[AliG]

To all DIY experts, I am trying to figure out a phenomena - "hum buildup" 

It happens like this, if I only have my amp turned on, my speakers are dead quiet. Now if I turn on a preamp, which is connected to the amp via RCA interconnect, I can hear a constant small "hum" if I place my ear near the 7" midrange driver (I have a 3-way speaker, the woofer doesn't hum). This hum do not increase in loudness if I turn up the volume on the preamp.

I have tried using cheater plug on my amp only , or preamp only , or both with cheater plugs together, the hum still persist. What's interesting is that after some 30mins of music playing, the hum gets louder and becomes audible 4 to 5 ft away - hence I call it "hum buildup"

My suspicion is some kind of DC leaks. I was wondering, can a oscilloscope be used to check against this?

Thanks for your time.
barry

[pbrstreetgang]

does it happen when just the power amp is on and connected to the speakers without being attached to any other component?

[AliG]

No..it doesn't happen when ONLY the power amp is on.

I first turned on my amp - no hum.

I then turned on my preamp - hum!

That's enough to convince me where the problem lies. I'm just trying to figure out what's in the preamp that's causing it.

does it happen when just the power amp is on and connected to the speakers without being attached to any other component?

[pbrstreetgang]

But you need to disconnect the RCAs that are connecting them

[AliG]

I have tried that too, and the hum is there. I have even tried the following:

The amp is connected to a dedicated line and switched on, the preamp connected to a battery backup power supply totally isolated from the household electricity circuit, as soon as I connect the preamp and amp via RCA interconnect, I can hear the hum.

But you need to disconnect the RCAs that are connecting them

[jules]

A small amount of hum, audible when you put your ear next to the mid range/tweeters is normal. It's 50 or 60Hz, depending on where you live that being the frequency of the mains AC supply. If you have an output cap on the pre-amp and/or an input cap on the power amp, DC shouldn't get through. It's really hard to say what the source of the hum might be but you're using tubes and they do warm up over the period you're talking about. Maybe if you have a choice you could try some tube rolling just because it's a fairly simple thing to do.

jules

you posted while I was writing ... are you saying the backup power supply is DC or is it inverted to AC at mains voltage?

[AliG]

A small amount of hum, audible when you put your ear next to the mid range/tweeters is normal. It's 50 or 60Hz, depending on where you live that being the frequency of the mains AC supply.

Jules, I thought about this one. Now, if it's the frequency of the mains AC supply, then the hum should be on when I only have my amp turned on right? But there's no hum when I only turned on the amp.

Regarding the battery backup..it's inverted back to AC.

thanks

[jules]

Something else you could try ... switch on the source and pre-amp and play something for 45min. After 45 min, turn on the power amp and see if the hum is there straight off. If so, it narrows things down to the pre-amp. If the hum still takes 30 min to develop then the power amp is playing some part in it.

jules

[kyrill]

hum is not DC
you suggest hum on both channels?
try to bring the preamp in an "open place" no other wires or ac chords in the neighborhood, away from its original place
and connect it to the cold amp
it is not a good habit to turn yr preamp ON while the power amp is ON
then put the power amp on
still hum you know the rca is not picking up hum, so it is the pre amp, hum in the preamp can have many sources and is a real PIA to find out what it is

i would try other interconnects just to make sure it is not the rca itself

[JoshK]

Can you plug the preamp into the wall's AC instead of the batter backup? 

I am 98% sure this is a ground loop hum, and just because the AC is regenerated from a battery pack (DC) doesn't mean that ground loops are going to be avoided.  The regen-AC has its own virtual ground that isn't at the same potential as the earth ground in the amp or other components, then there is going to be current flowing through the ground in the interconnects, which has some finite resistance, creating a voltage drop = hum.  Unless that virtual ground is referenced by a very low resistance path to earth ground there are going to be ground loops. 

[AliG]

Josh, is there a way simple way to minimize such current flowing in the interconnect? From what I know, the preamp is not grounded to the chasis.

Can you plug the preamp into the wall's AC instead of the batter backup? 

I am 98% sure this is a ground loop hum, and just because the AC is regenerated from a battery pack (DC) doesn't mean that ground loops are going to be avoided.  The regen-AC has its own virtual ground that isn't at the same potential as the earth ground in the amp or other components, then there is going to be current flowing through the ground in the interconnects, which has some finite resistance, creating a voltage drop = hum.  Unless that virtual ground is referenced by a very low resistance path to earth ground there are going to be ground loops. 

[NewBuyer]

I'm not Josh, but I'm thinking a Jensen CI-2RR, or perhaps one of the EbTech products (Hum Eliminator, HumX, etc) might help here... unless of course your gear is outfitted with balanced interconnect options (which you should use in that case). Since the hum appears to get worse during the first 30min or so, it is also possibly temperature dependent - which unfortunately indicates the hum might just be inherent to your preamp.

[jules]

barry,

have you talked to Moscode about this? Since the range of the hum is no greater than 5' it's almost "normal" for many systems so they might be able to suggest a fine tuning measure from their own experience.

jules

[Haoleb]

Josh, is there a way simple way to minimize such current flowing in the interconnect? From what I know, the preamp is not grounded to the chasis.

Can you plug the preamp into the wall's AC instead of the batter backup? 

I am 98% sure this is a ground loop hum, and just because the AC is regenerated from a battery pack (DC) doesn't mean that ground loops are going to be avoided.  The regen-AC has its own virtual ground that isn't at the same potential as the earth ground in the amp or other components, then there is going to be current flowing through the ground in the interconnects, which has some finite resistance, creating a voltage drop = hum.  Unless that virtual ground is referenced by a very low resistance path to earth ground there are going to be ground loops. 

If you want to just give it a try, Radioshack sells a ground loop isolator cable for 17 dollars. If it works then you can look into getting something of higher quality. I bought a set of the cables to use between my computer and stereo and it works like a charm. I did not notice any loss of quality but I probably would be looking for something nicer for use on my cd player or anything I do real critical listening to. For the computer it works fine though.

http://www.radioshack.com/product/index.jsp?productId=2062214&cp=&sr=1&origkw=ground+isolator&kw=ground+isolator&parentPage=search

[AliG]

Well...Jules...I really don't want to bother George Kaye.. I can turned on the Moscode forever (all other components off) and there's no hum. One of the differences between my preamp and the other "normal" ones, is that this "hum" does not increases when I turn up the volume. I'll try to play with different grounding mechanisms first.

barry,

have you talked to Moscode about this? Since the range of the hum is no greater than 5' it's almost "normal" for many systems so they might be able to suggest a fine tuning measure from their own experience.

jules

[lonewolfny42]

Barry....Any luck with "the hum" ?