Quad 405-2 limiter circuit - diyAudio
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Old 16th January 2006, 10:13 AM   #1
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Default Quad 405-2 limiter circuit

All,

A Quad 405 for repair needs to have its PCBs re-done and I thought that I might as well put the 405-2 variant in there for the same amount of trouble: audio reviewers claim the 405-2 is definitely better than the 405-1, which I will be happy to verify once a board is built. I have read the warnings in another thread on how to avoid oscillation -- I will use Quad's PCB layout (M12565 Issue 7), parts list etc. I'm not sure about copyrights, but I suppose one is probably allowed to repair an existing unit for which the warranty has expired since many years...

The current limiter circuits (shown as N1 and N2 in the circuit diagram) raises a few question marks. Though it is not necessarily required, it might be worthwhile to have it in the finished version.

I suppose its functionality will be OK if I materialise it as a concrete classical small PCB and not as a thick film circuit. The circuit diagram, however shows a PNP transistor with its emitter connected to pin 4 while it is labeled as a BCW60 -- which is an NPN if I'm not too much mistaken. From its position in the general circuit, I would assume an NPN is to be used...

Is this just a 'typo error' in Quad's circuit diagrams (it appears like that in all diagrams I have found) and can I use an NPN (e.g., a BC550C)?

My 'soldering' experience is more in the area of digital circuits and the heaviest end amplifier I ever built so far was a solid-state 15W RMS thing, so any other caveats & usful warnings will be appreciated as well.

Thank you for sharing your insights,

--
Jacques
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Old 16th January 2006, 11:39 AM   #2
Zombie is offline Zombie  Sweden
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Hello Jaques!
Are you aware of the Berndt Ludwig mods to the 405(-2)?
Please have a look here:

For some iimediate improvement:
- dump the TL071 opamp at the input, use an OPA134 or similar, add suitable bypass at the supply pins, see spec sheet
- change all elcos, increase the big ones to 15000uF.

What concerns the current limiters, gently desolder N1 and N2 and politely say goodbye before they enter the garbage bin (you are not using ESL57 are you?)...

Check Net-Audio for more commercial upgrades to the 405 - link

Good luck!
Thomas
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Old 16th January 2006, 01:18 PM   #3
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At one stage I knew people at an EMI military factory building the 405, so they were probably to blame for the hybrid.

I thought that the triac on the output is the protector for the electrostatics, they are extremely peak voltage sensitive unlike normal speakers which are only power dissipation sensitive.
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Old 16th January 2006, 01:24 PM   #4
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The triac on the output is to prevent potentially damaging dc offsets from appearing at the output. The output feeds through an R-C filter so that if a large enough dc offset appears at the output, the triac crowbars and short-circuits the output.

So, if you have removed the current limit circuitry on your 405, perhaps it also makes sense to remove the output triac boards as well!
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Old 16th January 2006, 04:10 PM   #5
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Hello Thomas,

Yes, I know about Bernd Ludwig's recommendations for improving a 405 and I intend to apply at least the op-amp improvement (with its decoupling capacitors) and some other things which I have yet to sort out. The power supply capacitors need replacement as well, they look as if they will explode any moment.

I want to replace the existing 405-1 boards (which appear to have been treated badly) with new 405-2 ones, with all new components, hence my question about this limiting circuit. At least I can re-use the T-profile heatsink mounting stuff...

I understand that the N1/N2 circuits have a different purpose than the X1 shortening link (to be applied when using Quad 57 ESLs): they limit the current into bad load impedance (below 3 Ohm), hence 'protect' the output transistors whereas the X1 link limits the output voltage at maximum 26V, protecting the vintage ESL57 speakers one may have.

The N1/N2 current limiting circuits are supposed to be an improvement over the older T5/T6 circuitry; reviewers in the audio press (early 1980s) claimed that the difference was audible.

Best regards,
--
Jacques
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Old 18th January 2006, 08:30 AM   #6
Zombie is offline Zombie  Sweden
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dunno if the current limiter does anything to protect the components...a short circuit blows the output transistor(s) and the fuse...
anyway, there's a company on the net selling modded 405 cards at a cost. Don't remember the name, but I'm sure a search will help, the're mentioned on this forum, too
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Old 18th January 2006, 07:20 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
the triac is shown across the output to return.

Can a pair of these triacs be connected from Vcc and Vee to return and simply blow the PSU supply fuses?

Would this still save the speakers and not damage the output stage?
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Old 19th January 2006, 06:46 AM   #8
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If the 405 still has the original current-limit circuitry, the output triac 'crowbar' circuit will not damage the output transistors. The possibility of damage is if the crowbar clamps the output on a board in which the output current limit has been increased to allow the 405 to drive 100W into 4R loudspeakers.
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Old 19th January 2006, 10:22 AM   #9
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As you say, the BCW60 is NPN, and if you look to the right of the PNP one, there's another BCW60 correctly drawn. So it's just an error with the diagram, using the wrong symbol.
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Old 19th January 2006, 11:01 AM   #10
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Quote:
Originally posted by Zombie
[snip]What concerns the current limiters, gently desolder N1 and N2 and politely say goodbye before they enter the garbage bin (you are not using ESL57 are you?)...
[snip]

These are not current limiters. These are safe operating area limiters. One of the main differences btween the -1 and the -2 is in the improved protection system. The -2 system limits the operation area of the output stage to protect blowing up the output devices. They do not just limit current to some arbitrary value. Throwing them out is a bad idea; they will not limit the power under normal conditions, but will protect the amp in case of shorts and prolonged overload.

Jan Didden
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