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Old 7th February 2002, 01:30 AM   #1
PassFan is offline PassFan  United States
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Default Make Amps Sound Better

Does anyone think amps sound better when they warm up. My A75 got a good burn in this past weekend. I listened to it for about 4 hrs while working in the garage. It got up close to 50 - 55 deg. C and stabilized there and really sounded good, really detailed and smooth. When I turn it on cold it doesn't seem to have the same liveliness. Is this explainable or is it my ears deceiving me?
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Old 7th February 2002, 01:54 AM   #2
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no your ears probably aren't deceiving you.... as temperature changes so can bias, DC offset, and a number of other characteristics..... and they often change rather rapidly upon turn on until the amp reaches it's operating temperature.
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Old 7th February 2002, 01:57 AM   #3
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Setting aside questions of bias settling in as the unit warms up, there's also the matter of electrolytics--particularly new ones or those which have been gathering dust--needing to get up to voltage.
I've heard of other possibilities, as well, but don't have the energy to start a ruckus this week. Perhaps next time around...

Grey
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Old 7th February 2002, 02:04 AM   #4
PassFan is offline PassFan  United States
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Would it then be a good idea not to use bleeder resistors on the power supply caps ? Do they effect the charging of the cap in a negative way?
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Old 7th February 2002, 02:28 AM   #5
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I don't use them, myself. Certainly not for a solid state piece, given the lower voltages. On a tube design, you could argue that it's a good idea...but I still don't. I'm jealous of each and every little electron. I want them to reach their destiny later on in the circuit.

Grey
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Old 7th February 2002, 02:32 AM   #6
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I agree Grey, what with the usually horrendously low efficiency of the designs we commonly use, i want every little electron possible to get to where it can do some useful work.
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Old 7th February 2002, 04:21 AM   #7
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Default Bleed resistors

I do use bleed resistors, especially when I'm still prototyping the design.

Why? Well, after discovering the 50,000uF caps on the +25V and -25V rails for my Aleph 30 monoblocks have easily enough instantaneous current capacity to spot-weld a stray wire to the ground plate, I decided to discharge those suckers with a 220R 5W resistor from each cap to ground.

With the mains power removed, the bias current drains the caps down to 3-4V or so, but then the output mosfets will turn off. A stray lead while I was modding the wiring and ZAP!

Come to think of it, I haven't removed the bleed resistors from the finished boxes. Perhaps I'll open the boxes up and take them out.
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Old 7th February 2002, 06:51 PM   #8
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Most of these parts, MOSFET and Bipolar, increase
their transconductance as they warm up a bit, and
their junction drop Vgs or Vbe alters with temperature,
so we always do our adjustments and evaluation after
they have warmed up for an hour. Over that period of
time you can measure very significant performance
changes, not to mention any subjective perceptions.
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Old 7th February 2002, 07:18 PM   #9
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Default Warm Up

Solid State amps in particular seem to need to warm up -- i have found some stuff seems to get better over as long as 48 hours -- so i just never turn my hifi off. I like the music happening all the time anyway.

dave
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Old 7th February 2002, 09:42 PM   #10
PassFan is offline PassFan  United States
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Keeping stuff on would be a disaster in Florida. Were the lightning capitol of the world. I am going to remove the bleeder resistors though.
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