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Projects => Power amplifiers => Topic started by: peranders on August 12, 2005, 04:30:46 PM

Title: QRP02 DC out
Post by: peranders on August 12, 2005, 04:30:46 PM
To disconnect DC servo, just short R20. If the offset goes down to low values the problem is around the servo, if not sometihng elsa is wrong.

The servo has +- 12 volts?

Pin 5 and 6, IC1  are at zero volts?

When you have +32 volts out, do you have -10 volts out from the DC servo?

Give me some more measurements and I'll may be able to figure out the problem. Have you built one or two working units? If two, are the alike, both putting out 32 VDC?
Title: QRP02 DC out
Post by: AudioSpirit on August 12, 2005, 05:00:42 PM
I short r20 ,didn't help.Both boards have a same problem.

I have 0 v on pins 5 and 6 ,but on servo out i have just 0,9 v instead
of -10v. Also when I just touch with hand pin10, voltage going down from +32
to +28 volt.
Title: QRP02 DC out
Post by: AudioSpirit on August 12, 2005, 05:01:04 PM
I short r20 ,didn't help.Both boards have a same problem.

I have 0 v on pins 5 and 6 ,but on servo out i have just 0,9 v instead
of -10v. Also when I just touch with hand pin10, voltage going down from +32
to +28 volt.
Title: +32v
Post by: AudioSpirit on August 12, 2005, 05:05:39 PM
I short r20 ,didn't help.Both boards have a same problem.

I have 0 v on pins 5 and 6 ,but on servo out i have just 0,9 v instead
of -10v. Also when I just touch with hand pin10, voltage going down from +32
to +28 volt.
Title: QRP02 DC out
Post by: peranders on August 12, 2005, 05:09:28 PM
Do you really have zero ohms between pin 10 of LM3886 and ground?

Since the servo is out of feedback you must have plus or minus 0.6 volts at pin 6. Did you have +32 volts out?
Title: QRP02 DC out
Post by: AudioSpirit on August 12, 2005, 05:11:03 PM
Yes,definitely 0.Just short draht between pin10 and earth
Title: QRP02 DC out
Post by: peranders on August 12, 2005, 05:14:49 PM
If you can touch the LM and things happen, check very carefully for floating pins.
Title: QRP02 DC out
Post by: AudioSpirit on August 12, 2005, 05:16:00 PM
All pins is soldered well.
Title: QRP02 DC out
Post by: peranders on August 12, 2005, 05:17:44 PM
How about pin 1 IC1? Zero volts?

Can you get proper signal out when you have removed the offset? If, yes, something is floating.

Back later after 19.00
Title: QRP02 DC out
Post by: AudioSpirit on August 12, 2005, 05:22:29 PM
On pin1 is +33v .What mean remove offset?
Title: QRP02 DC out
Post by: peranders on August 12, 2005, 05:23:24 PM
Note pin 1 IC1 not IC2. When the output voltage goes away (when you touch it), does the amp work?
Title: QRP02 DC out
Post by: AudioSpirit on August 12, 2005, 06:43:39 PM
pin1 ic1 has +10 volt on it
Title: QRP02 DC out
Post by: peranders on August 12, 2005, 08:14:45 PM
Check very carefully all parts around the input buffer, IC1. Check supply voltage. Pin 3 should have DC-path down to ground and pin 2 should have connection via R5 and down to ground via R4.

Both pin 2 and 3 should be at zero volts, and so the output pin 1.
Title: QRP02 DC out
Post by: AudioSpirit on August 12, 2005, 08:16:44 PM
I was able to bring up one of chanells.Sounds ok,now playing.Will compare with not working chanel,it may will help me.If not I will let oyu know.Thank you.
Title: QRP02 DC out
Post by: peranders on August 12, 2005, 08:51:35 PM
What was the trouble?
Title: QRP02 DC out
Post by: AudioSpirit on August 12, 2005, 09:04:37 PM
I installed r19 and r20 not correct.Moved them to other direction.First chanell is fixed,but second still doesn't work.I work on it right now,hope I've not burned lm3886:)
Title: QRP02 DC out
Post by: AudioSpirit on August 12, 2005, 09:42:33 PM
Ok,I fixed all.On second board was bad Ad8620 .I replaced ic and now all is ok.
Title: QRP02 DC out
Post by: AudioSpirit on August 12, 2005, 10:01:22 PM
Actually I like AC level ,it is 0,000V now,under my Fluke 112 measurement level.Unbelivable!
Actually all will be bilded in dual monoblock design inside of Technics SA900 body and hope will looks extremely attractive.Will post more details later.
Title: QRP02 DC out
Post by: peranders on August 13, 2005, 08:51:08 AM
You will probably experience 0.02-0.08 mV(!) offset. Good to hear that you managed to get it working... I suppose you didn't build as a wrote in the instructions. It's smart to build and test piece by piece just because SMD is a bit harder to troubleshoot.

So fix first the PS and +- 12 volt, then the IC1 and last the LM3886.
Title: QRP02 DC out
Post by: AudioSpirit on August 13, 2005, 10:44:09 AM
Ooops.I awake today,bringed amp up und... BUMMM from speakers.I fast disconnected speakers. ad8620 died. I replaced to other now it is again working.But here one dangerous thing which I like to fix.
Here is no defence against AC from IC 1.Can I replace r6 1 kOhm with capacitor? Even tought about I can burn my loudspeakers make me very nervous...
Title: QRP02 DC out
Post by: peranders on August 13, 2005, 12:58:45 PM
Are you complete sure that the +-12 V is working properly? When everything work perfectly you'll have a silent and smooth startup and also power down.

I have never distroyed and opamp in such position and nor has the other builders.

I'll guess something isn't solder properly or simply forgotten or some parts are located wrong.The AD8620 has no startup transient what so ever and below 12 volts the LM3886 is shut down so both the servo and the input buffer are working proberly before the LM3886 is.

I suggest that you measure the startup transient at pin1 IC1.

Since you have 1 kohms as R6 a coupling capacitor must be pretty big. Sure you can have it but it's a bit a waste in the same time.
Title: QRV-02
Post by: AudioSpirit on September 12, 2005, 10:15:05 PM
Ok amp is up and running for over 2 weeks now,and I have only positive emotions. Very nice and clean sound,excellent dynamic. I never expected such quality from LM3886! Highly Recommended. 10/10
Title: QRP02 DC out
Post by: peranders on September 13, 2005, 06:48:42 AM
Good to hear that everything are working alright.