I just received one of these kits - unfortunately it came already assembled with no documentation 🙁
I am able to trim the 2k pot (schematic shows it should be 1k) for one channel and get a 2.5v drop across the 1k5, and able to get 0v dc offset on the output by trimming the 500R pot. All good. However, the other channel, I get a constant 1.27v dc offset on the output. The lowest drop I am able to get across the 1k5 resistor by trimming the 2k pot is 2.97v - but can go higher. The 500R pot does not seem to vary the dc offset at all - no change. Very odd. The dc voltage from the supply is +/- 13v. I have not modified any components from original kit. The two channels look identical from component placement. Any ideas what might be going wrong? Thanks!
I am able to trim the 2k pot (schematic shows it should be 1k) for one channel and get a 2.5v drop across the 1k5, and able to get 0v dc offset on the output by trimming the 500R pot. All good. However, the other channel, I get a constant 1.27v dc offset on the output. The lowest drop I am able to get across the 1k5 resistor by trimming the 2k pot is 2.97v - but can go higher. The 500R pot does not seem to vary the dc offset at all - no change. Very odd. The dc voltage from the supply is +/- 13v. I have not modified any components from original kit. The two channels look identical from component placement. Any ideas what might be going wrong? Thanks!
My guess would be the Idss of the JFET's isn't very well matched, but I could be wrong.
The kit I received from Jim's Audio, which doesn't use K170/J74 devices, worked fine after I assembled it as far as biasing and offset are concerned.
I tried building the same kit, with supposedly matched K170/J74 devices from another seller, and never could get the offset below a certain unacceptable level on one channel despite using 100 ohm trimmers like Mr. Curl suggested.
Maybe I should have checked the JFET's Idss myself before soldering them in place.
I finally got tired of playing around with it and threw it in my junk box. A half-working preamp isn't going to do me much good.
The kit I received from Jim's Audio, which doesn't use K170/J74 devices, worked fine after I assembled it as far as biasing and offset are concerned.
I tried building the same kit, with supposedly matched K170/J74 devices from another seller, and never could get the offset below a certain unacceptable level on one channel despite using 100 ohm trimmers like Mr. Curl suggested.
Maybe I should have checked the JFET's Idss myself before soldering them in place.
I finally got tired of playing around with it and threw it in my junk box. A half-working preamp isn't going to do me much good.
Thanks ammel68, quite possible the JFETs are not matched, but I would not know.. it came pre-assembled.. and who know if there was any quality control! I would like to get it working to eventually merge this, or some other design, with arduino land. Perhaps I should purchase new matched JFETs but would like to know any potential other issues that might be introducing the dc offset.
Same board, pre-assembled but not via eBay.. Aliexpress vendor. But I think all the vendors from there get it from the same place. But its essentially the same board. Not my preferred design, as would prefer to see the PSU not on the same board - isolated elsewhere in the chassis.
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