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Old 28th June 2003, 01:29 AM   #1
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Question extra output trans'rs in parallel?

hi

what is the effect of adding say,an extra 2 output transistors in parallel with the existing ones?/
(eti480 or sc480)

i am thinking of how to run a 4ohm load, while bridging 2 amps,that are only rated for 4ohm load,

when bridged they wil 'see' 2ohms wont they..

im sure there is formulas and things for this..

i know that in parallel it wil allow more current,does this do what i want it to do then?? allow bridging for 4ohm speaker load ?

i have a 300va toroid so that isnt a problem

can other things in the circuit be cooked from the higher amps than normal anyway. eg needs uprated power resistors/?


thanks from an amp newbie

i have searched for the 'eti 480' kitset amp,there is some good technical articles for it ,i just cant see which ones apply,link from my site

http://homepages.paradise.net.nz/quadroph

'jbl2226h' eti480 amp link..



_mike
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Old 28th June 2003, 07:48 AM   #2
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Hi Mike,
Post the schematic, so we can look at it and give you some advises..
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Free Schematic and Service Manual downloads www.audio-circuit.dk, Company: www.dupont-audio.com, Joint venture: www.DupontMantra.com
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Old 28th June 2003, 09:52 AM   #3
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Yes, when you bridge two amps they each see half of the load. So a 4ohm speaker will appear like a 2ohm speaker to each amp. This is because the speaker current is doubled for the same voltage swing per amp.

So you need to consider what impact a doubling of current will have on each amp. Note that power dissipation is proportional to the square of the current and so the power demands on each amp will be quadrupled compared to the unbridged case.

> Will each amp allow a doubling of current? What about the current limiting circuits...they may need adjusting.

> Will each amp's psu supply double the normal current?

> Maximum power dissipation will be quadrupled - how will you ensure sufficient heatsinking for the output transistors?
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Old 28th June 2003, 10:31 AM   #4
djk is offline djk
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"Hi Mike,
Post the schematic, so we can look at it and give you some advises.."

He did at his site.

For the terminally lazy I will link to it here:

http://www.alphalink.com.au/~cambie/

Mike, if I remember right your toroid is 28-0-28V 300VA?

That is barely adequate for 200W8R out of a pair of ETI480 in bridge.

The JBL is 8R so what is the deal?

You can always scrape everything off the circuit board and start over.

What are you out? Two ETI480 boards come with about $10 of worthless transistors.

Build them up, dump them on somebody to recover some of your cost, start over with a clean sheet of paper.
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Old 28th June 2003, 10:49 AM   #5
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Default thanks djk

Quote:
The JBL is 8R so what is the deal?
yes it is,i was wondering in future for a shiva etc..


i mainly wanted to know if it is at all possible to put into practice..

i was thinking if i uprated the existing amp,i wouldnt need to buy a new toroid such as 40-0-40 300VA for some other kit that isnt bridged..

BUT i see ,,it is easier to simply get a decent amp from the beginning...

oh yeh! 300VA= 150watts rms in bad (easily normal) conditions of power factor !

argh

i think il stick with my efficient jbl/jbl basshorn ideas for now
-or change to 'music' power instead of RMS lol!

all this started because dick smith only offers eti480s,not even sc480s, and i didnt check siliconchip for a better amplifier design before hand lol..


tis all good!

[b]no worries[\b],il sell this crap and get something better ,when i can :-D i got a mate who wants a sub+amp(1300$ sony 5.1 with no sub,it says 4000watt pmpo on the front haha)
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Old 28th June 2003, 11:09 AM   #6
AJT is offline AJT  Philippines
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"i am thinking of how to run a 4ohm load, while bridging 2 amps,that are only rated for 4ohm load,

when bridged they wil 'see' 2ohms wont they.."

not really, it will see whatever load you will connect to it...it will not change a 4 ohm load into a 2ohm load by itself...
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Old 28th June 2003, 04:21 PM   #7
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In bridge mode each amp must be able to source or sink 2 times the load current compared with single-ended drive: this is because the instantaneous voltage across the load is doubled by the bridging action.

Since each individual amplifier is only providing the same output voltage swing as it would in single-ended drive, we get

(same V) divided by (2*current)

looks like the load resistance is halved as far as eac amplifier is concerned.
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Old 28th June 2003, 04:27 PM   #8
PGW is offline PGW  United States
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Adding "two extra transistors in parallel" isn't the same as bridging.

If you are going to take an existing amp and try to boost its power output by adding an extra pair of output transistors and do nothing else, you'll probably run into trouble:

the driver stage may not have enough current output to drive the extra pair of ttransistors, leading to current limiting of your output stage; you may also encounter stability problems with the interaction of the driver stage and your now lower impedance output stage;

the transformer will probably not be rated for the extra current drive required.
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Old 1st July 2003, 10:41 PM   #9
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"In bridge mode each amp must be able to source or sink 2 times the load current compared with single-ended drive: this is because the instantaneous voltage across the load is doubled by the bridging action. "

in a bridged mode the instantaneous voltage across the load is doubled by the bridging action, that is why the power is quadrupled theoretically, depending on your power supply. in practical terms you can get at least double the power.
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