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Old 6th June 2002, 07:32 PM   #21
trwh is offline trwh  United Kingdom
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Aud_Mot, When Chris describes the Velleman kit he has built as "soft start", he means that it connects the speakers to the amp after a short delay - not that it limits the inrush current when the power supply is switched on. I don't know whether he has included a soft start circuit in his PSU as well. Anyway, the mains fuse should remain a slow-blow type: even with a PSU soft start circuit. The supply rail fuses should blow before the transformer primary fuse.

Despite what you say, the DC protection on the Velleman kits is very fast-acting, and negates the need for speaker-line protection fuses as well, I suggest you remove them. Of course, you MUST still include the 5A fuses in the amp supply line. They won't stop the output transistors being destroyed in the event of a fault, but they will prevent any serious damage such as a fire, etc.

Hope this helps,
TRWH.
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Old 6th June 2002, 08:12 PM   #22
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Default Inrush current

Sorry Trwh and Aud_Mot that I was not clear in my last post.

I have actually built a delay circuit posted by Kristijan-K and Panos called mains switch delay from Panos PCB which should handle the inrush current.

And I also purchased 2 Velleman K4700 kits that has a delay prevents switch on clicks and DC protection on the output of the amplifier.

My questions was that is it possible to not using the fast blow fuse at the amplifier output.

Now I have one more question how should I really connect the two delays in relation to the regulated PSU transformer?

Mains from the wall to the inrush current delay -----> the K4700 protection kit ----> PSU transformer

Thanks for your comments
Chris
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Old 6th June 2002, 08:24 PM   #23
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Sorry I have forgot to attach the description of the mains switch on delay posted by Kristijan-K.
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Old 6th June 2002, 11:06 PM   #24
trwh is offline trwh  United Kingdom
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Chris, the Velleman kit must be connected to the mains supply before the inrush limiting (soft start) circuit, directly after the mains switch. The Velleman kits should have their own fuses, 80mA slow blow types. The main (transformer) fuse should be placed in the live line as soon as it enters the chassis, so that it protects all the internal wiring. You do not have to worry about the loading effect of the Velleman kit power supply - it is negligable. The schematic looks like this:

Mains inlet -> main fuse (as per transformer rating, slow blow type) -> mains switch -> velleman kit(s) via one 80mA slow blow each, AND soft start circuit - > transformer.

Also, you should set the delay on the soft start circuit so that it switches out before the Velleman kit. That way any pops due to the supply rails coming up to their full operating voltage are avoided.

And, again, you may remove the speaker line fuse now.

See ya,
TRWH.
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Old 6th June 2002, 11:43 PM   #25
trwh is offline trwh  United Kingdom
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Chris, the fuse value (80mA) I gave you for the Velleman kits is only correct for 230V operation. There's more info on the fuse value and type to use in one of the leaflets supplied.
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Old 7th June 2002, 01:33 PM   #26
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Thumbs up Thank you very much!

Trwh,

Thanks for your help, clear as mud.


Chris
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Old 9th August 2002, 09:55 PM   #27
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Default I'm not an expert but...

...can you not just use fuses and an RCD on the rails?

This way when one or other fuse trips the residual current to earth will trip the RCD and cut the power until it is reset?

I may be compeletely wrong as I have only limited knowledge

Any answer to this would be great as I'm just about to make my first diy amp (purely by coinsidence a dual P3A configuration with P88 preamp) and I stumbled across this concern about the fusing issue.

Also any feedback on the sound?

I'll be using it in a passive bi-amp setup (I would go active, but I'm a wee bit scared to rip appart my beautiful B&W 603-S3's and invalidate the warranty).

Also (and this is the last question) can I just split the output from the P88 preamp (conventional voltage feedback opamp design) and feed this signal to both P3A's in order to biamp or do I need a buffer stage (opamp based?) to stop the inputs of both power amps interacting in some strange way I don't yet know about (oscillating???)?


...where is Peter Daniel when you need him, lol...
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Old 10th August 2002, 01:23 AM   #28
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Does RCD mean residual current detector? Is this the same as a ground-fault interrupt? In North America, a GFI is a special type of circuit breaker.
Anyway, if I understand you correctly your idea is pretty much what Geoff suggested earlier in this thread, except his was based on DC offset voltage.
The 3A sounds great to me, but I don't have a Class-A to compare it against. Blows the doors off anything else I've had, even with inexpensive speakers. I never would have thought an amp would make that big a difference with cheapo speakers.
Passive biamping loses a lot of the biamping benefits.
Anyway, should be no problem driving 2 P3A amps from a P88 preamp. Output impedance of the P88 is close to 100 ohms, input impedance of the P3A is something like 22K (11K for two in parallel).
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Old 10th August 2002, 02:03 AM   #29
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I was going to start a thread about protection schemes, but you beat me to it =)

This circuit should cut the power on the rails if either rail goes dead, or if the 3V or so across JP1 go away. It allows power control and detection (if a rail goes out, the JP1 circuit will as well) for a preamp I'm designing. I also have one that uses optoisolators, which I can post if anyone wants it. Note: I haven't tested these, and I'm not sure if the gates on the mosfets would stay open if the protection circuit is on a capacitive load. It should shut the circuit off, but the prototype of this omitted the diodes and stayed live until the caps discharged (bad). This was especially bad because it drew the current backwards from the opposite rail. =)

Will

Note: the mostfets in the schematic are not complimentary pairs. My eagle libraries didn't have the ones I needed, so I used an approximation. They should (theoretically) be both IRF530/IRF9530s or IRF540/IRF9540s. Also, the amp would connect on the left side, and the PSU on the right.
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Old 10th August 2002, 03:32 AM   #30
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I always say, "If you're tired of your 5amp fuses blowing, wire a 4amp fuse in series with it."
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