My Cambridge 640a keeps over heating when using headphones, the circuit protection mode triggers but when I feel the top it is extremely hot. It always seems to happen when using headphones. It is worse with my Grado SR60is.
Anybody know what is causing this? There is no obvious problems with it :( I have an old Rotel pre amp so I may have to use that to drive these headphones or buy a headphone amp :(
The thing is amp used to work perfectly with this headphones so I am not sure what is up.
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This is from a friend in Plymouth ~
If it's getting hot the obvious answer is that it's driving too hard, into too low an impedance, which is strange as he says the headphones were OK b4. Maybe they're faulty (a partial short in the cable) Though I'd have thought the amp would have had a thermal cut out? He sez something about an overload circuit cutting in? Whats that about?
Tell him to TRY SOME OTHER HEADPHONES FIRST!
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Remembering my old electronics, certain classes of amp would run hotter when on a lighter load (headphones compared to speakers).
Your amp probably depends on covection to cool it.
Might be worth removing the covers and giving it a gentle vacuum. ISTR you've posted pics of computers blocked up with dirt.
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Rattle I'm firmly stuck with 20-30yr old classic hi-fi gear but I do keep an eye out on the modern scene now and again to see the latest wheel re-invention. I notice your amp has something of a reputation for running hot and triggering its protection cut-out even when only driven at modest levels. Seems to be quite speaker/load critical.
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>> Remembering my old electronics certain classes of amp would run hotter when on a lighter
>> load (headphones compared to speakers).
>>
>> Your amp probably depends on covection to cool it.
>>
>> Might be worth removing the covers and giving it a gentle vacuum. ISTR you've posted
>> pics of computers blocked up with dirt.
>>
Yes, Class A amplifiers for example draw the same amount of power irrespective of load, so tend to run hot. I agree checking for dust is a good place to start.
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>> It always seems to happen when using headphones.
What's inside the headphones?
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What's inside the headphones?
Rattle.
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That was my first thought. But I didn't reply as I thought the mods would think it cruel and un-necessary. :-)
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One of the words I can never read correctly is "shopfitters"; my brain always reads it as "shoplifters".
I'm having the same problem with the title of this thread - Amy is apparently overheating. I'll go no further.
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My Grados are hand built in New York as a result build quality suffers slightly. I used my 20 year old Rotel BX870 pre amp to drive the headphones last night and there wasn't a problem, I wiggled the wire etc too and dosn't seem to be a break anywhere.
My old Goldring headphones snapped so I have been using my audiophile pair (the grados cost nearly £100) as a main set which is silly. I do have some old Berydrnamics but they sound a bit lifeless for TV. My Senhsier PX100 sound fine for TV and are extremely convertable but lead won't scretch to my amp so I may be an extension cable for it.
I seem to listening to more and more music so my amp seems to be left on a lot more, there is noise coming from my soundcard (only slightly) and if my amp is set to that channel I suspect its not heloping the cooling.
There is no obvious signs of dust inside the amp.
I think I have come to the conclusion that the amp is finding it too hard to drive the headphones.
My amp never cuts off with my brilliant Wharfdeale Diamond 9.1s.
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Beware, I know nothing of what you write, but can you attach some sort of sub-woofer-type power sink? I know nothing... but SFAIK a sub woofer is put in series with speakers in order to protect them from blowing. A silent, but high impedance power-sink might have the desired effect of generating the heat outside the amp.
Otherwise, what about installing a fan, or drilling larger ventilation holes in the box. Clearly the amp is no use for your purposes, so you might as well either (1) sell it, or (2) bin it, or (3) attempt to fix it.
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Headphone impedance is too low.
If you more or less exclusively use headphones, consider a dedicated headphone amp. Normal amps are designed to deliver their rated loads, not 1/1000 of it, and I doubt you're going to be betting the best sound quality this way.
Good headphone amps range from £20 - £200.
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My Rotel amp sounds ok via the headphones. It does also sound a bit better quality but not played any music through it yet. I have been thinking of getting either the Project headphone amp (£90) or a Creek at £140.
Money is a little tight so I will wait till after my holiday.
www.superfi.co.uk/index.cfm/page/moreinfo.cfm/Product_ID/2445
www.superfi.co.uk/index.cfm/page/moreinfo.cfm/Product_ID/2229
I also check out ebay although it seems to be full of potential dangerious Hong Kong imports.
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Stick a heat sink on top of the output power transistors, and stick a small fan in the back.
You must have all the required thermal paste, heatsinks and fan in your pc spares cupboard.
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The amps pretty much one big heatsink as it is :).
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According to the interweb, the headphones are 32 ohms nominal impedance, and the amplifier is quoted for 4 or 8 ohms (providing up to 100 or 75 Watts).
Maybe the amplificateur needs the load - how about a pair a nice beefy 6.8 ohm shunts to keep it happy?
It would certainly be a conversation piece, and maybe keep you warm too.
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And if I understood that lot I wouldn't be a Builder.
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If I understand that I would be able to do more with electronics than build silly synthesisers using a 555 timer!
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the impendence for the amp at 4-8 ohms are for the speakers, cambridge amps do run hot cos its a class, if its got a headphone socket then it is suitable for headphones, rattle check that the amp top cover or air vents arent blocked.
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Its not a class A as far as I know. It is 70w per channel into an 8ohm load though.
There is about 3inches of clearence from the top of the amp to the top of the shelve so that may be the problem. I have solved the problem for now by using the output on my Rotel BX870 pre amp.
The full specs are here
www.cambridgeaudio.com/set_territory.php?TID=1&Redirect=/summary.php?PID=13
Edit it seems is a class A I never even realised :( Poor ventalation is the problem then!
PS Sajid welcome to the other side :).
Last edited by: RattleandSmoke on Sat 3 Apr 10 at 12:43
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Your amp is not likely to be true class A... it just uses some section of its circuits that can be called class A... a marketing ploy that has been used many times over the years. The heating issues you are suffering are entirely a know problem with this amp as it can be thrown by unusual loads.
None of this negates the fact it's a well reviewed amp but you just neeed to accept its foibles.
As a matter of interest I have four hi-fi amps here reaching back to 35yrs old and all will drive headphones only with no issues so it's not a common problem across all makes.... as you've found with the Rotel.
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The Rotel was a fantastic buy, I had a proplem of running out of inputs a few years back on a Marantz PM4000 (parents have this amp now) so somebody of the net sold me the Rotel for £15.
I use for extra inputs, headphone output and phono stage :). I think its circa 1993 but I was very surprised to find that inside its a very nice little pre amp.
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>> Its not a class A as far as I know. It is 70w per channel
>> into an 8ohm load though.
>>
Having read the specs. it does indeed say class A follower driver stage which implies it isn't the output stage, but a max. power input of over 600 Watts max. suggest it isn't terribly efficient overall.
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Well it was a cheap amp so all they really cared about is getting the best £ per sound. I am sure the EU will soon put a stop to it :(
I only paid £150 for it new and the sound makes my old Marantz PM4000 (£130 at the time) sound lifeless.
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I had the budget classic NAD3020 for about 8 years, that was lost in a burglary, I replaced it with a second hand Rotel RA840, I've still got that in the loft IIRC but it started distorting on one channel so I replaced it with a new Kenwood KA3020se, that probably still works fine but I haven't had a 'proper' stereo set up for about 5 years now so that's sitting in the loft as well :-(
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I tried out a Sugden A21 some time ago, which is meant to be class A - but only 15-20 watt output. Decided against it because of the heat it gave out!
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Sorry, I'll try to be a bit less obtuse.
If you wire a resistor of 6.8 ohms in parallel with each of the headphone feeds, the amp will be presented with about 5.6 ohms load.
( 1/6.8) + (1/32) is approx = (1/5.6)
The resistors would need to be quite high wattage, as they will be taking most of the output of the amp, and they would need to be non-inductive too.
Being in parallel, and taking most of the load, they are a "shunt".
I had a look at the service manual here:
www.scribd.com/doc/23336802/Cambridge-Azur640A-Pwr-Sm
which is very fiddly to zoom into for sufficient detail to read, and doesn't help a lot, other than confirm it is class B and give detail of the output change-over relay which mutes the speakers when you plug the headphones in.
As a complete aside, there are some people having problems immediately after unplugging the headphones. Quite what happens to the amplifier as the output change-over relay operates is anyones guess.
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Amp keeps overheating - Dog
