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In the Forum: Audio Discussions
In the Thread: Small SET’s bass, besides everything- is it about power?
Post Subject: Small SET’s bass, besides everything- is it about power?Posted by Romy the Cat on: 1/12/2009

It is difficult to generalize but I will. As much as ribbons with each hertz down sound more and more unpleasant the low power SET amps with each hertz down in bass region more and more and more loosing ground. Ok, I am not taking about the real bass – SETs do not have it. I am talking about something that I call midbass, let say 50Hz – SETs do it but I would say they do it with a LOT of challenge and mostly, let call the things as they are, they do it badly – the acoustic pressure is there but spare confidence to maintain the pressure is not there.

OK, let to think a bit deeper. The typical conflict in SET amps between sufficient inductance (LF) and minimization of capacitance (HF) is always there and all SET amps fight with it to a different degree of success. So, the bass is always a challenge with SET. There is nothing new in this fact. It is also not a new that some SETs do have very serious midbass and some of them even very serious bass. A contradiction? Not really. Look at the output power of the “good bass SET amps”. You will not see among them low-powered 1-8W amps. All low powered systems I heard had very poor bass, with bass absolutely not able to deliver “size” and “space”, it was always what I call “suffocated bass”. Is it a pattern in it?

Well, the first pattern is that most of the people that have seen use the low power 1-8W SET with so called full range speakers that in my view is not a declaration of idiocy but a factor that disqualifies a person form rights to express any audio judgment. When I see a person who runs his low-powered SET into a pair of back loaded Lowthers-like crap and who tall me stories that we juts paid $1500 for a set of some kind of kinky 1.5W tubes from 1927 because those tube have “wonderful” bass” I always feel sorry the his mama did not make abortion. It juts sounds a complete idiocy to me. The 1.5W shell go with over 105dB sensitivity, very small room and most like in DSET configuration, otherwise… otherwise we hit the second pattern.

The second pattern. From here is the section why I write this post.

Among what western person would understand I never had those low-powered amps. I arrived in US in beginning of 90s and in the mid of 90s I was running BAT electronics with VK-60 being the less powerful. In the end of the 90s I switched to Lamm electronics and from 2000 I was running Lamm ML2.0 sets. In addition to 20W that ML2 had I used it in multi-amp configuration, playing with loading. In 2004-2005 I switched to my Melquiades that also has a powerful output triode. Since then I stay with multi-amping and DSET that immune from bass limitation of SET amps. So, how come that I did not use any low power SET and I feel that I am in position to express a generalizing opinion?

Well, I had a strange feeling lately. I was running my test DHT amp (San Audio 2A3 kit with Milq’s battery-biased driver). The amp was driving the Tannoy REDs in custom enclosure. I was playing that magnificent Andante from Mahler 6 (where Mahler enters his Bruckner mode)  and suddenly I caught myself on the feeling that I inadvertently have the mid-bass very similar to all those Lowther’s freaks, where the drama of bass instruments sound like a belch from a sewer pipe.  What surprised me was not the low quality of midbass but the similarity of my midbass with all mid-basses I ever heard in the systems that run low-power SET. I ask myself: “If the sound we get is the same then it is because we all have bad amps of there is something else is in common there?”

I power up the 15W Melquiades and played the same fragment with a full-range injection channel. The “sewer pipe” effect gone completely and the stinky Tannoys were pressuring the room with poise and required forcefulness.  Switched to 2A3 – the “sewer pipe” came back. OK, RED are 16R, the 6C33 in injection channel loaded with 1200R and it does ~13W. The 2A3 is load with 7K. With this load I take from 2A3 perhaps 1-2W. During the normal operation the amps output a few hundred millivolts and very little current. Is possible that the low currant the low power amp pumps is the key why all out low-power SETs have the same midbass character?

Let look into it. The bass drivers have own suspension and being bass drivers the have a relatively heavy cone mass and relatively  low compliance of suspension. The suspension is mechanical devise does not give a damn about signal and it blindly reacts upon excursion of the driver. However, since the bass suspension of mass-centric divers is “hard” then it minimum current necessary to make the suspension to react is relatively high as well. A few dozen of milliamps from low-power SET might not be accustomed but the bass suspension but they might be squashed by the suspension reactance, the reactance that I would note has no familiarity with character of the signal and the driver returns to default position by suspension not by the amp current. This would be true with exception when amp is driven by negative output impedance where the force of suspension is contra-compensated with output impedance of amp. However, we never see not only negative but even low positive output impedance in low power SETs. Those with any feedback have a few ohms and then the “minimum current of suspension” becomes a huge factor.

So, what might be the “second pattern”? I think it as simple as power – the low-power SETs juts do not have power to successfully battle the “minimum current” requirements of bass drivers. Those few people who went DSET can deal with it to a degree by playing with custom loading of dedicated SET but a person who runs low-power SET full-range is really at the mercy of … God know what, complimented with “minimum current” and the “sewer pipe”.

There are a lot of talks out there about the “magic of SET” and the “flea-power amps” but I never seen that those people besides running their mouth with sexy adjectives (not own but those that were invented for them to use) were able to demonstrate a serious sonic result. I did not have any low-power DHT but now what I do and when I drive 93dB sensitive speaker with it I have the very same ugly midbass that I experienced with countless setups of other people. I think it is a pattern. Also, regardless how good or bad the Sun Audio’s Tamura output transformer but I think that 150Hz-50Hz is still the region where they operate, I did not measured where Tamura begin to die, but I will.

Anyhow, I do not insist that I am right, it is juts my today’s view. I do not care about the validity of this view as I feel anyhow that any SET, not to mention the low-power SET is just a tool for DSET application. However, those of you who run low-power SET full-rage it might be an opportunity to think again. If you SET does not clip, if your driver does not run into grid current and if you have enough volume of sound then it is not an indication that you SET is enough to maintain the midbass none-asphyxiated tone.

The Cat

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