Bil Christie's LM12 140W amp hum problem

[Tottman2]

Hi all,

I've built Bill Christie's LM12 140 watt amp as described in Audio Express. I just got it up and running and I think it sounds real good, but there's a hum/buzz that is too loud for my taste. I usually build tube projects. If there's hum, I have a better idea of what to do. Like add a choke, another filter stage, rectify the filaments.

I've built my own chassis that is somewhat larger than Bill's to try to avoid the the problem I'm now having.

Any recommendations for making this amp more quiet?

Thanks in advance for your help,
Tom

[BWRX]

Hi Tom. I'm assuming that you are referring to the hybrid LM4702/tube output stage amp in the January 2008 issue. If the amp sounds fine but has hum, it's probably either a wiring issue or a grounding issue. Do you have any photos of your amp layout?

[Tottman2]

Hi Brian... Thank for your comments.
This project is from the August 2007 issue of AudioXpress. It's titled Cascoding National's LM12 for 140W by Bill Christie.

I left my camera at my son's house over Christmas, so I don't have a picture. But the layout is just like the picture in the article, except wider and taller. I wanted to insure low or no hum problems with the big toroid.

I learn much more from problems than successes. I'm now reading National's technical notes for the LM12. Bill's circuit was taken from this document... he just added a OPA134 as an input stage to the cascode circuit in this document. I don't completely understand all of what's said, but it seems clear that I should keep the LM12 wiring as close to the package as possible.

If anyone is interested, I'll post my progress.

And I'm still interested in anyone's advice on OPamp operation.

Thanks again for your interest,
Tom

[longthrow]

hi,

are you referring to this chip?

http://pdf1.alldatasheet.com/datashe...C/LM12CLK.html


im interested with your project...
can you post the schematic?

[peranders]

Why don't you write the original link?

http://www.national.com/mpf/LM/LM12CL.html

[hydra]

I've also been considering building this amp. AudioXpress has pdfs of the chassis wiring layout, parts layout, and amp and power pcb.

For example:
http://www.audioxpress.com/magsdirx/...g_on_frame.pdf

Tom, any progress on reducing hum? Also were you able to locate the Aavid heat sink (79815) used in this project? I've tried all the major parts sellers (digikey, allied, mouser, etc) without success. Aavid rep has also not responded.

[gainphile]

If it's a grounding issue you're may be in luck. I just posted similiar issue:

Gainclone HUM, THUMP problems - The solution

[gootee]

Originally posted by Tottman2
Hi all,

I've built Bill Christie's LM12 140 watt amp as described in Audio Express. I just got it up and running and I think it sounds real good, but there's a hum/buzz that is too loud for my taste. I usually build tube projects. If there's hum, I have a better idea of what to do. Like add a choke, another filter stage, rectify the filaments.

I've built my own chassis that is somewhat larger than Bill's to try to avoid the the problem I'm now having.

Any recommendations for making this amp more quiet?

Thanks in advance for your help,
Tom

Hi Tom,

I downloaded the three PDF files that I found, for that amp, from the AudioXpress website. I noticed that on the "Chassis Wiring Diagram", it says "See Heatsink Wiring Diagram". But I did not see a heatsink wiring diagram for download, and can not find it in any of the PDFs that I downloaded. Also, is there a copy of the article, or construction notes etc, availanle for download, anywhere?

At any rate, assuming that the original design does not hum like yours, what, if anything, have you done that is already known to be at-all different from what Christie shows and describes, besides just changing the size of the enclosure?

Did you use his PCBs, for example?

[Speedskater]

The August 2007 (#8/07) article in AudioXpress magazine has no wiring diagrams.
Just 2 schematics, 2 mechanical drawings, 6 photos and parts lists.

[gootee]

Originally posted by Speedskater
The August 2007 (#8/07) article in AudioXpress magazine has no wiring diagrams.
Just 2 schematics, 2 mechanical drawings, 6 photos and parts lists.

Thanks, Speedskater.

The three PDFs that I was able to find and download include one mechanical drawing, entitled "Chassis Wiring Diagram", two PCB patterns (amp board and power/gnd distribution board), one PCB component-layout diagram (for the amp board, which also includes active-device part-drawings and type-designations plus all passive component values), zero photos, and no parts list or descriptive text.

The "Chassis Wiring Diagram" mechanical drawing does include details of wire connections to the power/gnd distribution board, and to chassis-mounted hardware (with types and values of chassis-mounted components). But it does not quite show the details of the connections to what I assume are vertically-mounted amp boards, on the sides of the case, although they are probably obvious-enough to easily figure out, since the board drawings do have labels for all of the connection points. The "Chassis Wiring Diagram" drawing also includes, along each side, the notes, "See Heat Sink Wiring Diagram", which are in 'text-boxes' that obscure the areas where the wire-connection details would have been shown.