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Slight bit of hum normal? (Read 6036 times)
MarkBlair
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Slight bit of hum normal?
02/11/11 at 16:22:39
 
I use my SE84C+ with nearfield monitors, so I'm usually sitting roughly 30 inches away from the speakers.  And there's always a slight 60Hz power harmonic hum that comes out of both speakers (doesn't vary with the SE84C+ volume knob, which I leave fully clockwise so my preamp acts as the volume control, and it's there with the preamp off, too).  So it seems be be something coming from the SE84C+'s power supply.

And if I put my ear up to either of the speakers, the hum is very obvious.  But it's only normally noticeable to me when no music is playing -- which unfortunately happens quite alot while conducting a recording session.

Also, I swapped rectifier and input tubes -- no difference.  Didn't swap output tubes because the hum is the same on both speakers, and Occam's Razor doesn't like two separate sources for one problem! Wink

Anyway, has anybody else noticed this?  And is there any easy way to tell whether this is normal, of if there's perhaps an issue with the amp?
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ZYGI
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Re: Slight bit of hum normal?
Reply #1 - 02/11/11 at 18:19:25
 
Mark,

Slight hum? It would depend on how efficient your speakers are, Using 108dB efficient speakers would make a huge difference compared to 90dB speakers.

While I have had hum issues over the years, Its never been the amp itself, its either what the amp is plugged into or what is plugged into the amp.

A hum could come from a dimmer or any other electrical item on the same circuit as the amp, all the way back to the circuit breaker panel.  It could also be from a source whether it be at the pre amp or even farther upstream, like a CD player.

First just listen for the hum with the Zen by it self, nothing plugged into it, just the speakers, if the hum still persists, try moving it to another plug, hopefully not on the same circuit, or better yet not on the same leg in the breaker panel.  If its quiet then, you'll have to go back, and start plugging in everything upstream one at a time and see  what the first thing that makes the amp hum. Then try to figure out why, or what is causing the hum. You could also get a cheater plug (the little guy that changes a grounded plug three terminal plug to a two terminal plug) and lift any ground loop.

I've had several hum issues, but all were resolved and even on 100dB plus speakers, I hear no hum at the listening chair and barly any with my ear up to the horn.

ZYGI
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MarkBlair
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Re: Slight bit of hum normal?
Reply #2 - 02/11/11 at 19:37:51
 
Great points!  I guess I also need to see if I can figure out how to check that I have no more than the 1.5mV of hum/noise that the manual states for the specs.

My speakers are 90dB (PMS TB2S+), so it would be a lot more noticeable with more efficient speakers.  But it's certainly not a total show-stopper -- just annoying with nothing playing (see my post above for full details).

I'm all geared up to start troubleshooting if necessary, but my real question is -- how can I tell how much hum is normal?

I don't want to troubleshoot a non-problem, even if it is slightly annoying.  Also, I have a lot of other recording-related systems (tried other amps, different circuits, etc.) -- this is the ONLY thing that exhibits any hum.  So the odds are it's something in the amp itself...

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ZYGI
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Re: Slight bit of hum normal?
Reply #3 - 02/12/11 at 15:59:22
 
Mark,

The one thing I hate more than listening to a Solid State amp is HUM!!!  I hate it, period.

If you can hear it at the listening position, with 90dB speakers,  its to much.

Someplace on the Decware site, there is a Hum troubleshooting guide, which unless I looked over it, could not find it.

Finding what is causing the hum can be tricky, I have one amp, Torii-I that will not hum, no matter what's hooked up to it. Or what its plugged into, it just doesn't hum. I had a Torii-II which would hum when I had the CSP2 plugged into it, only when I used one particular source hooked to the CSP2 (I never did figure out what that was) When I first built my listening room, I made it a point to run a dedicated line from the circuit breaker box to the stereo set up and a different line run for all lighting. I had HUM in everything I did.  Then Steve told me to check and sure enough I had both circuit breakers for the room on the same leg of the breaker box. I switched the circuit breakers so they were on different legs, problem solved.

You may also check your wiring, as having interconnects running parallel to power cables is a sure fire way to induce hum. distance from each other is best but if that can't happen running them at 90 degree angles will sometimes do the trick.

When you do check the amp by itself, either by measuring with a meter of listening test, make sure it is by itself, and away from all things that could induce the hum.  

ZYGI  
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ski bum
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Re: Slight bit of hum normal?
Reply #4 - 02/13/11 at 16:39:50
 
Hello Mark and welcome to the forum!

That is quite an exacting system and use for the Decware amp.  Any chance you could rearrange your listening room so the distance to the speakers is 4-5 feet?  

While I find my SE84's do hum, it is imperceptible to me beyond a few feet, with speakers more sensitive than yours.  With your extreme near-field setup, you're cutting it pretty close, and even an extra foot of distance may ameliorate the problem.

Keep us posted.  

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MarkBlair
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Re: Slight bit of hum normal?
Reply #5 - 02/13/11 at 20:28:14
 
ZYGI & ski bum -

Thanks for the replies!  Objectively, however, I may be too nit-picky about this hum.  I set up my SPL meter on a tripod at my nearfield listening distance (about 30 inches) last night, and the ambient room level was a bit over 41dB.  Then, with only the amp (no preamp) turned on, the SPL measured about 44dB.  Which -- to be fair -- is still pretty low!  And, in all firness, the hum is just barely audible with nothing else coming from the speakers.  Incidentally, it read the same with the peramp on, too.

My challenge now is to figure out whether this is actually within limits (the manual just mentions a max 1.5mV of noise + hum).  But it's not a show-stopper, so I probably won't adjust my listening distance or even expend too much time troubleshooting it further -- unless either of you two (or someone else) see's a problem here.

My room is not an ideal size for recording/mixing -- just 10'x12'x8' high.  And I'm currently in what's considered by many acoustic "experts" to be the best listening location (3/8 the lenght of the long wall away from the short wall -- so I'm now 54" back from the center of the short wall).

Plus, to form a roughly equilateral triangle with the speakers, while still keeping them far enough from the walls (my PMC TB2S+'s have a rear-pointed Advanced Transmission Lines and need some room for the bass to "breathe"), I don't have any real maneuvering room.

So, I'm kind of "locked" into my current listening location.  Also, with 90dB speakers, I don't want to ask the SE84C+'s 1.8 watts RMS into 8 ohms to go past its limits, either.

And changing my PMC TB2S+ speakers for something more efficient at this time is non-negotiable (google them and you'll see what I mean).

So, it's probably onto recording and mixing for the time being.  After all, humming a tune is a lot more interesting than tiuning the hum...
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ski bum
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Re: Slight bit of hum normal?
Reply #6 - 02/13/11 at 21:42:54
 
Sounds like you have some tight constraints.  What sort of room treatment are you using?  It seems that a room that small would load up the bass pretty quickly, which would exacerbate 60hz hum.  Plus, treatments may make placement a bit more flexible.

I've heard PMC's before.  Love that transmission line bass; tight, dry, fast, and very natural sounding.  Good stuff.  
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MarkBlair
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Re: Slight bit of hum normal?
Reply #7 - 02/14/11 at 01:17:27
 
Believe it or not, no "real" room treatment.  But don't scoff too quickly.  Unfortunately, I get an upload error trying to attach a graphic of my room response from the IK Multimedia Advanced Room Control (ARC) software -- which is a standard vst software plugin for Digitial Audio Workstation (DAW) software.

It measures the amp/speaker/room response (using test tones and a calibrated microphone), and then digitally "corrects" it within the DAW.

Nevertheless, even the "before" graph for each speaker shows a relatively flat response down to about 150Hz, where there's a 6dB peak, and then a somewhat worse dip at about 70-80Hz.  Anyway, I'm using the ARC software in place of room treatment -- since the room is surprisingly good for it's size, expecially when you consider that some world-class recording studios are only flat to within 5dB.

But again, the hum isn't a show-stopper.  And it's certainly nowhere near bad enough to require me to shift from such a workable setup.  The IK Multimedia ARC software isn't really having to do anything too drastic (for such a small room), so I really don't want to do anything aside from -- if necessary -- address the source of the hum.
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dank
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Re: Slight bit of hum normal?
Reply #8 - 02/15/11 at 00:17:22
 
Mark

Just put a digital multimeter, set to ac volts, on the speaker terminals when no music is playing.  You should be able to read how much hum you have.  If you haven't got a meter, I would suggest buying one - they're fairly cheep and have many, many uses.  The cheapest one that will read down to the millivolt in AC should be fine ($10-20 I would guess).  This one would work:
http://cgi.ebay.com/LCD-Digital-Multimeter-Voltmeter-Ohmmeter-Ammeter-8813-/3206...

1.5mv = 0.0015 v   If something is wrong, it will probably be more like 0.01 or higher.

Dan
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MarkBlair
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Re: Slight bit of hum normal?
Reply #9 - 02/15/11 at 18:38:23
 
Thanks, Dan -- good point!  My voltmeter must not be working (that was actually the first thing I tried), becasue I got no reading (on AC or DC voltage).  So I need to troubleshoot my voltmeter, or maybe get a new one.
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MarkBlair
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Re: Slight bit of hum normal?
Reply #10 - 02/22/11 at 03:10:53
 
Haven't gotten a new meter yet.  However, I decided to swap in my old amp (a Parasound HCA-750A), and absolutely no hum!  However, I use to think that amp sounded really good (and perhaps it does) -- but I couldn't believe how bad it sounded to me after a few weeks of listening to the SE84C+ while burning it in.  Again, the hum is barely audible at a distance of 30" with nothing playing.  And perhaps I need to do a few more comparative listening tests.  But the SE84C+ seems to be a very impressive amp so far...
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GaryAR
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Re: Slight bit of hum normal?
Reply #11 - 03/20/11 at 18:27:45
 
I've had an annoying hum (SE84CS + bastanis prometheus), it's been the only thing that I don't absolutely love about the new Decware amp. I checked the a/c voltage at my speaker terminals and it's about 0.015V. I'm glad to know that's not typical for the amp, and I will definitely try to find the cause now.

gary
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MarkBlair
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Re: Slight bit of hum normal?
Reply #12 - 03/20/11 at 20:17:56
 
I still haven't gotten a new voltmeter -- but I get the same amount of hum with or without my preamp hooked-up to the SE84C+, and the volume knob position doesn't affect it, either.  So it seems it's internal to the amp itself.  But I'd be curious if plugging it into some sort of isolation transformer might make any difference.  Nevertheless, I'm so overall pleased with the sound that I don't feel compelled to solve the problem right now (although problem is probably too severe a term).

In fact, my main concern right now is that I don't get enough bass -- but I've done some tests and I'm confident that's more of a speker/romm issue.  Nevertheless, I'm more focused on integrating a subwoofer right now than spending too much effort on this hum issue.  Plus, a powered subwoofer should take some of the bass load off of the SE84C+ if I do it right, which should give me more overall volume out of my 90dB speakers.

But the hum issue is going to stay on my to-do list!
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Mark Williams
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Re: Slight bit of hum normal?
Reply #13 - 03/20/11 at 21:35:01
 
Mark,

I was told a slight bit of hum was "normal" too when I bought this used 84c+.      I run Klipsch La Scalas and the hum, although slight was driving me crazy.

I refused to believe NOTHING could be done so I replaced the original Zen second filter cap with an 80uF Sprague Atom 450V one.

This killed ALL the hum!    I have to walk over now and see if the amp is really running and I'm now so pleased with the Zen.

Since I am a total neophyte with electronics I have no idea whether this is recommended or wise for the Zen, but it works, and has worked for many months now.    

I don't know your level of experience with electronics but if you send me a private e-mail I'd be happy to help you.   Starting with a photo of what I did.

You will be happy with the results.   And you don't have to accept this "normal" hum.

Regards,
Mark




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GaryAR
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Re: Slight bit of hum normal?
Reply #14 - 03/21/11 at 00:22:33
 
Mark -

I don't have enough posts yet to send PM's, but I'd appreciate it if you could post a picture of the mod.

Thanks
Gary
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