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Old 1st May 2011, 09:50 AM   #1101
AndrewT is online now AndrewT  Scotland
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make it switchable or add shorting pins (0.1" pitch) across the output resistors.
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Old 1st May 2011, 09:57 AM   #1102
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Knight,

Since you are working in Eagle you should check missing junction between Q1/R6/R7 and C6/C9/ground.
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Old 1st May 2011, 11:15 AM   #1103
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Hugh, have you control over power-on and power-off bumps? 15 volts DC may damage a low ohmish earphone and might be very unpleasant for your ears.
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Old 1st May 2011, 01:58 PM   #1104
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Not checked yet, P-A, to my discredit.

One thing we should add is a 10R series resistor at the output with about 10nF to ground at the load end. This will ensure doubly that RF is not an issue, and will certainly reduce power somewhat for 24R cans, but for the majority will have no effect and will ameliorate any thumps.

I had hoped by now that someone might have built and tested one, but can't complain since I did not do it myself.

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Old 1st May 2011, 06:46 PM   #1105
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What about a protection/delay board? Placed at the output, providing a optional impedance switch, 1 second delay, optional zoble network, and even a push button on. Does that appeal? It fixes all the issues without having to redesign the entire amp.

Corrections made to schematic, thanks again aparatusonitus
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Old 1st May 2011, 10:46 PM   #1106
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I think the prototype should first be built in ratsnest to see if there are switch on thump issues. Strictly that should be done before the board is completed. If there is a problem, then a protection board is a good idea, otherwise, it almost doubles complexity.

A second option, worth considering, is to redesign for single rail and cap couple the output. This offers magnificent protection, thought there may still be a small thump as switch on. Kurt, do you have a relay in mind for protection? A good one should use gold plated contacts; audio is extremely touchy about relays.

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Old 2nd May 2011, 04:26 AM   #1107
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You could also have a relay which shorts instead, then it will not be in the circuit when it's activated.
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Old 2nd May 2011, 10:23 PM   #1108
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I'll bread board the amp with generic parts, BC557/547 will be BC556/546 since that's all I have. And MJE15035 instead of MJE15031. Hopefully I'll have time to get it done by thursday.

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Originally Posted by peranders
You could also have a relay which shorts instead, then it will not be in the circuit when it's activated.
Hopefully we won't need delay, but I favor this way.
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Old 2nd May 2011, 10:29 PM   #1109
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Take your time, Kurt, there are no deadlines, maintain the interest but don't wear yourself out. Your board is a very good one, but these operational issues should first be resolved with a breadboard. I breadboard every circuit first; otherwise, you finish up with lots of pcbs you can't use, which represents an appalling waste of resources.

The relay idea is not often used to my knowledge with HPA. I think it should be possible to design out the turn on thump.

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Old 5th May 2011, 10:36 PM   #1110
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I managed to create a abomination of the amps output stage, with a 1.95volt LED and no DC adjust. I managed to almost burn myself on Q4 with my small little heatsink, does anyone know how much heat Q4 is theoretically puting out? The heatsinks that I used on the board layout might be too small. Please note I didn't use the spected heatsinks for the test, they were a much smaller slid on heatsink.

I'll hopefully have time to refine the railspliter the simple two resistor virtual ground circuit next week as well as install a proper DC adjust
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