I/V opamp techical help

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Hello i need some techical help, i dont have electronics background.
I have an old pcm1702 dac with the classic opamp iv and opamp buffer after.
If i want to use only the i/v part direct to rca bypassing the buffer, do i need a resistor in series with signal or a resistor to ground?

Right now i using an lt1028 as i/v opamp with i/v resistor and a parallel cap only from the output of the dac to the output of the opamp, and conneted the output direct to rca without any resistor in series or to ground.

Next i dont have preamp only have Amplifier with a 2.2uf input coupling capacitor.

Do i need any resistor in series or to ground from the iv opamp output to rca?

Tks.
 
One would usually put a small resistor between the op-amp output and the RCA connector to ensure that the op-amp stays stable when you connect a cable.

By the way, the LT1028 is not unity gain stable. Hence, it may oscillate even without any cable if you did not include any external compensation.
 
One would usually put a small resistor between the op-amp output and the RCA connector to ensure that the op-amp stays stable when you connect a cable.

By the way, the LT1028 is not unity gain stable. Hence, it may oscillate even without any cable if you did not include any external compensation.
I know but in reality i tested some opamps and felt that lt1028 sounded the best of them, very emotional and realistic sound.
I also liked the ad797 and used the recommended circuit on the datasheet that uses also another resistor in series with the feedback cap.
Is there anyway to see if the opamp is oscillating? What type of scoope do i need?.

I have always the cable connected before connect the dac power up. In that case do i need the resistor?
Tks
 
If it oscillates, it is probably at a frequency of 25 MHz or less. An oscilloscope with a bandwidth of 20 MHz or more should show you that straight away when you connect it to the DAC output. (I know 20 MHz is less than 25 MHz, but the response of an oscilloscope usually rolls off gradually.)

Regarding the output resistor, the order of connecting things has nothing to do with it. You can do without the resistor if the capacitance of the cable and amplifier is small enough - which means that the cable has to be short, among other things.
 
Is there anyway to see if the opamp is oscillating?
Check for increased power consumption, if you don't have better tools.
I have always the cable connected before connect the dac power up. In that case do i need the resistor?
Tks
Yes. This is a good practice for protecting internal electronics from static electricity. Unrelated to the recommendation you have received.
 
If you haven't got a scope but have a multimeter, you can check for excessive offsets, weird bias points and hand effect (bias points changing when you only point at the circuit). Connect a resistor of 10 kohm or so in between the circuit and the multimeter to ensure that the meter and meter leads don't cause the instability by their capacitance.

Usually parasitic oscillations keep growing until something starts clipping and limits the amplitude. If that clipping happens asymmetrically, which it usually does, it will also shift the bias point.
 
If you haven't got a scope but have a multimeter, you can check for excessive offsets, weird bias points and hand effect (bias points changing when you only point at the circuit). Connect a resistor of 10 kohm or so in between the circuit and the multimeter to ensure that the meter and meter leads don't cause the instability by their capacitance.

Usually parasitic oscillations keep growing until something starts clipping and limits the amplitude. If that clipping happens asymmetrically, which it usually does, it will also shift the bias point.

Ok MarcelvdG, i have a multimeter, if you help me with some guidance, i think i can test the circuit.
What should i measure and where??
also have the 10kohm resistors at home...
 
For one thing, the DC voltage between the + input and - input of the LT1028 (pins 3 and 2) should be very close to zero. So please connect a 10 kohm resistor to pin 3 and one to pin 2, and connect the leads to your multimeter and the multimeter itself to the other sides of the resistors. Then measure DC voltage.

Anything greater than a couple of millivolts is very suspicious. Theoretically the voltage should even be less than 0.15 mV, but maybe the meter will add some millivolts of offset of its own.
 
For one thing, the DC voltage between the + input and - input of the LT1028 (pins 3 and 2) should be very close to zero. So please connect a 10 kohm resistor to pin 3 and one to pin 2, and connect the leads to your multimeter and the multimeter itself to the other sides of the resistors. Then measure DC voltage.

Anything greater than a couple of millivolts is very suspicious. Theoretically the voltage should even be less than 0.15 mV, but maybe the meter will add some millivolts of offset of its own.

ok, with my multimeter on milivolts scale i just measured 00.1, sometimes 00.0 dc between pin 2 and 3.
 
OK, so far, so good! Does it make any difference whether you connect or disconnect the cable to the amplifier to the DAC's RCA connectors?

i just the measure with the rca cable connect from de dac to amplifier but the amplifier was powered off during measurements. Also i was no playing anything. Just the dac powered on.

is this ok or should i play a tone... ?

also i measured temperature and each one is around 40/43º celsius
 
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