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Old 19th August 2011, 05:44 PM   #11
jaycee is offline jaycee  United Kingdom
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A microcontroller is unneccesary really. You can achieve the same result with a few transistors
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Old 19th August 2011, 07:45 PM   #12
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All amps should have some kind of battery offset/supply error based shutdown.

I like the micro controller idea for a few reasons.
1. Your power switch can be the micro controller power switch. It can be in charge of turning on the amp and switching the output relays.
2. It can easily run a shutdown routine that will switch off the output and then shutdown the amp.
3. It can monitor the rails in real time. Threshold shutdown..

It also sucks for a few reasons.
1. It's expensive.
2. Needless excess.
3. While space and battery life isn't a concern in a desktop amp. It is in a portable.

There is a need for a competent design for something that is in charge of the condition of the battery/supply. Ground channels and rail splitters are like using a sledge hammer for finishing nails. I have always thought this was a better solution.
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Old 19th August 2011, 08:06 PM   #13
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If you use 'phones designed to the IEC recommendation for a 120 Ohm source impedance (Senns, most Beyers etc), then simply using that 120R in series with the output will probably prevent this off +/-12v supplies.

Into 120ohm phones you'd get 0.25W dissipation - slightly outside most phones 200mW maximum, but very likely survivable (use 1/8w resistors and they will smoke first!) Higher impedance phones would see slightly more, low impedance phones rather less. Unfortunately it is low-impedance phones that often require a lower output Z to shine.

Just a thought...
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Old 20th August 2011, 12:10 AM   #14
jaycee is offline jaycee  United Kingdom
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Yeah I thought that. In sim I got a dramatic rise in THD with the 120 ohm resistor, and the amp sounded better without it. The Beyers are rated for 50mW max.

Mine got hit by 1.8W if I've calculated correctly! I'm surprised the voice coils are even intact - the 'phones still work but they sound absolutely awful now, especially the right channel. I knew I was screwed when i turned it on and they went CLICK.
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Old 21st August 2011, 06:56 AM   #15
jaycee is offline jaycee  United Kingdom
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OK so I have been trying out various DC detect circuits. So far, I haven't found one that's very reliable with low supply voltages - ideally I would like one that trips with only 1V DC as this is plenty to destroy headphones. The biggest problem I have found with most detector types is that they are asymmetrical.. they may trip with +4V for example but then need eg -9V to trip. Not very good.

Any suggestions?
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Old 21st August 2011, 12:09 PM   #16
Mooly is offline Mooly  United Kingdom
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I think you would need a purpose designed circuit for this... it's going to end up more complex than the amp itself. And if it does trip what does it act upon... shut the PSU, disconnect the phones?

If you are determined then I think maybe you need to look at a window comparator.
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Old 21st August 2011, 01:38 PM   #17
jaycee is offline jaycee  United Kingdom
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Quote:
Originally Posted by Mooly View Post
I think you would need a purpose designed circuit for this... it's going to end up more complex than the amp itself. And if it does trip what does it act upon... shut the PSU, disconnect the phones?

If you are determined then I think maybe you need to look at a window comparator.
I was going to put a relay on the output as I have plenty of suitable ones - i would also think that at this sort of level, small signal relays would work very well.

I have seen circuits based around opamps and comparators so I guess that is the way to go - i can certainly see how using a comparator with a very low reference voltage would trip quickly, and it could easily be made to latch which would be even better. I've seen allsorts of really curious setups used for the detector - from diodes building a rectifier to transistors in all sorts of odd combinations!
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Old 21st August 2011, 04:23 PM   #18
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Quote:
Originally Posted by Rembrant View Post
It also sucks for a few reasons.
1. It's expensive.
2. Needless excess.
3. While space and battery life isn't a concern in a desktop amp. It is in a portable.
1. If You call 2$ Expensive (See Attiny 25)
2. It is my hobby, so I do not mind, and it needs no external components to work except the relays and buttons!
3. SO8 package isn't exactly big. Attiny25 works on 1,8-5V and can power from a CR2032 battery for a year if Your program only activates in about every 1s only. (periodic checks, You need a backup battery to use a bistable relay to save energy)

It is really a concept. If there is a better solution with a few opamps or comparators, I will try that too but I am not an expert in those.

Last edited by Earfanatic; 21st August 2011 at 04:26 PM.
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Old 21st August 2011, 06:00 PM   #19
Mooly is offline Mooly  United Kingdom
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You want simple really...

Just ideas... use a relay to short the output across the phones if DC appeared. Use a low value fuse in the rails somewhere. Transistor/IC/FET whatever to turn the relay on when anything DC wise appears... at least 0.7v of course for a simple transistor based detector.

Use FET's instead of relay if the signal is to pass through the relay... recent thread on this, you've probably seen it.

This is a comparator I designed but never built for real other than on a breadboard. If you kept the top input at 0 volts it would have the -/+0.1v trip threshold. It was really designed to compare input/output differential of amp to eliminate the integrator delay.

Do you know what I'd do Jaycee... build your amp on a PCB and accept the risk. If the designs good, components good and build good it's never going to fail is it ?
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Old 21st August 2011, 11:29 PM   #20
jaycee is offline jaycee  United Kingdom
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Quote:
Originally Posted by Mooly View Post
This is a comparator I designed but never built for real other than on a breadboard. If you kept the top input at 0 volts it would have the -/+0.1v trip threshold. It was really designed to compare input/output differential of amp to eliminate the integrator delay.
I'll play with that next but it is pretty much similar to what I was thinking about! I believe the Velleman speaker protector is also similar to this circuit.

I also remembered a comparator circuit of sorts from an Arcam amp. I simplified it a bit and this is how it turned out. It seems to work fairly well.

Click the image to open in full size.

In the above schematic R2 represents the relay coil.

Click the image to open in full size.

Notice the relay is across both supplies - so if one supply fails, the relay will shut off anyway!

Quote:
Originally Posted by Mooly View Post
Do you know what I'd do Jaycee... build your amp on a PCB and accept the risk. If the designs good, components good and build good it's never going to fail is it ?
Never say never! Though yes, in this case the amp only "failed" because it was on breadboard. I should have at least built it on Veroboard I suppose. I was working on a PCB layout but have little free time, and now the money that was going to pay for the case, toroid etc is going to have to be spent on new headphones
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