OP AMPS QUESTION

[daverave]

ive tried the LT1028 in the surf pi but i cannot get the voltage from pin 6 to zero using the offset control...it will only go down to about 0.6v

[daverave]

i think your right kt315....noise is something very hard to get rid off in a pulse machine cause coil untuned and very broadband.

[DIMI4321]

use a tantalum capacitor 1uf in series 1k resistor of inverting input to the ground .this way you make a high pass filter from 159hertz and down
and get rid of the nulling circuit because this way is auto nulling.
You will get rid a lot of noise this way

[6666]

get rid of the nulling circuit

What are you calling the nulling circuit ?

[DIMI4321]

the trimmer in pins 1 and 8 of NE5534

[Tibuck19]

KRinAZ used an LM4562 for increased sensitivity -> http://www.geotech1.com/forums/showt...586#post206586

Can the Lm4562 also be used to replace the 5534 on the Surf pi 1.2 ?

[6666]

Can the Lm4562 also be used to replace the 5534 on the Surf pi 1.2 ?

Swapped U5 (NE5532) with LM4562 for better sensitivity.

The 4562 is a dual op amp there fore not a direct plugin replacement for 5534, but if you were to modify the pcb, technically it could be done, you could also make up a small daughter pcb to plug in to the 5534 socket.

[6666]

use a tantalum capacitor 1uf in series 1k resistor of inverting input to the ground .this way you make a high pass filter from 159hertz and down
and get rid of the nulling circuit because this way is auto nulling.
You will get rid a lot of noise this way

get rid of the nulling circuit because this way is auto nulling.

This does not seem to work ?

[DIMI4321]

you mean this is not working.it is working with surf pi circuit it is tested. it is also tested years and years before with other op amp circuits, this
way the input is a hi pass filter

[6666]

you mean this is not working.it is working with surf pi circuit it is tested. it is also tested years and years before with other op amp circuits, this
way the input is a hi pass filter

.
The auto null does not work, but will try again

[6666]

.
The auto null does not work, but will try again

Tried again , Nope it dont work as soon as you put a cap in series with the 1K the preamp stops working.

[green]

Tried again , Nope it dont work as soon as you put a cap in series with the 1K the preamp stops working.

Don't know if the problem. Resistor goes to coil, capacitor to amplifier input and back to back diodes to resistor capacitor junction.

[6666]

use a tantalum capacitor 1uf in series 1k resistor of inverting input to the ground
and get rid of the nulling circuit because this way is auto nulling.

What I did after reading the post above was I removed the nulling parts from the opamp from a surf pi
and added a 1uF cap in series with the 1K that goes from the inverting input thats pin 2 to ground
but when you do the preamp stops working, am i misunderstanding the above post ?

It would be handy if this auto nulling idea worked

[green]

What I did after reading the post above was I removed the nulling parts from the opamp from a surf pi
and added a 1uF cap in series with the 1K that goes from the inverting input thats pin 2 to ground
but when you do the preamp stops working, am i misunderstanding the above post ?

It would be handy if this auto nulling idea worked

My error. I thought maybe the input went to the negative input of the op amp. Looking at your schematic the amplifier out offset should be bias current times 1 million.

I Just tried your schematic with spice, not working maybe some thing I'm doing wrong. I'll look at it some more. Worked with spice with coil going to -input

[DIMI4321]

you are not misunderstand I just forget to mention that this circuit it is used to phono preamps so the input signals are in the order of 20 millivolt maximum .In the surf pi I used a rail to rail input op amp with very little offset the opa350 and worked with 1000 amplification.
After this I used convetional circuit with 2000 amplification and now I am receiving the whisper of a 2 euro coin in vertical position 27 cm in the
earth.Anyway with opa350 and 2000 amplification I have only 70 millivolt offset without nulling circuit.
ps it is the best op amp for this application it has very fast recovery time from overload .