JLH 10 Watt class A amplifier - Page 265

Puffin · 7th December 2012, 07:33 AM

I have finally finished my amp using the Silicon Ray boards. I have undistorted sound on both channels, but I cannot get the dc offset lower thad around 7v (yes v - not mv) on either board. VR1 and VR2 don't seem to have any adjustment left in them (fully clockwise). Should I fiddle with VR3 (current) and then adjust VR1 & 2? It seems to me that if I turn VR1 and 2 anti-clockwise the DC offset just goes up (it started at around 15v) Oddly enough there is no turn on or off thump and no cone excursion.

AndrewT · 7th December 2012, 09:50 AM

The speaker MUST NOT be connected until AFTER you have completed all your tests and the amplifier has "passed" all the tests.

philpoole · 7th December 2012, 10:20 AM

Don't plug speakers into that! And be careful without them plugged in. Something's obviously wrong with your circuit, and you might find your quiescent current could be too high - which could cook your output transistors. Do you have very low resistance in the power rail (I think my JLH 96 has 0.33ohm power resistors - they were in the original circuit). You could check the voltage across those and calculate the Iq to rule out it being too high.
Check the circuit. Are you happy the variable resistances are wired the right way? Are all your transistors wired up as you expect?

Puffin · 7th December 2012, 10:47 AM

Thanks chaps. Will do some checking.

Rob.

Puffin · 7th December 2012, 02:25 PM

Looks like the orientation of VR3 may be wrong?

Puffin · 9th December 2012, 08:09 AM

Quote:

Originally Posted by Puffin

Looks like the orientation of VR3 may be wrong?

Anyone?

It seems from the mask that the adjusters are linked to the series resistors?

mindutis · 9th December 2012, 08:34 AM

I do not get it, the actual board this guy soldered is not the same as in the picture. The output transistor pins are not in a line as in the soldered board. On the PCB layout picture output pins are in the triangle, and not in a line.
Lets hope here are no mistakes after all.

Same is with AmpCampAmp board. Does not match with the schematic given

P1 goes to + of output Cap on the PCB, but not the schematic. Sorry for off topic. I hate mistakes

Check what you have got.

Puffin · 9th December 2012, 09:17 AM

Is the above reply meant for me? If so, if the answer to my question is yes, does it matter at present for the mask not to follow the schematic?

AndrewT · 11:26 AM

Note that all three adjust pots have two leads/legs connected by a trace.
This converts all three pots to variable resistors. Post2647 is different !!!!!
When the pot is a variable resistor then the direction of rotation of the adjust screw changes with the orientation of the pot.
The designer can, if he/she wishes arrange for the resistance to increase with clockwise rotation of the adjuster.
Or, just as valid, that the parameter that is affect by the adjustment increases with clockwise rotation of the adjustment screw.

Equally the designer could swap the pots around for anticlockwise to achieve the same.

The Builder has a choice:
a.) do it the way the designer shows.
b.) do it the way the designer intended.
c.) do it the way it makes sense to you as the builder/operator.

Puffin · 9th December 2012, 11:38 AM

Thanks Andrew. I was going to originally ask if the orientation of VR1 - 3 made any difference. However I assumed by post 2643 that it did. So it seems you cannot get it wrong? If that is so, then I have problems elsewhere.

Rob.

9th December 2012, 08:34 AM	#2647
mindutis diyAudio Member Join Date: Apr 2008 Location: Sweden	I do not get it, the actual board this guy soldered is not the same as in the picture. The output transistor pins are not in a line as in the soldered board. On the PCB layout picture output pins are in the triangle, and not in a line. Lets hope here are no mistakes after all. Same is with AmpCampAmp board. Does not match with the schematic given P1 goes to + of output Cap on the PCB, but not the schematic. Sorry for off topic. I hate mistakes Check what you have got. __________________ My new website is relocated and released...

9th December 2012, 11:23 AM	#2649
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Note that all three adjust pots have two leads/legs connected by a trace. This converts all three pots to variable resistors. Post2647 is different !!!!! When the pot is a variable resistor then the direction of rotation of the adjust screw changes with the orientation of the pot. The designer can, if he/she wishes arrange for the resistance to increase with clockwise rotation of the adjuster. Or, just as valid, that the parameter that is affect by the adjustment increases with clockwise rotation of the adjustment screw. Equally the designer could swap the pots around for anticlockwise to achieve the same. The Builder has a choice: a.) do it the way the designer shows. b.) do it the way the designer intended. c.) do it the way it makes sense to you as the builder/operator. Last edited by AndrewT; 9th December 2012 at 11:26 AM.

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7th December 2012, 07:33 AM	#2641
Puffin diyAudio Member Join Date: Feb 2006 Location: U.K	I have finally finished my amp using the Silicon Ray boards. I have undistorted sound on both channels, but I cannot get the dc offset lower thad around 7v (yes v - not mv) on either board. VR1 and VR2 don't seem to have any adjustment left in them (fully clockwise). Should I fiddle with VR3 (current) and then adjust VR1 & 2? It seems to me that if I turn VR1 and 2 anti-clockwise the DC offset just goes up (it started at around 15v) Oddly enough there is no turn on or off thump and no cone excursion.

7th December 2012, 09:50 AM	#2642
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	The speaker MUST NOT be connected until AFTER you have completed all your tests and the amplifier has "passed" all the tests.

7th December 2012, 10:20 AM	#2643
philpoole diyAudio Member Join Date: Oct 2004	Don't plug speakers into that! And be careful without them plugged in. Something's obviously wrong with your circuit, and you might find your quiescent current could be too high - which could cook your output transistors. Do you have very low resistance in the power rail (I think my JLH 96 has 0.33ohm power resistors - they were in the original circuit). You could check the voltage across those and calculate the Iq to rule out it being too high. Check the circuit. Are you happy the variable resistances are wired the right way? Are all your transistors wired up as you expect?

7th December 2012, 10:47 AM	#2644
Puffin diyAudio Member Join Date: Feb 2006 Location: U.K	Thanks chaps. Will do some checking. Rob.

7th December 2012, 02:25 PM	#2645
Puffin diyAudio Member Join Date: Feb 2006 Location: U.K	Looks like the orientation of VR3 may be wrong?

9th December 2012, 09:17 AM	#2648
Puffin diyAudio Member Join Date: Feb 2006 Location: U.K	Is the above reply meant for me? If so, if the answer to my question is yes, does it matter at present for the mask not to follow the schematic?

9th December 2012, 11:38 AM	#2650
Puffin diyAudio Member Join Date: Feb 2006 Location: U.K	Thanks Andrew. I was going to originally ask if the orientation of VR1 - 3 made any difference. However I assumed by post 2643 that it did. So it seems you cannot get it wrong? If that is so, then I have problems elsewhere. Rob.