Max voltages for transistors drivers

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Fedess

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Joined 2010
#1
Hello

I built an amplifier and tested it with a +/- 30VDC power supply and it finally works fine (I say finally because I had some trouble to get it working), but now I'm going to use it with a +/-56 VDC power supply that I'm building.

My question is if the drivers and pre-drivers transistors would resist that voltage. The datasheet says that Vce is up to 80V max, but at the amp's author web he suggest replacing the BD139, BD140, BC639 and BC640 when more than 40V is used. Unfortunatly he didn't mention if those 40V are AC or DC.

The author suggests using MJE340 and MJE350 instead (which I was recommended not to use them in this forum).

In this thread I posted the amplifier schematic if you need to see it:

http://www.diyaudio.com/forums/solid-state/171041-need-help-constructing-amplifier-s-sub.html

thanks!
 
#2
Hi
Not sure about what your question is or not reading the other thread. But to be safe consider the amps output can swing from rail to rail and sometimes even higher than the rails Vcc, esp at clipping into inductive speakers. so Vas, driver, and OP devices Vceo (min) normally should be 2x rail DC max. ~ 120 V or higher. Consider the driver needs to provide higher currents, look at the drivers SOA driving full output current at low speaker impedance and using OPT beta. MJE340 / 350 are rated for really low current!
 
#3
I put the link to the other thread just if you want to look at the schematic, which is in post #1. You need to read nothing from that thread as I already solved the problem presented there. 🙂


I didn't mean the output power transistors, just the transistors that are previous to them (Q6, Q7, Q13 and Q14 in the schemo; they don't connect directly to the speakers); I just want to know if the transistors showed in the circuite would be ok for a +/-56VDC power supply...

What I understand from your reply is that those transistors are at the limit as their Vce max is 80V and I will connect them to 56 VDC each, more than 80V/2=40V... did I understand well?
 
#4
No Vceo ~ 2*Vcc or 120V
YES the driver transistors are connected to the output load current! by Ipeak/beta of the OPT.
This seems to be related to the other thread unless you are totally ignoring things.
You are raising both the Vcc and the output power! so the drivers and OP devices power goes up exponentially.
 
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#6
Fedess said:
Maybe I'm missing some things... for what are you saying I understand that the transistors used won't work at 56V...
Click to expand...

The output, drivers and VA stages will all 'hit the rails' meaning 112 volts when driving to clipping with no load. Your devices better be capable of that or you'll let the smoke out. And, after checking transistors on a curve tracer I would recommend 20% above the rails. Don't forget the line voltage can vary too - meaning ANOTHER 20%.

G²
 
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#7
I don't know
If you can provide the links to the data sheets to the parts actually used. those are generic Thompson/ SGS things?

You should shoot for ~ 100W using a single pair of 200 - 250W devices. So using a 200VA transformer at no more +/- 56Vdc at no load, dropping down to say +/- 50V with 6 ohm (8 ohm woofer) load.
You should be very carefull about NOT driving this into clipping since there is no protection circuit and no output clamping diodes.
 
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#9
infinia said:
You should shoot for ~ 100W using a single pair of 200 - 250W devices. So using a 200VA transformer at no more +/- 56Vdc at no load, dropping down to say +/- 50V with 6 ohm (8 ohm woofer) load.
Click to expand...

100W @ 8 ohm That's the idea, power transistors are MJ15024 and MJ15025, transformer is 40+40VAC 400VA (for a stereo set-up), but I'm not sure about the drivers...
 
#10
stratus46 said:
The output, drivers and VA stages will all 'hit the rails' meaning 112 volts when driving to clipping with no load. Your devices better be capable of that or you'll let the smoke out. And, after checking transistors on a curve tracer I would recommend 20% above the rails. Don't forget the line voltage can vary too - meaning ANOTHER 20%.

G²
Click to expand...

What I understand is that NPN drivers will drive current from +56VDC, and PNP from -56VDC; so:

56VDC * 1,2 * 1,2 =80,64 ;

According to data-sheets those drivers are within (but at limits) tolerance (theyt are rated 80V for Vceo)

Please correct me if I'm wrong...
 
#16
Fedess said:
What I understand is that NPN drivers will drive current from +56VDC, and PNP from -56VDC; so:

56VDC * 1,2 * 1,2 =80,64 ;

According to data-sheets those drivers are within (but at limits) tolerance (theyt are rated 80V for Vceo)

Please correct me if I'm wrong...
Click to expand...

Your power transistors will be getting the _full_ power supply of 112 Volts if driven to clipping with no load. The MJE340/350s are 300 Volt transistors and will be just fine - provided you don't exceed the current and thermal limits.

G²
 
#20
Thanks AndrewT I will consider that so I don't blow the woofers!

Today when I was peparing things to mount the bridge and filter cap of the new PS I was just wondering if also it's needed to change the others transistors, (Q1 to Q5, Q9, Q11, Q12, and Qt). I believe this is not needed as they are not directly connected to +V and -V...


Edit:
Just to post a direct link to the schematic:
http://www.pcpaudio.com/pcpfiles/proyectos_amplificadores/ampsub/etapa.pdf

thxs
 
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