Good site for Audio Op-Amp Education
http://www.tonmeister.ca/main/textbook/electronics/12.html
(unacknowledged link, so thanks to the site owner)
http://www.tonmeister.ca/main/textbook/electronics/12.html
(unacknowledged link, so thanks to the site owner)
mhennessy said:
Because the inverting input is a "virtual earth"
I really recommend getting hold of a copy of "The Art of Electronics" by Horowitz and Hill, and reading through chapter 4. It's a really good non-mathematical explanation of basic op-amp theory. I don't mean to be patronising, but this will answer your question and teach you lots more at the same time.
I'll attempt to sumarise - due to the action of negative feedback, the two inputs of the op-amp are always at the same voltage. This is one of the "golden rules" of op-amp operation. Believing this is the first step to understanding. Now, as the non-inverting (+) input is tied to ground, it follows that the inverting (-) input will always be at ground. This is why it is called a "virtual earth".
This is only part of the story, but the book will tell it better
Cheers,
Mark
thanks for the explination. how do i know how low i can make this resistor for my preamp (or cd walkman in this case) to work?
Matttcattt said:
Mostly you can't, unfortunately. Some equipment will list the minimum recommend impedance in the specifications - all my old Technics gear recommended 10K, and Arcam recommend 5K
Don't sweat it - just go for 10K
Matttcattt said:
Again, you often don't. It is sometimes quoted in the spec's if you're lucky. But, it doesn't really have much bearing on things if you stick to the lo-Z out, hi-Z in principles. Output impedances range from 50 to 600 ohms, although you sometimes see as low as zero, and as high as a few K (for older kit). Input impedances can range from 10K up to several hundred K
If you want, you can measure output impedance. You'll need a test CD with a recording of a tone. You'll also need a way of measuring the level of the tone - a 'scope is best.
Connect the CD player to your measuring device, and record the level. Now add a resistor between the signal and ground - the level will drop. Experiment with different resistor values until you've found the resistor that makes the level drop to exactly half the origional level that you measured with no resistor.
The output impedance of your CD player is equal to the value of resistor that makes the output drop to half it's origional value.
This is because you've made a potential divider consisting of the output impedance and the load resistor. I'm assuming you understand how potential dividers work, but if you don't, feel free to ask here, or check The Art of Electronic book that I mentioned before...
Cheers,
Mark
More quickly you open the lid of the device and check the resistor that's in series with each RCA output.
Sometimes it's easy, because they are right next to the RCA, but sometimes you have to follow the ciruit because they are far away.
See what value is that resistor.
It's the output impedance.
Sometimes it's easy, because they are right next to the RCA, but sometimes you have to follow the ciruit because they are far away.
See what value is that resistor.
It's the output impedance.
carlosfm said:More quickly you open the lid of the device and check the resistor that's in series with each RCA output.
Sometimes it's easy, because they are right next to the RCA, but sometimes you have to follow the ciruit because they are far away.
See what value is that resistor.
It's the output impedance.
Carlos,
How many portable CD players have you taken apart? And then sucessfully put back together again
I'm joking with you - it's a good suggestion. But, this issue is complicated by several factors, including hard-to-read surface-mounted parts. Also, I've seen plenty of designs that have more than one resistor in series (due to multiple muting transistors). And, the series resistor is only part of the story - what about any resistors to ground?
When I built my preamp, I wanted exactly 50 ohms. So, I used a 51 ohm resistor, but with a 2K55 to ground. This works out at exactly 50. Small difference I know, but we are fanatics here
I you want to know the output impedance of something, measuring it is the only real way to be sure. But for the applications being discussed here, I don't think it's worth worrying about
Cheers,
Mark
right
Mark, you're right, I was joking too, but I opened many portable devices, including CD players.
The last portable was a (good) Aiwa walkman I changed the belt.
Anyway, as a rule you can start at 10K for input impedance.
I have 50ohm output impedance too on the preamp I'm making right now.
Mark, you're right, I was joking too, but I opened many portable devices, including CD players.
The last portable was a (good) Aiwa walkman I changed the belt.
Anyway, as a rule you can start at 10K for input impedance.
I have 50ohm output impedance too on the preamp I'm making right now.
Re: right
I've taken apart a few Sony MD recorders - these are nicer because they use decent metal cases.
Cheers,
Mark
Yes, I do a reasonable amount of that sort of thing - the trouble is knowing where the plastic bits are clipped together. It's just too easy to end up with screwdriver-sized dents in the plastic seamscarlosfm said:
I've taken apart a few Sony MD recorders - these are nicer because they use decent metal cases.
Sounds interesting - have you got a webpage or some details?carlosfm said:I have 50ohm output impedance too on the preamp I'm making right now.
Cheers,
Mark
Re: Re: right
No web page.
I'll post some pictures when I finish it, on a new thread.
It's playing some beautiful sounds.
The only thing that's missing is a nice box.
Now it's playing with the boards laying on the floor.
Anyway, does OPA627 with BUF634 sound good to you?
To me it sounds like a million dollars.
mhennessy said:
Sounds interesting - have you got a webpage or some details?
Cheers,
Mark
No web page.
I'll post some pictures when I finish it, on a new thread.
It's playing some beautiful sounds.
The only thing that's missing is a nice box.
Now it's playing with the boards laying on the floor.
Anyway, does OPA627 with BUF634 sound good to you?
To me it sounds like a million dollars.
Re: Re: Re: right
why dont you make one? its really simple. i learned HTML from www.htmlite.com and i host my site with www.freewebs.com
carlosfm said:No web page.
why dont you make one? its really simple. i learned HTML from www.htmlite.com and i host my site with www.freewebs.com
Sounds Good!
On my long list of things to do is try some op-amp experiments. Have you any recommendations to try instead of the OPA2134 and NE5532's used in my preamp? Is there a dual version of the OPA627?
Cheers,
Mark
Sounds good!carlosfm said:Anyway, does OPA627 with BUF634 sound good to you?
To me it sounds like a million dollars.
On my long list of things to do is try some op-amp experiments. Have you any recommendations to try instead of the OPA2134 and NE5532's used in my preamp? Is there a dual version of the OPA627?
Cheers,
Mark
Re: Sounds Good!
It depends.
OPA2134 is a good op-amp, if the circuit is well made.
Or else it doesn't deliver it's full performance.
Bypass them well and check how it sounds.
You may try LM6172 on that position, it's very good too.
But well bypassed! It's a very fast op-amp!
You don't specify what the NE5532 is doing there.
Phono preamp?
If so, you'll be surpised if you change it for OPA2228.
If not, use one of the above.
It doesn't exist a dual version of the OPA627.
Crème de la crème only in single versions.
mhennessy said:
Sounds good!
On my long list of things to do is try some op-amp experiments. Have you any recommendations to try instead of the OPA2134 and NE5532's used in my preamp? Is there a dual version of the OPA627?
Cheers,
Mark
It depends.
OPA2134 is a good op-amp, if the circuit is well made.
Or else it doesn't deliver it's full performance.
Bypass them well and check how it sounds.
You may try LM6172 on that position, it's very good too.
But well bypassed! It's a very fast op-amp!
You don't specify what the NE5532 is doing there.
Phono preamp?
If so, you'll be surpised if you change it for OPA2228.
If not, use one of the above.
It doesn't exist a dual version of the OPA627.
Crème de la crème only in single versions.
Ah!
You were faster than me, because I've just talked about them.
I like them in phono stages.
pinkmouse said:I have been mucking around with some bits for a sort of strange pre/buffer type thing, (don't ask, it's far too complicated to explain ), and I have got some quite nice sounds out of a BB OPA2228. It is probably never mentioned 'cos it's not fashionable, but I am quietly impressed
You were faster than me, because I've just talked about them.
I like them in phono stages.
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