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Old 21st July 2003, 01:57 AM   #1
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Default Is anyone here familiar with Butler Audio amplifiers?

Hi,

Apparently B K Butler is a pretty well respected name in the car audio world, and has recently moved to home audio. They make a hybrid amp that they claim is different from the way everyone else makes hybrid amps, so a friend asked me to take a look at the patent.

http://patft.uspto.gov/netacgi/nph-P...0&RS=6,507,240

I believe this is public domain information, so it should be OK for me to post it here. If not, I'd request a moderator to please remove it.

I finally managed to get to the portion that is in plain english, and then got to the schematics (you'll need QuickTime to view the images). It does seem unusual, the tube is connected in a fairly strange way. The grid and plate are connected to each other (directly or through a resistor), the cathode is connected to the base of the power/output transistor, and the input signal is applied to the grid/plate. So, the varying signal voltage changes the current through the tube, which changes the current through the transistor. What I thought was unusual was (a) with grid and plate connected, the bias voltage is the same as the plate voltage (or pretty close to it), right? The patent says that the tubes are operating near saturation. Also, it's the current through the tube that controls the transistor, instead of the output voltage of the tube, which seemed interesting. And then the whole thing is repeated for the other half of the PP pair, this time with the grid being connected to the cathode.

Anyway, I would appreciate it if someone could take a look and let me know what they think. I couldn't find a way to post the schematic, or I would have.

Oh, and I think we can ignore the claims of how it's a brave new idea to fit a blue LED into the base of the tube, because then the tube will start glowing as soon as the amp is powered on, and the user won't have to wait for the filaments to warm up for him to know that the tubes have power to them. Yes, it does actually say that, pretty much in those words.

Thanks in advance,

Saurav
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Old 21st July 2003, 10:38 AM   #2
EC8010 is offline EC8010  United Kingdom
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Default Tosh

This would be a joke if it wasn't published as a Patent!

Once you've cut away the silly claims, the spurious references to add integrity (Hamm) and Edison nonsense, there's little left. Bipolar junction transistors are conventionally voltage driven, but their exponential relationship between applied Vbe (base-emitter voltage) and Ib (base current) means that they distort. If you were to add a device in series that had an inverse law, you might be able to make the base current more linear.

If we apply enough Vak (anode to cathode voltage) to a thermionic diode, we strip the electrons away from the cathode so fast that a space-charge is unable to form around the cathode. Unsurprisingly, this condition is known as space-charge limited. It takes progressively more voltage to strip the last electrons away from the cathode, so the diode curve takes the shape of what I believe Americans call a "lazy S".

If we add the space-charge limited part of a thermionic diode curve to a semiconductor diode curve, we can cancel the two to improve linearity. In practice, the effect may well reduce the amplitude of low order harmonics, but it is most unlikely to reduce higher order harmonics. Cancellation will not remain stable as the valve wears out and cathode emission falls.

In summary, it's primarily a marketing exercise. Oh, and Studer used the LED in the valve base ploy years ago - which should have been revealed when the patent was examined.
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Old 21st July 2003, 02:52 PM   #3
John B is offline John B  United States
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Interesting scientific explanation there EC. In layman's terms does it mean this amp is badly designed? I'm gathering from what you said that you think the high end frequencies will come out distorted? And over time this amp will degrade in it's audio quality? Is that the gist of it?
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Old 21st July 2003, 04:00 PM   #4
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So in essence they're using two entirely unrelated transfer functions (that happen to be similar) to cancel each other out?

I remember reading a post on an online forum where he said that he'd come up with a new way of retaining a tube amp's sound, i.e. the tube still interacted with the output load in the way it would with an all-tube amp, but "through a transistor", which allowed higher power output and all the other transistor advantages. Or something to that effect.

The amps use 6SL7GTs, I think. So it's a triode converted to a diode and operated in its space charge limited region. Would this still sound like the triode originally did (we're back to "the sound of a tube", but let's ignore that for now). I'm not sure how a diode "sounds", since there's no voltage amplification taking place.

Thanks for replying.

Saurav
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Old 21st July 2003, 04:23 PM   #5
SY is offline SY  United States
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Weird.
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Old 21st July 2003, 04:26 PM   #6
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Default Butler audio sounds great...

Jeeves, turn the LP over, and bring me a cool drink.
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Old 21st July 2003, 04:47 PM   #7
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"Tube Illumination Internal BLUE LEDs (Trademark and Patent Pending)"

I heard one of the original Tube Driver amps.
Of the 5 people listening, all of them preferred an Adcom car amp (no longer being made), followed by a McIntosh for the money.
The more musical was of course a Milbert tube amp, but only 30W if I remember.

Perhaps they have been improved, but IMO a quality design will not have to resort to marketing "gimmicks".
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Old 21st July 2003, 04:55 PM   #8
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The first time I read the website, I thought they were saying that shining the blue LED on the tube somehow helped with the thermionic emission. Thankfully, the patent makes no such claim.
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Old 21st July 2003, 05:17 PM   #9
EC8010 is offline EC8010  United Kingdom
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Default Badly designed?

It all depends on what you mean. Ultimately, the amplifier is designed to be sold and enhance the manufacturer's profits. In that respect, it's very well designed. Adding the LED in the valve base implies that the valve is going to be on display. A double diode such as an EB91 would do the job perfectly well, (only a little current is required), but from a marketing point of view, a 6SL7GT is far better. It looks far more impressive and is known to be an audio valve with a good reputation. (Although it would need the LED to shine into the top, rather than the base.) Seeing the valve, uninformed observers will come out with all sorts of stuff about the history and usage of valves, and the product will become great by association.

From an engineering point of view, the valve won't last all that long before its characteristics change and it no longer cancels distortion effectively, so it will need to be replaced. The manufacturer will be perfectly correct in stating that the consequent poor sound is due to valve failure, and that valves do not last for ever. Perhaps the manufacturer will point to the fact that Hi-Fi valve amplifiers require periodic valve replacement too.

In layman's terms it's a marketing exercise. Although the valve can cancel some of the distortion generated by a transistor, it will only cancel the lower harmonics, which are arguably innocuous, and can easily be reduced by negative feedback. Further, the design operates the valve at its very limit, which will certainly reduce its life substantially.

Distortion cancellation is a bit like a see-saw, you can balance quite large weights, but a small change disturbs the balance. For distortion cancellation to work, you either need both transfer functions to be unchanging (transistors) or changing at the same rate (push-pull valves). Here, we have cancellation between an unchanging transfer function and a changing one. And that makes it a bad engineering design.
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Old 21st July 2003, 05:28 PM   #10
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Quote:
It looks far more impressive and is known to be an audio valve with a good reputation.
That was my question - the way they're using this tube, does that reputation even matter? Aren't the tube's behavioral characteristics totally changed by hooking it up this way? That doesn't matter in the eyes of the buying public, I understand that. But from an engineering point of view, the 6SL6GT isn't a 6SL7GT any longer, in terms of its audio performance reputation, is it?
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