350.5 modding the amp to work in more class A - diyAudio
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Old 27th February 2007, 04:20 AM   #1
jfrago is offline jfrago  United States
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Default 350.5 modding the amp to work in more class A

Has anyone tried to increase the amount of class A that the amp works in to 90 watts?

How hard would it be and would it cause damage to the amp due to the increase heat it would generate?

Thanks.
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Old 27th February 2007, 04:30 AM   #2
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There's more to it than just the heat dissipation.
--Can the power supply supply sufficient current? In this case, probably so.
--What effect will increasing the bias have on the SOA of the output devices? That's a little more risky.
--And, of course, the heat. Actually, I'd say that this is the least of your problems. Use fans and you can get rid of quite a bit of heat.

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Old 27th February 2007, 01:17 PM   #3
moe29 is offline moe29  United States
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Why would you want to mod a $9,500 amp? Why not just buy a XA 160?

Or better yet, why not build an Aleph 2? (i love mine)
That would give you 100 watts of lovely Class A.

Sorry if i sound like a jerk, but messing with such a beautiful amp is
a bit puzzling to me.
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Old 27th February 2007, 04:11 PM   #4
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Better to burn up than to fade away.

Expect at least 10C higher heatsink temperature and worn down knee caps in 10 years, without counter measures.
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Old 27th February 2007, 04:39 PM   #5
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With the X350.5, or indeed any of the 0.5 versions, you will also need to consider how much of the class A is delivered in SE. At the very least this will complicate the situation. Why exactly do you want to change the factory default?
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Old 27th February 2007, 06:24 PM   #6
jfrago is offline jfrago  United States
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Default 350.5 in class a

Ian - good point. I don't know how much of the amps output is SE Class A and how much is regular Class A. If anyone knows please let me know.

As far as why, for me and this is just my opinion but the Aleph series is a bit polite for me and I find the XA series lacking as well.

Maybe I am just nostalgic I find the old Threshold designs of Nelson's - like the SA-12e or the SA-4e series to have a terrifficly engaging quality but they lack in the transparency of the X series. I do find the X 350.5 to be the most engaging of the X series but I would still like to try to increase both the SE class A and the regular class A output to get it closer to amp that would combine both of the qualities that I stated above. Mybe the increase doesn't need to be that much?

Thanks for your thoughts.
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Old 28th February 2007, 08:06 AM   #7
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I think the answer to how much power is available in SE Class A can be found in another thread on this forum (or at least I know I read it somewhere). From memory it is around 12W.
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Old 1st March 2007, 12:47 AM   #8
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The X circuit depends on maximum linearity in the circuit, and
really shines in limited feedback situations.

Bias is very important to these amplifiers, and we like to run it
high, mostly limited by the hardware heat dissipation.

Of course there's nothing to keep you from cranking up
either/both the SE (just lower the value of the power resistor
drawing SE current) or PP bias (just turn the pots up).

You can get away with a significant increase, but you must take
care to keep the amplifier cool, and this might mean the
addition of fans. Alternatively, you can accept the shorter life
which comes with higher temperature - the amp may fail in 10
years instead of 20.

Also, you can reduce the AC incoming voltage, which will reduce
the dissipation (and power rating) but allows for more bias
within a given dissipation.

As a practical matter, if you keep the sinks well ventilated you can
think in terms of a 25% increase in bias, and this will give you
56% more Class A. If you drop the AC line to 110V, you can take
it up 70% more or so without penalties.

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Old 1st March 2007, 01:34 AM   #9
jfrago is offline jfrago  United States
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Mr. Pass,

Thank you for your reply. That is so cool that you took the time to write. Thank you.

What I was looking for would be about a 50% increase in the amount of the class "A" . Which, please correct me if I am wrong, would bring it to around 80-85 watts of class "A"? That should be good enough to cover most of the frequencey range for most speakers in class "A" - at a moderate playback level. What do you think?

Again, thank you for writing Mr. Pass.
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Old 1st March 2007, 03:45 AM   #10
jfrago is offline jfrago  United States
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Mr. Pass,

I am sorry I multiplied incorrectly - the amount of class A would be more around 60-65 watts not 80-85 watts.

What's the lower ressitor value and where should I install it to get the 56% increase in SE class A?

Where are the bias pots? How much would I need to turn them up to also get the 56% increase?

Where should I measure the temprature of the amp, would it be at the heatsinks and what temp is acceptable? It may shorten the life but I don't want to do any damage?

Thanks, again.
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