increasing the output power - advice needed

Hello,

First of all thank you in advance hence I have very limited knowledge of how an amp works. Luckily, this work will not be done by me, but I want to learn whether it is a good idea.

I have an old Phonic Mar 4 power amplifier and recently while trying something, accidentally two speaker bare wires were touched each other and voila, the amp is in a coma now!

I dropped it to one of the electronics repair shop and today I have learned that transistors were gone (both channel).

I asked him whether it is possible to increase the output power while changing the transistors because I want to use this amp to power 2x 12" Infinity subwoofers (300w rms - 4ohm). He said yes, adding additional transistors would be OK (he mentioned that the toroidal transformer is sufficient).

Do you think there will be any detrimental effects like distortion, overheating, etc (for subwoofer usage)?

NOTE: You can find the service manual here: phonic-mar2-mar4-mar6-service-manual-475292.pdf - Google Drive

Best,
Onur
 
The power you can obtain from an amplifier is near VCC² / Rl, being VC the final stage power supply and Rl the load impedance. So, replacing or putting more transistors in parallel to those existent WILL NOT INCREASE THE POWER OUTPUT. This guy lied to you.

If the amp is capable of working on loads as low as 2Ω, the you CAN increase power output putting speakers in parallel down to this value, if you decrease further the amp load, it will blown again.
 
When I want more power out of an amp, I usually upsize the heatsink and/or increase air flow from the fan.
I can also replace output transistors with more modern higher current soa version. I got my ST120 up to 70 W/ch for 5 seconds at a time by replacing 40636 (special 2n3055) with NTE60 (soa same as MJ15003). And adding heat sink and fans.
I don't have schematic of your old amp, but in your part of the world 2sa1943 pnp 2sc5200 output transistors were pretty common. These can be replaced with MJL21193/4 or MJW1302/3281 from ON which have more soa @ 80v. But getting real On semi parts in your part of the world may be problematic. If the 2sa/sc parts are very close together, the On parts might not fit. Really, more heat sink & fan increases the time you can run the amp at high power. Many amps are designed to melt down if you run them at 100% power more than an hour, or in some cases, 3 seconds. The 100 % 24/7 amps were 1990's era PA amps, Crown, Peavey, QSC, Yamaha: According to repairment on the PA thread. Some inflation of watts ratings happened about 2000 with 8 transistor/channel amps being rated 1000-1500 w/ch instead of 450-650 in the nineties.
BTW in future use speakons (best) or dual banana jacks & plugs for hooking up speakers to amps. Much harder to short out.
 
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Two 4 ohm speakers - one per channel? Or are you using bridged mode?

Basically is pointless trying to increase the power output of a commercial amp, they are built down to the specifications, because over-engineering means no profit margin.

You might, however, increase the robustness of the output stage by adding more devices, particularly if you are driving highly reactive speakers which eat into the SOA.

These amps are supposed to have short-circuit protection, you should to check if the protection circuit is at fault.

And you'll have learnt your lesson about bare wires.... If there's room in the cabinet consider adding SpeakON connector(s) to prevent this issue in the future.
 
Thank you all, your responses are much appreciated!

Tomorrow I will call and tell him not to add additional transistors, just replace the dead ones.

Now, if possible would you recommend what/how many transistors to use to make it better? Indianajo mentioned some make/models, I will look into them but some other options would be good to know (I believe in the first post the service manual has schematics -mine is the Mar4).

Best,
Onur

Note: by the way amp can run on 2ohm load so perhaps after the repair I can use the bridge mode and connect 2x 4ohm subwoofers in parallel, as suggested.

Note 2: ON MJL21193/4 transistors are around 5 dollars each(wow, expensive)
 
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Ketje

Member
2012-12-19 7:24 pm
Flanders
Note: by the way amp can run on 2ohm load so perhaps after the repair I can use the bridge mode and connect 2x 4ohm subwoofers in parallel, as suggested.
If you connect a 4Ω speaker to a bridged amp, each amp "sees" 2Ω and the power is double.So do not use a 2Ω speaker (nor two 4Ω parallel).
More output transistors need more drive current, without modifing the driver it's useless.
Mona
 
Obviously. Again, with the same load and with the same power stage bus voltage, there is no way to increase the power output, and changing or increasing the number of output transistor paralleled, only increases current capability but not the output power. Perhaps you can gain a pair of watts replacing bipolar by MOSFET's by its lower VCEsat, but above 15-20V it is negligible and need re-engineering of the driver stage.
 
Perhaps I couldnt make it clear. I will tell him just to replace the stock (dead) transistors. But here is the caveat, I bought his amp as a second hand and probably it was fiddled with before. I would like to know what size/type and how many transistors would be needed to have factory settings (I believe the number/specs can be found in the schematics, but I dont know what to look for). Best.
 
Obviously. Again, with the same load and with the same power stage bus voltage, there is no way to increase the power output, and changing or increasing the number of output transistor paralleled, only increases current capability but not the output power.
What you get with more heat sink, air flow, & O.T. soa is ability to put out max power for longer time without burning up.
With more transistors, you would need more driver current & soa, probably upgrading heat sinks on those. too. I did when I hot-rodded my ST120. ST120 came to me burnt up, went up in a ball of flame 3 months after I "repaired" it, has horrible reputation online as "worst amp ever made". Now 7 years since upgrade for more power heatsink & airflow I use ~ 14 hours a day.
To get actual power increase you need a transformer with more current & voltage capability than needed for rated wattage. My hot rod unit had that. 72 v @ 6.25 A is pretty capable transformer for a 120 W household market amp.
Actually the biggest bang for a buck is adding heat sinks to the driver transistors, which consumer grade amps usually leave off. If drivers are TO92, upgrading to TO126 transistors with heat sink capability is usually a big reliability increase.
Of course don't neglect to ask for speakon or dual banana jacks for speaker connection. That was the cause of your problem. Speakons are off patent, there are low $ copies out there now.
 
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This is the true reason of more power.
Heat sink improvements help too. The OEM ST120 at only rated power had a tendency to short the output transistors and set the driver resistors on fire. See above where it did it to me with NTE181 O.T. which are Vceo 140 v transistors instead of OEM 60 v ones.
Adding bat wing heat sinks pn 506007B00 to the flat plate "heat sinks" and putting a PCAT ATX supply fan on each one stopped that tendency. Drill, tap, add heat compound, screw new sinks on, heat flows out the fins.
Another case where heat inadequacy might have been the problem. PV-4c I bought "for repair" had shorted O.T.'s which were very capable MJ15024/25's. Initials on heat sink indicated this was the fourth set. Could be an inept roady crew shorting speaker wiring over & over. However, added 4 each 7019BG bat wing heat sinks to each existing heat sink, put a dust filter in front of the fan. 6 months as my TV amp, so far so good, no problems.
 
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