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power amp experts, thoughts on this 1000w beast!

that's very fair...to be honest, I am not stressing, and I had to switch around something, either a cable or the sockets as the hot and cold were reversed. Next step to do that precisely and decide if it's any good and worth spending more time on it.
 
I am joking, of course! While I do have a lovely pair of SBLs in walnut spare, I also have an old speaker I used to use in the kitchen, I plan to use as a 'stunt speaker' :D
 
davidismynaim said:
Alan, thank you for this link, I have wanted to read this excellent article before, but the previous searches/links have been dead. I will follow this and your advice and split the link between pin 1 and 3. Attach shield of the singal internal cable to pin 3 and attach pin 1 to chassis.
Click to expand...

Here is the other useful article by Rane for various cable connections between balanced and unbalanced equipment
https://www.ranecommercial.com/kb_article.php?article=2107
Alan
 
quick update, replaced all 4 XLR inputs, sex changed and switched hot and cold, so now conforming with modern standards. Connected up with a stunt speaker, and played directly from pre (SE) and just the bass from dbx crossover (balanced) 3 channels good, have to put my ear up very close to speaker to hear the faintest hiss. Channel 1 is bad, playing, but much louder buzz/hissing sound. Hoping its a bad cap, I haven't changed over the 44 ROE caps on the 4 boards, so that will be next job, plus making up 3 more speaker cables with XLR connectors. Not able to do any critical listening, but a fairly positive start.

I've kept the pin 1,2, chassis ground connected, as that was how it was originally, but will try breaking link to pin 3 to see if that reduces the tiny amount of hum, but fairly sure this will not make any difference.
 
end state, ideally yes. I would like to end up with a 6 channel active system. DSP dbx venu360 managing crossovers. This amp driving a pair of subs (considering Ripoles or Linkwitz LXmini+2's) 20-60Hz and the TL bass picking up to 230hz in my innersound isis Hybrid ELS. I have a Sanders ESL Amp to drive the panels of the ELS. I have a pair of monoblock ncore500 nords, that I would sell on, if this amp beats them, or indeed gets close, to reduce box sprawl mostly and recover some funds.
 
sounds like a good find, the transistor selection reminds me of 70s-80s quad and many others.
no reason it won't sound good.
I'd be inclined to measure at least the frequency response, distortion and noise with some of the free software and a soundcard. I'd use RMAA in this case,
at least then you have a baseline of performance and you know there are no faults
there is a great article on PC measurement here, https://audioxpress.com/article/pra...ds-for-data-acquisition-in-audio-measurements
strangely the link to all parts is only on the first page.

probably the hard worked electrolytics would be first for replacement, and maybe some low noise newer versions of the small signal transistors (as common on quad 33 restorations)
make sure you know how to reset the DC point and Bias before changing transistors.
 
Grog said:
and maybe some low noise newer versions of the small signal transistors (as common on quad 33 restorations)
Click to expand...
Please don't. It's a high power DC coupled circuit. This means that any imbalance between small transistor pairs translates to a much bigger offset at the output and potential tragedy. The offset pot is a fine adjustment and won't compensate for problems elsewhere. In addition, the design is stable using the specified transistors and there's no guarantee that it will still be so with alternatives. Messing about with big poweramp design needs know how and test equipment that's good enough to let you see what's going on at all frequencies and load conditions.

As for wholesale capacitor replacement, I would get everything working properly first. The more variables you add, the harder it is to find the fault.
 
Grog said:
sounds like a good find, the transistor selection reminds me of 70s-80s quad and many others.
no reason it won't sound good.
I'd be inclined to measure at least the frequency response, distortion and noise with some of the free software and a soundcard. I'd use RMAA in this case,
at least then you have a baseline of performance and you know there are no faults
there is a great article on PC measurement here, https://audioxpress.com/article/pra...ds-for-data-acquisition-in-audio-measurements
strangely the link to all parts is only on the first page.

probably the hard worked electrolytics would be first for replacement, and maybe some low noise newer versions of the small signal transistors (as common on quad 33 restorations)
make sure you know how to reset the DC point and Bias before changing transistors.
Click to expand...

The small signal transistors are no different from today's and largely irrelevant to the amps performance. Leave them alone!.
There are plenty of reasons why it may sound rather lacking in fact.... but could be plenty good enough for driving woofers.
 
If it's going to be used to drive subs then I'd also leave the front end LPF filter unadjusted (ie retain teh 4k7 input resistor, and not drop it to 2k7 or 2k2 as suggested earlier in thread))

Yes that limits the top end to c 23khz as designed - but that's actually a good thing for noise and stability. And one less unchecked-change made before being sure the thing is working properly.
 
martin clark said:
If it's going to be used to drive subs then I'd also leave the front end LPF filter unadjusted (ie retain teh 4k7 input resistor, and not drop it to 2k7 or 2k2 as suggested earlier in thread))

Yes that limits the top end to c 23khz as designed - but that's actually a good thing for noise and stability. And one less unchecked-change made before being sure the thing is working properly.
Click to expand...

Martin, Thank you, glad you stepped in with this secondary advice, as I had planned to change the value, but as you say, leave it alone, due to my plans to purpose this as a bass amp. Well, actually bass and sub/ripole bass amp :)

Not sure re the one noisy channel to start swapping caps out maybe 2 at a time and retest, a. to ensure the swaps are ok and b. to see if the noise goes away that I have actually replaced a bad cap. The fault could of course be elsewhere. I don't have an ESR meter, just a MultiMeter. Unless anyone has a better suggestion that is my plan to start the overall cap swap out, and to try and fix the one faulty channel.
 
I would start with the basics. Swap the input cable with one of the good channels just to make sure it is the amp that's at fault. Check that there's continuity through the bridge/split switch. From what I can see on the schematic, it looks like both signal and ground are switched here. Look for anything that's been disturbed while you were working on the input sockets. Also, it's not clear on the schematic but it's just possible that your modifications
davidismynaim said:
I will follow this and your advice and split the link between pin 1 and 3. Attach shield of the singal internal cable to pin 3 and attach pin 1 to chassis.
Click to expand...
have left something floating when it needs to be connected to ground. Try reconnecting pins 1 and 3 and see if that improves anything.
 
taken some closeup of the caps on channel 1: and one is very clearly cracked, the other not looking great either. I do plan to replace all, but will start with these two. will give this board a better clear too!!
YWle8aG.png

prUfh9P.png
 
Ok, so it's been a while, but this amp has had a big makeover. I ended up working with an amazing engineer who specializes in Pro work, he does not normally do domestic work. He was sufficiently intrigued by it and initially took on the work to fix the mechanical hum on the toroid transformer. As you can see from the photo's, he went much further. Including added soft start, DC protection, balanced input, new transformers, new main caps, more cap replacements on the amp boards (I had done some already), redoing the connections at the back (I had done some, but he re-did it, to a much higher standard) and a significant re-working of the earthing to further improve the amp performance.

Incredible standard of work and is an amazing guy to deal with. We must have exchanged many hundred emails and I am looking forward to meeting up with him one day, shame we are 100's of miles apart.

Suffice to say, it is not the same amp, it is dead quiet mechanically, really nothing at all. Also very quiet electrically too, have to put my ear almost onto the drive units to hear the faintest hiss. It is driving the bass drivers on my hybrid ELS, and my own (homebuilt) ripoles, which are 4 12 inch bass drivers, all via DSP.

It measures well now, and all I can say is I am very pleased.

Before and after shot:

qekRjEJ.jpg


vrtlndf.jpg


QbmmiJx.jpg


lpUT1z2.jpg


And here it is in its place (amp on the right), just need a bigger room, so I can have the amp's facing the right way, haha.

FrArzJW.jpg
 
Those bleeder resistors look "more than adequate". Assuming the new PSU has the same 56V rail voltage as the original schematic, the power dissipation in the resistors is only

56*56/5600 = 0.56W

So those parts should last OK. ;) This site says it'll take about 3 minutes to drain down to 10V

https://www.digikey.co.uk/en/resour...version-calculator-capacitor-safety-discharge

And my HiFi brain keeps looking at the layout, seeing 2 toroids, 4 caps, and telling me IT MUST be a classic dual mono, split rail configuration. Despite knowing it isn't. But it looks like one!

BugBear
 
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