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Old 27th December 2009, 03:12 AM   #6491
a.wayne is offline a.wayne  United States
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Thanks for the reply Zen , after 160 pages of searching I had to ask
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Old 27th December 2009, 03:14 AM   #6492
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by a.wayne View Post
Thanks for the reply Zen , after 160 pages of searching I had to ask
naah ......

what's 160 pages between friends

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Old 27th December 2009, 03:18 AM   #6493
labjr is offline labjr  United States
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I've always felt that if you add outputs some of the midrange magic may be lost since it's virtually impossible to have perfectly matched parts.
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Old 27th December 2009, 03:25 AM   #6494
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I don't have any evidence that midrange magic is a
function of matching. Perfect matching reduces 2nd
harmonic. Those SE tube designs with the coveted
midrange are supposed to have more than their share of
2nd harmonic.

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Old 27th December 2009, 09:01 AM   #6495
sangram is offline sangram  India
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I set the thermistors on the outputs with a drop of epoxy.
I'll look at doing something similar, I have some thermal tape but the thermistors have no flat surfaces for adhesion, so maybe Arctic Alumina will have to be sourced.

I have three follow-up questions

1. We have a huge variation in weather conditions where I live. Will I need to readjust offset in summer or will the amp simply run a little (lot!) hotter and still have the offset low enough? We typically see 15 degrees all around between the hottest summers and coldest winters, and between day and night during most of the seasons - right now we're at 23/11 degrees, a typical summer day will be 38/24 (all Celsius).

2. I seem to have a pretty good thermal headroom thanks to the large sinks, can I use one heatsink for two channels/balanced F5/higher bias for a little more Class A power?

3. And the imminent question - what is the maximum (theoretical) undistorted output voltage from a single-ended F5? I have way to directly measure it as of now. 20V? 30V p-p? 50W/2 ohms (clipping point mentioned in the pdf) is ~10 Volts, so that can't be right as the 8 ohm output is significantly higher voltage...

Thanks
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Old 27th December 2009, 10:06 AM   #6496
AndrewT is offline AndrewT  Scotland
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the whole amplifier will run 15Cdegrees hotter when your day time temperatures are @ 38degC than when it is set up for daytime temperatures of 23degC.

If the Ts=50degC at set up then it will run @ 65degC when Ta=38degC.
From there you will find that Tc and Tjmax will both be 15Cdegrees hotter as well.

The thermistors will trim back the hot running current slightly, that is why they are there, but they cannot temperature compensate for big changes in Ta.
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Old 27th December 2009, 10:48 AM   #6497
sangram is offline sangram  India
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Well, I'm expecting it to run ~20-22 degrees hotter, given that heatsink efficiency would drop off with increase in ambient, and not linear either.

I wanted to know if the offset could expect to undergo some change as well, and whether it was enough to worry about.

In summer I might even be using the F5, come to think of it. My room is a furnace then.

So, Q2/3 were keeping in mind current running conditions - though I could fire it up and see how it goes. I just seem to have large enough to take a wee bit more power, I guess.
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Old 27th December 2009, 12:20 PM   #6498
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Quote:
Originally Posted by sangram View Post
what is the maximum (theoretical) undistorted output voltage from a single-ended F5?
Strange thing to ask.
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Old 27th December 2009, 12:46 PM   #6499
EUVL is offline EUVL  Europe
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I want to ask an old queation before the end of the year....

Has anyone actually have a Spice Model for the Fairchild devices (FQA19N20C, FQA12P20)?
If not, what are you actually using for all these sims (IRFPs?) ?


Thx,
Patrick
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Old 27th December 2009, 01:19 PM   #6500
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FqB19n20/fqB12p20
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Last edited by jacco vermeulen; 27th December 2009 at 01:21 PM.
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