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Old 14th August 2005, 03:49 PM   #1
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Default LM3875 dual mono kit - a few questions.

I have bought 2 dual mono 3875 kits from madaboutsound.com to make a 4ch amp.

I finished the first 2 channels yesterday but I'm getting 78mv at one set of terminals and 144mv at the other set.

I have triple checked everything and both channels are identical - any idea what I may have done wrong?

I have read that I'm supposed to ground the PSU board but can't see where to ground it so only the amp boards are grounded to the chassis via cg.

Now... This morning I put the other kit together for the other 2 channels, I switched the amp on and smoke started pouring off one of the rectifier diodes... Not good...

Again, I triple checked everything and it's all put together properly. I haven't ever built anything like this before so I have been working very carefully to make sure I do everything properly and don't make any mistakes.

The transformer I'm using is a 500 VA with two 25-0 secondaries, the PSU boards are connected AC1 to +25, AC1- to 0, AC2 to +25 and AC2- to 0.

What would cause the diode to smoke?

On the bright side, I listened to the first amp for a few hours first thing this morning ( I have spare drivers for that particular pair of speakers ) and it sounds exceptional, I'm well chuffed.
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Old 14th August 2005, 05:11 PM   #2
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This is not unusual values.

According to datasheet the output offset with low ohmish resistors in the feedback and low DC resistance at the input, it can be 1-10 mV times the gain which is 30, 30-300 mV that is. If you them have non optimal resistor values the offset can be even higher. (therefore do I use DC-servo which gives me 0.07 mV(!) offset, every time!)
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Old 14th August 2005, 05:23 PM   #3
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Default Re: LM3875 dual mono kit - a few questions.

Quote:
Originally posted by quickshift
I have bought 2 dual mono 3875 kits from madaboutsound.com to make a 4ch amp.

I finished the first 2 channels yesterday but I'm getting 78mv at one set of terminals and 144mv at the other set.
Are those measurements without source connected, and if connected, are you sure that source does not generate any DC offset?

BTW, 144mV is very unusual value. Out of 300 chips I measured, I incountered such high offset only 3 times. Chips producing more than 100mV offset are usually less than 10%.
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Old 14th August 2005, 05:54 PM   #4
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Quote:
Originally posted by Peter Daniel


Are those measurements without source connected, and if connected, are you sure that source does not generate any DC offset?

BTW, 144mV is very unusual value. Out of 300 chips I measured, I incountered such high offset only 3 times. Chips producing more than 100mV offset are usually less than 10%.
The measurements are with no source connected. I have also checked the output from the PSU and it's +/- 35.1V on both boards.

Should I be worried that they values are so different?

Thanks peranders

<<QS runs off to look up what a DC-servo is... >>

I think this was with a previous revision of the board but Db Dungeon says 28mV DC offset is about right. As long as it's not going to cause a problem that's OK.
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Old 14th August 2005, 05:59 PM   #5
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You have to also consider, that depending what is connected to the input, those values of the offset may decrease, as offset depends on input shunt resistance as well.

So, if it's active preamp, with no coupling cap, the offset may drop to 20mV or so. If you use a pot at the input, the offset will vary with rotation (of the pot). I find that anything less than 70 mV (or even 100mV) is fine for the speakers, but personally prefer if the offset is less than 50mV. If the offset is excessively high, the chip should be replaced.
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Old 14th August 2005, 06:12 PM   #6
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If you short circuit the input you will see how much voltage offset you have got.If you measure the output and divide it with the gain (32 in your case) you will see if the value is below 10 mV which is technically OK but I presume you will get 3.6 mV.

If you then want to get it lower you may in sert a pretty big capacitor in series with the 680 ohms but this is not a good idea. You can't lower it further without adding any trimming network. Maybe the best solution is to change the LM3875 but then you may mess up the board in the process.
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Old 14th August 2005, 06:14 PM   #7
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It is very hard to desolder the chip from the board, I don't even try it. I always cut all the pins first, and remove them one by one, the high offset chip is no good anyway (for my applications)
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Old 14th August 2005, 06:18 PM   #8
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One way to remove the IC is to cut all pins first. Then heat up one hole at the time and then very fast blow (with your mouth) away the tin. Check very carefully afterwards for tin stains.
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Old 14th August 2005, 06:27 PM   #9
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Quote:
Originally posted by Peter Daniel
You have to also consider, that depending what is connected to the input, those values of the offset may decrease, as offset depends on input shunt resistance as well.

So, if it's active preamp, with no coupling cap, the offset may drop to 20mV or so. If you use a pot at the input, the offset will vary with rotation (of the pot). I find that anything less than 70 mV (or even 100mV) is fine for the speakers, but personally prefer if the offset is less than 50mV. If the offset is excessively high, the chip should be replaced.
Thank you.

I have measured it with the preamp connected, it's active but I have no idea if it has a coupling cap. Sorry. So much to learn...

I haven't used a pot in the amp.

It's now reading 55mV and 19mV which is more sensible. I'll call the shop tomorrow and have a chat with them about it.

I still can't believe how good £35 worth of parts sounds, I can't wait until it's run in and all 4 channels are working.

These go pretty close to the top of my "things I wish I had found out about some time ago" list.
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Old 14th August 2005, 06:38 PM   #10
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Quote:
Originally posted by peranders
If you short circuit the input you will see how much voltage offset you have got.If you measure the output and divide it with the gain (32 in your case) you will see if the value is below 10 mV which is technically OK but I presume you will get 3.6 mV.

If you then want to get it lower you may in sert a pretty big capacitor in series with the 680 ohms but this is not a good idea. You can't lower it further without adding any trimming network. Maybe the best solution is to change the LM3875 but then you may mess up the board in the process.
Thanks.

I have measured it with the inputs shorted, the high side reads 57.6mV and the low side reads 13.1mV. If it's down to the chip I'll get a replacement, would it also be worth getting premium components with tighter tolerances?

I can't see any other way to remove the chip apart from cutting the pins, even removing an LED from a board by the "see saw" method is a pain.
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