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
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 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
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
hi,
are you referring to this chip?
http://pdf1.alldatasheet.com/datasheet-pdf/view/8630/NSC/LM12CLK.html
im interested with your project...
can you post the schematic?
are you referring to this chip?
http://pdf1.alldatasheet.com/datasheet-pdf/view/8630/NSC/LM12CLK.html
im interested with your project...
can you post the schematic?
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/ax/addenda/media/chassis_wiring_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.
For example:
http://www.audioxpress.com/magsdirx/ax/addenda/media/chassis_wiring_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.
If it's a grounding issue you're may be in luck. I just posted similiar issue:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=118735
http://www.diyaudio.com/forums/showthread.php?s=&threadid=118735
Tottman2 said:
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 said:
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.
Tottman2 said:
Hi Tom (if you're still around),
It looks like Christie did a very good job with the star-grounding scheme. So at least that might not be the problem.
You probably already know all of this, but, "just in case":
If your layout is "exactly" like it is shown in the "Chassis Layout Diagram" in the PDF file from Audio Express's website, then that might be the problem.
Maybe someone who has a copy of the actual article can look into this, but, it appears that the layout diagram was drawn more with clarity in mind, rather than as a guide to how the wiring should actually be done.
For example, it seems like you would definitely want to twist-tightly-together each wire pair, and would want to route all audio wires as far away as possible from the toroid and the other AC stuff. The center axis of the toroid, in particular, is usually where any radiated fields would be the strongest. So you might not want to have any wiring passing over the toroid, unlike what is shown in the diagram.
You will also want to make very sure that all input and output jacks are completely isolated from the chassis. For example, typically any RCA jack's barrels must not be connected to the chassis. (However, I can't quite tell from the wiring diagram what that is, between each set of banana jacks, and whether or not it's connected to the chassis. And I also don't see any connection between chassis and earth ground, unless those are it, since earth ground apparently connects to the star ground. I see no disconnect network there. But there ARE 10 Ohm resistors between the star ground and whatever is between each set of banana jacks.)
You should probably twist the input and output pairs together separately. But keep them close to each other, to minimize 'enclosed loop area'.
Please let us know how it goes.
The August article does not have any wiring diagrams. It does have some small photos of the inside of the amplifier (but not very helpful). In the November issue letters to the editor, Mr. Christie responds to a letter, the he adapted his design from National Semiconductor's application note AN446.
From Christie's Chassis Wiring Diagram: The yellow object between the banana jacks is the the input RCA jack. It appears that the input is routed with shielded cable. The pdf displaying the parts superimposed on the pcb shows a ground hole adjacent to the input pad and I'm also assuming that the shield is connected at the RCA jack end (and then again maybe not?). Anybody know the purpose of the connection that runs from the input RCA jack ground (10 ohm resister to the pcb star ground) and why this doesn't cause a ground loop. And what is the purpose of the 10 ohm resister? (Or is the input wire shield only connected at the pcb side to avoid a ground loop?)
Good observation that the pdf Chassis Wiring Diagram doesn't show a connection to chassis ground. In fact it appears that every pad on the power pcb is used. I'm imagining that the input earth connection is run from the power entry module (Corcom) to chassis ground and then up to the pcb star ground? Does this sound reasonable or would there be a better way to implement this?
Also I'm still looking for a source for the Aavid 79815 heat sink.
http://www.aavidthermalloy.com/cgi-...th=3&airflow=57.2&CType=Natural&AirUnits=LFM#
Any suggestions for buying Aavid heat sinks or a comparable heat sink are appreciated. The heat sink will have 3 TO3 packages mounted to it.
-------------
Don
LM12 project pictures
I apoligize for not responding more quickly. My wife deletes lots od unwanted mail and my notifications must have gotten deleted too.
Thanks everyone for your interest in my LM12 project.
The latest on my hum problem... when I short the inputs to the LM12 amp, there is no hum. So I seem to have a ground-loop problem. I have installed a 10ohm resistor in parallel with a .1uf cap from the circuit ground to the power input ground... the ground connection of the AC power cord. The hum is still there.
No sure what to try next.
Since this circuit has an alot of gain, I may try putting a 50k pot on the input to ground, taking the input from the wiper arm and eliminating my pre-amp. Maybe my ground loop is related to my pre-amp.
LM12 price -- yeah $55 is expensive. The LM3886 is much cheaper ($6 or so), is easier to heat sink and may even have lower distortion.
PC boards: I copied Bil's artwork and ordered my PCB's from ExpressPC.
Tom
ps - I tried to post pictures but they have too many bytes to post. I anyone wants to see them, let me know and I'll email them to you.
I apoligize for not responding more quickly. My wife deletes lots od unwanted mail and my notifications must have gotten deleted too.
Thanks everyone for your interest in my LM12 project.
The latest on my hum problem... when I short the inputs to the LM12 amp, there is no hum. So I seem to have a ground-loop problem. I have installed a 10ohm resistor in parallel with a .1uf cap from the circuit ground to the power input ground... the ground connection of the AC power cord. The hum is still there.
No sure what to try next.
Since this circuit has an alot of gain, I may try putting a 50k pot on the input to ground, taking the input from the wiper arm and eliminating my pre-amp. Maybe my ground loop is related to my pre-amp.
LM12 price -- yeah $55 is expensive. The LM3886 is much cheaper ($6 or so), is easier to heat sink and may even have lower distortion.
PC boards: I copied Bil's artwork and ordered my PCB's from ExpressPC.
Tom
ps - I tried to post pictures but they have too many bytes to post. I anyone wants to see them, let me know and I'll email them to you.
lm 12 is no tube
to my understanding you have a ground loop issue meaning that there is somewhere some mistake you have made .....
your approach trying to eliminate the noise adding things is wrong .... you have to see/find what you ve done wrong ....
possible cause is :
first no matter what the schematics say the ground of the box has nothing to do with the ground of the sound .....
look if somewhere on the way signal ground is connected to the box ( it can simply be that the rca jacks you use are not isolated from the box )
then simply lift the input cable from both amp boards to see what changes .....
if you still have noise with no signal cable connected in the amplifier boards it can easilly be the power ground ....like you have from star point ( normally very close to filter capacitors ) one cable goes to one amplifier and only that and then another cable to the other amplifier and goes on ......
in this star point should also exist one cable from center tap of the transformer and absolutelly nothing else NO CABLE FROM STAR POINT TO THE GROUND OF THE BOX !!!!!
last couple of things ..... in the rails should only exist the same topology like from plus out of psu on lead to plus of one amplifier board and another starting from psu plus lead to amplfier board 2....
exactly the same fro minus rail ....]
and thats it !!!!!
if you connect.... one resistor to plus lead in serious with a led and serious with a resistor to create a power on indicator that is enough to create a noise .....
if you have any auxiliary circuits that operate in the same box with other suplly tthen care should be taken on how you handle the local grounds
and finally very common mistake anyone can do is very possible that one of any pcb ground is connected to the box with a screw ...so its actually some kind of mechanical error that does all that ....
the topology starting from plus lead from example to the first amp and then to the second amp is not working !!!!! the same apply for all power cables and all ground cables
best regards sakis
to my understanding you have a ground loop issue meaning that there is somewhere some mistake you have made .....
your approach trying to eliminate the noise adding things is wrong .... you have to see/find what you ve done wrong ....
possible cause is :
first no matter what the schematics say the ground of the box has nothing to do with the ground of the sound .....
look if somewhere on the way signal ground is connected to the box ( it can simply be that the rca jacks you use are not isolated from the box )
then simply lift the input cable from both amp boards to see what changes .....
if you still have noise with no signal cable connected in the amplifier boards it can easilly be the power ground ....like you have from star point ( normally very close to filter capacitors ) one cable goes to one amplifier and only that and then another cable to the other amplifier and goes on ......
in this star point should also exist one cable from center tap of the transformer and absolutelly nothing else NO CABLE FROM STAR POINT TO THE GROUND OF THE BOX !!!!!
last couple of things ..... in the rails should only exist the same topology like from plus out of psu on lead to plus of one amplifier board and another starting from psu plus lead to amplfier board 2....
exactly the same fro minus rail ....]
and thats it !!!!!
if you connect.... one resistor to plus lead in serious with a led and serious with a resistor to create a power on indicator that is enough to create a noise .....
if you have any auxiliary circuits that operate in the same box with other suplly tthen care should be taken on how you handle the local grounds
and finally very common mistake anyone can do is very possible that one of any pcb ground is connected to the box with a screw ...so its actually some kind of mechanical error that does all that ....
the topology starting from plus lead from example to the first amp and then to the second amp is not working !!!!! the same apply for all power cables and all ground cables
best regards sakis
Hi Sakis,
Thanks for your responce.
"look if somewhere on the way signal ground is connected to the box ( it can simply be that the rca jacks you use are not isolated from the box ) "
My RCA jacks are mounted to fiber board. I run a seperate ground wire to the input pc board. One for each channel.
"then simply lift the input cable from both amp boards to see what changes ....."
I'll try this... but how is this different from when I grounded both input RCA's? What I did, after disconnecting the preamp from the LM12 amp, was connect the outside of the RCA jacks to the central pin... separately on each channel. When this is done there is no hum. In my experiance, if you turn-on a power amp with no input connection, there will be hum or buzz.
"in this star point should also exist one cable from center tap of the transformer and absolutelly nothing else NO CABLE FROM STAR POINT TO THE GROUND OF THE BOX !!!!! "
The only wire that is connected to my metal chassis (only the side plates are metal) goes to the ground wire on my power AC cord. I also connect the circuit ground thru a 10ohm resistor to this AC power ground. I've tried lifting this ground... there is little, if any, effect on the hum.
"last couple of things ..... in the rails should only exist the same topology like from plus out of psu on lead to plus of one amplifier board and another starting from psu plus lead to amplfier board 2...."
exactly the same fro minus rail ....]"
That's what I've got!
"if you connect.... one resistor to plus lead in serious with a led and serious with a resistor to create a power on indicator that is enough to create a noise ..... "
Good point. I'll try removing my power-on LED. Thanks.
"if you have any auxiliary circuits that operate in the same box with other suplly tthen care should be taken on how you handle the local grounds "
There are no auxilary circuits.
"and finally very common mistake anyone can do is very possible that one of any pcb ground is connected to the box with a screw ...so its actually some kind of mechanical error that does all that ...."
I'll double check this... I did use a "ground plane" on my PC board. It could connected! Good point. Thanks.
"the topology starting from plus lead from example to the first amp and then to the second amp is not working !!!!! the same apply for all power cables and all ground cables"
I don't understand what you are saying here. Sorry.
Thanks again for your help. When I've made the above checks and tests I'll report back.
Tom
Thanks for your responce.
"look if somewhere on the way signal ground is connected to the box ( it can simply be that the rca jacks you use are not isolated from the box ) "
My RCA jacks are mounted to fiber board. I run a seperate ground wire to the input pc board. One for each channel.
"then simply lift the input cable from both amp boards to see what changes ....."
I'll try this... but how is this different from when I grounded both input RCA's? What I did, after disconnecting the preamp from the LM12 amp, was connect the outside of the RCA jacks to the central pin... separately on each channel. When this is done there is no hum. In my experiance, if you turn-on a power amp with no input connection, there will be hum or buzz.
"in this star point should also exist one cable from center tap of the transformer and absolutelly nothing else NO CABLE FROM STAR POINT TO THE GROUND OF THE BOX !!!!! "
The only wire that is connected to my metal chassis (only the side plates are metal) goes to the ground wire on my power AC cord. I also connect the circuit ground thru a 10ohm resistor to this AC power ground. I've tried lifting this ground... there is little, if any, effect on the hum.
"last couple of things ..... in the rails should only exist the same topology like from plus out of psu on lead to plus of one amplifier board and another starting from psu plus lead to amplfier board 2...."
exactly the same fro minus rail ....]"
That's what I've got!
"if you connect.... one resistor to plus lead in serious with a led and serious with a resistor to create a power on indicator that is enough to create a noise ..... "
Good point. I'll try removing my power-on LED. Thanks.
"if you have any auxiliary circuits that operate in the same box with other suplly tthen care should be taken on how you handle the local grounds "
There are no auxilary circuits.
"and finally very common mistake anyone can do is very possible that one of any pcb ground is connected to the box with a screw ...so its actually some kind of mechanical error that does all that ...."
I'll double check this... I did use a "ground plane" on my PC board. It could connected! Good point. Thanks.
"the topology starting from plus lead from example to the first amp and then to the second amp is not working !!!!! the same apply for all power cables and all ground cables"
I don't understand what you are saying here. Sorry.
Thanks again for your help. When I've made the above checks and tests I'll report back.
Tom
in your previous
post its mentioned PC .....
didnt really exactly understand .....
is your amplifier connected to a pc ?????
because if it is there should be absolutelly no hum while is not connected to PC
all this theory about groubd handling is academical bull sh****et in case you are connected with a pc .....
first !!!!!
nobody can guarantee you that ground is handled properly from your pc and of course there is absolutely no reason to do so since the litle green out of your pc is not a line out !!!!!!
this out is designed to drive pc speakers ( do not require exactlly line level signals ) and headphones....so ground cable of headphones could be power ground after all ....
so please make all your tests with a cd player and as about the pc i can tell you what to do exactly after that
post its mentioned PC .....
didnt really exactly understand .....
is your amplifier connected to a pc ?????
because if it is there should be absolutelly no hum while is not connected to PC
all this theory about groubd handling is academical bull sh****et in case you are connected with a pc .....
first !!!!!
nobody can guarantee you that ground is handled properly from your pc and of course there is absolutely no reason to do so since the litle green out of your pc is not a line out !!!!!!
this out is designed to drive pc speakers ( do not require exactlly line level signals ) and headphones....so ground cable of headphones could be power ground after all ....
so please make all your tests with a cd player and as about the pc i can tell you what to do exactly after that
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.