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Old 4th December 2007, 01:17 AM   #1
jfrago is offline jfrago  United States
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Default sa-3.9e & sa-6e monos

Does anyone know if the sa-3.9e is a stereo version of the sa-6e mono's? And how different do they sound?
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Old 4th December 2007, 09:42 PM   #2
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More or less. If you don't need the power, the 3.9 is
one of my favorites from the Threshold era.

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Old 4th December 2007, 09:49 PM   #3
jfrago is offline jfrago  United States
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Nelson,

Is that a picture of Zappa you have posted? I use to see Zappa and the Mothers at the Filmore East in they late 60's. I miss the way they jammed back them. I am sure in the bay area you had a great deal of bands that were into jamming.

What I was wondering if the SA-6e were mono versions of the SA-3.9 but it seems like you like the sa-3.9e the best which is why I was wondering if they might be your second favorite?

jfr
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Old 4th December 2007, 09:53 PM   #4
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Quote:
Originally posted by jfrago
I am sure in the bay area you had a great deal of bands that were into jamming.

What I was wondering if the SA-6e were mono versions of the SA-3.9 but it seems like you like the sa-3.9e the best which is why I was wondering if they might be your second favorite?
All I had to do was stick out my thumb.

The 3.9 enjoyed a higher bias figure in proportion to its output,
and I only needed a few watts anyway. That's why my ears
still work.

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Old 8th December 2007, 04:38 PM   #5
jfrago is offline jfrago  United States
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Nelson,

What do you mean the bias was higher on the sa-3.9e?

I thought class a was class "a"?

Can transistor amps behave like tube amps; class "a" triode pp or class "a" pentode pp or class "a" s.e.t.by altering the bias?

If transistors amps biasis can be changed, say I take the bias on the sa-3.9e and then take a sa-4e 100 watt class "A" but have the bias set to the bias setting of the 3.9e. Would that be too much heat for the amp or should I adjust the bias on a set of sa/6e monos to the sa/3.9e level just to spread the heat around?

Here's something you might enjoy - I saw a preview screening of the "the Dewy Cox story; Walk Hard". Van Dyke Parks and Marshall Crenshaw did the soundtrack. It's funny as hell and the music is great!

I think you might like it. It comes out Christmas. I had nothing to do with the picture - just wanted to pass it along.

jfr
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Old 8th December 2007, 11:55 PM   #6
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Hmmm...how to answer...class A, strictly interpreted, means that the output devices do not turn off at any point during the signal. Now, to achieve class A, you have to know a couple of things. One of them is what load you want the class A to work into. (Yes, I know that was an awkward sentence...deal with it. I'm not wearing my author hat at the moment, I'm wearing my "drinking good wine" hat. You have been warned. Merry Christmas, by the way...) By that I mean that for the same voltage swing--which is determined by the rail voltage--you have to apply more bias in order to allow the amp to continue to deliver full power into lower and lower impedances. This is simple Ohm's Law stuff:
IR=E...thus,
E/R=I
and so it is obvious that (always wanted to say that),
for a decrease in R (to be interpreted broadly as impedance, meaning that we're including inductance and capacitance), you need to increase I (the current) in order to meet a given E (the voltage). Amongst other things, you can see that amp spec sheets that show, say, 100W into 8 Ohms, but deliver only 150 or perhaps 180W into 4 Ohms when you would legitimately expect 200W, aren't biased heavily enough to double the way they should.
Okay, so what the hell does this have to do with your question?
One, you have to ask the question, "Class A into what load?" Two, and of more interest to DIY folks, is can the power supply and heat sinks handle the extra bias? And more particularly...once you've achieved class A into a given load, is there any benefit to biasing the amp harder still?
Well, oddly enough, there is.
There are any number of technical parameters that you can argue about, but the thing is that devices in general and MOSFETs in particular perform better if you push the dickens out of them. To give you a concrete example, I've got a pair of Magneplanar Tympani IVs which represent a lowish, but otherwise pretty tame load. Okay, they're heavily modified, but that's a distraction at the moment. We can come back to that if necessary. The crux of the matter is that I was playing with the bias on a pair of home-grown Aleph 2s (thanks, Nelson) and ran the bias up and down and all over town, just for fun. To boil it down, the results were that the amps were warmer and more open sounding with higher bias. The stock circuit is far from lean, but adding more bias still makes images rounder and more distinct. The sound was more life-like, less hi-fi. Now, the amps aren't 'hi-fi' sounding in their stock state, but raising the bias made them more liquid, still.
The downside? Increased demands on power supply and heat sinks, both expensive...unless you go water cooled, but that's another distraction.
It's your choice, but if you can handle it, try increasing the bias. In a word...Yum!

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Old 9th December 2007, 08:47 PM   #7
jfrago is offline jfrago  United States
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Thanks for your answer.

Now things make more sense and Merry Christmas back at you.

Just have to figure my speaker load, throughout the signal spectrum, and then define how the amp behaves in feeding class "a" accross that load to see how much to push the output devices to maintain the class "A" bias.

You're right the more bias, less high-fi, yummmy!

jfr
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