aleph hum

lowfi

Member
2002-03-22 10:15 pm
hey all,
i know that grounding amps has been beaten to death, but i searched and couldn't find a relative answer to my current delima. maybe the answers were there, i just couldn't recognize them. sorry if that's the case.

here goes:
when i fire up my amp, i measure 27 mV and 49 mV on the outputs of my channels. when i hook up speakers, all's quiet.
when i introduce my preamp, it hums like crazy and i reak 340 mV on the output.

current ground scheme:

earth point on chassis to ground on outlet and my input neg leads

star for each channel with pcb ground, including output ground, center taps and that channel's filter cap grounds.

same for other channel.

i think it is something obvious, but i'm not comfortable enough with this to start hooking wires up... or can i? is there any danger in using a wire to hunt down the cause of this loop?

and is it a ground loop? is that what's giving me the increase dc at the outputs?
thanks for any input,
scott
 

rtirion

Member
2000-11-24 12:02 am
Could it be that your preamp is DC coupled and showing some offset at its output?

If so, put a capacitor at the input of your aleph, say a 2.2 uF MKP or something like that. Check if the offset/hum problem at your aleph's output is solved. Please use a test speaker before hooking your aleph to your main speakers.

Later you could take a look at your preamp and see if you can do something about the offset.

Regards
 

lowfi

Member
2002-03-22 10:15 pm
no,
i measured the preamp and no dc at the outputs. oh, and don't worry about the spkrs, i learned the hard way a while ago. i have a throw away spkr that i use for testing. once bitten....

what's weird is that it used to work with this pre and others really well. very quiet. i redid the amp boards and now tha i have it up, sort of, i tried to improve the grounding. however, i went back to the old way, that was quiet and now i get this bothersome hum/buzz.

thanks,
scott
 

rtirion

Member
2000-11-24 12:02 am
Star grounding should help you prevent ground loops. But connecting it to the ""outlet ground"" could cause trouble again.
I suggest diconnecting this temporarly and see if there is any change.
I had some ugly experiences in the past with ""dirty grounds on an outlet"".
Please try my previous suggestion as well (input cap), since it only takes a very little DC to cause all sorts of mishap.

Regards.
 

lowfi

Member
2002-03-22 10:15 pm
my star ground is floating, for each channel. so, in other words, each star has:
pcb ground
output ground
centertaps
and that channel's ps filter cap ground

the only components hooked to earth are
input gound
chassis

the cap, how exactly do i install it, in line with the input? please advise and i'll give it a try.

thanks alot!
scott
 
Hello Lowfi,

seems to me you´ve got a nice earth loop.
To remove this you can do the following:

Connect your central earth point(s) via a resistor(10 ohms) or ntc to the chassis ground.

Connect the input ground only to the pcb or to the central earth point, NOT to the chassis. (you will have an earth loop cause you preams output is probably also at chassis ground level)

I´ve build all my amps this way (after experiencing the same problems you have) and never have any "humm" problems.

william
 

lowfi

Member
2002-03-22 10:15 pm
william and all,
i connected the two star grounds for the channels and the dc at the outputs dropped right down to 20-30 mV. but that hum is there, it's everywhere! i removed the input ground from the earth and started touching it to various parts on the amp to see if i could get the hum down. no go, it's jus this loud, clean hum. it does get louder when i touch it to the pcb ground, but not by much.

any thoughts?
thanks,
scott
 
lowfi,

If you are using BrianGTs PCB boards and not isolating the ground trace from chassis you could be getting your ground loop from this. If you are using metal standoff and they are hooked to chassis, guess what, ground loop. You could use plastic standoffs or cut the traces around the mounting holes.

BDP
 

lowfi

Member
2002-03-22 10:15 pm
i filed a break in the ground traces to isolate the boards from the chassis.
the pcb grounds are isolated, at the moment, they are floating along with the ps cap grounds and xformer center taps.

i have taken the input ground wires and "touched" around trying to eliminate the hum with no luck. they, too are just floating now.
 
lowfi,

if I understood this correct your amp doesn´t hum when not connected to your preamp, right?

If so, what happens when you connect a cd player directly to your amp? Does it also hum?

If not have a look at your preamp and what´s connected.
Do you have a tuner connected? Disconnect the antenna and try again. If it´s gone now you need a filter to avoid dc-coupling to the antenna (don´t know the englisch name right now)

happy trying,

william
 
Try connecting all your AC powered equipment to the same
power strip connecting to the same AC wall out let. Remove
the all center ground return's ( the center ground on AC cord).
You can purchase cheater plugs-connectors that gives you the
option of connecting these grounds. This normally this solves
most 60Hz hum problems.

The biggest problems comes from AC home runs to you AC
house circuit breakers box. Also, cable TV boxes, cable modems
and computers in a system can inject 60HZ hum. These components must have ground return isolation.

Remember that hum is caused by circulating ground currents paths and missing grounds connections.

Good Luck
 

lowfi

Member
2002-03-22 10:15 pm
well,
this is the problem, it is very quiet with nothing at the inputs. it's when a pre or cd is hooked to the inputs that the hum is loud. i went to a different circuit and plugged it all in with the same results. if you haven't yet, take a look at the "amp. oscillation" thread i started just before this one, it may help to understand the full scene that i have going on here.

it's really strange, and i don't have the knowledge/experience/tools to really analyze this, that's why i'm hoping someone out there can show me the light.

thanks!!:)
scott
 
Hi,

Well, if the hum appears when you connect the amp to a source, it IS a ground loop (unless you have another amp that hums with the same source).

Without any input connected, try and temporarily connect the ground leads together at the input terminal (using a screwdriver, paper clip or just anything that leads). If you hear a hum, it IS a ground loop.

One solution to this (as I fixed it on my Leach Amp) is to avoid the loop. On the Leach Amp each (mono) board has two ground connections. One for the input stage/signal and one for the output. These grounds are separated on the board with a small resistor. What I did (which was recommended on Prof. Leach's web-site) was to disconnect the signal ground on one board from the ground post and connect the grounds together on the input post, letting the input stage on one board have the same ground reference as the other board. My hum disappeared right away.

Is the hum there when you have only ONE input connected, or is it there ONLY when you have TWO inputs connected?

Best regards,
//magnus
 
Hummmmm!!!!!

Hmmmmmm, sounds exactly like my problem with a 3 channel Leach Amp. Hummed only when ANY source is connected (preamp, CD player etc), absolutely silent without any input (input floating). Have tried everything that is suggested by Prof. Leach but to no avail. May have to do a total rebuilt just to satisfy myself that everything has been done correctly. If only I have the time.......
 
Have you tried this, the star ground connects to input ground and then ground it to chasis and then finally to mains earth
star- input- chassis-mains.
If you need further explanation try reading douglas self homepage on the topic of grounding. The method used is describe clearly with drawing at the page and also the reason that this method is used.

Also, i haven't seen the aleph ps but in general don't used the star grounding at the power caps, something to do sharp spikes due to charging or discharging. Read this somewhere
 

lowfi

Member
2002-03-22 10:15 pm
you know,
before, when all was quiet, is used 14awg stranded wire from the inputs to earth. now i have 22awg wire, is the gage THAT critical? i'm at work now, but i'll try DSelf's method with as large dia wire as possible.
thanks for that tip, D Self's site is excellent.
cheers,
scott