one of these auto-transformers is employed as suggested in the link you referred to. It is a pity these items are too expensive to allow, for me at least, testing this with Bruce's OTLs.
I will be very interested to see what Bruce comes up with in the way of a a new transformer coupled amp.
Reply with quote  #3 
yes, the sudden jump could become a problem depending on how different pure OTL vs transformer OTL will sound and depending on how accurate the threshold can be set. Because I think if it really kicks in only at peak levels and fast enough, the sudden increase in volume which the amps could deliver with the transformers would mask for the ears minor sound differences. One would rather be impressed by the sudden increase of power from the same amps at higher volume levels.
Testing the transformers in general is the first step and no risk. They offer a 60day trial period and shipping in the US is only $8.
Of course I wouldn’t want any interaction between the OTLs and the circuit that triggers the switching which could influence the SQ. That’s why I think in the meantime  it’s not a good idea  to use the first signs of clipping as trigger because of necessary interaction with the amp circuit.
It’s a lot better and easier to implement  to have something that works like a compressor with 3 controllable parameters: threshold, attack & release. The threshold could come from a fast reacting high resolution volume meter circuit which monitors the input parallel to the signal that actually drives the amp (no interaction). The threshold could be set with a selector switch for the steps between -3dB to 0dB for example and differently for different music styles. With the attack time I would set how fast the circuit responds to the trigger (volume meter) which would make sure that it doesn’t react to every little peak, and with setting a proper release time I would make sure that it doesn’t flip-flop back and forth and keeps “on” long enough after it has been triggered. It would be a stand- alone unit with three controls, RCA input, and speaker/switch output for each channel.
But first things first. Let's find out how these transformers sound!

Regarding transformer coupled amps: I surely would be interested to hear one of Bruce's new designs and may be this would cure my frustration with these kind of amps. SE opt amps sounded always kind of duller in the midrange  and "slower" than OTLs. The only positive exception in my experience have been Audio Research amps that could kind of satisfy me (but they are not cheap). But they also didn't have all the midrange details of an OTL and could sound kind of artificial in the treble (to my ears). As the opt is the last link to the speakers and really good sounding opts are expensive I cannot imagine how Bruce could design something  which would sound really different at low cost. But we will see. I am open for any positive surprise.

Kelvin Tyler
Reply with quote  #4 
I had noticed the 60 day trial offer on the Anticables site. The problem here, living in the UK, is that the shipping costs are higher and we have to contend with customs issues. It obviously depends on how the 60 day period is counted, but I generally avoid the complications of trial offers from the US.
Now for the amusing bit. Looking at these auto-transformers and reading the 'experiences' of others on the site, a phrase from John Broskie came back to me. A while back he said, in a general discussion about auto-transformers, "If all speakers were equipped with one, then 300B OTL amplifiers would be more popular!" So, after the deepest possible 5 minute contemplation, I decided that these were essential items to have in my armory, and I ordered a pair![smile] My Christmas present to myself as far as the wife is concerned.
I understand your approach with a 3 parameter external switching setup. My concerns remain how to actually do this switching in a transparent way, and not introduce any unwanted transients or distortions. I find it difficult to imagine what would be the effect of suddenly switching any type of transformer into and out of circuit like this. I generally go for simple solutions with tube electronics; I am not sure about this having a simple answer. Of course, I may well be wrong on this.
I take it you live in the US. Will you be taking up the 60 day trial? I will let you know how I get on in due course.
Capt. Z
Reply with quote  #5 
Hmmmmm. OTL with auto-transformers?????[confused][confused][confused][confused]

That would be no longer a OTL design.
Reply with quote  #6 


yes, I live in the US and I also ordered a pair. It will be interesting how we will experience the effect of these autoformers in our sound systems.

I was hoping that the switching could be masked and wouldn’t be audible by selecting a very loud passage for the transition with the perfect setting for attack and release time. But I am not able to make any prediction without practical experiment. Let’s find out!



Some thousand feet of copper wire between tubes and speakers is no longer OTL design. That is true. But then again it’s also a misunderstanding of the original OTL design if we connect low impedance/low sensitivity speakers to OTLs and force them to play. OTLs are simply not made for that and their wonderful sound qualities are wasted. So what’s the best compromise if we are attached to these kind of hard to drive speakers and are the proud owner of  OTLs? It seems that autoformers have at least some minor advantages compared to tube output transformers due to their different construction. But that’s all theory. In the end only the audible results count. Chances are that OTLs with autoformer sound in the end better and more like OTL again  than OTLs that cannot drive the speakers properly.

Kelvin Tyler
Reply with quote  #7 
I really enjoy listening to music through one of Bruce's excellent OTL amplifiers when connected to a quality speaker system of appropriate impedance. Quite magical. Very hard to beat for such a modest outlay. I do not like thinking too much about the system details when in this pleasurable state.
In other circumstances I am, and always have been, an inveterate experimenter in matters audio and electronic. It is clear that Wolfgang is also a keen 'what if? - let's try'  advocate. I totally concur with his comments re trying out these auto-transformers (A-Ts) to see if they are able to mitigate some of the problems modest power OTLs have in driving low impedance loads, and without undue loss of the OTL character. We are not talking about a full blown Beast amp here. This is the first logical step before considering the more elaborate switching scheme he was thinking about.
A-Ts do have some real advantages over conventional OPTs. In the proposed application, the turns ratio seldom exceeds 2:1 (impedance increase of 4x) and the lessening of leakage inductance gives greater bandwidth. This can be seen in one of the images on the Anticable web site. Some folk just consider these devices to be an impedance adjusting part of the speaker system. I am not sure if they add as much as 1000ft of extra wire, but many speaker crossover systems are not devoid of extra wire, albeit to a somewhat lesser extent.

Reply with quote  #8 
Autoformers are very nearly free of the degradation that OPT do to the signal, because the iron is not really in between the amp and the speaker.

I have experience with Autoformers between 300B SE OTL and speakers as well as with the Slagle Autoformers in front of the Masterpiece for volume attenuation.

Bottom line is that they are *almost* transparent.

When I changed to better speakers this year - where phase coherency is the prime development principle - I took them out again, because the smidgen of degradation the auoformers bring became more noticable on these more perfect speakers.

I guess as usual it all depends on the system and your tastes.

Wolfgang, as to your idea of an additional circuit to fork dynamically two different feed paths to the speakers - hard to imagine how all that detection and dividing circuitry should remain more transparent than the Autoformers themselves.

I think a better way would be to find the spots on the impedance curve of the speakers that cause the current bleeding that brings the amp into clipping and compensate those spots in the speaker network.

Another approach with additional circuitry could be found in Broskie's suggestions with regard to impedance multpliers and similar:

Could well be that after trying the Anticables autoformers the question becomes moot, as they turn out to be just transparent enough.


Reply with quote  #9 


my goal is to listen to pure OTL, no parallel, in series, or booster amplifier or anything else that could influence the original sound in any audible form while at the same time critical peaks can be handled without distortion. I don’t think we can achieve this if we limit our thinking to electronic solutions only. We always get something on one side but lose something on the other side.

MP3 is a good example how high data reduction could be achieved without sacrificing too much SQ by using an algorithm that deals with certain frequency bands where the human hearing is less sensitive and with others where it is more sensitive in a different way. In the same way it could be possible to use an autoformer only when we really need it (on demand) but to be able at the same time to  “hide” any audible influence. At peak levels or when a flea watt OTL (or a bad speaker –OTL combination!) starts to clip distortion is naturally higher and we tend to rather accept it there than at lower levels. Our hearing/discrimination power is  less sensitive while we adjust to sudden dynamic changes than if we listen at the same level at lower volumes for a longer period. Both switching and possible audible influence of the autoformer can be placed in this specific range and we would rather feel overall that it’s a big  improvement (less distortion) even if the autoformer would add a little something. But at all other times the autoformer would be completely disconnected.

The signal that drives the amp is just monitored for level changes; its integrity is not touched and not influenced by this process. The switching between autoformer and OTL without autoformer is the most critical part. Everything more than 1:2 would be audible I guess. If we place the switching in an area where the amp already starts losing power while the signal is still rising the “jump”  during a rising signal phase could be completely inaudible. That’s at least the theory.

I would need this solution with an autoformer only for my Klipsch bass/Lowther PM6A midrange BLH combination (4 ohm) and for my active near field monitors with the Scan Speak Illuminator. So I am pretty relaxed about the outcome of this experiment.

Reply with quote  #10 
Say you have 4 output tubes OTL - how about splitting them up, and connecting 2 tubes' output to the speakers direct, and 2 tubes output via the autoformer to the speakers? Should give you best of both worlds.

I noticed that there is even more refinement and definition in the Pinnacles' tone when I run them on 2 tubes only.

Otherwise I could only think of frequency dependent splitting of the signal paths, say low frequencies across the autoformer to the speakers, high frequency direct. Power splitting will always require active circuitry, won't it?
Kelvin Tyler
Reply with quote  #11 
Maybe I am misunderstanding your first suggestion, but with an asymmetric power drive like that, what effective impedance would the 'direct' pair see with a 4 ohm speaker connected?
Reply with quote  #12 
The directly connected tubes would see the speaker impedance as is, the autoformer-connected ones would see augmented impedance in accordance with the autoformer tapping.

The question is if this arrangement would cause some harmful backfiring into the feedback loop of the amp. And where then one would have to tap the feedback sensing. As the autoformers do not radically change matters, it might be ok. Thinking it through, the autoformer path should in fact calm down the feedback amplitude, as the load becomes more benign.

I could be wrong of course.
Reply with quote  #13 
Achim,I would guess that if you split the output tubes in the Pinnacles without further adjusting the circuit that this would also affect the output impedance of the amp, power and damping factor. Kelvin must have the detailed answers. It would be basically an active speaker system with active x-over but using only one amp. Would save a lot of money if it works.

The refinement of the tone which you experience might come from somewhere first guess would be less interference between tubes if the tubes are not perfectly matched in the actual amps! Parallel tubes have a tendency to hog current, the stronger ones draw more and show higher Ip than the weaker ones. That leads to an imbalance and is most audible during warm up if you don;t use mute circuits or shut the tubes down with higher neg. bias during warm up.

Did you ever measure your mains power? Is it pretty stable(1-2V fluctuation) or very unstable depending on day/night time and also in general like mine (1-5V). That could also influence the amps especially as they don't use regulated B+. The Ip would react to higher mains power fluctuations and that would also influence the sound if you use tubes that are not perfectly matched or if some are more worn out than others. I use a P3 power plant for all my amps that solves this problem.  No need to interfere with the B+ power supply (besides, I don't like how regulated B+ sounds..) If you hear the difference you will be impressed.

"Frequency depending splitting" is basically using an active x-over ...but I would use 2 amps as I already do with my SE-OTLs and NHT monitors. And,yes, that 's exactly what I have in mind with my Scan Speaks...bass (87dB)with autoformer, tweeter (91dB)without and perfectly aligning the phase  and matching the drivers with the Behringer Ultradrive.
Kelvin Tyler
Reply with quote  #14 
The output impedance of the Pinnacle is about 2 ohms. An A-T with an impedance ratio of 2:1 would reduce this, from the point of view of the speaker, to 1 ohm. Splitting the output would, I calculate, give 1.3 ohms. This presumes the same degree of NFB in each case. So using an A-T either on its own or in a split arrangement improves the damping factor. The power output would be roughly doubled with the A-T ( actually a bit less), and the split configuration would be roughly equivalent to 3 tube pairs, or 1.5 times or a bit less.
Reply with quote  #15 
Achim, the 4 tubes are better matched. In the SE version it is easy by measuring cathode voltages (input of current sinks) and swapping the tubes for minimal difference. For me only swapping between 2 amps brougth worthwhile improvement. Maybe it is not possible in the push-pull version.

Btw. the slight DC offset of a push-pull amp might prevent (?) connecting an autoformer. The primary has low resistance, maybe 1 Ohm only. I own the P. Speltz' autoformers, but had no itch trying them with the 300B amp yet. It sounds good as is and I don't like fiddling with cables. Better to solder something... But I might try them although then I loose biwiring option of my cables (superior to single wire)
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