Simplistic NJFET RIAA

[Salas]

You mean there is no drop over the setting resistor near it? Because with Vgs=4V its certainly on.

 

[zeeuw]

yes, there is a drop over the resistor, but only IxR
U is at drain and source the same

I expect a drop in voltage

or not?

 

[Salas]

You should have Vin-Vout across the CCS Mosfet. Can you set the output at all by turning the voltage setting trimmer?

 

[zeeuw]

just measured
Vin 44v
Vout 43,5v
50mA (0,5v over the 10ohm resistor)
when turning the pot to 100mA
Vin 43,5v
Vout 42,5v

possible mosfet is blown out?

 

[Salas]

Check Q2,Q3,Q5 for being OK first.

 

[zeeuw]

checked
are ok
measured also other channel,same reading

so I can set the mA, but no voltage change

 

[Salas]

Do both Mosfets show VGS reading? How much is the voltage drop across the series resistor under the trimmer? Is the trimmer correctly wired as a rheostat?

 

[zeeuw]

yes, Vgs = 5,37 v
voltage across r = 20,5 v
and is reostat

 

[Salas]

That means there is 6.12mA running in the resistive Vref and normally you could set between 21 to 43.5V. If the trimmer is indeed changing Ohmic value when you turn, then I would be very suspicious of Q2 and Q3 first when having VGS on Q4. Does Q3 show healthy Vbe?

 

[zeeuw]

the 5,37 was on Q1
on Q4 there is Vgs 2,2v
Vbe = 0,62v
still I do not understand
what should be normal readings on tippoints in the sence circuit?

 

[Salas]

2.2V is little. The sense circuit must be tied to the force circuit terminated with a load consuming much less than the CCS circuit provides. It should show Vout at its tips too. Is it configured like that now you measure?

 

[zeeuw]

the load is the phono stage
but the BF245 can only take 30v, so maybe broken

 

[Salas]

Should only see VGS across it when all is working well. This version has been built successfully many times in this thread, you will surely debug it.

 

[zeeuw]

replaced Q2 (BF245) with 1k, nothing changes

 

[Salas]

See about the output Mosfet orientation and/or health then.

 

[zeeuw]

thx for your help, will work on it

 

[zeeuw]

Found it!
I build the shunt as double mono
so I needed to increase R1
I will finish this weekend case
and than listening........

 

[Salas]

OK. Post some pics when finished to show us. Good luck.

 

[buzzforb]

Tomorrow, it begins. Here are the boards and prereg.

 

[Rush]

Tomorrow, it begins. Here are the boards and prereg.

What boards? Thought this was supposed to be PTP???

Rush