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Old 15th November 2006, 03:25 PM   #1
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Location: Great Yarmouth, UK
Default Fuse blew

Well, I have built myself a GC based on carlosfm's non inverted schematic and "snubbered" regulated PSU along with a soft-start from Rod Elliot.

I've got a single channel hooked up, it all turns on an off just fine. The relay click happens very shortly after I flick the power switch. Probably more than 100ms, but definately less than 1s... lets say 500ms .

I checked the offset, with no input signal, using a 10R 7W resistor, before hooking up a test speaker, and I got 92mv (a little high but probably nothing to worry too much about at this point?).

So next I plugged in the speaker, still without an input signal. Silent turn on, strange high pitch click on turn off. Speaker didn't seem to be blown, but it made that strange click on turn off every time.

The moment of truth. My current amp, a Cambridge Audio A5, has a "PRE OUT", which appears to mean it's the output of the preamp. Seems obvious enough, so I assume that's exactly what it is and I was hoping to use it purely as a pre-amp for now. I plugged one channel in to the pre-out and then plugged the other end in to the GC input. Sat hesitating for about a minute, and, while playing music, turned on the beast with the volume control WAY down. It was LOUD, very loud, and as far as I could tell the volume control was doing nothing at all to quieten it. It was, however playing music. I turned it off, checked everything... and tried again, it was still loud. I think the fuse blew when I turned it off that time.

What worries me is that it was so damn loud and the volume control was doing sweet FA. I measured the pre-amp output and it only seemed to be ~300mv, BUT this figure didn't seem to be changing when I twiddled the volume control either I suppose this is something wrong with my current amp. I'll worry about that later I guess.

The fuse I used is a 3.15A quick blow, and I have a 500VA toroidal transformer (originally was going for 300VA, but the 500VA was cheaper at the time!!) which might not be enough I guess? I have a 5A slow blow, but I am a little worried about turning it on with that though!?

Oh, also, as it's not in a case I haven't connected anything to the safety earth pin yet ( yes yes, call me an idiot but what am I supposed to connect it to at this point? ). Might this also have something to do with it, or not?

For what it's worth I didn't seem to see or smell any smoke, it just wouldn't turn on so I checked the fuse... and it seems the waxy stuff inside melted. Nice and green in there now Hopefully this is a good sign!
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Old 15th November 2006, 03:34 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
how often do we need to tell everyone.
Use a mains light bulb wired in series with the live mains feed to test the unit from the mains, to avoid catastrophic blow ups.

It is cheap and potentially saves a fortune in damaged components if something has been mis-wired, transformers and caps particularly.

Output offset is measured with the output open circuit and the input short circuit. Do it again correctly and if it is still near 100mV then fix it first.

Come back with the correct offset reading while it is still connected through that light bulb.
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Old 15th November 2006, 03:41 PM   #3
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AH! I misread / misunderstood something somewhere then. I thought the output was just supposed to have a "load" to test the offset.

I guess just a wire from input +ve to input -ve on my board is OK? That would be what shorted means right?

By the way, I know about the light bulb thing. I just dont have a spare to do it with. My own risk and all that, yes, I know!

PS thanks for your really quick reply!!

EDIT: still ~90mv
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Old 15th November 2006, 03:49 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
can you post your schematic?
We should be able to reduce that output offset substantially.
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Old 15th November 2006, 03:51 PM   #5
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Ah cool, I used this schematic by carlosfm...
Attached Images
File Type: png cfm_amp4.png (12.2 KB, 254 views)
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Old 15th November 2006, 03:57 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
first glance shows that Carlos is using mixed AC & DC coupling. That is not good for output offset and offset stability.
I recommend fully AC coupled or fully DC coupled, not mixed.

DC coupled means DC input at the non-inverting input pin AND DC feedback at the inverting input pin.

AC coupled means DC blocking capacitor in the input line to the non-inverting input pin (as shown in the schematic) AND DC blocking capacitor in the lower leg of the NFB loop.

Mixed AC & DC coupling means one of the DC blocking caps is omitted.
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Old 15th November 2006, 04:04 PM   #7
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By DC coupling you be refering to the 3.3uF cap between the power rails, I assume? Or the 300nF between input pins? Or even both of these?

Regardless, I do seem to recall carlos saying his DC offset with that circuit was < 50mv. Probably partly to do with my crummy PCB layout?
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Old 15th November 2006, 04:12 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
the non-inverting input pin sees a DC resistance of 330R + 15k =15330r to ground. All the input offset current has to sink (or source) through this resistance generating a +ve input offset voltage.
The inverting sees 100r//2k=95r2 ohms to ground. This generates a different (lower) -ve input offset voltage.
The sum of these voltages are multiplied by the chip amp gain (21Times) to give the output offset.
As the two input offset voltages are brought more equal in value the output offset reduces.

The easy solution is to insert a DC blocking cap into the lower leg of the NFB loop. but this will still leave 2k vs 15k3 unmatched input impedances. It will be better but not optimum.

Can anyone confirm that this chipamp is not internally compensated to reduce input offset currents? If it were so then balancing impedances is unlikely to offer a solution.

The secondary advantage of balancing the input offsets is reduced thermal variations in the output offset. With a chip amp thermal variations are enormous compared to discrete and very clever component layout is required inside the chip to minimise this critical parameter. Keeping the chip cooler and with as little temperature variation as possible will help.
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Old 15th November 2006, 04:21 PM   #9
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Quote:
Originally posted by AndrewT
Hi,
the non-inverting input pin sees a DC resistance of 330R + 15k =15330r to ground. All the input offset current has to sink (or source) through this resistance generating a +ve input offset voltage.
The inverting sees 100r//2k=95r2 ohms to ground. This generates a different (lower) -ve input offset voltage.
The sum of these voltages are multiplied by the chip amp gain (21Times) to give the output offset.
As the two input offset voltages are brought more equal in value the output offset reduces.

The easy solution is to insert a DC blocking cap into the lower leg of the NFB loop. but this will still leave 2k vs 15k3 unmatched input impedances. It will be better but not optimum.

Can anyone confirm that this chipamp is not internally compensated to reduce input offset currents? If it were so then balancing impedances is unlikely to offer a solution.

The secondary advantage of balancing the input offsets is reduced thermal varaiations in the output offset. With a chip amp thermal variations are enormous compared to discrete and very clever component layout is required inside the chip to minimise this critical parameter. Keeping the chip cooler and with as little temperature variation as possible will help.
All a bit over my head, but I think what you're trying to tell me is that I should be trying to get the feedback down to 2k, the same as the input?

I don't know...

I just tried it again with a quiet song, and it comes out pretty darn loud. The speaker still works though I'm thinking maybe the fuse is just stupidly under rated for the toroid. The loud(er) song I played before must have put it past 3.15A?

It sounded really "airy". I might try with one of my spare speakers at this point. At the moment it's just a cheapo full range driver sitting on my bed, not in an enclosure or anything. I haven't even listened to that driver before!
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Old 15th November 2006, 04:35 PM   #10
Nordic is offline Nordic  South Africa
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you could probably drop the gain a bit...or double check the resistor value you used there... also double check the pot with a multimeter to make sure it is working and not dead shorted inside... I have never needed more than a 10k pot.

In fact if the pot is doing nothing, that is what is wrong, cause when set to 0 it is suposed to be very close to a ded short to ground over the input.

Also full range drivers outside of any "box" tend to be very efficient(read loud), could give you even 100db off a single watt in theory.
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