[analog97]

I am experiencing a low-level "hiss/buzz" in my speakers.  Someone suggested I measure the DC offset at the speaker terminals.  I did and I was getting a fluctuating reading from about -30 mV to +50 to 60 mV DC.  This reading was taken with my Clarinet on and my amp on.  When I switched the Clarinet off, the reading was very low, about 1 mV DC.  Does anyone know if this level of DC is capable of producing the slight buzz?  From my reading, it appears a DC offset of >100 mV is bad, but I don't fully understand this.  Any thoughts appreciated!!   

[dnewman]

On my somewhat high sensitivity speakers (98 dB), I find 20mV to be very noticeable and very annoying.  My current amp runs around 200 uV (microvolts, and yes my dmm -- Fluke 88V -- can measure in that range).

One way to measure is to short the amp's input at an RCA jack and then measure right at the corresponding speaker output terminal.  Repeat on the other terminal.

Dan

[analog97]

One way to measure is to short the amp's input at an RCA jack and then measure right at the corresponding speaker output terminal.  Repeat on the other terminal.

Thanks, Dan.  Not sure I fully understand.  I did not short anything and made my measurements across the speaker INPUT terminals.   I did this with only the amp powered up, then with the Clarinet pre-amp and amp powered up.  No source music was playing.  That is what I read as a correct way to measure DC offset.  My aberrant readings were made only when the Clarinet was added. 

Again, I'm just trying to figure out why I have this low level speaker buzz.  Now that my system sounds so good, I just want to achieve that "stone-cold" silence people talk about.  The buzz does not interfere with music, but it must be there with the music playing and it is, as you say "annoying".

Thanks for your response.  I am hoping someone can comment on the way I measured DC offset and can tell me whether I may have a problem with my Clarinet.

[dnewman]

There's many ways and steps in tracking down noise in a system.  And, of course, the most common source is ground loops between components.

What I was referring to is how I go about seeing what the inherent noise is of just my amp.  I look at just it, not connected to anything else, and short its inputs so that I know it is seeing a 0V input signal rather than something which is floating (and hence at who knows what voltage).  To short the input signal, I connect the center pin of the RCA jack to its outer shield.  (I actually keep an RCA plug which does just that.)  I then measure the voltage at it's speaker terminals.

For looking at your entire system, you can still start by looking at just the amp.  Short both inputs to the amp, and hook up just one speaker to the left channel. Power on the amp.  Do you hear the hiss?  Turn off the amp, disconnect the speaker from the left channel and hook it the right channel.  (I use the same speaker each time.)  Again, do you hear the hiss?  Turn things off again.  Now, short both inputs on the pre-amp, and plug in the pre-amp's right channel to the amp's right channel.  (Leave the amp's left input shorted.) Turn things on -- amp and pre-amp.  Same hiss?  More hiss?  Different quality to the hiss?  Turn things off and repeat with the left channel.

Using such a procedure as above, you can get a good idea as to which component is adding the hiss: keep on adding components until the hiss/noise manifests.  Once you know which component, you can further investigate whether it is just one channel or both channels.  If just one channel, does it move with any tubes dedicated to that channel.  (E.g., if the noise is just on the left channel, what happens if you swap a tube between the left and right channel? Repeat for each channel-dedicated tube.)

As to 1mV: that shouldn't be too noticeable unless you have high sensitivity speakers (>100 dB).  The fluctuating -30mV to +60mV: that's hard to comment on since it sounds like you didn't have the pre-amps inputs shorted to ground.  Thus, they were floating and you were amplifying that floating signal in both the pre-amp and amp.  Having fluctuating voltages at the amp's outputs is then to be expected.

Here's a link to what the Bottlehead folks recommend for tracing down noise in their kits (which are largely point-to-point wired and hence have more opportunities for builders to introduce noise when they build the kits).

http://www.bottlehead.com/loosep/troubleshooting.htm

Cheers,
Dan

[amandarae]

I am experiencing a low-level "hiss/buzz" in my speakers.  Someone suggested I measure the DC offset at the speaker terminals.  I did and I was getting a fluctuating reading from about -30 mV to +50 to 60 mV DC.  This reading was taken with my Clarinet on and my amp on.  When I switched the Clarinet off, the reading was very low, about 1 mV DC.  Does anyone know if this level of DC is capable of producing the slight buzz?  From my reading, it appears a DC offset of >100 mV is bad, but I don't fully understand this.  Any thoughts appreciated!!   

Hello,

First off, are you using a tube amp?  If so, then you should not measure any DC at the output terminals because the Output transformer should not pass DC.

I will do what Dan suggested. 

Let me add that if it was me, I will disconnect the preamp to the amp, leave the amp connected to the speakers, short the input of the amp if possible.  Turn the amp On, any noise, instantaneously or after say 5 minutes?  No, then it has to be the preamp.  Also measure the AC in mV from the speaker output terminals( + of the meter leads to 16/8/4 ohm tap, minus on the GND terminal) and write it down.  This is the output coming from the amp at idle with no input.

Heres my take.

For the preamp, lets take one channel at a time, you can see that the output of the Clarinet is cap coupled (C303)...




... it should not pass any DC.  With the preamp ON and disconnected to the amp, measure the DC voltage from the top of R312 to GND (same as the output pin of the RCA), there should be zero or very close to zero.  If it is large (I hope not close to the 145 DC minus the voltage drop at R311 on the cathode of V301A) then maybe your cap C303 is shorted or leaking.  Do the same for the other channel.

If both channels measures close to or at zero volts DC, then, the buzz you are hearing might not be coming from DC but  might be AC .   Is it really a buzz or a hum?  If it is a hum, it could be a ground loop. 

With both preamp, amp Off, connect the preamp output to the amp, and amp to speakers.  Turn On preamp first, wait a couple of minutes, then turn On the amp.  If the noise you are referring to still exist, measure it with AC on the speaker terminals and compare it to the recorded reading you have.  You can tell how much is the difference and if so, can use it to trace it on the amp preamp circuit path.

Note:  Do not connect/disconnect input signal wires or speaker terminal wires when power is On on the amp.



Goodluck!

[analog97]

First, thanks to Amandarae and Dan. I am using a SS amp, a Parasound HaloA21 driving Polk SDA 1.2Tl's (~90db I think).  Pretty sure this is not a ground loop.  A ground loop in my experience always is a very loud hum.  The "hiss" I have is something I have heard over many years with many different components.  Simply put, it is the sound that eminates from the speakers with nothing playing.  I have never understood this nuisance noise and am trying my best to get rid of it.

I will try to solder a wire inside an old RCA jack and perform the tests suggested by you guys.  First, I will try it on a cheaper amp.  I always worry!!   Thanks again.

[amandarae]

You are welcome.

The "hiss" I have is something I have heard over many years with many different components.  Simply put, it is the sound that eminates from the speakers with nothing playing.  I have never understood this nuisance noise and am trying my best to get rid of it.

This is a different scenario.  If you already tried changing the tubes in the Clarinet with the same results, then it is possible it is not a hiss as a result of a tube rush especially if it does not increase in volume as the vol selector is turned up without any source playing.   Since you have this nuisance on every component you tried, was assuming even with a different amp, maybe you should check how your components are connected to the wall outlet. 

Have you tried using just one power strip and the power strip connected to just one socket on the wall, the other socket if it is a duplex receptacle not powering anything?  Do you have any neon/flourescent lighting or any regular bulbs equipped with a dimmer switch?  CATV/cable/DirecTV demodulator connected to the same power line you are using for the amp or a receiver that has a connection to your preamp in any way?

FWIW, IMO, take your time to pinpoint the source of this noise that you hear and I am confident you should be able to locate where it is coming from especially if you can still hear it  even if the speakers are 90dB sensitive because it means that it is loud.  Also, you said, always there even with different components.  That is why I suggested to look at the power connections this time since by process of elimination, it tells us that its source can't be from the components anymore.

Goodluck!

[hagtech]

You might just be measuring a low frequency wander of the 120V ac line.  It is not rock solid at 120V, but has some low frequency (sub Hz) fluctuations with various load in the house (fridge, lights, etc).

The Clarinet does not use voltage regulation on the B+.  It merely filters out to subsonic levels.  Hence, it passes not only dc, but also low frequency content.  If the ac line amplitude is changing, this will show up at the audio output, but rolled off as a function of the high pass filter, defined by the 1uF capacitor and the input resistance of the following amplifier.

As in LP warps, a phonostage or linestage that passes too much low frequency content will affect things further down the chain. 

jh

[timind]

How close do you have to get your ear to the speaker to hear the hiss?

[analog97]

How close do you have to get your ear to the speaker to hear the hiss?

I can hear it from 6-8 ft away with no music playing.  It does not interfere with listening at moderate-high volume.  At low volumes, within 2-3 feet of the speaker (both equally) I can hear it thru the music.  I am using a dedicated 15 amp circuit (12 guage Romex) that is split into 2 duplex outlets.  Thanks for your suggestions and to JH for an explanation of what I might have been measuring.