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RF at Ona wrote:

Romy, I might have overlooked something on your site but I understand that your ULF amp is driven directly by the preamp line stage. If that is the case, you may want to consider an old technique for integrating subwoofers and drive your ULF amp with the attenuated output of the midbass amp. Perhaps this is an idea you considered and discarded but sometimes it helps to state the obvious. Frequently with a deep bass speaker we want to bypass the low frequency limitations of the main amp and drive a separate bass amp directly from the pre-amp. But in your case, the midbass SET amp is optimized for the low bass so using it to drive the ULF solid-state amp might help restore some tonal system integration. This technique should also help to improve the phase alignment of the ULF and midbass speakers since the ULF would see the phase variation of the midbass low-pass crossover and any phase variation from the SET amp/transformer. You may need to adjust the ULF/midbass crossover afterwards and pay attention to any phase inversion of the amps.

Hm, interesting. I did not even think about the option to drive ULF from out of the midbass amp, partially because my midbass channel is high-passed by Milq’s inner-stage capacitor at 20Hz. I tend to unload unused bass from the midbass horn in order to prevent the unnecessary excursion of the driver that do not lead to bass able to go out of mouth. I know, it is not what people do – people load as much bass to midbass horns trying to get “more”. A horn is a delicate devise and if not blow into a horn more than it might handle then it handles what it can handle with more grace.

I am not sure that the minor problems I observe are in the realm of “restore some tonal system integration”. You see, the ULF does not output anything that resembles tone. The third order at 25Hz is not “sounds” but rather some LF bumps on the road. So, I might be mistaken but I do not think that I deal with tone but rather with shaping the sharpness of the bumps.

If I use midbass with no highpass then considering that I have monster LF out transformer in there I might be a good idea to have some kind powerful current SS buffer and to drive ULF from there. It is not how my playback organized however. In my playback the midbass channel drives midbass horn with line level, inner-state level and speaker level filter. The ULF power amp source signals from preamp via a line level filter of third order. Since the current at line level is relatively low and the shunting capacitance in my RCRCRC 25Hz low-pass filter before ULF amps is relatively high the ULF filter is “slow” itself. This slowness of ULF file compensates the advantage of the super fast rise time that my ULF power amp has over my tube Melquiades. My ULF power ams is Yamaha B2, that is full DC amp with no capacitance in signal and high current. Those amps have very fast rise time and if I use it directly then I would probably shunt the am with come capacitance to keep it in rise time in the same par as my Milq has. The harmonic difference between my SS ULF amp and Milq is totally different subject however but I hope that at sub 25Hz the harmonic differences might be not so prominent… I hope….

Actually the idea that I feel very enthusiastic is to use the same type of 1:1 output transformer that I use for my midbass but only the SS amps and my ULF section, I mean literally to bring the high current SS output signal to the transformer code. Do not forget that at sub 25Hz all coupling is coming via core. Furthermore, since the amp is DC amp (the preamp is also DC preamp) I might put some gap into the 1:1 transformer to deal with DC. I am not sure that the problems I hear are not DC related. I have drove my bass with DC amplification. Before Milq I drive my bass with a dedicated Lamm ML2 what was a SET with output transformer…  So, I am very interested to try the transformer approach but I am just too cheap to commission people to have those transformers to made just for sake of experiment. If I have borrow such a transformer and make experiments without paying around $1K then it would be more sensible solution. Does anybody have a 350H-400H, 250W-300W, 1.25:1 transformer with 100-150mH gap that I can borrow for a week or two? Preferable for me do not pay $1000 for the shipping of that thing…

RF at Ona wrote:

I hope people reading about your progress appreciate that a highly refined integration of your ULF and midbass sections is not a straightforward exercise. The midbass uses a transformer coupled tube amp, the ULF a direct coupled solid-state amp. The midbass is horn loaded, the ULF a direct radiator. They are positioned on opposite sides of the listener and their traveling waves moving in opposite directions. The pass bands are narrow allowing the primary high-pass phase/amplitude responses of the loudspeakers to mix with the responses of the crossover filters in unintended ways. Even if the ULF output is not supposed to overlap the midbass output I suspect it does so in practice - and of course the ULF distortion spectrum and resonances surely factor in the midbass if they are audible at all. And as I mentioned in an earlier post, there may be unwanted crosstalk effects between left and right channels because of close placement of the midbass horns. Be happy the results have been so pleasing and the only need is fine-tuning. Even from afar I have found this a provocative technical challenge.

Yes, what you describe is sound like what it is it but in reality it is not, well, not all of it. The “behind” positioning of midbass horn, the biggest problem that I was afraid is turned out to be not a problem at all. The key with operating of this horn was to find a VERY precise balance between the out of midbass and upperbass. I have locked it with less than ¼ db precession (ask me how and you will be laughing) and believe me or not it stop to be a subject of my concern. Even without running ULF the midbass does not call to itself, it’s location and it’s output are very fine in my estimation and I have no needs to do anytime with it. The incorporation of ULF of different topology and driver by different amp is of cause a task but frankly as I dumped my ULF channels where they are now then I got right the way a VERY serious integrated bass. BTW, it is not a reference of any kind for me but since then I had a few of audio people in my room and no one reported any dissatisfaction with bass integration. So, with all complexity of my playback it is happens that it is not so complex to manage the things. still, in contrary to the people who visited me I do feel that there is “something” in my ULF that is not the way how I would like it to be. it is very minor and it is more like harmonic or dynamic differences but I do not think that it is harmonic or dynamic. I still am trying to figure out what it is….

RF at Ona wrote:

P.S.:  If you are going to reposition your ULF towers you might consider facing them away from the listener seat in an effort to reduce by redirection any higher frequency distortion or resonances that may have some (small) beaming to the front. The frequencies you want the towers to reproduce are very omnidirectional so there may be just a slight benefit to turning the towers around fully or even somewhat from the listener.

I did have some VERY interesting and very surprising results last night while I was moving my ULF but I had very limited time to do it and to think about it. as I will do it again I will report the results.

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