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The BA-3 as preamp build guide
The BA-3 as preamp build guide
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Old 27th July 2014, 09:49 PM   #31
delecoy is offline delecoy  United States
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Zeroed offset with both meters connected. When I removed the meter from R10 offset jumped to 4 volts?
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Old 27th July 2014, 09:50 PM   #32
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The BA-3 as preamp build guide
cool of

proceed tomorrow , post schematic with voltages at important nodes

be sure that DMM isn't with probes in current measuring position
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Old 27th July 2014, 10:08 PM   #33
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Old 1st August 2014, 03:38 AM   #34
delecoy is offline delecoy  United States
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Alrighty. After a few days off, and new batteries for the meters here's what I have:

Right channel:
Q1: D to Ground = 21.6V, S to Ground = 68.1 mV
Q2: D to Ground = -21.2V, S to Ground = -60.1mV
R10: Current = 55 mA, voltage = .50
Offset at the output is bouncing around between 0 and 10 mV. Never stops moving.

Left channel:
Q1: D to Ground = 21.7V, S to Ground = 68.6 mV
Q2: D to Ground = -21.7V, S to Ground = -57.6mV
R10: Current = 54.8 mA, voltage = .50
Offset at the output is bouncing around between 0 and 10 mV. Never stops moving.
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Old 1st August 2014, 03:43 AM   #35
delecoy is offline delecoy  United States
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And some pictures:
Attached Images
File Type: jpg photo 1.JPG (152.9 KB, 1849 views)
File Type: jpg photo 2.JPG (115.6 KB, 1785 views)
File Type: jpg photo 3.JPG (147.0 KB, 1763 views)
File Type: jpg photo 4.JPG (134.0 KB, 1757 views)
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Old 1st August 2014, 03:47 AM   #36
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Quote:
Offset at the output is bouncing around between 0 and 10 mV. Never stops moving
I had a similar problem, then found the center ground or 0v at the power supply was incorrectly connected. I inadvertantly connected to the stand off holes.
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Old 1st August 2014, 04:09 AM   #37
delecoy is offline delecoy  United States
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That's interesting. Those pictures show where I took the ground off the board with the green wire to the thermistor. I'll try adding another connection to the thermistor and see if that settles the offset.
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Old 1st August 2014, 04:40 AM   #38
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That seems to have narrowed the range and slowed it down, but its still moving. Thank you . Is it safe to hook up and test?
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Old 1st August 2014, 04:41 AM   #39
delecoy is offline delecoy  United States
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Alos, offset is very high when it first starts up ( over a volt ). It takes about 30 seconds to settle towards zero. Should this be a concern?
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Old 1st August 2014, 05:28 AM   #40
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The BA-3 as preamp build guide
When you say "offset at the output" you mean before the cap or after? 10mV before the cap is fine.

Everything else looks even, so what does it sound like?
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