The Opamp thread

[Charminbaer]

 
Quote:

 
The pinouts for the 8 pin package are on the front page of the AD797 datasheet. They are the same for DIP and SOIC.  Pins 4 and 7 are the power inputs.  There is no pin for ground - you will have to find the nearest ground (zero volts) point on the circuitboard.
I suggest starting with the power supply decoupling which are the 2 pairs of 4.7uf Tantalum and a 0.1uf ceramic capacitors in parallel that you mentioned from the datasheet.  That may be enough.  Otherwise, the capacitor from pin 2 to pin 6 will be for bandwidth limiting in the feedback loop.
This can be more difficult if you are using chip adapters, as the bypassing components should be as close to the chip as possible.  Some adapters have facility to mount bypassing caps on board.  I like to solder the bypass caps to the underside of the PCB beneath the op-amp socket, directly from the socket pin to the ground point.
Some design techniques work for all op-amps in general (e.g. power supply bypassing), while others are specific requirements of individual chip designs.  Pin 8 on the AD797 is one such individual feature which is outlined in the datasheet. The LT1028 has pin 5 for tuning.

No i didnt mean that. What i wanted to know is if i can solder the 2 bypass caps from pin 8 and pin 4 of the dual socket to ground. I dont want to kill the opamp during soldering, therefore i would prefer soldering on a replaceable socket. Will i need other caps in that case , since im bypassing 2 opamps instead of just one directly? I would not like to solder to the board directly if possible.
The Datasheet says the benefits of Pin8(single Soic) are mainly for high gain or and/or high frequencies : "Use of this feature improves distortion performance when the closed-loop gain is more than 10 or when frequencies of interest are greater than 30 kHz."  
Would it bring me any benefit if i would like to use my amp with lower or equal gain?
 
Quote:

sometimes you will find that the pins that are labelled NC or 'not connected' are tied to ground

 
If i look at the layout of the  "2xSoic to 1xDip Adapter" i can see no grounding. Isn´t there another way to get to ground without soldering directly to the board?
 
 
So many Questions.

Thanks to you guys for helping me ,I hope i m not using up too much of your time.

 

[dlaloum]

Maybe you can use a multimeter on continuity test mode, try each pin compared to ground (pretty much anywhere on the board) if one of the pins comes up positive you are in business.... if not, you can always run a ground strap.
 
The Audio-Gd discrete opamps often do that - they plug into the socket but also have a wire to connect to ground, which is a fairly simple thing, and may even be doable without soldering depending on the amp. (well without soldering anywhere except the socket that is!)

 

[qusp]

the adapter has no ground pin, in fact it has no pins labelled at all of any type, pinouts are different for everything that uses it. no, what I mean is, most opamps, if you look at the datasheet and the pin assignments for the opamp, it will have a couple of pins that are not used for anything; these are usually labelled NC, for No Connection, these pins, whatever they may be for the chip in question are sometimes tied to ground to avoid them acting as an arial and picking up noise. it will depend on the designers choices. just saying that if you can check those pins with a meter you may find that they are rather conveniently placed grounding points...or you may not....

 

[qusp]

when I make amps that will be rolled, I always install a sip socket pin so that the decoupling can be plugged in and disconnected without soldering. I suggest you do the same if there are no pins on the dip tied to ground and no, not 'pretty much anywhere on the board' as close as possible to the socket, installing an aerial to pick up noise and shunt it to ground is not such a good thing

 

[Charminbaer]

 
Quote:

Maybe you can use a multimeter on continuity test mode, try each pin compared to ground (pretty much anywhere on the board) if one of the pins comes up positive you are in business.... if not, you can always run a ground strap.
 
The Audio-Gd discrete opamps often do that - they plug into the socket but also have a wire to connect to ground, which is a fairly simple thing, and may even be doable without soldering depending on the amp. (well without soldering anywhere except the socket that is!)

okay i will try this as soon as i can get myself to stop listening , soundquality of the AD797BR is amazing, even with noise from the high gain

 
Quote:

the adapter has no ground pin, in fact it has no pins labelled at all of any type, pinouts are different for everything that uses it. no, what I mean is, most opamps, if you look at the datasheet and the pin assignments for the opamp, it will have a couple of pins that are not used for anything; these are usually labelled NC, for No Connection, these pins, whatever they may be for the chip in question are sometimes tied to ground to avoid them acting as an arial and picking up noise. it will depend on the designers choices. just saying that if you can check those pins with a meter you may find that they are rather conveniently placed grounding points...or you may not....

 
I very well understood you. But even though there are such pins on the AD797, how can they be grounded if they are not even connected via the "2xsoic to 1xDip adapter"?

 
 
Quote:

when I make amps that will be rolled, I always install a sip socket pin so that the decoupling can be plugged in and disconnected without soldering. I suggest you do the same if there are no pins on the dip tied to ground and no, not 'pretty much anywhere on the board' as close as possible to the socket, installing an aerial to pick up noise and shunt it to ground is not such a good thing

Damn, so i have to solder to the board in the end.
That socket is a very good idea. I will try that.
 
EDIT:
 
I found out, that a screw in direct proximity to the opamp socket is grounded. So i don´t need to solder, i can use the screw to secure the wire.

 

[Charminbaer]

hm... i did it, but i hear just a loud hum. Unplugged the socket with the caps, now everything works as before. Strange.
Could this be because i did not directly solder it to the opamp?
 
Edit:
 
Just checked the temperature of the topside opamp , it´s 70°C without cover on the m-stage

Is that because of oscillation or is it normal ? Seems too hot for me.
the HA3-2525-5Z merely reached 40°C.

 

[murrays]

Quote:

 
Just checked the temperature of the topside opamp , it´s 70°C without cover on the m-stage

Is that because of oscillation or is it normal ? Seems too hot for me.
the HA3-2525-5Z merely reached 40°C.

 
That's definitely not good.  That one certainly failed the "finger" test.
Most chips won't get much more than luke warm.  You should be able to hold your finger on it forever.

 

[qusp]

yep, oscillation city!! ..not good. i'm saying the 797 is not liking your circuit at all. there could be all manner of issues at hand here mate, what sort of caps did you use, was the screw really at power supply ground, or some other ground potential? its posible the opamp is using a completely different reference voltage for its 'ground'  you may have reversed the polarity on one of the caps, you may have created a ground loop, etc etc.  very difficult for us to help here, without a schematic, or pictures etc nigh impossible. 

 

[Charminbaer]

 
Quote:

 
That's definitely not good.  That one certainly failed the "finger" test.
Most chips won't get much more than luke warm.  You should be able to hold your finger on it forever.

 
Yes that for sure, i am happy i didn´t try that in the first place 

 
Quote:

yep, oscillation city!! ..not good. i'm saying the 797 is not liking your circuit at all. there could be all manner of issues at hand here mate, what sort of caps did you use, was the screw really at power supply ground, or some other ground potential? its posible the opamp is using a completely different reference voltage for its 'ground'  you may have reversed the polarity on one of the caps, you may have created a ground loop, etc etc.  very difficult for us to help here, without a schematic, or pictures etc nigh impossible.

 
I used 4.7uf tantalum and 0.1uf ceramic, don´t know which brand.
I measured the screw against the ground pin of the power jack.
As for the polarity, i used the "+ side of the tantalum" on V+ and "- side of the tantalum" on ground, the opposite for V- ( v- >- tant + > gnd)
Ceramics had no marking on them and as far as i know it does not matter, or am i wrong ?
I tried to get a schematic but i did not find one. Wrote some emails to coolfungadget and the Matrix online store. Got no reply so far.
I will try to do some pictures tomorrow.
 
 
I talked to a electrical engineer i know and he wants to measure the opamp with an oscilloscope to exactly know whats wrong. He advised me to use a choke on the feedback loop.
Is a choke ok to use , since tangent advised to use polypropylene caps ?
What do you guys think of that?

 

[qusp]

yes, but what I mean is the opamps power supply ground may be sitting at a different level to actual ground, ie there may be a rail splitter or other virtual ground being used for signal ground.you are correct that its pretty unlikely the ceramics are polar. I personally wouldnt use a choke in the feedback loop, they are too nonlinear I would stick to polypropelene or NPO ceramics. there is no reason why these techniques shouldnt work, there is something else up with your power supply or circuit; the 797 needs 30pf or so in the feedback loop for bandwidth limiting as its much wider bandwidth than we need for audio and can oscillate without it. have you done this? make sure you bring the datasheet with you if you go over to see your friend, hard to know what else to say without more info or pics.
 
good luck

 

[murrays]

Quote:

I used 4.7uf tantalum and 0.1uf ceramic, don´t know which brand.
I measured the screw against the ground pin of the power jack.
As for the polarity, i used the "+ side of the tantalum" on V+ and "- side of the tantalum" on ground, the opposite for V- ( v- >- tant + > gnd)
Ceramics had no marking on them and as far as i know it does not matter, or am i wrong ?

I talked to a electrical engineer i know and he wants to measure the opamp with an oscilloscope to exactly know whats wrong. He advised me to use a choke on the feedback loop.
Is a choke ok to use , since tangent advised to use polypropylene caps ?
What do you guys think of that?

The ground point for bypass caps must be the closest point on the circuitboard ground plane. It must be a very short path to the main ground point as seen by the amplifier circuit.  If you are using a screw by the power jack it may be too far and not the best ground.
 
Ceramic capacitors do not have voltage polarisation.  Tantalums DO, and can be very sensitive to reversals (BANG!!).
 
Taking measurements with an oscilloscope will help.  It's the best way to see oscillations, as they generally won't be audible.
 
As for components to help stabilise the circuit - as others have said, this is dependent on the individual circuit design.  Your electrical engineer may be able to help you, otherwise you may be in for some lengthy experimentation.  This is a problem beyond normal audio range and into HF/VHF range, so it needs to be looked at from that perspective too.  The circuit design and layout may be fine for the original chip but may just have too many issues to allow the new chip to run in a stable manner.
 

 

[teamanon]

Hi there fellow audiophiles. I just joined head-fi so I could post in this thread and ask for assistance. I have the ASUS Xonar Essence STX and In it i have (2x) OPA2111KP's and I love them. See the problem is that I still have the stock OPA in the other slot (pic related) and I was wondering what do you recommend to go with my (2x) OPA2111KP's?
 
I am looking for one that has amazing clarity in the mid-high range. Any suggestions?

 

[murrays]

Quote:

Hi there fellow audiophiles. I just joined head-fi so I could post in this thread and ask for assistance. I have the ASUS Xonar Essence STX and In it i have (2x) OPA2111KP's and I love them. See the problem is that I still have the stock OPA in the other slot (pic related) and I was wondering what do you recommend to go with my (2x) OPA2111KP's?
 
I am looking for one that has amazing clarity in the mid-high range. Any suggestions?

Hi, Welcome to Head-Fi. 

While you wait for those in the know, it might pay to do a search on that particular card as I seem to recall it being the topic of many previous discussions. Even just narrow it down to this thread, you should find some postings.
 

 

[teamanon]

Thanks for your reply.  I was hoping someone could just recommend a good one to go with my (2x) OPA2111KP's.  I read some of this thread, but it is 200 pages long and I don't have the time to read all of it.  I am only looking for one (x1) OP AMP to go in the single slot in my ASUS Xonar Essence STX soundcard(pic related).  Something that makes the mid-high range of the audio clear and crisp preferably.

 

[Charminbaer]

Unfortunately we had not enough time to measure every frequency for oscillation, but we found the opamp to oscillate at around 4Mhz. My friend said it could be that it oscillates at a higher frequency but it shows up at 4Mhz. He put some 22pf ceramics in the feedback and oscillations at 4Mhz disappeared.
At home i listened for a couple of hours and it sounds much better at 0db gain. Nothing like before.
But the opamp temperature is still at 63°C. An improvement of 7°C. But this is still too high, isn´t it ? Could it be that it still oscillates?
 
 
Quote:

yes, but what I mean is the opamps power supply ground may be sitting at a different level to actual ground, ie there may be a rail splitter or other virtual ground being used for signal ground.you are correct that its pretty unlikely the ceramics are polar. I personally wouldnt use a choke in the feedback loop, they are too nonlinear I would stick to polypropelene or NPO ceramics. there is no reason why these techniques shouldnt work, there is something else up with your power supply or circuit; the 797 needs 30pf or so in the feedback loop for bandwidth limiting as its much wider bandwidth than we need for audio and can oscillate without it. have you done this? make sure you bring the datasheet with you if you go over to see your friend, hard to know what else to say without more info or pics.
 
good luck

Yes, you´re right it was a different ground. My friend showed me the right source to ground. They are pins as you would have done it. I will  try to bypass power supply, this time to the right ground

. I will use smd caps my friend gave me, he said they do a better job than ceramics in terms of power supply bypassing.
What do you think about "Glimmer" Caps for the feedback loop?

 
Quote:

The ground point for bypass caps must be the closest point on the circuitboard ground plane. It must be a very short path to the main ground point as seen by the amplifier circuit.  If you are using a screw by the power jack it may be too far and not the best ground.
 
Ceramic capacitors do not have voltage polarisation.  Tantalums DO, and can be very sensitive to reversals (BANG!!).
 
Taking measurements with an oscilloscope will help.  It's the best way to see oscillations, as they generally won't be audible.
 
As for components to help stabilise the circuit - as others have said, this is dependent on the individual circuit design.  Your electrical engineer may be able to help you, otherwise you may be in for some lengthy experimentation.  This is a problem beyond normal audio range and into HF/VHF range, so it needs to be looked at from that perspective too.  The circuit design and layout may be fine for the original chip but may just have too many issues to allow the new chip to run in a stable manner.
 

 
The ground pins are about 1,5cm away from the opamp. the screw was about 6cm away, and apparently the wrong ground.
It would be a shame if i couldn´t use the AD797, because it sounds so much better than every other opamp i´ve heard before. What do you think about noise cancellation feature on pin8 ? Would it bring any benefits or disadvantages ?