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Tube pre with SS HT- issues


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#1 of 47 OFFLINE   Yogi

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Posted April 21 2003 - 03:51 AM

Its been over a month I have been living with an ASL 2004DT tube linestage in my system. Its connected right before the SS amp. The sound is wonderful and dimensional but I still have this nagging hum issue. There is hum from both speakers when the tube pre is on. This hum is independent of volume level. I have tried cheater plugs, wiring the amp and the preamp chassis together, change of interconnects and change of power cords but that hum remians. Last night I noticed a funny thing and thats when I decided its time to get professional help.

I usually turn on the preamp and let it warm up before turning on the amp so I dont let any spikes and DC surges associated with tube warm up go thru my speakers. Last night I turned on the amp and there was a hum thru my left speaker even with the preamp turned off. Yes you heard it right even with the preamp off there was hum from my left speaker and left speaker only. The right speaker was dead silent. I disconnected everything else from the preamp and the hum was still there. I swapped the interconnects going to the amp and the hum came back on the right speaker. So it was definetly coming from the left channel of the preamp. Next I disconnected the power cord on the preamp and there was dead silence in both channels.

So here is the situation that prompted me to seek out professional opinion as its beyond me as to what could cause this: hum from the left channel of the preamp even when its turned off with the power cord connected. With the power cord disconnected there is no hum from any channel. What could be going on guys?

Sorry for the long post, but I need some professional help here.

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#2 of 47 OFFLINE   Brett DiMichele

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Posted April 21 2003 - 04:13 AM

Sounds like transformer induced hum. The transformer has
power all of the time except when the power cord is not
connected.

I get hum from my ASL Monoblock tube amps and the hum comes
directly from the transformers in my case. It's so faint
it does not bother me (and like your issue it's not volume
dependant) it's something I expect from tube gear that uses
old style transformers. If these units used Toridial design
transformers it probably would not be an issue but most
tube companies don't use toroid's.

It could be something as simple as a signal lead inside
the pre too close to the transformer or not properly sheilded.

Dunno what else to tell you.
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#3 of 47 OFFLINE   Yogi

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Posted April 21 2003 - 04:22 AM

But then why is it only on one channel? Please pardon my ignorance and be patient with me as I am learing something everyday.
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#4 of 47 OFFLINE   Brett DiMichele

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Posted April 21 2003 - 04:31 AM

Yogi,

I am thinking maybe there is a internal cable routed near
the transformer that could be the culprit.. ASL uses a lot
of point to point wiring in thier components.

It is possible that a line level signal lead is running
somewhere near the transformer causing emi to get into the
signal lead? That's just a guess but it could be the cause.
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#5 of 47 OFFLINE   Martin Rendall

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Posted April 21 2003 - 04:55 AM

Quote:
The transformer has power all of the time except when the power cord is not
connected.

I don't know the design of the ASL's, but this seems very unlikely to me. Most often, the power switch is between the power cord and the trannie, for safety reasons.

Basically, hum is coming from one channel only, and only when the power cord is connected. I would hazard to suggest that one of the channels within the pre is not grounded properly. Specifically, the left channel. There is likely a ground loop within the pre itself. Does the hum go away when the pre is turned on, or does it remain?

Interestingly, I noticed that my well grounded pre also hums out both speakers when the power is off. I think I will try unplugging it as an experiment tonight, and report back what happens.

Regarding lowering the noise floor - try an inline RCA attenuator. One common brand can get -3dB, -6dB, and -12dB attenuation. You plug the attenuators in between the cable and the amp. As long as you have the headroom, this works great. Let me know if you want more details.

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#6 of 47 OFFLINE   Brett DiMichele

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Posted April 21 2003 - 05:30 AM

Martin,

Good point about the switch being between the transformer
and the cord. Some do keep the transformer energized all
the time and some don't. I guess we would need to see the
schematic for it.

How loud is the noise that you hear Yogi? Just curious..
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#7 of 47 OFFLINE   Scott Oliver

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Posted April 21 2003 - 05:46 AM

Yogi, you could probably tap into a few more knowledgable tubophiles by posting this at Audio Asylum in the Tube, Amp/Pre, or DIY Tube forums.

Joseph Lau also monitors that Tube forum, so you just may even get a response from the designer himself.

#8 of 47 OFFLINE   Yogi

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Posted April 21 2003 - 06:56 AM

Good suggestions all. The hum is loud enough to be heard 3 ft from the speakers during daytime and from 6ft away during night time. The hum is there in both speakers when powered on and only on the left speaker when powered off. No hum with the cord off. I'll look up the schematics tonight. Also the suggestion for a line attenuator is great. I think it will work great in my system as I seem to have plenty of headroom. The pre is at 9 O clock position for HT bypass. Also I have tried switching the 12AX7 with a 12AU7 for lower noise floor and loss in gain. So far thats been the best solution to my problem, but I would like to completely eliminate it assuming that such a thing as a quiet tube preamp exists.

BTW from the ASL website's schematic for this preamp:
http://www.tubehifi....cuit/AQ2004.JPG

It seems like the power switch is between AC-in and the transformer.
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#9 of 47 OFFLINE   Martin Rendall

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Posted April 21 2003 - 07:26 AM

Yogi,

From the schematic, the switch is before the trannie, so I think you can rule out induction from the transformer.

One certainly can persue a black background in many ways. Personally, I believe it to be a hugely frustrating task, especially if you want to do it "right". My advice is to go the attenuator route, and enjoy great music.

Buy this: http://www.partsexpr....=16208&DS_ID=3

The -12dB will get your hum audibility down to a quarter the distance.

It will also allow you to turn your pre volume up somewhat, which will change the input signal strength to the valves, which in turn might improve the sound.

Best luck,
Martin.
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#10 of 47 OFFLINE   Darrel McBane

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Posted April 21 2003 - 11:26 AM

I just added a Audible Illusions Mod3 tube preamp into my system. My choice was to keep the two channel separate from my HT system. I'm using the same amp. But, I only hook up the amp to the AI when I plan on two channel listening. That may help your situation as well. It is a little effort switching but, not much. And to my ears it is dead silent from anything from the preamp to amp to speakers.

Just a thought.
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#11 of 47 OFFLINE   Scott Oliver

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Posted April 22 2003 - 02:23 AM

Yogi, I definately think a quiet tube pre exists. A $399 tube pre is not going to be representative of all that is out there.

Last week I was able to hear a system that had Avantegarde Duos horns (100+db efficiency) that are one of the toughest tests around for electronics to not allow any hum or hiss through them. They were driven an Art Audio Symphony 300B SET and either a Hovland HP-100 or BAT VK-30SE (both tubed). The system was dead quiet, except of course if you put your ear right next to the tweeter horn, which all systems suffer from this minute amount of noise.

#12 of 47 OFFLINE   Brett DiMichele

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Posted April 22 2003 - 03:18 AM

Scott,

And I would hope a set of $25,000.00 horns hooked up to
what sounds like no less than $15,000.00 worth of front end
would be quiet. Hmmm I think relative quietness can be had
fot substantialy less.. Posted Image
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#13 of 47 OFFLINE   Scott_N

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Posted April 22 2003 - 04:47 AM

I've heard preamps from Cary,Conrad-Johnson,Rogue,Quad and VTL all of them tube and all of them quiet so I bet it's just a case of you get what you pay for. Try a Rogue Audio 66 Magnum. Very good preamp for the price. You have very nice equipment Yogi why go cheap on one component?

#14 of 47 OFFLINE   Yogi

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Posted April 22 2003 - 05:28 AM

Scott, I do plan to separate my HT from 2 ch system by putting together a system with Magnum66 and my Proceed AMP2. The ASL 2004DT will stay in my HT setup for the front mains. I do agree that I got what I paid for and for the price I am absolutely thrilled. I plan on going by Martin's suggestion of using 12 db line attenuators and that should get the hiss to well within acceptable limits. I have had my eye on the Magnum 66 for a while now. I just went to the ASL as a first baby step. I am sure many big steps will follow, many against my will and logicPosted Image.
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#15 of 47 OFFLINE   Chu Gai

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Posted April 22 2003 - 05:39 AM

Dum question, but I'm assuming it's under warranty, so what's ASL's position on this as this sounds like a defect.

#16 of 47 OFFLINE   Scott Oliver

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Posted April 22 2003 - 08:09 AM

Brett,

I know you can get no hum for a whole lot less than that, but that was my most recent experience with a tube pre and it certainly was not an easy task as highly efficient speakers will certainly be amplfying any amount of noise in the system. Plus no where in his statement: "assuming that such a thing as a quiet tube preamp exists" is there a cost qualifier.Posted Image

#17 of 47 OFFLINE   Yogi

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Posted April 22 2003 - 08:18 AM

Chu, this was bought over at audiogon so I dont know if its under warranty or not. My assumption is that since I bought it used its out of warranty.
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#18 of 47 OFFLINE   Brett DiMichele

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Posted April 22 2003 - 12:57 PM

Scott,

I know there was no cost qualifier.. I was just stating
the obvious that I would "hope" that much equipment would
be quiet.

ScottN,

Have you listened to anything from ASL? They don't win
golden ear awards for no reason. The company produces well
built products at non inflated "audiophile" prices. Unlike
some other well known companies. And you can go right over
to HarmonicDiscord and speak to the designer of most pf the
gear (Joe Lau).
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#19 of 47 OFFLINE   Scott Oliver

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Posted April 22 2003 - 01:08 PM

Well I am soon going to get to test out the quietness of a tube preamp myself. I just ordered a demo model of the Granite Audio 770fp off Audiogon for a pretty good price.

I hate having champagne tastes on a beer budget, as I am often reduced to having to order some pieces off of reviews and word of mouth only, but then again if I didn't do things this way then I would be severly limited in my options as I am a tube man and my local dealers just don't carry a lot of tube stuff or if they do it is very high priced stuff. Sigh.

Here is a review on the piece:
Positive-Feedback Granite Audio 770FP review

But anyhow now i will be able to finally add vinyl to my system. Woohoo! Now I just need to inherit some money to be able to afford a turntable, arm, and cartridge.

#20 of 47 OFFLINE   Brett DiMichele

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Posted April 22 2003 - 04:18 PM

Scott,

I can sympathize completely.. Only for me it's more like...
I have Omega 24K DeVille tastes and a Seiko Kinetic Budget
(not much of a driker but I also love watches..)

As far as noise concerns.. That's the main reason why I
chose to run a S.S Pre and use Tube Amplification. I am
comming out of my Pre fully balanced into a linestage tube
equalizer (Behringer) and it's dead silent. It uses a pair
of Sovtek 12AX7's in the linestage with complete control
over the "amount" of tube saturation you add to the signal
and it's also defeatable as is the parametric EQ'ing functions.

I guess some high enders would scoff at me running a tube'd
equalizer in the signal path but that's ok.. It's 100%
defeatable if I want to run Pre direct to amp and I will be
using the direct outputs of my Parasound Halo P3 which bypass
the Tone Shaping network.

It should be dead quiet..

As for my tube amps they do hum.. But it's not the speakers
that make the hum it's the massive 300V Input Transformers!
Sometimes we have to live with the limitations of the technology
I guess.

If ASL used encapsulated Torodial's I am sure there would be
no hum. But the hum I am getting is barely inaudible when
there is no source playing.. Heck my central air is more
noisy than the hum by a factor of at least 3!
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