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Old 20th January 2011, 02:39 PM   #10021
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I need 2 x transfomers for my Dual Mono F5 with 70W/8 ohm - 140W/4 ohm.

I have gotten several offers from a company, I can either get 2 x 800VA standard transformers OR 2 x 500VA Audio grade transformers, for the same price.

Audio grade gets you this :

In audio grade, noiseless transformer price You'll get transformer wound on high inductive, selected and measured core. Core and all the windings will be impregnated. Transformer will also has electric and electromagnetic shields, epoxy filled interior and mounting pads in price.

Is it worth it? Lower noise is always a big plus...
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Old 20th January 2011, 03:03 PM   #10022
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Hi regireg122

Well in my case yes the of the top of my wooden head they are 2.2K used to slowly bleed power off when the amp is switched off.

But not sure if this is relevant in regard to the topic we were discussing.

Neutrality was proposing to interrupt the supply to the amps in case of a fault (Mosfets short circuit) the bleeders are somewhere else in the circuit.

Just to recap from what we were on about.

IMO big IMO

To place fuses or relays in series with the speakers is certainly an option in my case I could not cope with Her telling me I told you so if I blow the Polk (besides customer services is not the best there is as far as I can tell for now at least here in the UK)

The fuses have timing issues the relays which must be rated for DC current (DC ark between the contacts and is much worst than AC in this regards just see the rating for the relays not many give you the contact breaking capacity for DC) and they are either expensive or really cheap the contacts are made with what not and can be sonically worst than a fuse.

There is a much better written explanation of this in Bob Cordell book, which was also quoted here, word by word in another post under different credentials...

So with Neutrality we were discussing the option of fuses in Neutrality view to disconnect the supply to the Amp modules.

This sound to me like a better option as it should not effect the sound too much.

At this stage and I think we should give this it time to circulate
Neutrality view is to place fuses on the output of the capacitor banks.

F5 is a fairly constant load on the supply and this is certainly an option

My question was (and I need help with this please) was to use a mosfets as a switch
Question: what is the heat load that this solution would present the mosfets would be in the saturation region so the resistance seen in the circuit is very small would the power dissipated be small also?

I got plenty of spares that did not make it trough the matching so it would be cheaper than a fuse and ancillary and it could sit on a corner of the heat sinks

I had another question relating to the control circuit for the switching mosfets

My thoughts where as the musical signal is made up with waves the voltage at the speakers AC in nature will cross the zero line say for a 10Hz sine wave every 25 milly seconds (is my math correct 1000/(10 X4) )

So I was thinking to use a LM339 to detect the signal voltage rising edge and set a timer a second comparator to detect the falling edge and reset the timer before it reaches the time out of say 50 mS the comparators with inverted functions can perform the reverse on the negative branch of the supply

If the timerís time outs they latch remove the supply to the gates the mosfets stop conducting speakers safe

It will also work as DC offset protection maybe as the 339 set point can be set to work allowing a tolerable DC offset

So I am big enough now to play with a 555 (maybe) but I donít think it will be good enough so any one to help with this please.

Tanks
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Old 20th January 2011, 03:24 PM   #10023
AndrewT is online now AndrewT  Scotland
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detect the zero crossings.
Send a pulse for each zero crossing.
Use a 555 as a missing pulse detector.
Trigger protection on missing pulse.

However, I would use some kind of "and" to detect the average signal after rectifying. If the average of the rectified signal is >threshold AND missing pulse detector is active then activate protect.

Some music signal is not symmetrical, I suspect this asymmetrical signal could trigger on no zero crossing too often to be tolerated and eventually the user turns off the protection.

Whereas with a zero crossing/missing pulse AND and excessive average DC (either +ve or -ve) the threshold and/or the duration could both be adjusted at build stage to eliminate most, if not all, of the nuisance triggering.
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Old 20th January 2011, 07:25 PM   #10024
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Quote:
Originally Posted by Neutrality View Post
I need 2 x transfomers for my Dual Mono F5 with 70W/8 ohm - 140W/4 ohm.

I have gotten several offers from a company, I can either get 2 x 800VA standard transformers OR 2 x 500VA Audio grade transformers, for the same price.

Audio grade gets you this :

In audio grade, noiseless transformer price You'll get transformer wound on high inductive, selected and measured core. Core and all the windings will be impregnated. Transformer will also has electric and electromagnetic shields, epoxy filled interior and mounting pads in price.

Is it worth it? Lower noise is always a big plus...
So, any opinions about what to choose?
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Old 20th January 2011, 07:33 PM   #10025
6L6 is offline 6L6  United States
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I would probably choose the 2 x 500va 'audio grade' but only to have 2 PSU pcb, and make it dual-mono.

With 2 PSU PCB (such as Peter Daniel's) you would be able to tune the amount of capacitance easily.

The shield and epoxy may have a nice benefit as well.
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Old 20th January 2011, 07:40 PM   #10026
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Quote:
Originally Posted by 6L6 View Post
I would probably choose the 2 x 500va 'audio grade' but only to have 2 PSU pcb, and make it dual-mono.

With 2 PSU PCB (such as Peter Daniel's) you would be able to tune the amount of capacitance easily.

The shield and epoxy may have a nice benefit as well.
Well, my plan was already going dual mono and I have made my own PSU PCB design. Just need to get it manufactured.

Getting a pair of "audio grade" transformers does seem tempting, but a transformer can never be too big so getting the big 800VA has its merits as well. 60% more current capability is nothing to sneeze at.

Maybe I should just pay the extra money and get 2 x 800VA "audio grade" transformers. Sometimes its better to buy the size you want from the beginning. On the other hand, for a 70W/140W in 8/4 Ohm amplifier 500VA pr channel should be more than adequate.

Decisions, decisions, decisions.

Last edited by Neutrality; 20th January 2011 at 07:46 PM.
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Old 20th January 2011, 09:36 PM   #10027
tinitus is offline tinitus  Europe
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well, yeah a rather smallish kids bass
man, this heatsink is so heavy I can barely lift it
bought in a moment of insanity

oh, and my supply caps arrived today, yeah

wrote to Mr Schuro in Germany about trafos, but no reply
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Old 20th January 2011, 09:48 PM   #10028
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Easy to cut with circular wood saw.

Quote:
In audio grade, noiseless transformer price You'll get transformer wound on high inductive, selected and measured core. Core and all the windings will be impregnated. Transformer will also has electric and electromagnetic shields, epoxy filled interior and mounting pads in price.
Toroidy?
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Old 20th January 2011, 10:42 PM   #10029
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Tinitus

Not fair yours is bigher than mine

Only joking realy made up for you
If you don't mind How much?

Going to get them anodised?

If you do make shure you got a hole or fit a bolt where you are shure it will not be seen as they need an electrical contact

Me plonker toke them to the shop in a rush and now they are scrathed just in the wrong place

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Old 20th January 2011, 10:52 PM   #10030
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Neutrality

I go with 6L6 advice

But then do they make the 800VA with the same specs?
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