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Old 6th January 2013, 03:46 PM   #1
clm811 is offline clm811  United States
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Default 63vdc rails with two pairs OP devices?

Making 150w monoblock amps with only 2 pairs instead of 3 pairs OP Trans.

I have a pair of 300VA 45-0-45vac transformers(63vdc rails, measured), and will be using two each of the beefier ON Semi MJL4281-4302 transistors, mounted on large 350mmx150mmx50mm heatsinks (Conrad MF35-151.5).

My speakers are rated at 4ohms (Dynaudio Focus), and therefore present a lower impedance than most.

Please note that I'm using two 4281 and two 4302 on one heat sink per channel, and my main supply caps are 36 mF per rail.

(I do have the option of using a pair of small (200va) 40-0-40vac transformers I have O.H., but don't think they're up to supplying the big reservoirs on these mono amps...)

I'll be using for Jazz & Classical music, primarily.

Any reason I couldn't get away with this arrangement?

-Chas
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Old 6th January 2013, 04:24 PM   #2
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150W/ 4 ohms is 35v only!
40v rails would do.
63v rails are too high considering a 4 ohms reactive load.
You should aim for 2 ohms worst case.

Last edited by bobodioulasso; 6th January 2013 at 04:29 PM.
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Old 7th January 2013, 12:46 PM   #3
djk is offline djk
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I would do the 40-0-40VAC and two pair of MJ2119x, the plastic will not hold up.
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Old 7th January 2013, 02:11 PM   #4
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Still too high.
Even considering the rails sagging under load that will give more than 200w/4 ohms
How many watts into 2 ohms?
Three pairs minimum.
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Old 7th January 2013, 05:48 PM   #5
Vostro is offline Vostro  South Africa
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Just to share my small amount of experience.

Im using 3 pair of MJ2119x on 45Vac-0-45Vac (63Vdc-0-63Vdc).

When driving a 4R load the Transistors struggle to pass enough heat from the die into the heatsink. (normal use for the amp irl is 6R and 8R depending)
My temp contolled small fan kicks in fast, the transistors seem to always stay to much warmer than the heatsink. they dont burn out, but with another pair, it would be able to transfer heat a bit better.

Ofc this means I have to use a MUCH bigger heatsink if I dont want the fan.

Regards

Last edited by Vostro; 7th January 2013 at 05:57 PM.
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Old 9th January 2013, 08:42 AM   #6
clm811 is offline clm811  United States
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OK
Looks like I'll use the 40-0-40vac (+/-56vdc rails) to power the two pairs of
MJL4281/MJL4302 on 11x5x2" heatsinks.

I'm using a UPC1237 circuits for protection, just in case. I understand these can provide both speaker protection and amp protection, although the kits I got appears to be just for speaker protection against DC:

High Quality Power Supply DIY Kit PSU Rectifier UPC1237 Speaker Protection | eBay

Anyone familiar with how to add amplifier overcurrent protection feature?

Thanks for the input.

-chas
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Old 9th January 2013, 09:30 AM   #7
djk is offline djk
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"Three pairs minimum. "

For plastic, for metal two pair will be fine. This assumes you have an adequate heatsink in both cases.

"struggle to pass enough heat from the die into the heatsink."

Good insulators are essential, and the metal devices can operate with a junction temperature around 100C virtually forever, and at that temperature have twice the wattage rating of the same device in plastic due to the difference in maximum junction temperatures.

The Leach LSR&D 101 has three pair of MJ1501x, a 1KVA transformer, and a really good heatsink. The transformer gets hotter than the heatsink, even driving a 2Ω load with a headbanger on the volume control. Sometimes he drove it so hard into clipping the foldback current limiter would kick in and you could hear it 'farting'. That would have wiped out any normal tweeter, but his JBL tweeter drivers have 4" voice-coils (2441) and it just sounds bad. He does back it off to only 6dB or so into clipping when he hears the 'farting' sound.

"the kits I got appears to be just for speaker protection against DC:"

It has no current sensing or current limiting if that's what you mean, it's just a DC sense and relay driver.
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Old 9th January 2013, 03:29 PM   #8
JMFahey is offline JMFahey  Argentina
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Quote:
150W/ 4 ohms is 35v only!
Only on Simulator World, not with real parts on a bench.
+/-40V just approppriate for 100W/4r .

FWIW I use +/-65V rails , 2 pairs IRFP250 for 300 (measured) RMS Watts into 4 ohms *Musical Instrument* speakers , which are a tough load.
My power transformers are very good, and have much higher regulation (lower drop) than usual.
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Old 9th January 2013, 04:03 PM   #9
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Quote:
Originally Posted by JMFahey View Post
Only on Simulator World, not with real parts on a bench.
+/-40V just approppriate for 100W/4r .

FWIW I use +/-65V rails , 2 pairs IRFP250 for 300 (measured) RMS Watts into 4 ohms *Musical Instrument* speakers , which are a tough load.
My power transformers are very good, and have much higher regulation (lower drop) than usual.

Agree.

Have a nice amp here, with +/-57V rails. The manufacturer said 125Watts/4 Ohms back then, mid seventies. Sounds good still
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Old 9th January 2013, 04:06 PM   #10
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Quote:
Originally Posted by djk View Post
"Three pairs minimum. "

For plastic, for metal two pair will be fine. This assumes you have an adequate heatsink in both cases.

"struggle to pass enough heat from the die into the heatsink."

Good insulators are essential, and the metal devices can operate with a junction temperature around 100C virtually forever, and at that temperature have twice the wattage rating of the same device in plastic due to the difference in maximum junction temperatures.
Happy New Year DJK,

Triple EF + Grounded collectors + Metal devices + Large Can Capacitors + Parallel Class-G............

Needless to say anything else, but only you will understand.

Regards,
Kanwar
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