ST151 100W AMP

[john g]

HI GUYS
I DID THE ABOVE AMP AS PER THREAD (GETTING NO RESPNSES FROM THREAD)-SIXTEK.
SHEMATIC AND PIC PER ATT.
TRANSFORMER 32-0-32VAC - REG 50-0-50VDC
QUIESCENT CURRENT SET TO 20mv (tried 3.3mv) set with P1.
ONLY CHANGES MADE FROM THREAD GUIDENCE AS FOLLOWS:
OUTPUTS 5200/1943
C4 4.7pf
C13 150nf
PRE-AMP TDA 1524A AS PER DATA SHEET
TESTING SPEAKERS "200W" 4 OHM (NEW CAR SPEAKERS)
PROBLEM I HAVE, I GET A POPPING NOISE ON SPEAKERS WITH BASS TURNED HALF UP. OTHERWISE IT SOUNDS CLEAR AN CRISP.
I CAN GET HOLD OF A SCOPE AND GEN BUT WOULD NOT KNOW HOW TO TEST THE AMP.
ANY HELP WOULD BE GREATFULL- PLS
TXS JOHN

[tinitus]

hello John

nice one

but please use small 'letters'

'capital letters' is harder to read, and around here considered 'shouting'

good luck


moderation

[john g]

sorry, i do apologize

[djk]

Unhook T10, T14 and I bet the popping stops.

[jaycee]

To clarify what djk is saying - T10 and T14 are VI limiting transistors. These activate when the amp is working too hard, to prevent damage to the output transistors. The popping you are hearing is these transistors activating and limiting the current to the output stage.

The protection was designed for the original TIP35/36 transistors which are not as capable as (real) 2SA1943/2SC5200 transistors. You'd need to alter the resistors around the protection circuit to take this into account.

[Bone]

This popping noise could be due to insufficient smoothing/reservoir capacitance on the supply (ie motorboating). What capacitance are you using?

[djk]

The 'pop' is from the re-triggering of the protection circuit when driving an inductive load line.

[homemodder]

While I agree with what these other members here are saying there might be another factor you should be taking in consideration. 200w car speakers usually mean 50W RMS MAX, 200w usually being music power. From the look of the schematic this amp is capable of somewhat more than 50 RMS. You may be already be overdriving the speakers and car speakers make pop sounds through excessive cone movement before the coil heats up too much and finally fries. Try removing T10 and T14 like suggested by DJK, it will probably answer your question of where the pops are coming from, either the protection circuit needs adjustment or youre overdriving those speakers, I seriously doubt the speakers can handle the sort of power this amp is capable of espesially if they are of the 2 or 3 way type (95 % of car speaker manufacturers cant), subwoofers yes.

[wahab]

As "designed" , the current limiters will work once each power device conduct
2 amperes peak current , that is 4 amps peak for the whole output stage,
and of course , with a 4 R speaker , this will happen once the output voltage
is no more than 16 V peak, largely below the amp s supply voltages.

You can increase the output current capability by increasing the limiters
thresholds to about 4 amps / power device by connecting 220 R resistors
from base to emitter of both T10 and T14.

[AndrewT]

Once you are sure why the amp/speaker is misbehaving and you are sure it is a poorly designed limiter.
Better to add some transient signal tolerance to the IV limiter.
Keep the DC limit at or slightly above the 2Apk.

Add an RC filter to the input to the detection circuit so that fast pulses can pass without the limiter activating.
As slower/longer pulses pass the filter but attenuated you will find that the limiter will start to trigger but at a higher level than the DC sensitivity.

The device SOA is very much more tolerant of high current short period transients than the DC SOA. Use that margin to give you a better limiter.

Remember: a good limiter will never trigger on a valid Audio Signal passing into a valid Audio Load.