i/v stage problem

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maxpou

maxpou

Member
Joined 2006
Hi,
i built this stage iv for the PCM1798 or COD from twisted pear audio, but i don't understand why i have dc offset at the output ( 0.7V) while my j-fets are match paired. I have this problem when I put a cap in C7 and C8.

Someone can help me?

Thank you! Maxpou
 

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Ken Newton

Ken Newton

Member
Joined 2003
So, when C7 and C8 are removed there is no D.C. offset?

If yes, then measure the D.C. voltage at the drain of Q4 and the gate of Q5, with C7 and C8 in place, to see if they are symmetrical with respect to ground. If there's an imbalance, my first guess is that one of those 1,000uF e-lytic caps may be leaking D.C. Obviously, the offset should not be affected by C7/C8 which should function as open circuits at D.C.
 
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maxpou

maxpou

Member
Joined 2006
Hi Ken,
thank's for reply. my first test was to change the capacitors for 100uF, without success, 2nd I lowered the resistors R8 and R9 by half, without success and my last test was to change my j-fets without success too. :crying:

Maxpou
 
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Ken Newton

Ken Newton

Member
Joined 2003
maxpou said:
Hi Ken,
thank's for reply. my first test was to change the capacitors for 100uF, without success, 2nd I lowered the resistors R8 and R9 by half, without success and my last test was to change my j-fets without success too. :crying:

Maxpou
Click to expand...

1) Just to confirm, there is no D.C. offset with C7/C8 removed?
2) Also, what are the supply voltage readings at the drain of Q4 and the gate of Q5 with and without C7/C8?
 
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Z

Zoran

Member
Joined 2004
zorandimovski.com
Disconnect C2 at the point with jfet buffer. Leave 100uF and 100nF C in place.
(This will give You a chance to check IV stage and C. Because of the C not conducting DC should be 0mV at disconnected end. If You have any positive DC C is not good and should be replced.)

Anyway, check the ground connection from input resistor of the jfet buffer stage to central ground, where GND from power supply connected. And solderings too.
 
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M

Markw4

Member
Joined 2016
What if there is a high frequency oscillation that is throwing off your DVM readings? Wouldn't having capacitors to bypass the resistors help increase the amplitude of the AC output and possibly throw off the DVM even more? Maybe there is an oscillation or other problem or maybe there isn't, but if no scope to check then maybe all you can do are things like take both AC and DC DVM measurements at each node, and maybe see if changing capacitance to ground anywhere in the circuit changes the output offset. In other words, you might be able to get some better idea of what is going on.
 
Lampie519

Lampie519

Member
Joined 2006
What did you find adjusting the resistor value? No change at all?

Another question: Is the output loaded (maybe a small resistor value in series will help (100 Ohm). This will increase the output Z a little but is good practice as it helps protecting the transistors agains short circuit and HF oscillation.
 
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Ken Newton

Ken Newton

Member
Joined 2003
Lampie519 has the right sort of idea. Try inserting a 1K resistor in series with the gate of Q4. This will create an RC filter pole in conjunction with the gate terminal's parasitic capacitance, and is known as a 'gate stopper'. Solder the resistor close to the gate terminal of Q4.
 
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