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Matthewong 11th May 2007 08:50 AM

Hum Noise!
 
I have been making a few pre-amp and power amp and until now, there is still one problem that always occur, HUM NOISE. Have been trying to figure it out since my first project, why HUM NOISE always occur? There is only one way that I can decrease the HUM NOISE to the minimum level is using a bigger casing, try to put the transformer and power supply away from main tubes. Sometimes I want to make a more compact pre-amp and use a smaller casing, HUM NOISE will be audible, pretty loud. Does anyone have any idea how to make a compact pre-amp or power amp and the HUM NOISE in minimum level?

thomsva 11th May 2007 09:02 AM

Do you have AC-heaters? If you do, it's a good idea to reference them to a positive voltage instead of ground. This is probably a good idea also for DC-heater. Maybe you have allready done that.

Another thing you can do is optimise the transformer placement. If the output transformer is too close to the power transformer there will be hum. Also orienting the OT in a 90 degree angle from the PT helps. I have also used the "headphone trick" that I have leared from the AX84.com forum. It's a good way of finding out how to place transformers. It goes like this:

The "headphone trick"
-Build your amplifier, but dont attach the OT to the chassis.
-Wire a pair of headphones to the OT secondary
-Turn on the amp, and listen to the hum in the headphones
-Move the OT around while listening. Try to find the place where you hear the least hum.

Be very careful when you do this! Make sure to insulate all leads, as there will be high voltages on the wires you move around.

Salas 11th May 2007 09:07 AM

Use a regulated psu. It will help a lot too.

Tweeker 11th May 2007 10:42 AM

If its magnetic coupling between OPT and PT and placement cant cure, steel shielding may help.

Matthewong 14th May 2007 01:20 AM

Thank you for the advices. But how about for pre-amp? Pre-amp for me is the most difficult to minimise the HUM NOISE. I have been making a few pre-amp and only 1 pre-amp I seccessfully to minimise the HUM NOISE to unaudible level. It was using a bigger casing than any other pre-amp I have ever built. The others pre-amps were built into smaller casing because I wanted to make it more compact, but all failed.

jarthel 14th May 2007 01:32 AM

I have built headphone amps and I have found that using a ground-loop breaker can eliminate hum. I'm not sure how but it does in my experience.

I have used this guide: http://sound.westhost.com/earthing.htm

There was one DIYaudio member who has hum in his preamp and I suggested the same article to him. It helped him but it didn't remove the hum completely.

Jeb-D. 14th May 2007 02:37 AM

Hum caused by AC filaments is far over-exaggerated IMO. Improper grounding and inadequate power supply filtering is usually the case. The only time Iíve seen AC heater cause significant amount of hum is on cathode follower stages where the heater to cathode voltage difference is over 50V.

Not to say it's not possible but that the AC heaters is probably the last place I'd look.

refference 14th May 2007 03:53 AM

Hi Matthewong ,

Quote:

Originally posted by Jeb-D.
Hum caused by AC filaments is far over-exaggerated IMO. Improper grounding and inadequate power supply filtering is usually the case. The only time Iíve seen AC heater cause significant amount of hum is on cathode follower stages where the heater to cathode voltage difference is over 50V.

Not to say it's not possible but that the AC heaters is probably the last place I'd look.

I agree entirely with Jeb-D. He is correct !!

Obviously , if you use a DC regulated heater supply , will
be the best , even best if we are talking about preamplifiers .

But , even if your equipment is well constructed , there is an
IMPORTANT source of hum noise , that can never be ignored ,
The +B power supply .

There is two ways to eliminate hum noise from your projects :

1 Ė Use a +B regulated power supply . This is the best answer.
But in some cases this is very expensive and complex .

2 Ė Put in the +B rail ( after the rectifier and the first smoothing
capacitor ) one or more ( preferably two , three or even four
for a preamplifier ) RC filter , with a time constant as great
as possible , preferably greater than 0.5 second ( Time
constant = R x C ) . The 0.5 second is not a ďmagicĒ value
itís only a guess with base on my own experience , the actual
value can stay under or above it .
This is an amazing " hum noise EXTERMINATOR " . Itís cheap !!
It can be used in most cases !! The only drawback is that you
need a +B supply ( or transformer ) that can deliver a voltage
value a bit greater than the normal voltage, to compensate
the loss on the resistor (s) of the RC (s) filter (S) .

Regards ,

Carlos

cbutterworth 14th May 2007 04:18 AM

Matthew,

I agree with Carlos and Jeb to an extent. Mind you, I only have my own experience on which to base this, and there are other sources of hum, including PSU filtering, ground-loops, etc.

My experience with an Aikido preamp is that the switch to DC heating nailed what remaining annoying hum that I had in my system. Now, I have to listen closely to the speakers to hear anything at all.

Switching to DC heaters should not be dismissed. In fact, I think that a number of causes such as ground-loops, poor filtering, AC heaters work synergistically to bring hum levels up to the very audible.

The first two causes - ground-loop and PSU filtering probably should be addressed first, as they're symptomatic of underlying problems in design / layout. As for the ground-loop, I found that my TV (the s-video cable in particular) causes a ground loop, that even affected my fully grounded CD-player. Anyway, when I lifted the signal gnd for the TV and left is unconnected in my system, the hum dropped very significantly.

As for the PSU, my chokes are too big for such as low current draw, if I use a choke-input filter. Adding a "tuning" cap in front of the first choke compromises between choke-input and cap-input. Also adding a 20K power resistor across the final cap, increases the power draw of the PSU to something more in line with required power draw for 20H chokes at 300V. Again this helpd significantly.

I also added a ground lift using the circuit found at http://sound.westhost.com/earthing.htm using 20 ohm of lift. Again, I got good results.

Absolutely double-check that the GND parts of your RCA's are not in contact with the metal chassis.

Finally, in my case the switch to DC heaters did the final trick, although it does involve more circuitry.

I hope this helps, be aware that my experience is very limited.

Charlie

Jeb-D. 15th May 2007 01:01 AM

I forgot to mention rectifiers. Most common diodes will create a hash(spikes) in the supply that seem very difficult to get rid of. I never had much luck with paralleling a cap on the diodes. It didn't seem to ever help much. However, Hexfred type rectifiers work very well and are much quieter. Of course tube rectification is good but isn't practical for a lot of applications.

Didn't mean to give anyone the impressing that I think DC heaters is completely pointless, just that it is usually the last thing to do when you have a hum problem in an amplifier. DC heating is a defineate for pre-amps though (especially Phono) because your dealing with small signals and will have several gain stages following it. By the time the signal gets to the output stage of the amp, any hum from the preamp will be greatly amplified.


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