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ARC VS60 hum in speaker

GeneJazz

Member
2010-10-11 10:50 pm
NYC
I have a brand new ARC VS60. When I connect my Focal 1027 Be speakers to them, I have a very soft 60hz hum coming from them. I tried disconnecting the input of the amp and hum still remains. I also tried lifting ground and the hum is still there.

The hum is not loud at all but I am concerned if this is a defective amp. Should this amp be dead silent at the speaker or is a small amount of hum normal? For what it's worth, I upgraded from a solid state Arcam amp and that amp was dead silent.
 
considering what's written on their web page ARCDB - VS60 , I think that's time for you to call your dealer

don't do anything , because of warranty .

with this :

Hum & noise Less than 0.2mV RMS - 100dB beow rated output (IHF weighted, input shorted).
Power supply energy storage Approximately 166 joules.

there must not be any hum
 

GeneJazz

Member
2010-10-11 10:50 pm
NYC
After a quick search on "hifi strereo tube amp hum", this problem is not uncommon at all. Vast amounts of tube amp seems to emit a soft hum in the speaker from what I read. The hum is quiet and only noticeable when I put my ear to the speaker. However, I'd like to correct this if possible. I'll test it with the inputs shorted and see what's up.
 
No. Just unplugged. I've been told the Focal 1027 Bes are high sensitivity at 91db. Is this low? or high?


Speakers which are above 100db sensitive are known to make hum problems a lot worse. 91db is just above average.

Unless you actually measure the hum with the inputs shorted it will be hard to say anything for certain. And to what end? Is it worth shipping the amp back for a minute amount of hum? Not sure if ARC would be able to do much about it anyway.
 

GeneJazz

Member
2010-10-11 10:50 pm
NYC
I did just that. I took the amp to my bench and measured the hum with the inputs shorted. The published spec states 0.2mV IHF weighted.

I measured on the right channel, 0.25mV and on the left channel, 0.7mV. Nearly 3 x more hum. I did notice the left channel hum to be louder so I wanted to confirm. It is not the tubes as I swapped the R and L channel tubes and still got the same measurements.

I think there is something wrong with this amp and will call ARC for an exchange or repair. A brand new amp shouldn't have 3 x more hum in 1 channel, I would think.
 
I measured on the right channel, 0.25mV and on the left channel, 0.7mV. Nearly 3 x more hum.



A friendly advice: unless you intend to listen exclusively on headphones, ignore the hum. I don't believe it can actually interfere with yours or anyone else's enjoyment of music.

As an academic exercise it is certainly interesting to analyse the source of it. Without knowing anything about the circuit i would venture that it is most probably a design oversight, rather than a defective component. As HT and heater supplies are common you can remove the PS as a culprit. Which mostly leaves poor ground layout or inductive coupling as possible sources of hum. Not likely ARC will fix either of those but a good technician, after investing some time and effort might.
 

GeneJazz

Member
2010-10-11 10:50 pm
NYC
A friendly advice: unless you intend to listen exclusively on headphones, ignore the hum. I don't believe it can actually interfere with yours or anyone else's enjoyment of music.

As an academic exercise it is certainly interesting to analyse the source of it. Without knowing anything about the circuit i would venture that it is most probably a design oversight, rather than a defective component. As HT and heater supplies are common you can remove the PS as a culprit. Which mostly leaves poor ground layout or inductive coupling as possible sources of hum. Not likely ARC will fix either of those but a good technician, after investing some time and effort might.

VS60 is a power amp so for headphone listening, I would leave it off. You are right with your insight. From a practical point of view, the hum is not too objectionable.

On the other hand, my tech interests wants to get to the bottom of this. Who knows, it might be as simple as using a chopstick and shifting some wires a bit.

I'll speak with ARC again. Perhaps they can send me another one and if that one is good, I'll do an exchange. These amps aren't cheap and I don't think one channel should be so out of spec.
 
Who knows, it might be as simple as using a chopstick and shifting some wires a bit.


Quite possibly. Or moving a grounding wire to a different ground point on the pcb. It is also possible there are separate for each channel PS decoupling caps for the input tubes - one may be defective. A pic or two taken from both sides of the main pcb could point towards a solution if you really are interested. I'll be quite curious what ARC will have to say about this.
 
Since the amplifier is new and under warranty I would just get it exchanged. Don't doing anything that might void the warranty. Generally the big ARC amplifiers I have worked on (not recently I'll add) have handily met or exceeded their published specifications when working as designed. These guys aren't noted for their design oversights or poor QC..
 

GeneJazz

Member
2010-10-11 10:50 pm
NYC
Quite possibly. Or moving a grounding wire to a different ground point on the pcb. It is also possible there are separate for each channel PS decoupling caps for the input tubes - one may be defective. A pic or two taken from both sides of the main pcb could point towards a solution if you really are interested. I'll be quite curious what ARC will have to say about this.

I did a bit of chopsticking. No change. The wires are done very well and quite sturdy.

It would be helpful if I can find a schematic for this amp but there is none out there. ARC is a bit reluctant to pass out schematics or so it seems.

I'll take some photos of the main pcb soon. I've got the amp back in my system and it does sound quite good. But I gotta get the hum in spec! :bomb: They claim 0.2mVAC with input shorted. IHF weighted. I am seeing 0.7 to 0.8mVAC on the left channel. I can live with the right at about 0.26 to 0.4 depending on the voltage. My listening room is 123 vAC and my bench is 117vAC.
 
I did a bit of chopsticking. No change. The wires are done very well and quite sturdy.

It would be helpful if I can find a schematic for this amp but there is none out there. ARC is a bit reluctant to pass out schematics or so it seems.

I'll take some photos of the main pcb soon. I've got the amp back in my system and it does sound quite good. But I gotta get the hum in spec! :bomb: They claim 0.2mVAC with input shorted. IHF weighted. I am seeing 0.7 to 0.8mVAC on the left channel. I can live with the right at about 0.26 to 0.4 depending on the voltage. My listening room is 123 vAC and my bench is 117vAC.

You didn't mention the IHF weighting curve, you might want to go have a look at that.. If they are using the old IHF A weighting curve which is more less identical to curve A shown in the link below - then your amp more than meets spec.. Ouch.. If IHF C then it does not meet the specification on one channel.

A-weighting - Wikipedia, the free encyclopedia

The A weighting filter is down ~24dB relative to 1kHz, so you could have about 3mVrms of hum on the output and still meet the specification. Most manufacturers used the IHF A method because it produces great looking numbers and the argument was that the filter accounted for perception of noise by the average listener. IMHO C weighting is a more honest choice at least as far as perception is concerned..

I don't believe in using weighting for electrical noise floor measurements, and for reasons which should be clear...
My amp has <0.5mVrms unweighted broadband output noise and is quiet with >100dB efficient speakers..
 
Last edited:

GeneJazz

Member
2010-10-11 10:50 pm
NYC
You didn't mention the IHF weighting curve, you might want to go have a look at that.. If they are using the old IHF A weighting curve which is more less identical to curve A shown in the link below - then your amp more than meets spec.. Ouch.. If IHF C then it does not meet the specification on one channel.

A-weighting - Wikipedia, the free encyclopedia

The A weighting filter is down ~24dB relative to 1kHz, so you could have about 3mVrms of hum on the output and still meet the specification. Most manufacturers used the IHF A method because it produces great looking numbers and the argument was that the filter accounted for perception of noise by the average listener. IMHO C weighting is a more honest choice at least as far as perception is concerned..

I don't believe in using weighting for electrical noise floor measurements, and for reasons which should be clear...
My amp has <0.5mVrms unweighted broadband output noise and is quiet with >100dB efficient speakers..

Thanks for your response. I do not know what curve ARC is using on the spec sheet. It just states IHF weighted. You can see it here:
http://www.audioresearch.com/VS60.pdf

Also the ARC spec of 0.2mV is rms. I just measured using using my Fluke meter with the inputs shorted. Usually, rms means lower than actual measurement on a meter, I think. So perhaps I am just obsessing too much?

So if it is using A curve, the amp meets spec by quite a bit. I spoke with ARC and the rep, Calvin, told me some hum in the speaker is not abnormal for this amp. Now, if it was something really loud from like 3 meters away, that would be concerning.

I am thinking of using PS Audio's Humbuster III. PS Audio told me it would most likely quiet the amp.

I don't want to exchange the amp if it is in spec and another one will probably show the same hum...

Your amp sounds really quiet. I'll have to check it out. I used to live in Boston. Now in NYC.
 

GeneJazz

Member
2010-10-11 10:50 pm
NYC
You didn't mention the IHF weighting curve, you might want to go have a look at that.. If they are using the old IHF A weighting curve which is more less identical to curve A shown in the link below - then your amp more than meets spec.. Ouch.. If IHF C then it does not meet the specification on one channel.

A-weighting - Wikipedia, the free encyclopedia

The A weighting filter is down ~24dB relative to 1kHz, so you could have about 3mVrms of hum on the output and still meet the specification. Most manufacturers used the IHF A method because it produces great looking numbers and the argument was that the filter accounted for perception of noise by the average listener. IMHO C weighting is a more honest choice at least as far as perception is concerned..

I don't believe in using weighting for electrical noise floor measurements, and for reasons which should be clear...
My amp has <0.5mVrms unweighted broadband output noise and is quiet with >100dB efficient speakers..

I read up on the different weighing curves. I have a feeling ARC does use the A curve. Which would show great numbers deceptively for hum... I have left a message asking ARC about which weighing curve they use. ARC has been working with me to solve this problem but since I am not the only customer, it does not go as quickly as I'd like.

With the inputs shorted, the hum through the speaker is minimal although I can hear it. The left is louder than the right which my measurements confirm.

If the hum was at the level when the amp inputs are shorted, I can be happy. However, no one uses the amp like that and when it is connected to my preamp (ARC LS25 MKII), the hum is much more objectionable. I can actually hear it from my listening position faintly if the room is dead quiet.

The gear is plugged into a PS Audio Quintet power conditioner. Any other hints on what can possibly be causing this? I thought it might be the CATV even though it already has MAGIC in it's path. I disconnected the cable but the noise remains.