robert
Rank: Soloist
Posts: 18 
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Post by robert on Mar 24, 2016 21:56:14 GMT
I need some help as my amp is under the weather so heres the background briefly.
Bought a new pair of speakers about 2 weeks ago (tannoys precision 6.4), plugged em in and noticed a slight hum, which with my previuos speakers wasnt there before.
As thing trundled along, i noticed it was slightly louder, plus if you put you ear near the dual concertic driver there was also a slight hiss. (This is from both channels) So
Checked the obvious, thank goodness im a serial tube roller so have alternatives to swap over, systematically swapped out the valves including power tubes and replaced, hum still there. So i cant be the valves
If you play music you cant hear the hum, stop the music its there.
So as i had a day of today and after chatting with mates last night i thought right, dismantle the whole lot out and put it back together. All done clean polished and back in situ, switched the amp on ( dac was not switched on) powered it up, valves power up, gentle thud(thats always been there) as the tranformers come to life, very loud hum and for want of a better word scratching and buzzing sound, so i switched the amp back off.
Bugger bugger bugger, apologies for the language.
The amp has been modified, caps upgrades etc.
Now sadly im no technician so wouldnt even attempt to tackle this issue or know where to start, but im think, leaky or dodgy or dieing cap?
I really need someone who knows what there doing to take a look and ideally repair. Happy to pay fortime components etc etc, but some suggestions or pointers wouldbe greatly appreciated.
Cheers
Robert*
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Post by dsjr on Mar 24, 2016 22:20:40 GMT
Just a thought from afar and may not be relevant. The Tannoys may possibly be far more sensitive than the speakers you used previously and the concentric tweeters can be a bit, well, assertive, in some models. As a result, hum and hiss may be more noticeable - possibly... I don't know what valve amp you have, but hopefully the mods have been done right and, if the speakers are nearer 4 ohms (I can't remember), you know what that will do to the frequency response when transformer-coupled valve amp driven  (the Stereophile reviews of similar amps shows you what happens most often). of course it could be something totally stupid like a screen come off an interconnect - cough - ... |
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robert
Rank: Soloist
Posts: 18
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Post by robert on Mar 24, 2016 22:54:20 GMT
I dont think there a sensitivity problem, the tannoys are 8ohm and the plugged into the 8 ohms taps on the amp.
All mods were carried out by someone who knew what they were doing so there no concerns there^. The amps a jolida 502brc
The noise thespeakers are throwing out when the amp is switched on aint good. So i not taking any chances. Previuosly the amp was used with Martin Logans with no issues and certainly nohum problems
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Post by Greg on Mar 25, 2016 1:10:14 GMT
Robert, where abouts do you live? Depending on that, I may be able to point you towards a nice price proper engineer. For sure it is nowt to do with your speakers and all to do with what's happening in your amp, although of course if your new speakers are more sensitive than previous, hum not previously revealed certainly would now be. I would recheck your revised hook up as the problem became worse after you disconnected and returned, Did you make any other change to your connections at the time?
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Post by Eduardo Wobblechops on Mar 25, 2016 4:56:31 GMT
All valve amps will have hum and hiss to some degree, this will be more noticeable if you use more sensitive speakers. It's unavoidable I'm afraid.
I'm hardly the world's expert, but could take a look. Problem is, I already have another member's amp to hopefully fix, which I still haven't had time to look at as I've been working away from home, so it might be a wee while before I could get round to it..
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Post by Chris on Mar 25, 2016 5:53:45 GMT
I've certainly noticed a difference in hum,little crackles,the sound of tubes warming up etc etc as I've switched between speakers. Always shuts up though.
No technical insight at all but I can certainly vouch for Eduardo - lovely guy who is very knowledgable and helpful. A real asset to the forum.
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robert
Rank: Soloist
Posts: 18
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Post by robert on Mar 25, 2016 6:57:23 GMT
Thanks for all the feedback everyone. Sadly, due to my messin about with valves ive had a couple go, so have an idea of what kind of thing to expect soundwise. I also thought about interconnects cables etc so i checked the as i put everything back together, couldnt see anythiny obvious.
Ive taken every pair of AX, AT and KTs i have out and replaced, thats why im sure its not the valves. Man you dont half get paranoid when something like this happenes, i even thought last night that when i moved the amp from its usual location to temporary rest on a coffee table yesterday whislt i cleaned a large glass lounge window behind the lowboard it lives on (well, youve still got to do your housework) that if it was a dodgy cap or even a loose connection on the cap, that movement may have been enough to make matters worse.
Im praying it aint the transformer
Still the lounge windows sparkling, not a streak to be seen, so every cloud etc
I in clackmannanshire just outside Stirling.
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Post by Chris on Mar 25, 2016 7:09:39 GMT
Don't think it's the transformer. Check the sensitivity of your new speakers against the old ones.
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Post by MartinT on Mar 25, 2016 7:56:30 GMT
It could be a capacitor gone leaky? Test for open circuit with a test meter after it settles. If a resistor is in parallel, test for that resistance. It's not perfect, but you have two channels to compare. Presumably there is a single shared PSU?
The fact remains that, since you pulled it apart, it's most likely to be contact in a valve base. Check carefully with a bright torch.
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Post by pre65 on Mar 25, 2016 8:44:19 GMT
I would also suggest the more sensitive Tannoys are making an existing noise more noticeable.
8 ohm is the impedance, not the sensitivity, which is measured in db, and the higher the db the more sensitive the speaker is.
Valves inherently suffer from two types of noise, Shot and Johnson. If you want technical info Google both, or try this. 
en.wikipedia.org/wiki/Shot_noise
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Post by Chris on Mar 25, 2016 9:27:04 GMT
Interesting link that. Think I'm going to have to read it a few times but interesting nonetheless.
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robert
Rank: Soloist
Posts: 18
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Post by robert on Mar 25, 2016 10:42:47 GMT
Apologies your correct 8ohms is the impedance, it was early this morning, when i replied to the post and i wasnt thinking straight due to worry and not sleeping all night and not eating since yesterday etc
My wife suggested that if i cleaned all the other windows in the house and did all, the other "little jobs" on "the list" it might help.
She also said standing looking at it wouldnt make things better either.
Thinking ahead myself i ducked as i replied "Oh all of a sudden every ones an expert"
I will have a read over the link, and will get the base off and take a look,
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Post by Eduardo Wobblechops on Mar 25, 2016 12:09:18 GMT
Likely a dodgy cap or loose wire is the culprit.
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Post by Eduardo Wobblechops on Mar 25, 2016 12:10:12 GMT
Or valve bases as Martin said.
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Post by Eduardo Wobblechops on Mar 25, 2016 12:18:58 GMT
I've certainly noticed a difference in hum,little crackles,the sound of tubes warming up etc etc as I've switched between speakers. Always shuts up though. No technical insight at all but I can certainly vouch for Eduardo - lovely guy who is very knowledgable and helpful. A real asset to the forum. LOL, dunno about the knowledgeable bit, but thanks for the vote of confidence! |
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robert
Rank: Soloist
Posts: 18
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Post by robert on Mar 25, 2016 16:35:53 GMT
Ok removed the base of the amp tested each cap with a meter all seems to be fine but i did notice a hairline crack on the case of one of the audience quricaps not sure it that has any bearing.
No to check the valve seats
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robert
Rank: Soloist
Posts: 18
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Post by robert on Mar 25, 2016 17:00:20 GMT
Check all the vavle seat a a few looked well more open the the others so i tightened them up, put all the valves back in powered it up, low level hum still there ( the sensitivity of the speaker is 90db) and the martin logans i previuosly used was about 86-87b.
Anyway i fired her up hum is much less and no horrific noises like yesterday so i will let the valves get some heat in the a give it a go.
Assuming i will have to live with the humm and the valve seats have been the issue thank to all for advice and assistance. You live and learn.
Failing that i will be back
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Post by MartinT on Mar 26, 2016 8:20:02 GMT
Great. Enjoy the music!
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robert
Rank: Soloist
Posts: 18
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Post by robert on Apr 9, 2016 9:32:09 GMT
Well i said i may be back, i hoped i wouldnt but i am.
Hum still there, but there's now an audiable intermitent buzz and and screeching type noise on both channels to accompany the humm (which as i said is much less), also tapped the AT and AX 7 valves starting on the left hand side of the amp with a pencil (just to check, but i know there ok). You can here it coming thro the speaker fairly loudly (the RH channel valves are the same, but not as loud) But when i did it again on the front left AX7, there was a very scary loud buzzing sound, so i switched the amp off.
I really need this looked at by someone who knows what there doing, ive tried a few people up here that are local to me, no joy.
So can anyone point me at someone who can try to get to the bottom of this, as im back to a small integrated amp and i anit happy and thats being polite.
As always any help would be appreciated.
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Post by pre65 on Apr 9, 2016 9:57:46 GMT
Nearest person to you that I would recommend is Philip Ramsey at Bluebell Audio in Dundee, about 60 miles away from Stirling.
Well worth giving him a ring, he is ace with valve amps.
www.bluebellaudio.com/
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(the Stereophile reviews of similar amps shows you what happens most often).
