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Old 6th February 2002, 05:49 PM   #1
SteveG is offline SteveG  United States
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Default paralleling mosfets in BLS

If I were to build a balanced line stage using two mosfets paralleled, two 80mA current sources, and 4 750 ohm load resistors per channel, would it lower the distortion? If so, about how much of an improvement would this make? I noticed that by paralleling two zen stages together the distortion is reduced, and in the pearl phono there are 4 paralleled jfets on the input for lower distortion. Would it be of benefit to quadruple the fets and current in a BLS?
Steve
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Old 6th February 2002, 07:02 PM   #2
grataku is offline grataku  United States
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Interesting question. What difference does it make if you double the devices but double the current too?
I posed a similar question before in another thread and nobody answered. Maybe we'll get lucky this time around.
There is a plot in NP article, the graph is THD vs. Vout. The plot shows three curves for increasing PS voltages. As the PS voltage increases the output distortion decreases and the minimum of such distortion curves moves to higher values of output voltage. The question is how low can you push the distortion while increasing the PS voltage? Basically, how much low distortion gain can one squeeze out of this circuit?
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Old 6th February 2002, 07:45 PM   #3
SteveG is offline SteveG  United States
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From my understanding of it, the distortion goes down as the power supply voltage rises due to increased bias current through the fets. There would obviously be an upper limit to this, as the dissipation would exceed what the FET could handle, and you couldn't use a bigger FET because the increase in capacitance leads to more distortion. Maybe I'm going about this the wrong way; maybe I need to use lower power devices (maybe even JFETS) and run them in quads. I don't know that paralleling the FETS used in this circuit would help much...??? Maybe somebody could help us out here. I'm kinda puzzled by the whole thing, and I'm not sure what question to even ask, you know? How about, is the input capacitance of this circuit directly related to the distortion of the circuit?
Steve
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Old 6th February 2002, 08:04 PM   #4
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All other things being equal, parallel MOSFETs is the
same as a bigger MOSFET except that it is already
nicely matched. The original Aleph P use IRF240's
as the gain devices, and they worked great. You
can really bias them up, say as high as 1 amp
with 25 volts across them.
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Old 6th February 2002, 08:08 PM   #5
SteveG is offline SteveG  United States
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OK. I've been reading a little, and I think I know what the deal is now. Paralleling dosen't decrease distortion, just noise. Correct?
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Old 6th February 2002, 10:31 PM   #6
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I've build the BLS almost the way you plan (just two mosfets, two active current sources). In the prototyping phase, I've tested the design with different bias currents, and found that IMHO the more you increase it, the better the sound. I stopped at 80 mA/Mosfet (IRF610) mainly for the global dissipation issue. I'm more than happy with the lush sound it provides, but I still wonder how it would have sound with an heavier bias . I think you should give it a try before paralleling.

I still haven't found a clear explanation of the "bias increase"/"sound improvement" relation - apart from distortion decrease. Any hints ?

Regarding the number of //ed JFets in phono stages, I think it first addresses the noise issue (decorrelated noise is divided by the square root of n, n being the number of fets). Am I wrong ?

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Old 7th February 2002, 03:13 PM   #7
SteveG is offline SteveG  United States
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I think that is what I will do also- increase the bias current. I suppose that it lowers distortion because the fet is in a more linear range. Nelson explained in one article (can't remember which one) that this is why MOSFETs aren't good to drop into class AB circuits- they aren't linear near cutoff.
I may also try paralleling two MOSFETS to get a little better noise performance. The only thing that I worry about is the increase in gate capacitance. I wonder if that will be alright with the source?
BTW, what load resistors did you end up using with your 80mA current sources? I was running a simulation with 100mA current sources at +/-50V, and I came up with 220ohm load resistors. I changed the resistors on the power rails from 22.1 to 10, because they were dropping a lot of voltage at the higher currents.
Anyone know if it is possible to cascode this circuit?
Steve
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Old 7th February 2002, 03:23 PM   #8
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Hi,

Don't have my schematics handy, but I think the drain resistances were 500 Ohms (Caddock MP915, TO-126 case). The positive rail is at +85V, and negative one at -40V (to reduce dissipation in current sources - MJE15030 BJT + Led)

I'm sure you can cascode the circuit. Never tried, but seems quite possible.

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Old 7th February 2002, 04:21 PM   #9
SteveG is offline SteveG  United States
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Default Will this work?

Here's a circuit I was trying on my simulator. Will it work?
Steve
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Old 7th February 2002, 04:26 PM   #10
SteveG is offline SteveG  United States
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I think that the cascodes are screwed up... don't think that I have the right voltages on the gates. What is a good way to turn them on?
Steve
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