speaker protection - fail again - help please

[bigpanda]

Hi

This is the third sp protection I 've been trying but I never had any one working fine. This is from :

http://users.otenet.gr/~athsam/protection_2_EN.htm

It is a modification of protection section of the PA300 with two slight difference :
1. an extra cap. C5 which is not present in PA300
2. R11 – 120k instead of 47R in PA300

I was not able to get the KTY81-122 so I did a slight alteration also :

I had the R3 and RTH(a NTC-1k instead of a PTC) swapped and I had a VR in place of R2 and R1 so that I may be able to fiddle around the voltages feeding into +- input of IC1A and hence the cut-off temperature. I tried to warm the NTC with my hair dryer and D5 did go on, signifying that the as the temperature goes on, IC1a out will go low but I don’t have the exact temperature. I think I can adjust it but I will do it when I had the other parts working also.

I measure only 11.5-12V on the zener (1n4744 which is suppose to be 15V) but I think this should be fine, isn’t it?

D6 is always on, so I had pull out R15 and R16, isolating the output of IC1b. I measure the voltage across C4 and it’s only about 0.65V where as –ve of IC1B reads about 1.2V. I try changing R13 to 1k thus lowering –ve of IC1B to about 0.7-0.8V. I change C4 to about 100u and it read about .68V when power on for about 30 sec. What can I do now? Keep making C4 larger? The time of charging will be getting longer even if the voltage can get over the –ve of IC1B, right?

Can somebody give me some help on this please. I need to have a sp protection circuit for the power amp. I’ve built (3 already and the next one is coming up)

Tks

[jaycee]

Here is a simple circuit that does work:

www.darkmatter.myby.co.uk/dcprotect.png

L1 is the relay, can be any 12V type. Signal from speaker output enters at R1. You can probably use any small signal transistors for Q1-Q3 (such as 2N5551), and replace Q4/Q5 with a Darlington transistor.

The R5/R6 and C3 arrangement implements a simple 2/3 second power on delay. If not needed you can remove R5, R6, R7, C3 and Q3, and connect R4 directly to +12v and the detector should still work.

I will probably rework this circuit to run from a bipolar supply, perhaps +/-15V so that it will work from the preamp supply.

[janneman]

Unless you want to start with circuit # 4 (and the problems THAT will give you), I would disconnect all the stuff having to do with the DC protection (Q1, Q2 etc).
Since the basic temp stuff seems to work, work your way from the output of IC1a to the relay stage to see where the bug is. It is pretty useless to swap or change components at random if you don't know what the problem is.
This is not a very complex circuit, but you need to be methodical and do some thinking.


Jan Didden

[jaycee]

Originally posted by janneman
Unless you want to start with circuit # 4 (and the problems THAT will give you)

Are you referring to the schematic I posted?

edit: didn't notice the original poster wanted a temperature protection. This is easy enough to add, using any old opamp (a 741 will do) configured in comparative mode, with a thermistor on one input and a fixed resistor/preset on the other. Connect it up so that when the temperature point is exceeded it pulls the output low. Connect this to the base of Q5 via a 10K resistor.

[janneman]

Originally posted by jaycee


Are you referring to the schematic I posted?

edit: didn't notice the original poster wanted a temperature protection. This is easy enough to add, using any old opamp (a 741 will do) configured in comparative mode, with a thermistor on one input and a fixed resistor/preset on the other. Connect it up so that when the temperature point is exceeded it pulls the output low. Connect this to the base of Q5 via a 10K resistor.

I have not looked at your schematic, that's not the point. If you give up on three after a problem, chances are large that you give up on #4 after a problem. Unless you sit it out and fix it.

Jan Didden

[bigpanda]

Hi janneman,

I totally agree with you : start another circuit, get into other or even more problems. I'll stay with this one (for I have spent quite a bit of time, putting them togher and even lay a PCB for it), Now that I've pulled out Q1, Q2, thus leaving the input of IC1B to the output of IC1A and the delay circuit. I still don;t get a voltage big enough to 'turn on' IC1B (it should be at least 1.4V according to the shcematic because at -ve input of IC1b, the volage is 1/11 x 15V = 1.36V). How to do so? Should I take out D1 also (I don't think it's necessary, but what's your opinion?) to isolate IC1B from IC1A?

I've made amp. from 1 pair of o/p's to 4 paiirs. I had been all the way successful but drowned in simple circuits like this!!!!!!!

[janneman]

This circuit has three functional inputs: the temp through IC1A, then the offset through Q1, Q2 and then the stuff through D2. You need to tackle each in turn to be sure it does what it needs to do in case of catastrophic conditions. You already took out Q1, Q2. Now still IC1B doesn't switch, my hunch is that Q3 is hard on. Take it out.
Now go back to the beginning:
Verify that V+/IC1B = 15V, and V-/IC1B = abouth 1.5V.
Heat the NTC, put a voltmeter on IC1A to make sure it switches low. If Vo/IC1A goes down towards zero, it should pull V+/IC1B down through D1 to below 1.5V, and Vo/IC1B should change state. Check that.

BTW, if your zener only gives 11.5-12V, that indicates that the series R from the supply is too high. So R17 may be too high, but that depends on your supply. Try lowering R17 to 2.2k or something to see if you can get a stable +15V supply. This is important, because other wise the nominal +15 varies with the circuit actions and that can upset the bias voltages in the opamp inputs.

Jan Didden

[bigpanda]

I've changed R17 to about 2.2k and now the voltage across zener is about 14.85V. I desolder 'C' of Q3 and when I throw the switch on, there is a delay of about 2 sec before o/IC1B is turned on.(D6 is on at start and stays for about 2 sec). But I was unable to change the delay time (I think 4-5 sec should be more appropiate)

I check the voltage at 'C' of Q3 (desolder from circuit) which is about 2.5mV

NTC is working fine : I heated it and D5 turns on but I don't have the temp. I felt quite hot with the NTC.

[Centauri]

Seems like Q3 is being turned on - is point B connected to the AC side of the amp's rectifier? - is D2 around the right way? There should be a NEGATIVE voltage at junction of C3 & R9 similar to amp's negative rail. The voltage at the base of Q3 should also be negative, thereby turning it off.

Reducing R11 should increase the turn-on delay - I would suspect the value of 120k is some sort of misprint (given the VERY bad English translation of the text, this would not be surprising).

Cheers
Graeme

[destroyer X]

Can be some solution.... at least...for a while, when you go studying your problem.

Before the electronics developed into our days sophistication, some basic rules could be obbeyed, as a straight line is the smaller distance we can have between two points.

Well, each one of us with our own preferences...be happy!

regards,

Carlos