Max voltages for transistors drivers

[Fedess]

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:

Need help constructing amplifier S-Sub

thanks!

[infinia]

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!

[Fedess]

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?

[infinia]

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.

[Fedess]

Maybe I'm missing some things... for what are you saying I understand that the transistors used won't work at 56V...

[stratus46]

Originally Posted by Fedess

Maybe I'm missing some things... for what are you saying I understand that the transistors used won't work at 56V...

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

[infinia]

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.

[Fedess]

BD139 and BD140 are SGS,

Here I copy the links:

BD139 Datasheet pdf - NPN SILICON TRANSISTORS - SGS Thomson Microelectronics

BD140 Datasheet pdf - Tranzystor ma³ej czêstotliwo¶ci du¿ej mocy - Ultra CEMI

BC639 Datasheet pdf - NPN Silicon AF Transistors (High current gain High collector current) - Siemens

BC640 Datasheet pdf - PNP Silicon AF Transistor - Infineon

[Fedess]

Originally Posted by infinia

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.

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...

[Fedess]

Originally Posted by stratus46

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

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...