Cambridge Audio A75 in the House

[Qwin]

I’ve always owned Integrated Amplifiers, the current one being a Sugden A21a.

I’ve been thinking of moving to separate Pre/Power along the lines of the Croft Micro 25 with built in Phono stage and a power amp which could be the croft Series 7 or maybe something else.

I’m not exactly flush with funds so the Sugden may have to go to help fund the move and like many people making this change, I will be buying the Pre/Power elements with a period of fund raising between the purchases. Which ever one I buy first it will leave me in a position where I can’t play music.

So for a while I’ve been looking for a cheap power amp that could be used with the intended Croft, as a temporary measure. I spotted this Cambridge Audio A75 on Fleabay.


http://www.jkwynn.co.uk/Pics/A75_01.jpg


I used to own a Cambridge Audio P40 Integrated from the same era and always liked the way it sounded so I made the opening bid which turned out to be the only one and picked it up for sixty quid plus postage.
It fired up ok using the Pre Out on the Sugden to feed it, sounds nice, I think its 75w into 8 Ohms and 100w into 4 Ohms per channel.


http://www.jkwynn.co.uk/Pics/A75_02.jpg


I opened it up anyway and gave things the once over. Some of the screws holding the power transistors were loose so I nipped them up to make sure they made good contact with the heat sink. I also cleaned the contacts for the fuses, other than that it looks pretty minty. It has the switch on thump which all of the Cambridge amps of the period had but I recon it was well worth what I paid for it and will bridge the gap nicely while I decide what power amp I should settle on and raise whatever Wonga is required. :eyebrows:

[istari_knight]

Interesting, it looks well enough Ken !

Nice simple design, slightly surprised to see the transformer lead out wires "exit point" facing the circuit though... I thought it was good practice to have them facing away from any areas potentially susceptible to noise pickup. In my DIY Naim clone I rotated the toroid whilst turned on & the noise/background hum varied as it went 'round... Being a Stan Curtis design [I think ?] I'm sure its fine.

[Qwin]

When I first saw it advertised I did a bit of reaserch and the name of Stan Curtis came to my attention, he certainly has a good reputation as a designer and yes it is one of his.

The A75 is often refered to as a dual mono design, but when I looked at the schematic one of the toroidals supplys the + rail on the board and the other supplies the - (50v).

Simple layout like you said, it should do till I decide what power amp I will finally pair with the Croft Pre/Phono I am saving for at present.

[dowser]

One of my big regrets was selling my p/a75 combo when I migrated to Oz - should you ever want to sell, let me know :) thanks, Richard

[walpurgis]

You certainly haven't gone wrong for sixty notes. That's a decent unit. I prefer the Cambridge power amps of that era to the pre-amps, which I thought were not so good.

[Qwin]

I think it is amazing value, the unit is really clean and the board looks untouched.

I had been looking at what was on offer in the second hand market from Rotel, Nad, Arcam, Modern Cambridge Audio, etc which were all bringing relatively highish prices, then this oldie but goodie came along.

It certainly wont disgrace itself in my system while I work out which power amp I would like, I'm thinking Chord or Audio Research as a used buy at present. Still saving for the Pre-amp at the moment so I can't get too carried away.:D

dowser - this is a temporary fix, so I will keep you in mind when I am ready to part with it.

[dowser]

I think it is amazing value, the unit is really clean and the board looks untouched.

I had been looking at what was on offer in the second hand market from Rotel, Nad, Arcam, Modern Cambridge Audio, etc which were all bringing relatively highish prices, then this oldie but goodie came along.

It certainly wont disgrace itself in my system while I work out which power amp I would like, I'm thinking Chord or Audio Research as a used buy at present. Still saving for the Pre-amp at the moment so I can't get too carried away.:D

dowser - this is a temporary fix, so I will keep you in mind when I am ready to part with it.

Thanks :)

[Qwin]

For safety sake as much as anything else, I decided to replace all the electrolytics in the amp.

http://www.jkwynn.co.uk/Pics/A75_re_cap.jpg

The four big power supply smoothing caps were originally 6,800uf 50v.
As the power rails are 50v I thought this a bit marginal so upgraded them to 10,000uf 63v items.

For the smaller values on the board I used Nichicon FG and Elna Silmic for the one value not available from Nichicon.

The job didn’t go off without some drama, when I powered it up on the bench a dodgy old solder joint on one of the leads from a transformer blew were it connected to the board. :rolleyes: I cleaned and re-made the solder joint, plus to be safe, I also re-made the others from the two toroidal transformers and it then powered up fine.

The new caps have improved the sound quality slightly, not quite as lush, the top end being cleaner and not as blurred in comparison, a very good sounding amp brought back to its full potential. :)

[Paul Lange]

I suspect that you have a very good, solid and underrated amplifier there. It doesn't look very sexy though.

[DSJR]

Stan the Man did some awful things in his time, but his successes were vast and this era was one of them IMO - he's also been very sympathetic with the current Quad stuff too, keeping these venerable designs up to date and still competitive :respect:

It was commonplace to drive supply caps hard as it saves a few quid on the retail price. I have tales of one well known manufacturer using the voltage tolerance margin of a cap to use over-run caps thereby saving money. Some like Naim, sail VERY close to the wind, using 40V caps on a mean 37V rail. Another manufacturer I've grown to know well in recent times and who claimed never to have cap issues (alright, NVA), uses 63V supply caps when the transformer is a 25V one, offering around 30 to 35V off the secondaries before rectification/regulation. If Naim had done this from the off IMO, a lot of the hugely expensive servicing necessary on their gear wouldn't be necessary - in my opinion....

[istari_knight]

Those [infamous] maroon ROE 'lytic caps have a distinct sonic signature & do tend to degrade quite badly over the years IME, they may pass a visual inspection but are quite possibly down 50% in value with goodness knows what ESR.

I don't have a service manual for this model but from what I can make out looking at your picture the ROE's are in the feedback position so will be having a huge affect on the overall sound of the amplifier... It also appears the Nichicons are rail decoupling [fine but Panasonic FC/FM would be better] & the Elna Silmics input coupling [I can highly recommend replacing these with MKT/MKP, even if the originals were 10uF or so you can safely replace them with 2.2uF/3.3uF... The corner frequency will still be below what you speakers can reproduce.]

All good fun regardless :thumbsup:

[DSJR]

Those ROE caps were regarded as really good back in the day I believe. As with all old caps, I think it's the way they were driven in the circuit that determines how long they remain stable.

The 40V 7000uF Mallory supply caps in my little Crowns measured above this value after forty years.. The supply rails were 30V approx. and the amps are always very cool running (for use 24/7 in a 19" rack). Didn't stop me fitting 22,000uF ones though (same physical size), but the main sonic difference is longer shut off as they discharge, rather than a 'musical' one.

[Qwin]

Didn't realize the ROE parts were lytics. Thanks for that.

I have a scematic, but no parts list so no idea what the values are and the markings 220/6 U2 don't mean anything to me, 220v 6uf perhaps?

Panasonic seem to be changing a lot of their cap ranges at present and Farnell/RS both seem to be waiting for deliveries around mid December which would suggest this could be new range stuff as lead times are not usually that long.

EDIT: From a quick search it would appear the ROE caps are 220uf 6vdc and bi polar. If that makes sense?

[istari_knight]

As Dave says they were highly regarded back in the day, used by both Naim & B&O ;) But that was a long time ago in capacitor terms...

220uF/6v sounds right for feedback as they'll have less than 500mV across them & as such polar/bi-polar won't make any difference ['lyrics are generally forgiving of reverse bias up to 1v/1.5v.]

[Qwin]

James – Would the caps I am proposing below be suitable.

For the 2.2uf 100v Elna Silmics: http://www.ebay.co.uk/itm/10-X-CAP-FILM-PET-2-2UF-100V-RAD-Part-WIMA-MKS2D042201N00KSSD/271444905301?_trksid=p2045573.c100033.m2042&_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1% 26asc%3D20131017132637%26meid%3D9b07cbabeadc40e1a7 24404090ef4629%26pid%3D100033%26prg%3D201310171326 37%26rk%3D2%26rkt%3D4%26sd%3D350819437946

For the ROE 220/6 the 220uf 16v version of this: http://www.hificollective.co.uk/components/nichicon_es.html


Cheers

[istari_knight]

That'll be fine Ken, I've used Nichicon ES in feedback positions myself on occasion.

I buy most of my films from here: http://stores.ebay.co.uk/HFO-Elektronik/Capacitors-/_i.html?_fsub=510193019&_sid=1093613689&_trksid=p4634.c0.m322

Works out much cheaper & they always have lots of interesting stuff thats difficult to find over here.

[Qwin]

:thumbsup:

[dowser]

Great work Qwin - my offer stands :)

[Qwin]

.
I’ve fitted the additional 2 caps per side as advised, the picture below shows one of the identical halves of the board.

http://www.jkwynn.co.uk/Pics/A75_03.jpg

Caps fitted were:

A). Samwha 10,000uf/63v electrolytic.

B). Nichicon FG electrolytic 100uf/63v.

C). Nichicon ES Bi-Polar electrolytic 220uf/6.3v

D). WIMA Film Cap (PET) 2.2uf/100v

It’s only had a couple of hours for parts to bed in, but it's sounding even better in the top half of the range, especially the very top end, so much more detailed, this is one damn good amp, very easy to forget about the equipment and enjoy the tracks being played.

Thanks for that advice James.

:thumbsup:

[istari_knight]

Nice one :)