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Old 10th October 2006, 06:29 PM   #1
dinu is offline dinu  United States
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Default Higher power A75.

The original purpose for this thread has taken a whole new direction. I finished my first A75 which was built for a friend and had it at my place for just enough time to get tragically hooked. The amp has a tremendously open and clean sound that makes my current system sound muffled in comparison. I grit my teeth every time I listen now.
I have to build my own and this time am going all out. There have been alot of improvements added by various builders to NP and Norm Thagard's original design. Thanks to all of you, Alain, Peter Daniel, and last but certainly not least Bob Ellis. (Mr Revise and Resubmit). Thanks for keeping me going.
I want to incorporate these improvements into a new board layout and, while I'm at it, why not more power?
You can follow some of the discussion to this point here a75 power supply problems
I'm putting together a new layout and will post soon. If any of you have a full version of Eagle and can help out please make yourselves known.
I'm starting by upping the output stage rail voltage to +/-63V. The output devices I used last time were TO247 packages. IRFP240's and 9240's. The amp requires a new power supply design, a soft start circuit, output protection, and Zener input protection. The finer points of the circuit improvements are something that I'm hoping Bob can elaborate on further since I have a very limited understanding and much shorter history with this design.
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Old 10th October 2006, 06:46 PM   #2
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Default Re: Higher power A75.

Quote:
Originally posted by dinu
Mr Revise and Resubmit
I prefer Captain Revise and Resubmit
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Old 16th October 2006, 12:59 PM   #3
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Default Auto adjusting DC Offset

I got bad news about my first A75. The best I can figure is, the DC offset is floating alot with breakin and temperature change. I'll be able to re-adjust it at thanks giving when I'll be in Florida but this made me think of a great upgrade to the A75 design.

Is must be possible to design an Op amp configuration that can keep adjusting P1 and P2 (I imagine they'd be some kind of small signal fets). I don't even know where to start researching such a mod but maybe someone out there can help.
This would also make it possible to effectively use R81 as a 3 position switch that offers 3 feedback settings, 100%, 50%, 0%. I made such a switch and it worked except for the fact that you can only adjust the dc offset for one setting. Once you changed the feedback setting, the DC offset went out of adjustment.
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Old 16th October 2006, 01:29 PM   #4
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What's the problem reported? Huge DC offset? It is more likely a component failure. Mine don't seem to drift with ambient temperature changes (other than when the top is off and a breeze blowing). They have been working without adjustment for a couple years now.

Easier than adjusting the CCS tails would be to make a DC servo. There are threads here discussing the design and implementation of servos.
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Old 16th October 2006, 01:42 PM   #5
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Well, he's been using the amp directly connected to his Ipod (no preamp). He was using it the night before for several hours, turned the unit off at the end of the day. I built a Velleman output protection kit into the amp. It shuts the output off when it sees >1VDC. It also provides turn on delay of about 6 seconds. Anyway, the delay never turns on because it sees >1VDC at the output. The strange thing is that both channels did this at the same time. The LED from the front end PSU turns on, and the fan, which runs off of the output stage PSU is running.
My guess is, that DC has been drifting with break-in to a point that is less than 1V when the amp is warm. When he turned it on in the morning and it's cold, the DC is way out of whack so the protection circuit won't allow it to turn on. I think if he leaves it on to warm up, the DC will eventally drift below 1V and the amp will turn on. That's pretty risky though so I won't even suggest it. I'll see him in a month so I'll be better able to diagnose the problem then.

I'll look into the idea of a DC servo further because it would be nice to have that feedback switch.
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Old 16th October 2006, 02:03 PM   #6
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Default Found Link

This seems like exactly what I want. It may as well be in coded greek though. I don't see how to apply this to the A75.
output bias servo
I need the plug and play version.
It's also for tubes. Tubes are scary.
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Old 16th October 2006, 02:05 PM   #7
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More direct link.
http://geek.scorpiorising.ca/GeeK_Zo...p?topic=1728.0
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Old 16th October 2006, 03:06 PM   #8
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There are better circuits than that to copy from for your application. Try limiting your search to the solid state forum.

Tubes shouldn't scare you - if you mess up they don't work or the plates glow. Much more forgiving than solid state, but the voltages can be scary.
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Old 16th October 2006, 03:16 PM   #9
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I wanted reassurance that my theory re: "They can't both fail at the same time! NOT at the same time!" is valid. I just hope to hell none of the output devices blew. I installed little sockets for the TO220's and TO92's so they're very easy to replace. The output 247's are a totally different story.
I also have to confess, There are no heat sinks on any of the 220's. That may be bad. They are right in front of the fan though.
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Old 16th October 2006, 03:35 PM   #10
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Unless your protection circuits are totally independent, it is likely that one channel being out of spec is keeping both from activating.

No heat sinks doesn't guarantee failure, but it is not a good thing for longevity. The regulator devices see a lot of dissipation -0 nearly 2 watts.
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