What amp for £250 new or used?

My Cambridge 640A amp keeps overheating and people have suggested it is a fault with the range and that getting repaired will only be a temp measure.

I have decided to get shut and buy an amp for around £250 when I have saved a bit of dosh but what would you buy? I am considering used because none of my brand new ones have lasted.

I have Wharfdale 9.1 Speakers, Grado SR60i headphones, Marantz CD6000 OSE LE CDP, M Audio sound card, Project Debut II and I have a very old Rotel pre amp that Rory sold me I use for the phono stage.

So should I consider buying?

 

Does it overheat even when used at low volume, or only when driven loud?

 

Both but more so if it is at a loud volume. It does it on the headphones too (Grado). I moved it to top of the rack a few months back which solved the problem until now. It gets so hot that I could cook toast on it even when driving the headphones.

 

Sounds faulty to me. Its a Class A/B amplifier and shouldn't get that hot running headphones where it has sod-all current demand.

Anyway, used amp for £250 and fully featured with a headphone socket.
Audiolab 8000A . Get a late one (serial F onwards) and that should do the trick.

 

Yep I have had my eye on Audiolab. I mainly listen to classic rock and punk these days (though listening to Cranberries now). Seen a PM6010 KI for £150 which I may consider but I think the Audiolab will be in a different league.

The heat is on the right hand side near the transformer so could it be a fault with that although when I ran it topless the ehat seems to be on the right hand rail, I am not sure what could cause those transistors to get so hot?

The thermal paste from the transistor to the heatsink must be ok otherwise the rail wouldn't be so hot.

If a repair is possible then I would want to repair but I would like to know what the fault likely is first.

 

Sounds like bias gone too high. Wont CA fix it?

Consider a Cyrus 7. Headphone socket on the back.

 

for budget money I'd go with vintage quality every time.

yes, you run a risk that it might need repair in a few years time - but until then it will be a very high class amp, and may be repairable if the worst happens.

Decent vintage amps often have good quality headphone amps in them too.


Here is one that I'd strongly consider in your shoes - look at the quality inside with the lid off.... you just cant get that from something a handful of years old for the same money.

http://cgi.ebay.co.uk/Super-RARE-Vi..._HomeAudioHiFi_Amplifiers&hash=item5640b5ce82

 

Rega have just reduced the price of a Brio to ?250 until they have built everything they can from the parts on stock/order. It would be well worth a listen.
________
Buy marijuana seeds

 

I've got the 640a service manual if you want it.

 

130455465011 and 350415906732

 

?

 

Does it give figures for bias and instructions on how to check this (should do I hope).

 

Cambridge Audio Azur 640A Amplifier Bias Setup​

To set the bias on the 640A Amplifier:

Measure the voltage across R22 and R100 (Left channel) individually; they
should be adjusted using Preset R116 so that the voltage measured on each
resistor is greater than 13mV. This procedure should then be repeated for the
Right channel by measuring the voltage across R52 and R69 whilst adjusting
Preset R68 so that the voltage measured on each resistor is greater than 13mV.

I've drawn some arrows on the PCB where the parts are:

 

Excellent.

Amazingtrade - if you are comfortable doing the adjustments yourself and have a good DVM then have a go.

If not, take a print of the above and take the amp along to a TV/electrical repair shop and ask them to adjust it for you. Give the amp 15 minutes or so warm up but the adjustment will take them no longer than 15 minutes. Should cost very little.

It says 'more than' 13mv which isn't exactly helpful - probably means distortion hits a minimum at that point and going higher doesn't help.
Get them to set 15mv on each side and should be job done.

 

Never really done any live work so probably will leave it to a TV shop, there is one quite near me that has been there for decades. He did say he did HIFI repairs but I wanted a general idea what the problem could be. The problem is I ask him to correct the bias won't that get his back up?

 

Depends how you word it I suppose. Just say you have a suspicion the bias is out, and you'd appreciate it if he can check it for you.

Alternatively get a digital meter (do you have one?) and simply put one probe on either side of those big resistors. Let us know the voltage. Don't stick the probes anywhere else and it won't blow up. Don't touch anything near the transformer and it won't kill you.

At least if you can measure that voltage we can know if the problem is here. That voltage is tiny by the way, it won't hurt you, although the mains can if you are silly enough to touch it. Working on your car is more dangerous.

 

I have a cheap one I use to measure voltages of computer power supplies. I fix PCs for a living but the difference with PCs is the dangerious voltages are sealed inside the power supply.

I will open the lid up tomorrow and have a look. I know exactly what and how to do it from the instructions above it is just confidence. Just worried about my hands slipping with the probes.

 

It can help to use mini crock clips on the meter probes. Maplin sell them.

Attach them to either side of the resistor with the amp off, then switch on, wait about 15 minutes and see what reading you get.

If and when you adjust the bias posts - do it very gently and don't rotate them quickly.

 

Thanks, I may invest in a better multimeter anyway as it is something I do use a lot. I have been meaning to get back into electronics and soldering too, I have not touched anything like for ten years. I prefer to get wasted on a saturday night these days than mess about with 555 timers and the like.

I may buy the proper tool too to adjust the pots.

What will happen if I over do the adjustment?

PS Thanks for all your help, if I can repair it I would rather keep it and spend the money on my business.

 

Worth getting a decent meter - many come with a range of probes including the clips I've mentioned. Makes life a lot easier. Some also come with temperature probes which are useful for audio.

if you over do the adjustment you'll increase the the quiescent current and if you do so severely it might blow the output stage, though its not likely to happen instantly so you get some tine to back off.
Don't let this deter you - go slowly and you cannot do any harm.
Measure it first and lets see what is going on.