[proid]

I'm building a 2 channel PPAv2 in bareboard with something changed from original:
- Dual +-12 rail regulated power supply, no voltage divider
- Change PN4392 to 2n5486
- 2n5087/2n5088 change to BC327/BC337
- No third channel
- both opamp and output transitor are powered by +-12vol
I have just finished wiring 1 channel and measure the voltage drop between output resistor (2.35R) : 5ma then i measure voltage between output and ground: 4.3V!! then i turn the trimpot to get 22mV drop between output resistor and the DC voltage at output is too high: 3.4V. Can someone give me suggestions? Thanks 

 

[Avro_Arrow]

 
What is the voltage at the output of the op amp?

It should be working to counteract the DC offset.
Is the feedback loop broken?

 

[duncan1]

Regardless of the outcome do you realise there is a large difference in the spec. of the 2 fets? . The one you changed the original fet for has a much  lower voltage limit and current drain limit and is meant for VHF use in delicate hf situations.. I hope you have compensated for it in the circuit? If you havent you will have a harder time balancing this.

 

[Avro_Arrow]

You can still get the PN4392...just in an SMD version.

 

[duncan1]

The  PN4392 is identical to the 2N4392-spec . is the same. A lot of the PN range are the same. --Mouser have them on sale at this moment-2N4392

 

[proid]

  Regardless of the outcome do you realise there is a large difference in the spec. of the 2 fets? . The one you changed the original fet for has a much  lower voltage limit and current drain limit and is meant for VHF use in delicate hf situations.. I hope you have compensated for it in the circuit? If you havent you will have a harder time balancing this.

 
I have read that these two fet are quite exchangeable , the problem of 2n5486 is that it can handle less than 30ma and i think that's fine in a PPAv2?

 

[tomb]

   
I have read that these two fet are quite exchangeable , the problem of 2n5486 is that it can handle less than 30ma and i think that's fine in a PPAv2?

 
I suspect your instincts are correct.  The only difference might be that you notice you can't bias things as high as you'd like - but it wouldn't cause offset issues.  We use the 2N5486 on the MOSFET-MAX and are able to bias all the way up to 125ma (I haven't tried a higher bias than that.).
 
P.S. Maybe Tangent will see this and help with the specifics.

 

[proid]

Or may be i solder the opamp to the socket wrong? I use opa1641 and it doesn't have the dot marking like normal:

So i found this picture and i assumed that the side of dot marking is the right side as the picture above?

 

[duncan1]

Tomb . I  take the comment you made about the bias being  able to take up to 125 MA as referring to the actual output of the circuit I was referring to the actual total current that can be used through a 2N5486 and that is certainly much less than 125 MA   in low figures. The original fet has a higher current load ability and voltage. So you are still putting in an active component that has not been designed for the original circuit--unless you have compensated for it by changing the circuit or you know the current load through the fet is less than the spec. of it.You can get away with putting in a lower voltage active device  in a circuit but whenever a fault occurs the safety leeway has been compromised and that component is more likely to  blow.

 

[Avro_Arrow]

 
@Proid

 
In both pictures, pin one is on the bottom, left.
 
@duncan1
 
Have you looked at the circuit where the part is used?

 

[nikongod]

Have you tried different op amps? *Never* run a circuit for the first time with a good op amp, especially if you have the op amp in a socket. Its worth keeping a small stash of something nice and cheap and unconditionally stable just for this... 
 
Assuming you verify that the op amp is not the problem:
Are the voltages measured at the input and output of the buffer equal, to within some reasonable measure of error? 
If they vary significantly, something is wrong with your buffer. 

 

[duncan1]

I took your advice and had a look at the circuit. I can see why he wants to use -2N5486 as its used at the output of the AD8610 along with  2N5484. But I take it he is using it as a replacement  to Q 4- This is a "different kettle of fish". The voltage difference at the output of the chip is low. But if it is Q 4 then that FET straddles across the  combined voltage of the power supply and is used to balance the amp. This is a critical position and personally I would fit the original or failing that there are plenty of equivalents available .It only takes a small fault in either active device at either end to blow the fet and if blown the wrong way would apply the maximum current available through the output. Perceived wisdom due to not just me but may projects on Electronics World Mag . Suggest by experience that the active devices on either side of it should be able to take the combined voltage load[+/-] > I have built plenty of loudspeaker power amps with approx the same biasing [theres a limit to the different types employed] and I always made sure the "amplified diode" type of circuit had an active device that had a working voltage of at least half  of the supply rail that VHF FET does not.

 

[tomb]

  Tomb . I  take the comment you made about the bias being  able to take up to 125 MA as referring to the actual output of the circuit I was referring to the actual total current that can be used through a 2N5486 and that is certainly much less than 125 MA   in low figures. The original fet has a higher current load ability and voltage. So you are still putting in an active component that has not been designed for the original circuit--unless you have compensated for it by changing the circuit or you know the current load through the fet is less than the spec. of it.You can get away with putting in a lower voltage active device  in a circuit but whenever a fault occurs the safety leeway has been compromised and that component is more likely to  blow.

 
Nope
 

  I took your advice and had a look at the circuit. I can see why he wants to use -2N5486 as its used at the output of the AD8610 along with  2N5484. But I take it he is using it as a replacement  to Q 4- This is a "different kettle of fish". The voltage difference at the output of the chip is low. But if it is Q 4 then that FET straddles across the  combined voltage of the power supply and is used to balance the amp. This is a critical position and personally I would fit the original or failing that there are plenty of equivalents available .It only takes a small fault in either active device at either end to blow the fet and if blown the wrong way would apply the maximum current available through the output. Perceived wisdom due to not just me but may projects on Electronics World Mag . Suggest by experience that the active devices on either side of it should be able to take the combined voltage load[+/-] > I have built plenty of loudspeaker power amps with approx the same biasing [theres a limit to the different types employed] and I always made sure the "amplified diode" type of circuit had an active device that had a working voltage of at least half  of the supply rail that VHF FET does not.

Q4 will not "blow."  It's in an isolated (so to speak) current loop that sets the bias in the diamond buffer.  If its IDSS is too low for the bias desired in the buffer, you will simply not be able to turn the bias current up anymore.
 
Use of the 2N5486 in a BJT diamond buffer is well documented and has been used for years:

This is the diamond buffer designed for the revMH Millett Hybrid by Steinchen.  Literally hundreds were built.  Bias can be set up to 60ma in this configuration with the 2N5486 (20-30ma was typical and I believe the setting recommended for the PPAV2).
 
You can explore all of this in gory detail at:
http://www.diamondstar.de/dDB_overview.html
 
There's also a helluva lot of detail and real-world applications at the Millett Hybrid support site.  Further, the Millett MAX and MiniMAX were originally designed for the 2N5486.  We went further than the typical 30ma bias with single output transistors (the above schematic has doubles in parallel) because we were using heat sinks (Steinchen's buffer went without them, as does the PPAV2).  It was only then that we went to the PN4392, but that was after we changed the current mirror to 2X - R4 and R5 is only 100 ohm in the MAX/MiniMAX.  That gives us the ability to turn the bias up higher than even the PN4392's IDSS limits.

 

[proid]

Hi all, i measured the output of opamp and it was 12V which is the same as power supply voltage +, the output transistor got no heat at all and Q32 was very hot, all other transistors were fine and Voltage at output socket is 9V now

 

[nikongod]

Which op amp are you using? 
 
Did you take the advice (given everywhere, actually) to run the amp with a cheap but known good op amp during the initial setup?