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Old 18th September 2013, 09:38 AM   #1671
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The offset sounds OK although 86mv could be considered high. I have tried lots of pots and to be honest find it difficult to hear any differences. Others will disagree. The only faulty component I have found with irregular offset is the IC itself. You could check that the resistors around the input are grounded properly (soldered).
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Old 18th September 2013, 08:23 PM   #1672
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Thiose offsets are perfectly fine and they mostly depend on the chips: Commercial Gainclone kit- building instructions
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Old 19th September 2013, 01:39 AM   #1673
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There you go JB, the master has answered. Now enjoy your music
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Old 2nd October 2013, 10:50 AM   #1674
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A little help needed please. Completed my 2nd LM4780 amp board, but it runs hotter than the first which only gets warm after 30 mins or so of use. Both amps are in individual heatsink cases (8"x2") boxes. With input loaded with 2k resistor or fed by Mezmerize, the supply quiescent current is about 165mA, rail voltages +/- 26 volts, output load nom 8ohm speaker. Sound is not distorted. Tried adding 10Uf and 0.1uF caps across each rail in addition to normal 1500uF caps,also 100pF between pins 15/16 & 21/22, but no change. Any suggestions or is it just a chip that wants to run at high quiescent current?

Last edited by BackinBlack; 2nd October 2013 at 10:53 AM.
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Old 2nd October 2013, 01:07 PM   #1675
udailey is offline udailey  United States
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DC Offset high?
Havent looked at pin out so maybe 100pf is across feedback resistor. Maybe try a .1uf just to test and see for sure if its oscillations or not.
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Old 2nd October 2013, 02:03 PM   #1676
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Quote:
Originally Posted by udailey View Post
DC Offset high?
Havent looked at pin out so maybe 100pf is across feedback resistor. Maybe try a .1uf just to test and see for sure if its oscillations or not.
Forgot to mention the offset, 4-11mA depending on input. Pins 15,16,21,22 are signal inputs, I'll give the 0.1uF caps a try.
BTW my Lighter Note is working a treat, amazing transparency!

Best
Ian
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Old 2nd October 2013, 03:44 PM   #1677
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Tried 0.1uF, still taking 165mA quiescent!!
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Old 2nd October 2013, 03:59 PM   #1678
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Offset is measured in Volts at the output of the amp. It will hopefully measure in low millivolts mV.
For oscillations the capacitor would go from output to inverting input allowing all high frequency oscillations to make their way back to the inverting input so that they may be removed. If its oscillating then the high frequency oscillation will see the capacitor as a short to the inverting input. The inverting input will flip that oscillation upside down or in other words make a negative copy of it so that when the output is added to the negative copy you have zero. If oscillation is your problem then this will do the trick or at least point to the problem.
DC offset - just put your DMM on Volts and measure the output on the speaker terminals on your amp. I do this with a speaker attached and music playing but you might have a lot of offset so detach your speaker and check and if its low then attach and check again.
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Old 2nd October 2013, 04:02 PM   #1679
udailey is offline udailey  United States
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Here is a thread on testing for DC offset. One of their explanations may pertain more to your amplifier than to mine
test for dc offset?
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Old 2nd October 2013, 05:16 PM   #1680
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Quote:
Originally Posted by udailey View Post
Here is a thread on testing for DC offset. One of their explanations may pertain more to your amplifier than to mine
test for dc offset?
The offset I quote is at the output, it varies depending on whether the input is shorted, open circuit or "loaded" with 2kohms. Tried 0.1uF caps across the inputs (no change) and across feedback resistors (motorboating). The amp is configured in parallel mode. Disconnecting the output paralleling resistors lowered the quiescent to 100mA, which is near the quoted spec of 110mA. Measuring the voltage at the inverting inputs revealed one input at -0.1V and the other at -1.2mV, so there is an imbalance in the feedback resistors. Probably means that the amps are fighting each other, new 0.1% resistors on order!
Many Thanks for your help.

Last edited by BackinBlack; 2nd October 2013 at 05:20 PM.
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