Complimentary power transistors in chip amp

[roshan101]

Almost finished constructing an amp using the schematic published in an old Elektor mag. Only the transistors remain to be mounted. Problem is - these BD249/250 are outdated and aren't available anywhere. Should have checked this out before constructing. Anyway, thought I'll substitute these using TIP3055/2955, - since packaging is similar to BD249/250 - so its easy to mount in the existing PCB & heatsink, except current handling & power is a bit less ...I guess.


Being a novice at amp bulding.. Here's my question. Did I make the right choice? Do I need to change values of other components? Will my amp blow???

Amp is supposed to deliver 100W at 4 ohm load from a power supply of +- 18V, 5A.

[roshan101]

here's the datasheet for BD249

[sayang001]

Futurlec has them, http://www.futurlec.com/TransPowerBD.shtml .

[sajti]

Originally posted by roshan101
Almost finished constructing an amp using the schematic published in an old Elektor mag. Only the transistors remain to be mounted. Problem is - these BD249/250 are outdated and aren't available anywhere. Should have checked this out before constructing. Anyway, thought I'll substitute these using TIP3055/2955, - since packaging is similar to BD249/250 - so its easy to mount in the existing PCB & heatsink, except current handling & power is a bit less ...I guess.


Being a novice at amp bulding.. Here's my question. Did I make the right choice? Do I need to change values of other components? Will my amp blow???

Amp is supposed to deliver 100W at 4 ohm load from a power supply of +- 18V, 5A.

The BD249/250 are the good old friends, but they are still in production. I can order them, and the cost is about 0.1USD...
I think the TIP2955/3055 is not strong enough for this application. I would use MJL21193/94...

sajti

[cunningham]

You might also try TIP35 & TIP36.

[Eva]

BD249C and TIP35C have nearly the same specifications, and also
BD250C and TIP36C are almost the same thing

BD249C and BD250C are still in production, at least from a company called Power Innovations

TIP35C and TIP36C are also in production from diverse companies like ST-Microelectronics, Mospec, KEC, On-Semi, etc... Try google for datasheets

These devices are pretty linear up to Ic=10A and with a single pair it's posible to drive a 4ohm load with +-40V rails without exceeding SOA. Their maximum ratings are Vce=100V, Ic=25A, Pd=125W

When using supply rails of +-40V or less these devices are a great and cheap alternative to the more expensive MJ15024/MJ15025, MJ21193/MJ21194, etc... They are also easier to mount due to its TO-218 case

[roshan101]

Originally posted by sayang001
Futurlec has them, http://www.futurlec.com/TransPowerBD.shtml .

Originally posted by sajti
The BD249/250 are the good old friends, but they are still in production. I can order them, and the cost is about 0.1USD...
I think the TIP2955/3055 is not strong enough for this application. I would use MJL21193/94...

Originally posted by cunningham
You might also try TIP35 & TIP36.

Thanks, but I prefer to purchase locally. Dont want to go through import/customs hassles.
Have already wasted enough time & energy Searched for the BD's every nook & corner. I was aware TIP 35/36 were the nearest substitutes, but even these were'nt available.

Originally posted by Eva
These devices are pretty linear up to Ic=10A and with a single pair it's posible to drive a 4ohm load with +-40V rails without exceeding SOA. Their maximum ratings are Vce=100V, Ic=25A, Pd=125W

Considering the fact the chip here has a max voltage at +-22V, Increasing supply voltage beyond this is out of question. So dont you think even by using transistors with lower SOA, the efficiency should almost be same? I might be wrong here...dont know.

Have already soldered the TIP3055/2955. Switched the amp for a minute and it seems to be working. Except gain is a bit lower than expected & the TIP's are pretty cold. Will need to play the amp for a while to confirm this. Think I also need to check emitter collector voltage to make sure Transistor's are conducting. Will do the test later this evening.

Sajti - why do you say TIP3055/2955 cannot hold? They're supposed to be 90W, 60V, 15A.

[Eva]

From the SOA point of view, TIP3055 and TIP2955 will work just fine [actually very happy] on that circuit since supply voltages are low and peak current is not likely to exceed 10A, however, be aware that :

- These transistors are slower than BD249/BD250 so check for signal-level dependent parasitistic oscillations and latching or other awful behaviors when clipping

- These transistors have a specified gain at Vce=4V of 100 for Ic=1A that drops to about 15 for NPN and 22 for PNP at Ic=10A, while BD249/BD250 show gains of 100 at Ic=1A that only drops to about 40 at Ic=10A. Lower gain means higher THD than expected, particularly at high frequencies

I don't understand why your local stores no longer sell the good old TIP35C/TIP36C/BD249C/BD250C but still sell more obsolete parts with inferior performance, like 2N3055, MJ2955 and '3055'/'2955' in diverse flavors

[roshan101]

High THD at high frequencies is not an issue. This amp is used to power a subwoofer. Hope I'm not loosing too much in terms of power. Remember, this is a bridge amp and ther's a complimentary pair on either side of the output. So I assume the 100W might be achievable.
This leaves just the oscillations you've mentioned that need to be checked.

I know its kind of frustating at times with the limited variety of components you get here. Any project you build has to be based on these standard set of components.

[Eva]

To get 100W@4 ohms +-30V output swing is required, so I think the article refers to regulated supplies when it mentions +-18V, since the own circuit will probably drop the extra 3V on each rail before clipping

For a unregulated supply I would use the highest supply voltage supported by the IC to account for sagging