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Old 20th June 2012, 12:47 AM   #931
Ruwe is offline Ruwe  Europe
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Quote:
Originally Posted by Jolylabs View Post
Did some simple sums 40V rail 4K7 resistor = 0.34W since the resistor looks like a 0.25W part I'm not supprised it's hot. perhaps you could replace with 0.5w or 1W parts guess that's just an oversite of the original designer. Also as the parts are being over stressed they will degrade anyway which will accelerate the problem.
Right on!
I would do the same - just replace the resistors and everything should be fine. Even catastrophic failure at the output would not affect those resistors if they are rated for enough power dissipation.
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Old 20th June 2012, 09:11 AM   #932
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Originally Posted by brianuk View Post
Maybe the protection wasn't for the customers benefit - if it somewhat limits damage?

The amp can certainly be made unstable if not looked after EG low capacitance high inductance cables are required.
Good point . Naim recommended 2.5 mm section cable twisted 2.5 times per metre in the early days as ideal inductance-capacitance compromise . The old QED 79 strand met this criteria if split and twisted . Not my cup of tea sonically . I like DNM cable and used it for years with my Naim amps .

The resistors shown do look to possibly be 0.25 W . I use 0.6W resistors in my work a lot . I usually use 300 VAC as an upper limit for testing when I can ( certainly 270 V ) . The circuit I am thinking of is 0.33 W per device . They will be happy at that forever if the genuine Vishay parts . I have tested them at 1 watt for ages . Perhaps after 3 days there is a mild discoloration . The other Vishay I listed are special high temperature devices and should be ideal . I see them in many designs where perhaps there was doubt about a previous resistor .

About the protection . As said I am dubious it does anything for anybody . However real Naim amps were fantastically reliable . Maybe I gave up too quickly with my version .

Look at the Quad 303 . It's output protection is very simple . It is equally useless for the application I was looking at . The Quad is two diodes . In fact the Quad is remarkable in all aspects of design . Some say the Naim 160 was derived from this amp ( is style of distortion ) . If ever an amplifier deserved a clone it is this one . The regulated PSU is a work of pure genius , genius is always about elegant solutions . The distortion of this amp is typically -80db for the second harmonic . The next harmonics in near perfect exponential decay . A work of total genius and in my opinion the most significant amplifier ever made by a mile . Remember it was coming alive in 1965 and was nothing like it's competitors . Neither was it stratigraphical expensive . If it was updated and perhaps put to +/- supplies who knows how good it would be ? There are methods to stabilize single transistors inputs for that as JLH shows . The current limit reset by using a different emitter resistor .

The 33 whilst modest is still very usable if the dynamical range is carefully calculated . If so it is surprisingly good ( it brakes every rule we now set hence my admiration ) . The phono side is not very special . No digital source I know of if not overloading it is inferior to it ( even 24 bit ) . Much could also be said the same of the Naim's of the period from a nowadays perspective .

When I tested the Quad method and that of Naim I found no wonderful difference ( I was told very confidently I would ) . If interested it was using BD135 in a filter ciruit . I foresaw a situation where a very large capacitor could be connected in error . Convinced a simple Naim or Quad style protector would work I said to myself how sensible . Neither worked in the way expected and vast current flowed in the initial period . A simple series resistor would have cured the problem . However I didn't want that as this was a limit of possible noise application . A simple constant current source made of 2 x BD136 cured everything . I could have used other transistors . However BD135/136( 9/40 ) have no equal . This device was an afterthought and therefore the possibility for problems was due to not asking he right questions in the initial design stage . I could also have used MPSA 42 and 92 after this modification which would probably be better . That would have been impossible if using the very best conventional over current limiters .

I probably overstate the uselessness of this form of protection . Both Naim and Quad were ultra reliable designs . I suspect what happens is the protection if lucky catches the perfect moment where sound is not ruined and yet current is within the surge ratings of the transistor .

One could argue that the the 303 probably helped itself by the current possibilities of the power supply . It's own response time was limited by the 2N3055 .

Perhaps one day someone will hybrid the Quad and the Naim . Looks like it would be dramatically excellent . All you have to do is put the Quad output stage and bias on the Naim driver stage . I would give it an 80% chance of instant success . The sweetness of the Quad , the punch of the Naim . I would use the Quad protection reset with 0R1 x 2 emitter resistors ( series with protection between the 2 ) . That would mean onset of protection at 6 amps . Use MJ series transistors .

Quad 33 and Quad 303
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Old 9th September 2012, 07:34 PM   #933
Spuds is offline Spuds  South Africa
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I thought I would post some of my efforts as a thread revival.

3 x stereo 140's done and ready to go into their new cases.

Click the image to open in full size.
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Old 9th September 2012, 10:35 PM   #934
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Nice work! You have plenty of cooling and power supply there. You may find the heat distribution is not as good as it could be to those tall sinks connected at the top edge with just a thin L bracket shown on the near, right side amplifier (assuming that is the position you intend to mount them in their case.) However, they seem more than sufficient size and it may not be an issue in domestic use anyway.

Are these Tubeshunter's or similar universal PCBs?.
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Old 10th September 2012, 05:59 AM   #935
Spuds is offline Spuds  South Africa
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Quote:
Originally Posted by Ian Finch View Post
Nice work! You have plenty of cooling and power supply there. You may find the heat distribution is not as good as it could be to those tall sinks connected at the top edge with just a thin L bracket shown on the near, right side amplifier (assuming that is the position you intend to mount them in their case.) However, they seem more than sufficient size and it may not be an issue in domestic use anyway.

Are these Tubeshunter's or similar universal PCBs?.
Thank you sir.

300va 35vac x2 per channel, around 50vdc after rectification. VERY quiet, even when assembled so messy.

The tall heat sinks were the ones I got first, and they do get very hot compared to the shorter one. Even more so under 4ohm loads. The cases they'll be going into will probably be made from 8mm alu on the sides, and then heat sinks bolted to the outside as well. I want them to generate as little heat as possible.

Boards are from someone in South Africa who did a run for people wanting to build these. I'm building them purely for my HT setup as I will be bi-amping everything.
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Old 10th September 2012, 09:56 AM   #936
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Hope this isn't off topic. I've built a couple of the NAP140 clones off ebay but can't get them to stop squegging. I'm running on +-30V. The scope shows only oscillation on the positive half of the cycle. The bias is around 40ma. Adjusting this can reduce the problem but not solve it. Any ideas?
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Old 10th September 2012, 02:15 PM   #937
Spuds is offline Spuds  South Africa
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Quote:
Originally Posted by grimy View Post
Hope this isn't off topic. I've built a couple of the NAP140 clones off ebay but can't get them to stop squegging. I'm running on +-30V. The scope shows only oscillation on the positive half of the cycle. The bias is around 40ma. Adjusting this can reduce the problem but not solve it. Any ideas?
Not perhaps something wrong with the rectification on the PSU side causing that? Same issue on all of them?
Same PSU design with the same components used?
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Old 10th September 2012, 05:23 PM   #938
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Lots of things to go wrong and the kit parts are still suspect as these clone substitutions and PCBs still evolve. It's quite possible to have incorrect pinouts or orientation of transistors even though you follow the overlay.

Set the output stage bias low at around 10mA and only measure current by calculating from voltage across the emitter resistors, not by putting a DMM in line with anything. If you have not used a safety device like a lightbulb, variac or resitor initially, something may have popped already. Test the base-emitter voltage across each transistor, which should be around 0.65V for small signal types, less for drivers and outputs.

Next, calculate the bias current shared by the input stage pair TR1&2 by measuring voltage across current source TR3's emitter resistor to the -ve rail and likewise, the emitter resistor of the VAS (voltage amplifier stage) current source, TR6. The currents should be of the order of 2 & 6 mA respectively. If these conditions are OK, there should not be anything too serious in the front end.

I have not seen clones with low (30V) supply voltages yet, so perhaps there is an issue here with low supplies to the limiters and low bias currents generally. Simply remove TR7 & 8 and check the sound - or should that be clipping that you can see on a scope? That would be strange but given some awful parts often supplied in these clones, who knows?

Last, if you have the fake Sanken kits, consider the odds of what you have there, given the relative sourcing costs of genuine parts. The much cheaper and very good 2SC5200/A1943 pair are typically more expensive! I would not even bother testing parts masquerading as Sanken MT200 types, even if they appear to work at low power. Even cheap TIP41 chips will do that. The likelihood of fakes is highest with expensive parts so you know what is suspect, even before you buy.

Good luck and come back with any results or comments
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Last edited by Ian Finch; 10th September 2012 at 05:29 PM.
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Old 10th September 2012, 05:26 PM   #939
Spuds is offline Spuds  South Africa
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Yes, the whole fake parts business is a real problem.

I'm using the On Semiconductor BUV20's for all mine.
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Old 10th September 2012, 07:50 PM   #940
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Thanks for the responses. Don't beleive it but I think I've solved the problem. Even in a lash up don't ignore proper grounding. I lifted the 10ohm output resistor in series with .1u cap and the output was stable. So I then used the PSU ground point as the star as in Naim to ground this output circuit and it is completely stable even up to clipping. I've read a few responses on these kits where they can't set the bias propery. It's probably because the circuit is oscillating at about 1-2Mhz!!

I originally used the Toshiba 2sc5200 supplied with the kits but am now using Sanken 2sc2922. I cant be sure their not fake but weigh 19.4mg as per spec. Some fakes weigh 10mg. They have the proper lettering on the stamped indents but who knows??

P.S. I hammered open one of the removed 2sc5200s and it looked like the die was the expected size.
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