reducing voltage

[lineup]

Originally posted by rtill
I just ran it the way it is...works fine. However, I have seen this done on several amps....I am surprise to see people post that "it doesn't work this way"

A power resistor to lower voltage from transformer/supply is no good.
Because the drop will change with the current drawn.
Say you have a 2 Volt drop at idle.
At max output you may have 20V or more drop across resistor.
Which gives the power supply voltage will be only +-20 Volt ......

A slightly better way is to use some power rectifier diodes.
Across each diode will be like 0.5 to 1.2 Volt maybe. Depending on current level.
This will stay more constant even at max current drawn.
------

But,
for Power Amplifier a difference of +-40 or +-42 Volt is nothing.
Because if the voltage is+-42 at idle, it may well be only +-36 Volt at full power output.
Anyway ...

[rtill]

Originally posted by lineup




A slightly better way is to use some power rectifier diodes.
Across each diode will be like 0.5 to 1.2 Volt maybe. Depending on current level.
This will stay more constant even at max current drawn.
------

...

that's very interesting! I appreciate the response, learn something new everyday I guess.

[sakis]

Originally posted by rtill



that's very interesting! I appreciate the response, learn something new everyday I guess.

music comes out of it it doesnt mean that is working properly ....

then again line up approach with the diodes is a start .....

but lets see it in another way

to my understanding there is no amplifier that is designed for operating with 40+40 volts that will not stand 42+42 ..... unless the amp is designed for 35+35 originally
post a schematic ...... be more specific about the application

[john65b]

There is the LM3886 which has a rail limit of around +/- 40VDC, but I have breached that with no issues.

May eventually burn out the chip, or cause other reliability problems...

[rtill]

here's the schematic....supposedly 200 watts, is this possible?

I have had a few issues since 1st posting. The tip142 wasn't secured properly, got too hot and exploded. Replaced it and now it blows fuses once it is turned on. What would cause this? I would imagine something else in the circuit is blown to now cause the fuses to blow.

[sakis]

looks like a very insecure thing

42 volts for this amp is not a problem but working with loads lower of 8 ohms is absolutelly oyt of the question .....

for shure not only tip 142 was blown from not being properly attached to the heat sink ...somehing else is gone too .....

aslo

there is a big issue about tip 142-147 and cousins bdv66-67 bdw 83-84 c...to work in an audio amplifier they need to be the same make and from the same production line ...IE you cannot use tip 142 from philips and tip 147 from ST ....usually it doesnt work

also this amplifier will produce about 60W at 8 ohms load but operating in lower load like 4 ohm will be stressing outputs like hell

you have to be very carefull with idle setting a bit too much idle will bring the smoke in to your amp

amplifiers working with these outputs may be simple but never stable upgrade option will be to use very nice metal can transistors from on semi MJ15015-16 diferent case but very reliable

amplifier needs a quiet big heatsink and vbe multiplier transistor 6, BC547 needs to be in very good thermal contact with the heatsink to monitor temperature and reduce idle respectivelly ....

amplifier like that if not made properly will oscilate like hell ...meaning that the usual methods like zobel with inductor decouplers bypassed here and there have to be installed .....

figoures of 200w as mentioned somewhere above is absoltuley out of any question .....you will never get them with an amp like that

regards sakis

[Corax]

Since the BC547/BS557 is only 'good' for Vce of 50V and depending on how the TIP142 blew up, there is a pretty good chance that more defective parts are involved and responsible for the (still) malfunction of your amp. The TIP142/TIP147 will withstand a Vce of 100V therefore no harm for them is expected with rail voltages of ±42V though.
A good point to start is the bias transistor T6. If he is blown and makes an open circuit both power stage transistors (T7 & T8) will conduct and draw a lot of current from the rails. Be in luck if they're not blown again and the rail fuse(s) always blow first. The current limiter T4 and T5 may be damaged too in the first place. Once they are blown, either open or short circuit, they will either never protect anymore or protect all the time.
Furthermore look for burned and/or discolored resistors and check for correct value(s) as stated in the schematic.

Good luck though.

[rtill]

Originally posted by sakis

for shure not only tip 142 was blown from not being properly attached to the heat sink ...somehing else is gone too .....

the .47 ohm resistors test ok. In the past when I've had a transistor blow, the power resistor follows, this is not the case this time.
in your experience what else could have gone?

[rtill]

Corax, your post I think answered my question, thanks. We must have been responding at the same time!

[Corax]

Yip - just with a 4 minutes delay (according to the time attached to each post). ;-)

I hope it helped at least a little bit. The finding/checking procedure for defective components is now up to you.

You may report your progress and what you'll be able to figure out.

Have fun.