JLH 10 Watt class A amplifier

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Mooly

Mooly

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Joined 2007
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The 0.87v across Q4 B-E suggests either a problem or that a fairly large base current is flowing.

Is there any voltage across R5 ? Normally there should be around 1.3 volts across here.

If there is no voltage across R5 then remove Q3 (or at least isolate the base lead) and recheck the voltage across R5. Has it come up ?
 
e_fortier

e_fortier

Member
Joined 2001
Mooly said:
The 0.87v across Q4 B-E suggests either a problem or that a fairly large base current is flowing.

Is there any voltage across R5 ? Normally there should be around 1.3 volts across here.

If there is no voltage across R5 then remove Q3 (or at least isolate the base lead) and recheck the voltage across R5. Has it come up ?
Click to expand...

For my measurement, I have an 8 ohm load and the input is shorted.

There is no voltage across R5 before and after isolating (cutting) the base of Q3.

You mentionned 1.3V across R5 which is a 8k2, this mean a current of 0.15mA...seems a little too low should I increase the value of R5..I wish I could simulate this circuit to see what value I should use for R2, R5 and R9.

R2 (base of Q4) is 15K but the majority of schematic I have seen uses 47k, can this cause the high Vbe ?

Thanks a lot.
Eric
 
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Mooly

Mooly

Administrator
Joined 2007
Paid Member
For testing don't have any load connected. Its to risky both for the load and the output transistors.

Yes, in normal operation there will be around 1.3 volts across R5. That's a non varying value and determined by the two B-E volts drops of Q3 and Q1 added together.

You can simulate it :) Here is the file that will run straight off in LTSpice. No extra models needed.

These two images show the normal voltages and the voltages when Q3 base was open.

The component refs in the sim are different to your picture
 

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e_fortier

e_fortier

Member
Joined 2001
Help please.

Well, you guessed it I'm back with some more issues with the second channel :D

I have attached the schematics with the DC voltage shown in red, these are referenced to gnd.

Q1, Q3 both have Vbe of 0V even though I just changed them.

Q7 is kinda odd with a Vbe of about 0.18 and it varies based on the value of VR2.

You want to laugh ? Have a look at the Vb of Q4, a whopping 6.4V...while input is tied to gnd. To top it off a nice 21V across R5 :)

If you have any idea what's happening please share your knowledge.

Thanks a lot,
Eric
 

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e_fortier

e_fortier

Member
Joined 2001
BOTH channels fixed !

Hi,

Well Mooly was right on once again. The problem was Q3..transistor was fine but the PCB trace between Q3 base and Q4 collector was broken therefore open circuit :D By looking at it the trace and solders looked fine.

Both channel now have nearly zero DC offset and are biasing nicely and stable at 1.3A/ch.

Thanks a million guys !

BR,
Eric
 
e_fortier

e_fortier

Member
Joined 2001
Mooly said:
Great to hear you have it up and running. Enjoy :)
Click to expand...

Hi,

It's working OK but I have the following potential issue I'd like to fix.

I'm using 2 power supply (switching) each rated with a single 14V, 3.8A output. These PS each have floating output so with 2 of them I'm able to create the required +/-14Vdc, 3.8A for my stereo set-up.

One of the SMPS (laptop type, brick style) is heating up uncomfortably while the second one is only warm.

Is there a way to share the load equally between my 2 power supply so 1 of them doesn't stop working prematurely ?

If that doesn't work, where can I buy a +/- 15Vdc, 120-150W power supply.

Thanks,
Eric
 
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