BrianGT 3875 kit troubleshooting

[ejn327]

2 amps in parallel share a rectifier and transformer.
How do I measure DC offset? I see it mentioned frequently on the forum here, but don't know how to measure it.

If I can, i will measure the amps and match the closest ones together.

[Peter Daniel]

You measure DC offset between Output and Ground (meter set to DC volts). You can do it either on the PCB or binding posts (to the speaker). Check this offset at your usual listening volume. If pot is istalled directly at amps input, use max pot setting.

[ejn327]

I can mail you out some of the new resistors (the ones from thecurrent kits), and you can try matching them more closely.

Are these the 1K ohm input resistors? I have access to some 1% that I should be able to get a hold of.

[BrianGT]

Originally posted by ejn327


Are these the 1K ohm input resistors? I have access to some 1% that I should be able to get a hold of.

I can mail you a full resistor set for all 4 channels if you want.
per channel:
4 - 22k
2 - 680ohm
2 - 1k

Drop me a mail and we can discuss this.

--
Brian

[ejn327]

Thanks for the offer Brian. I got have some 1% 1K resistors today that I am going to try tonight. I also want to measure the DC offset of each amps and match them together as best as I can. I'll let you know how it turns out.

[ejn327]

I measured the DC offset of all the chips, and one channel has 170 mV and 90 mV while the other channel has 35 mV and 45 mV.
I haven't had a chance to hook up the 1% input resistors yet, but could that be causing the wide range in offset?

[Peter Daniel]

While you were meausering the offset, how the inputs of the individual channels were connected to?

[ejn327]

I had the amps hooked up to a portable CD player. The signal to the amp is running through regular wire for now and soldered to some female RCA connectors.
I'm sure that shielded coax would be best for the signal, and I will change it before I hook it up for good.

When I played a signal to the amps, I made sure to keep the output level the same when I checked the offset. I checked them one channel at a time at first and the hooked both up. I got pretty much the same measurements both times.

I also unhooked the output resistors while checking everything

[Peter Daniel]

The reason I asked, is because when measuring DC offset, one has to make sure that no DC is also present at the input, otherwise the DC readings at the output might not be right. If your CD player has a DC blocking cap, everything is fine, but just to make sure, can you also measure what value of DC you are getting at amp's input (after RCA jack).

Since you did measure all channels from the same source, there is still substantial difference between the readings, and in case there is no DC passed from your source, 170mV is definitely out of ordinary and you have to replace the chip.

I measured at least 200 chips, and only once I encountered 170mV at the output (with 22k input impedance). The DC values of 120mV you can encounter occasionally (and more often with recent batch of chips), but on average it rarely gets past 70mV (with 22k input impedance; with lower impedances DC also goes down).