My LM1875, part 2


2006-04-27 11:50 am
Hi all,

This is mostly a progress report (and a question at the end...)

In this earlier thread I asked for som advice on a my layout for an LM1875 amp, and got plenty of very good feedback (thanks gootee and others!).

well, i incorporated most of those ideas and have finally managed to build it.

At the moment I have 1 channel built and tested, and the other one is due for testing tonight.

After some initial problems with massive hum and major dissapointment I eventually realized that my RCA input wasn't connected to ground at all. After fixing that it worked like a charm.

approximately 2mV of dc offset (i think my DMM isn't that accurate at these levels) and absolutely no noise or hum at all.
My PSU gives me 26.7VDC without load (from an 18-0-18 toroid) and that's as close to the maximum ratings of the LM1875 that i wanna go...

It sounds lovely, even though i have really only listened to it in mono yet. Initially i had planned to use this as an amp for my PC, but now i'm having second thoughts... first impressions tells me that it will really give my NAD314 a run for its money.

Basically, this is just a "thank you" post to those involved with providing me feedback earlier on, it really helped a lot.

If someone is interested i could make the eagle files available i guess. Here's the schematics and the layout:



So, the questions (if anyone is still reading):

Currently I have a 100k linear pot in front of this. Is that value too high or too low? I realise that it will affect both my input impedance, and probably also the frequency response, but I don't really know in what way.

Secondly, I might try this with an active preamp some time in the future. Should I add a 100k resistor to ground in front of C1 or is the input impedance ok as it is?