Issue with Trim control on 1073 style preamp

Potato Cakes · May 31, 2022

Hello, everyone,

I've built a dozen or so 1073 style preamps with various boards, all of them using the 5k trim control shown in the attached photo. It's just a shunt to ground between the 2nd and 3rd gain stages. This is has been successful for every build until this most recent one. During the last quarter turn of the trim pot, it goes from being an attenuator to a low shelf EQ around 1kHz. I would figure that the shorting the audio to ground (which I verified is happening) would cause all the audio to drop. The preamp functions properly other wise. Curious if anyone here as run into a similar issue on this or another circuit.

Thanks!

Paul

JohnRoberts · May 31, 2022

maybe check your grounds so there isn't an unintentional NF path...

JR

Potato Cakes · May 31, 2022

I'll go through and double check my ground connections. To see if I could find a way to kill the signal, I shorted the primaries of the output transformer together and got the same results. Currently the best I can come up with is putting the pot across the output +/- but at a certain point there is HF roll off and a 560R resistor has to be put in line to keep the lowest level of attenuation right at the point of HF loss above 20kHz. This gives about 12dB of range which is perfectly acceptable for what a trim control is supposed to do. But I will need to get to the bottom of this for future projects with these particular amp circuits. If I find an error (I find them all the time!) I'll report back.

Thanks!

Paul

Potato Cakes · Jun 3, 2022

Tried swapping out different boards with the same results. There might just be an issue with these particular PCBs. With the ez1290 and The Don Classics boards this function works as expected.

I am running these particular PCBs with a bipolar power supply where the ground reference is replaced by a -VDC (forward biased per Winston O' Boogie's recommendation). I then changed the power configuration to the traditional +24V/0V and still got the same results. I will double check my ez1290s to make sure I'm not more crazy than usual but I pretty certain at this point the issue has to do with the board layout with which I'm currently working.

Thanks!

Paul

radardoug · Jun 3, 2022

Are you sure you haven't just wired the pot in backwards?

Potato Cakes · Jun 4, 2022

It seems that it wasn't just backwards, but all three connections were incorrect. I had been working on this very late the last couple of nights and my eyes just could see the obvious problem. I had to draw out the schematic by hand before I found my errors. It's embarrassing when something you've built numerous time has a silly error which proves to be hard to find and then look silly by coming here with something that I should know. Thanks again for everyone's response. Maybe I just needed to come here before my brain would start working.

Thanks!

Paul