Looking for a solid state power amplifier project

[colin.hepburn]

Hi Stuart/All
yep I did have the Cyrus one amp back in the day and yes the switches were its weak point I recall it also had Quality capacitor’s in the single path and the split foil power supply caps as well
colin

[bikerhifinut]

I'm a fan of the Quad 405 and 405-2 so take that on board when I recommend one as a "do up".
Keith Snooks website is probably the definitive resource here.
However I feel you don't really get a bigger bang for your buck with most of the tweaks and "upgrades"
I'd concentrate on the reliabilty side of things. theres a couple of resistors that are too low in wattage, i cant remember which but it simply involves snipping out the 0.25W originals and subbing 1 or 2W replacements and standing them off the PCB to allow ventilation.
likewise on the 405-2 the horrible and nasty clamp/crowbar circuit is on the PCB. A common cause of failure is the 10uF non polarised electrolytic which tends to dry out and go low in value, its that brown thing on the clamp circuit. This can cause, I'm told, the crowbar to engage at lower power outputs which then starts a destructive cascade through the silicon and as often as not by the time one of the 4A power fuses blows you've fried the output silcon and pssibly the drivers too. So for me if its all working still, hang on to the original 2SD424 or similar output transistors if they work they're fine, if blown a good sub is MJ1502 but buy from a reliable source and get branded ones from anywhere but China.
The clamp circuits and on the 405 you'll find a separate clamp board on the speaker outputs, here I'd throw it away and get something like the velleman kit speaker protection relay board. Otherwise me and my pro designer pal have devised a tweak that simply involves replacing those 'orrible non polarised capacitors with a pair of polyprop 4.7uF in parallel, one on top of the board and the other tacked on underneath. they should never give any trouble again.
Avoid any suggestions to replace the other electrolytics with non polarised expensive audiophile tat. You could if sufficiently concerned use something like a Nichicon FG or Elna starget etc Low ESR jobs in the audio signal path and if really OCD bypass with a lower value Polyprop, I've got a 405 thats had hundreds spent on it with tweaks all over the board including a pointless feedback tweak on the input opamp that knocked the sensitivity down to about a volt, that amp actually needs a preamp with a decent bit of gain (12dB) and a fancy low noise op amp in the socket. I can't hear any difference between that and the refurbed 405-2 which had its output silicon replaced after a crowbar failure on one side. In fact if you had to put me on the spot I'd say that the more or less standard 405-2 has a nicer more relaxing sound but its too close to call.
It might be worth replacing the PSU reservoir capacitors if they're old and dried out, I'd check em before junking though. many people apparently replace the 10,000uF psu caps with 22000uF jobs, 63V are more than adequate here, I guess that will help on loud transients but with 100 real world genuine watts available each side at full chat You'd need to be using really insensitive speakers.
These are good amps, genuine High Fidelity and you'll find a lot of tatty looking ex pro audio ones on the used market that will have been hammered in sound reinforcement/PA applications in the past and by some big names in the business.
Their strengths lie in their easy serviceability so a repair technician can diagnose and fix a failure or problem realtively easily and fix it. Their unreliability as perceived by some critics is realy a bit unfair as all surviving 405 have way outlasted any "reasonable" service life and extra reliability mods are easily and cheaply at our hobbyist level acheived.
mains transformers can go noisy (buzzy rather than electrical hum) they are encapsulated and are unrepairable. The answer here is to source a decent 35 0 35V x 300vA toroid and if youre a half decent metal basher, fabricate a mounting bracket. There are kits out there but you can do it for literally half the price if you are prepared to do some simple metal work.

Pay less than £200 for a scruffy working one with a quiet mains transformer, I'd ask that question of any seller first, I had my fingers burned by not asking that question a while back,
Smart refurbished examples may cost you more Colin, again Caveat Emptor.
I would bet this weeks pension that Bobs 405-2 will be as clean and shiny as the day he bought it and would need relatively little to ensure its reliability for a couple of decades to come. So my comments on valuations will need an upward adjustment depending on how much it actually NEEDS doing.
I'm not au fait with the Quad preamps but the 44 i am told can be troublesome and not that good because of the CMOS swithing used in the signal path amongst other issues.
I use my 405 collection very happily with a modern rega Cursa3 preamp and also my valve CF buffer with good signal sources 500mV plus, and my tweaked Pre4 based valve preamp, both have a very low output Z so are fine for driving silicon amps.
Long post colin hope your heads not spinning.
Andy

[bikerhifinut]

Apologies if its a late post, I've been down a personal rabbit hole for a long time now

Andy

[Desmodromic]

What about a Quad 520f. I love mine, it's like a 306 but on steroids! Fantastic power supply, and pretty easy to work on. A very underrated amp.

[colin.hepburn]

Hi Andy/all
Thanks for the update on the quad 405-2 all things are on hold at the moment due to two things happening I was Diagnosed with angina mid covid and just had two stents fitted in March there and have now fully recovered thankfully

And my second issue is I just had an old boiler cupboard removed from the corner of my room and some asbestos was found in the ceiling so that was also taken out and now I am on a re-decorating frenzy so I have no hi-fi setup running right now but have a bigger better room size 15.6ft W10.4ft-H 7.5ft I have gained back the corner space about 6ft. 3in so after the decorating I will have a new room to play with I hope it works
So I can return to the hobby and enjoy the valve and solid-state adventures soon again
Colin

[bob orbell]

Sorry to read about your issues Colin, I have just posted on another knee replacement hope you get sorted soon Colin, remember, the hobby can wait, your life cant. Bob

[Richard]

Yes best wishes for a speedy recovery Colin

[colin.hepburn]

Hi Bob/Richard
Thanks for the concerns guys just did not expect this but feeling brand new again now I was told to keep walking and light uphill treks as much as I can well I have the perfect mentor to help me do that and he’s called Buddy the Border collie so yep going fine it's amassing what they can do now with these Stents injected them in through the vain in your arm job done in two hours while watching it live on the screen
PS I’m still redecorating