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-   -   is this realy stupid? (http://www.diyaudio.com/forums/class-d/70147-realy-stupid.html)

bob123 20th December 2005 10:05 PM

is this realy stupid?
 
I am planning on building a very high power class d amp. Would it be plausible to rectify direct mains? i have got a 270w, 1ohm resistor to prevent a 100000000000A surge when you turn the power on. This could prevent the need for getting a massive transformer.

I live in england so have a 240v RMS power supply with 340v peaks. This would mean that the supply rails would be at +-170V.

A current regulator could also be used to prevent excessive current at turn on.



Rob

poobah 20th December 2005 10:35 PM

Do you have a neutral?

bob123 20th December 2005 10:43 PM

yes

darkfenriz 20th December 2005 10:43 PM

Quote:

100000000000A
Hardly possible
if you have 340V rails than peak current at 'normal' operation (not hard-start) would be +-21A for 8ohms and +-40A for 4ohms. This may require 3-phase mains.

And Yes
putting 1ohm restricts 80% efficiency for 4ohm speaker. Except you will probably need a varactor and high current chokes for clean enough rails.

Quote:

+-170V.
For +-170 if your '0' has anything to do with physical earth or ground, than you're dead. A 'cold' wire in mains is often (in older hauses) the same as earthing. (I'm guessing you know about all these things better than me but many may read this thread). Bridge topology eith single rail may be a cure.

If you consider turn-on than it is absolutely OK to use soft start circuit. You may add a 10W transformer for it and a high current relay. That's not a problem. Problems are elsewhere. Fuses... protections... just think how hard it wll be isolate all these 'grounds' being 170V in your case.

bob123 20th December 2005 10:46 PM

ive just realised that the supply would be at +- 340v not 170v as it is 340 above and below neutral. Thats a bit much and i dont think it is worth doing as i would need to be working with high voltage components.

darkfenriz 20th December 2005 10:51 PM

It depends on rectification. If you use only one diode and a cap, than you have +340V, not +-
I assumed this.

poobah 20th December 2005 10:54 PM

+/- 170V Rails would give you 1800 Watts into 8 Ohms... a little nuts I guess... potentially dangerous too.

mzzj 20th December 2005 11:00 PM

330v single rail is what you get. Then you better go with full-bridge. As my 4.5kW project...

Safety issues become quite serious but it is possible with amplifier integrated to sub casing and using 4kV isolation spec line-input transformers(lunhdall, spell?)

woofer element needs also some special attention, consider woofer chassis to be part of live circuitry, ie separate metal grille in front of it etc. Horn/bandbass box desings are probably easiest to design safe so that toughing to element is not possible.

kartino 21st December 2005 12:40 AM

Hi, actually I am an electrical engineer.

Basically you must protect electric shock as standard equipment. Even you use isolated transformer, leakage current for all equipment are to be considered. Of course if you care this issue.

Use current leakage protection to protect you. All metal shall be earthed. Live parts shall be isolated, including your amp input. Speaker including box that may be in wet must be grounded. Use speaker binding that prevent to be touched. Careful to charged capacitor.

Yves Smolders 21st December 2005 06:17 AM

I thought anything considering direct mains was a no-no on this forum?


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