DC offset issues with GC - diyAudio
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Old 11th November 2009, 07:32 PM   #1
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Default DC offset issues with GC

i built a GC per Peter Daniels schematic with 1K input resistor. the schematic is on the first page: Commercial Gainclone kit- building instructions

with the inputs shorted, i get DC offset of (L)6.9mV and (R)0.3mV. with both inputs floating, i get around (L)22mV and (R)26.

my preamp has a DC offset of (L)0.5mV and (R)0mV.

when i connect the pre to the GC (no source connected), DC offset jumps to more than 500mV.

i get very little DC offset with a Benchmark DAC1 or an iPod connected to the GC.

any ideas?
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Old 11th November 2009, 07:47 PM   #2
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embarrassing as this is, i should also add that last week i fried a brand new pair off Fostex FF85K (10W max) when powered with the GC and iPod in a 24'x24' garage, after a "few" beers with a friend, and Weezer cranked way too loud. all my DC measurements were taken today.
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Old 11th November 2009, 08:19 PM   #3
piero7 is offline piero7  Italy
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watch here

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use ALWAYS a protection against CC
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Old 11th November 2009, 08:22 PM   #4
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First of all you should turn to Peter Daniel since he have sold you this product, still something isn't right. Is the preamp also homebrewed? What is the DC voltage from the preamp when it's connected to the Gainclone?

I think Peter should add some proper start-up procedure in his documention. How about inserting a small fuse? 100 mA when you are testing and/or a 2200 uF cap.

Power ground and signal ground together?
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Last edited by peranders; 11th November 2009 at 08:25 PM.
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Old 11th November 2009, 08:45 PM   #5
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this is not a PD product. i only used his schematic and don't wish my DIY efforts to reflect poorly of him. here's some pics from another thread detailing some instability issues i had, which were sorted: GC noise and heat issues

DC offset at the preamp is less than 1mV in both (L)(R) when connected to the GC.

both signal and power are tied to ground.

the preamp is detailed in the above linked thread. it's a actually a headphone amp serving both preamp and HP amp duties, and has an on-board DAC regulated at 5V by an LM317. all of my DC offset measurements were with no transport connected to the DAC. the headphone amp works fine in isolation.
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Old 13th November 2009, 06:42 AM   #6
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Quote:
I fried a brand new pair off Fostex FF85K (10W max)
Your amp is probably capable of doing more than 10W!
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Old 13th November 2009, 08:41 AM   #7
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Not many DC watts are required to destroy a speaker....
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Old 13th November 2009, 09:08 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by fishski13 View Post
i built a GC per Peter Daniels schematic with 1K input resistor. the schematic is on the first page:
what, exactly, have you fitted in the PD schematic?
Have you added any components?
Have you missed out any components?
Have you built the post3 schematic?
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Old 13th November 2009, 02:41 PM   #9
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R1= 1K
R2= 22K
R3= 1.8K (gain=13 dB, where 1+Rfb/R3=gain)
Rfb= 22K
Cs at V+ and V-= 1500uF

details of pictures:
-the twisted/stripped wire bridging the 2 terminal strips at the 2 outside end tabs chip is ground.
-white wires (from PS) carry PG+ and PG- and connect to bridged wire/ground at the ends terminal strip, at opposite ends.
-red wires connect the terminal strips ground to star ground near the IEC/chassis
-black wires (from PS) carry V+ and V- and connects with each Cs at the 2 center tabs of the terminal strips and to pins 1 and 4 of the chip. the other Cs lead connects to ground at the terminal strip end.
-R2, R3, IG and OG (white wire directly to (-) speaker binding post) are connected directly to the bridged wire/ground, in the middle, while the PG connections are at the ends of the terminal strips.
-R1 is directly in series with red input wire and covered with heatshrink.

i tried removing R1 and placing a 4.7uF non-polar cap in-series with input and measured a higher DC offset at 1.4V. right now, R1 is back in-series, with the coupling cap removed.

thanks!

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Old 13th November 2009, 02:58 PM   #10
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I suspect your pre-amp is home made too and does not include a DC blocking capacitor on its output. Then when you connect your amp, the DC offsets are all multiplied by the amp which also has no DC blocking or attention to DC conditions whatsoever, and produces an unacceptable output offset.
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