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Old 19th May 2011, 02:05 AM   #1121
AKSA is offline AKSA  Australia
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KoA,

There is always concern about offset. It's the last frontier before reaching Eldorado. If it's good, and it's stable, then we have ignition.

I'd now sit on your pcb design, doodle it for a couple of weeks to be absolutely sure, and then have it made. I use ourpcb.com, their service is good and they do excellent pcbs at moderate cost, and just get four done to begin. Your production run will inevitably need changes, so don't commit too early.

Finer points of pcb layout are very hard to detect at this stage. You really have to build it now, have the thing in your hand, turning it over and inspecting it whilst building, before you have the full, 3D picture.

Thanks for your hard work, nearly there.....

Hugh
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Old 20th May 2011, 03:52 PM   #1122
AndrewT is offline AndrewT  Scotland
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A short starting pulse approaching 1Vpk is going to eventually damage something attached to the connected headphones.
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Old 21st May 2011, 04:13 AM   #1123
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Here's something I've been thinking about, and I want to apply it to my speaker amps too;

Why not use a 3-pos switch to switch both main power and then connect your headphones? It's a small burden to have to wait a second before turning the power knob one more click 'round, and the time it takes you to turn from "ON" to the "Engage 'phones" might even be long enough for the amp to settle. Just make sure you buy a very nice switch.
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Old 21st May 2011, 04:25 AM   #1124
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I like that idea. The middle setting could simply be "mute".

We would have to demonstrate however that it was impossible for a human to flip the switch fast enough however.

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Old 21st May 2011, 04:38 AM   #1125
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I'll get my hands on a SPSP switch, I don't know why I didn't have a couple available here. In the meantime, I'll stare at the board layout intently. I've already got a couple things I want to iron out.
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Old 21st May 2011, 05:36 AM   #1126
Sheldon is offline Sheldon  United States
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Here's an automatic way to do it (although there may be better ways).

The bottom half of this figure is a filament supply and automated mute switch. The 8.8V output is just a regular filtered DC. Substitute the rail supply for this.

Now look at the two FET's. These are in series and both must be on in order to open the mute relay (not shown in this diagram). The relay coil is connected from the 8.8V output to the relay common.

The gate of one FET is powered by the two diodes that are outboard of the diodes for the 8.8V supply. It's done this way so that as soon as the power is switched off, this FET stops conducting immediately. You can change the timing by adjusting the relevant resistors and capacitor.

The other FET gate is powered by the 8.8V supply, through the 100k resistor. It must charge up the 100u cap before the FET will conduct.

So, upon turn on, the mute relay doesn't open until the amp output has had time to stabilize. But the relay closes upon power off, before the main supply droops low enough to create a transient at the output.


http://www.diyaudio.com/forums/attac...ono-supply.jpg

Last edited by Sheldon; 21st May 2011 at 05:40 AM.
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Old 21st May 2011, 05:57 AM   #1127
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Yes, and maybe something a bit more complete like a 'dc protection/delay' cct with a 'mute' option - probably require something more sensitive that the usual speaker protect ccts to enable a low 100mV trigger threshold - a Maxim 'Max 4236' chip could be adapted, for example, but many other ccts, probably simpler ....
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Old 25th May 2011, 02:14 AM   #1128
AKSA is offline AKSA  Australia
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Hmm, this is getting bigger and more complicated again.....

How about driving the cans through a coupling cap? That avoids many of the protection issues and permits use of a unipolar supply.....

Hugh
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Old 1st June 2011, 09:16 PM   #1129
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If preferred, you can ignore this post until any current issues you guys are working on get sorted out. Or, if anybody has a quick second they could be of great help to me. I have started practicing PCB design only maybe two weeks ago. I think I am getting the hang of it. I am pretty sure I want to build this headphone amplifier, and I do see that somebody is already cooking a PCB up for it, but I want a single layer one, so I will be making my own. Here is my attempt:

http://dl.dropbox.com/u/5430178/proj...fier%20PCB.png

The red traces are to symbolize jumper wires on the bottom of the board. Here is the schematic that I copied in order to make the pcb, in case I screwed up somewhere:

http://dl.dropbox.com/u/5430178/proj...%20Circuit.png

If this is not the first priority of you guys, I understand!

PS: I just want to know if this is OK. I know it probably isn't optimal or perfect, and that is fine. I'm not expecting anybody here to walk me step by step into optimising it and wasting you're time. Then again, if you have a real quick tip to throw at me, I won't mind.

Thanks,
Aaron.
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Old 2nd June 2011, 06:48 AM   #1130
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It might work but try to keep the output stage together. A pro wouldn't place parts like you have done.
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