Problems with LM833 Preamp

[soundNERD]

Hi everybody,

I haven't been around here in a while, I've been so busy lately, but I'm having some troubles with an LM833N preamp I have built.

The GC is two LM3875TF's running on a +/-35VDC supply (transformer from dead Logitech Z-680 speakers), and it's got a 3-pin neutrik XLR connector with +,gnd,- outputs to connect to the preamp.

Inside the preamp I have LM7815 and LM7915 chips to regulate the power down to +/-15VDC The inputs of both the preamp and amp have 500-ohm resistors on them in case a RCA wire comes loose or something (I know this is a bad practice, but at the time I built this, I didn't know that the cap from the negative input to ground prevented the large DC offset when no input signal was connected). The schematic I used in the preamp is identical to the one for the LM3875 (except I may have lowered the gain slightly, can't remember now). I also added a Bourns dual 10K potentiometer for volume control.

The problem I have with it is constant humming, and about half volume it turns to a buzz. This happens with no source connected or with a source connected but no sound playing. If the sound is playing, you can hear the humm if the volume is set low enough, and I could never turn the control halfway up to listen for a buzz - it would blow the fuses in my speakers.

Another problem is noticable distortion.

Any clues whats going on and if theres any way short of redesigning with a new schematic to get it working right?

I know you guys like pics so here they are (by the way, my camera loves to point out all the imperfections. Every spot it shows (uneven paint, missing paint, etc.) doesn't even show up in real life):

Thanks!

[soundNERD]

Picture two - of the inside

[soundNERD]

Last Picture - closeup of the PCB

[Leolabs]

try to post the schematic

[soundNERD]

Sorry I don't have one, but I can make one no problem.

As I said its just the plain LM3875 schematic on page one of the datasheet.

But I'll make one up anyway.

[wintermute]

High SoundNerd,

I might be missing something here, but shouldn't you be using a design for the specific chip used for the pre, rather than for the LM3875????? very different applications I would have thought.... you may need to put in compensation caps to stop the pre's op-amp from oscillating, what chip are you using for the pre??

edit: also if you are bringing + - and ground from the amp, and have signal ground as well, you may have created a ground loop. I'd think about a separate supply for the pre

edit 2: just noticed the chip is in the subject

Tony.

[Jocko Homo]

Why on earth did you design a preamp with LM833s. Do you have any idea how many of us have ripped those suckers out of Magnavox CD players?

But back to the topic.....

These 500 ohm resistors...........are these from "signal in" to ground? Is that right? Or are they build-out resistors?

How can the preamp schematic be identical to the LM-whatever amp chip?

Jocko

[wintermute]

OK after looking at the LM833 datasheet, it looks like no compensation circuitry is needed, so I guess if you have bypassed correctly and set the gain right then in this case following the layout on the first page of the LM3875 datasheet probably should be ok........

Gain required will depend on the lowest level input source you have, and the voltage required on your LM3875 to deliver full output power... these days most components have high level outputs like CD, DVD, Video etc.... in this case you can probably get away with unity gain (effectively a buffer), if you have lower level inputs then work out the max gain you need to get the required input voltage on the LM3875....

So I think possible problems are too much gain, and earth loops.

Tony.