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Old 1st September 2009, 10:16 PM   #1
Xoc1 is offline Xoc1  United Kingdom
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Default Maximum Voltage for Mosfet Output Stage?

I own a C-Audio SR 707
The voltage rails are 200vdc rail to rail (+100 / -100)
The output devices in the original mk 1 version were Hitachi 2sk135 / 2sj50 pairs. 160V drain to source
The output devices in the later versions were the Magnatec BUZ900 / BUZ905 pairs. Again 160V drain to source
There are 32 mosfets in total.
My Amp is the later version with the Magnatec devices.
Of the original 32 mosfets over 50% have failed!
Over many months I have been accumulating Hitachi 2sk135 / 2sj50 Fets
And now have enough to consider rebuilding the amp.
The transformer is multi tapped and I have the option to reduce the rail voltages.
Alternatives are
+100 / -100 for SR707 rated at 1175W / 2 Ohm, 825W / 4 Ohm, & 488 / 8 Ohm
+90 / -90 for SR606 rated at 750W/ 2 Ohm, 625W / 4 Ohm, & 390W / 8 Ohm
+75 /- 75 for SR404 rated at 500W /2 Ohm, 466W / 4 Ohm, & 283W / 8 Ohm

I am considering using the 606 option 180V rail to rail.
I assume that the maximum Drain to Source voltage would be 1 voltage rail plus the maximum output swing voltage of the amplifier.
Looking at the 8 ohm performance of the 606 Maximum output voltage would be 56VRMS so 79 V peak.
Adding that to the 90v rail gives a max drain to source voltage of 169V.
Obviously reducing the rail to rail voltage by 20V will reduce the chance of failure of the amplifier.
Questions.
Are my assumptions about how to calculate the drain to source voltage correct?
Will reducing the voltage by 20 V be enough to ensure the mosfets survive?
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Old 1st September 2009, 10:53 PM   #2
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You may as well assume the requirement of devices rated
to withstand the full rail-rail voltage.

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Old 1st September 2009, 11:12 PM   #3
Xoc1 is offline Xoc1  United Kingdom
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Nelson
By that I assume that you are telling me I am going to overstress the output devices by using 180V rail to rail.
I do have the option of aquiring 32 off 2sj56 2sk176 200V devices.
This would cost me about £250 British pounds.
I already have 32 off of the 160V part. But could utilise them elswhere.
My Assumptions probably arise from the fact that in its standard form this amp is so over voltage and me trying to justify that!
Regards.
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Old 1st September 2009, 11:41 PM   #4
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For 160V parts you have to go with +/-75V.
However, they are not necessary all break up on 160V, but I would always assume the worst case of the manufacturer's specs, except when I can allow myself to measure each device individually.
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Old 2nd September 2009, 12:00 AM   #5
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Have a look at the Exicon branded 200V devices from Profusion. Either that or limit yourself to the 75V rail setting.
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Old 2nd September 2009, 12:52 AM   #6
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Quote:
Originally Posted by Xoc1 View Post
I already have 32 off of the 160V part. But could utilise them elswhere.
I would tap the transformer down to the +/-75 figure and
use the parts you have.

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Old 2nd September 2009, 07:04 PM   #7
Xoc1 is offline Xoc1  United Kingdom
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Quote:
Originally Posted by Nelson Pass View Post
I would tap the transformer down to the +/-75 figure and
use the parts you have.

OK. I will try the amp at 150v rails. Not much to loose by trying!
I Have a complete schematic, and there are ony really a couple of resistors that are changed. These seem to effect the gain (feedback) and the clip indication circuit.

Quote:
Originally Posted by richie00boy View Post
Have a look at the Exicon branded 200V devices from Profusion.
The problem I have with the Exicon (Semelab) parts is I have been warned by 2 separate well respected designers, that these components are not really reliable when pushed to the limits. Unlike the Hitachi parts.

The Sr 404 used 24 output devices, but I will try fitting all 32 in place.
Hopefully this will allow me to run the amp as a 4 Ohm loaded mono bridge to get some useful power. Drivers are 2, off Fane XB 600w RMS 8Ohm These will take 900 to 1200w with some care.
Crossover is a tweeked Behringer DCX 2496 so the signal is well managed!

The other mod I will try is to change the gate resistors increasing the resistance from 560R to 680R for the P channel and 1k0 for the N channel to compensate for the lower capacitance.
The original amplifier had ceramic caps on some of the N channel devices but not all of them. You can see them in the photo in my first post.
I would also like to add some extra local decoupling adjacent to the mosfets, but will need to work out how to add a ground rail as there is no 0V track on the driver PCB.
Thanks for the input.
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Old 4th September 2009, 12:13 PM   #8
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Hi,
does the amp have a current limiting circuit?
This can cause extreme over-voltage back EMF into the output devices causing premature failure.

The standard protection for this form of abuse is to fit medium current diodes between the supplies rails and the output rail.
It is also worthwhile checking to see if diodes are fitted between the supply rails and power ground. Those 4 diodes/channel (1n4004) will cost cents/pennies.

32devices in one amplifier.
How are these arranged?
8pair per channel?
or
is each channel bridged leaving just 4pair per half bridge?

The amp options seem to show that the amp is capable of driving a 2r0 load. I see they use 2ohm in the power spec but we know they mean 2r0.
I do not see any duty cycle spec for this low resistance load.
I do not see any confirmation that the amp is capable of driving a 2ohm load (a real speaker). On the evidence of the 8r0, 4r0 and 2r0 power outputs I would NOT consider this amp in any of it's voltage forms as capable of driving 2ohms properly.
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Last edited by AndrewT; 4th September 2009 at 12:15 PM.
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Old 4th September 2009, 09:59 PM   #9
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I can not see any reference to, or any circuitry, that could be current limiting.
There are 2 In4004 between the voltage rails and the output rail, per channel. These are visible in the photo in my first post. Have not spotted any diodes on the supply rails to ground.
Amplifier topology is Class A/B - Balanced op amp input. Then first 2 stages (differential pair and VAS) are like the hitachi mosfet application notes, but with added constant current sources.
The driver section is more complicated, A pair of transistors, between each supply rail and the bias setting circuitry, are AC coupled to the feedback / output rail. Then a zener voltage limit, and finally a pair of driver transistors.
The spec talks of active headroom circuitry, that raises the driver voltage above the output stage.
The 2R0 loading is average continuous rating in the spec sheet.
The output devices are arranged in 8 pairs per channel.
This certainly isn't a toy amplifier. The weight is 29kg. The last list price I can remember was 1450 (last century money).
All I know is that when it worked, it drove my pair of Fane XB drivers in bridge mode, thats nearly 1200w per driver which is their peak rating.
Obviously this is pushing the SR707 too hard, as it broke!
The speakers are still fine!
Reasoning behind still using the amp in bridge mode is to try to extract some useful power from the amp, Otherwise I will need to invest in a new bass amp, and just use this one for the mids.
The lower voltage SR404 had less Mosfets, 24 in total. I going to fit all 32 and use the fets at a safe voltage, not 40V above their rating.
If it only makes the rated 1000w / 4R0 loading in bridge mode this is only 62 watts per mosfet pair. I have a feeling it should be capable of more than this.
Not sure of the VA rating of the transformer, but it has a soft start and a 15A internal AC fuse!
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Old 4th September 2009, 10:02 PM   #10
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The problem starts when you wind the amp up.

At zero sound the voltage across the transistors is 100 volts each.

As the sound level goes up one transistors sees less than 100 volts and the otehr more until you reach 160 volt across one transistor and they pop.
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