Background hum

[fullmetal]

Hey everyone, I have had a crack for about a month now, and since it is my first amp I am continually amazed at how awesome music sounds. However I have begun to notice quite a bit noise when there is no music playing. Is this normal? I usually cant tell when music is playing, unless it gets really low and then I can hear the humming in the background. Would this be fixed with the speedball upgrade? I use Decibel for a music player, HRT MSII DAC, blue jean rca, beyerdynamic dt990 600ohm.

[fullmetal]

Listening to it now it just started humming so much I could feel the vibration. I accidentally left it on all night last night, could that be a problem?

[Doc B.]

Leaving it on should never be a problem unless something has failed. Vibrating could mean that a tube is drawing too much current. Start by taking the voltage measurements and let us know if anything seems way off. As Grainger mentions it could be a solder joint that opened up, or it could be a tube going south.

[fullmetal]

Ok, I will do that. The hum is much more pronounced on the right side.

[Viktor]

Funny that I found this thread just now, I was thinking about the background hum in my amp as well. It is very low though still audible if there is no music at all and during silent passages with very few instruments. I don't think there is something wrong however, I just wanted to know if there is a hum present in these amplifiers or if they should be dead quiet? The latter seems a bit unnatural since I know how much my guitar amp hums.....

[Billyk]

My crack with speedball and stock tubes is very very quiet into shorted inputs, no hum at all. With some of the 6AS7 there is a tiny bit of hum, it can be made worse by adding a 6SN7. But even at it's worst it is still hardly discernible and when you ply music I do not notice it except between tracks or very quiet passages.

[fullmetal]

Is it normal for it to be more pronounced on one side or does that indicate a problem? When I get a chance I will go over it and check the voltage measurements and probably just buy the speedball upgrade.

[deltaunit]

I had a bad tube that had a raised noise floor on one channel.

Just listening to mine now I realised that if the source on my pre-amp was the iPod I'd get a slightly raised noise floor that isn't there with my dac. It could be a number of things until you've eliminated some steps - have you swapped the input cable around (l/r to r/l)? That at least narrows down the fault to the amp.

Dac > Speedball > 600Ω headphones is dead silent on mine.

[Doc B.]

Yes, to be sure of the noise floor you have to remember that it can be coming in ahead of the amp. So the best way to listen to the noise floor is with the inputs shorted with shorting jacks. That way you will only heard noise generated by the amp.

Fullmetal, have your tried different tubes? That would be one important test to make after you make the voltage checks. If a simple tube swap doesn't solve it - since your problem is dominant in one channel you should probably go over the solder joints concentrating on the connections on the side with the noisy channel. It makes the most sense to solve this problem explicitly rather than hoping that installing the upgrade will solve it.

[fullmetal]

I do not have another set of tubes to swap out. I hope that is not the issue as I just got the kit a month ago. What are shorting jacks? Thank you for the replies, hopefully I will get some voltage readings soon.

[deltaunit]

Would you believe it?

I just switched off my pre-amp power supply and power amp (both Naim) for the first time in months and bongo. One channel with a raised noise floor on my input tube... Grr, I only had that E80CC in for a few days as well, it was working perfectly. Both the Crack and Naim system are connected to the same mains but the crack was turned off at the time, could it be coincidence?



Luckily my faithful CV4003 is still working a-okay.

[fullmetal]

All voltage readings checked out. And Doc the reason I thought the speedball might be a good route is this is the only Crack I have ever heard so maybe the noise floor is normal.

[Doc B.]

I would suggest trying different tubes next. Contact Eileen tomorrow and she can send out a set to try. queen at bottlehead dot com

[Laudanum]

Maybe this can help...  I finished my build the weekend before this past one.   I built the Crack stock with the excpetion of using Mills resistors in place of the stock cement/wirewounds and used an Alps Blue Velvet volume pot.   The amp is, for all intents and purposes, silent.  I was and still am incredibly impressed with the lack of any preceptible background noise.  I havent shorted the inputs to check whether the background is actually "black", but in terms of normal listening, there is nothing there. For all intents and purposes, it's silent (Speedball must be really impressive).

My point is that the amp, in stock form, is capable of a near silent background during normal listening (including quiet passages and pauses in the music).  I use the word near as a qualifier only because an auidophiles description of silent may be different from mine, but you get the point I think.  So follow the advice of the more knowledgable folks who have offered suggestions.  

For the record, my main phones are Beyer 990's (250 ohm).

[Grainger49]

I expect that the sensitivity and impedance of the phones used will affect the level of noise you hear.  That is, in addition to the tubes as Doc mentioned already.