Opamp that can be dc coupled ?

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beppe61

Member
2005-02-08 11:00 am
torino
Dear Sirs,

I have been struck by these following words:
" All signal paths should be designed to be DC-coupled.
For example, the result of using a cap in the signal path is that it first destroys the phase integrity of the music by time versus frequency), which then adds unnatural colorations and causes changes to the soundstage, giving it a bloated perspective with larger-than-life instruments that are out of perspective with the soundstage.
With caps in the signal path, the edges of notes and the spaces between them are missing.
High fidelity? I don't think so "
Is he right ?

Then I am trying to select a opamp for a unity gain buffer that can be used without input an output caps following a good quality pot.
It must come in the minidip format (i.e. no smd)
Any suggestion ?

Thanks and regards,

beppe
 

beppe61

Member
2005-02-08 11:00 am
torino
Originally posted by ACD
I think we could fill many pages with suggestions, however normally Opamps doesn't create DC on the output (execpt for a very small offset Voltage)....
OPA134 and OPA627 would be good choices.
The problem layes in the DC output/Offset from your other equipment before and after your buffer.


Thank you very much indeed.
If I have a source without output offset can I put a pot after it and then the op-amp also without an input cap after the pot ?
Can I do this with any opamp ?
I need a buffered volume control to drive an amp that, unfortunately, is ac coupled with a cap at its input.

Thanks and regards,

beppe
 
If your source has no DC on the output, you will have no problem connecting a volume pot without a input cap between the source and buffer.

By the way...
Does your amp need this input cap or is it for protecting it for DC ?
If the cap is nessesary, then change it to a better type....
And caps aren't so bad as you make it sound !!
It is off course best to not have them, however the music you listen to has been through more than 1000 opamps and caps before written to your CD :D
 

beppe61

Member
2005-02-08 11:00 am
torino
Originally posted by ACD
If your source has no DC on the output, you will have no problem connecting a volume pot without a input cap between the source and buffer
perfect ! thank you very much again

By the way...
Does your amp need this input cap or is it for protecting it for DC ?
If the cap is nessesary, then change it to a better type....

I will try to do so but space is very little inside.

And caps aren't so bad as you make it sound !!
It is off course best to not have them,
however the music you listen to has been through more than 1000 opamps and caps before written to your CD :D

Your words calm me down, Thanks !
Actually there are a lot of people swearing for good sound from units like preamps that are indeed ac coupled at the input and at the output.
Excuse me may I ask which is your current choice of line preamp ?

Kind regards,

beppe
 

Gordy

Disabled Account
2006-11-02 6:15 pm
beppe61 said:
Dear Sirs,

I have been struck by these following words:
" All signal paths should be designed to be DC-coupled.
For example, the result of using a cap in the signal path is that it first destroys the phase integrity of the music by time versus frequency), which then adds unnatural colorations and causes changes to the soundstage, giving it a bloated perspective with larger-than-life instruments that are out of perspective with the soundstage.
With caps in the signal path, the edges of notes and the spaces between them are missing.
High fidelity? I don't think so "


Was that said on this forum?

Virtually every piece of audio equipment has at least one capacitor in the audio signal current loops. These might not be in the direct audio signal current loop (what some might ignorantly call the "signal path"), but they will certainly be in the power supply and hence directly affect the audio signal. Certainly this is so in all commercial equipment, which is most usually also AC coupled at the output via a capacitor.

Anyway, to remove the capacitors from the direct audio signal current loops requires some form of action to ensure that direct current generated from the circuit does not affect the output, such as using a servo circuit or transformer coupling (however they too are imperfect...).

For an op-amp the best way to achieve zero DC output is to exactly match the impedances seen by the inputs. For example, if the inverting input sees 10k Ohms, then arrange the circuit so that the non-inverting input also sees 10k Ohms. Also, choose an op-amp that draws little input current, such as an op-amp with a FET input stage.

(Matching the impedances at the two inputs might be a problem if you use a potentiometer directly before the op-amp!)

To assist with the issue of power supply capacitors affecting the signal you will need a circuit with either a very good Power Supply Ripple Rejection, or some very good regulators with a constantly low output impedance.


I disagree with the original statement, but that is a different matter.
 
Re: Re: Opamp that can be dc coupled ?

Gordy said:

[snip]I disagree with the original statement, but that is a different matter.

Me too. The statement that caps in the signal path lead to destruction of music coherence is bollocks. Observe: the voltage at the input of that cap will be exactly the same as source, it cannot be otherwise.

Now, what is the voltage at the other side, the output of the cap? If it is loaded with an impedance that is large with respect to the cap impedance, the bulk of the source voltage will appear across the load and the voltage across the cap will be almost zero. With the original source voltage at the input terminal of the cap, and almost zero across the cap, it cannot be otherwise than the signal at the output of the cap is almost exactly the same as the source signal at the cap input.

How do we make sure that the load is large with respect to the cap impedance? By chosing the cap large enough so that the relationship holds over most of the audio band. The point where the voltage across the cap becomes the same as the voltage across the load is called the 3dB point, and at that point there is indeed phase shift of 45 degrees. So, if we chose the cap large enough that the 3dB point is at, say 1Hz, we can be pretty sure that above 20Hz, the cap doesn't cause any appreciable phase shift.

Of course, the above has been known and practised by designers of high quality equipment for, ohh, maybe 60 years now? Probably longer. It is often dangerous to believe evverything read on the web unless you try to follow any logic behind it, which often, like in this case, isn't even there. Better yet, read a basic text.

Jan Didden
 
beppe61 said:
Dear Sirs,

I would like to thank you all for your kind and extremely valuable advice.
These words were taken by a review of a commercial unit that you can find here
http://6moons.com/audioreviews/tomevans2/vibe.html
Anyway there are so many other audio amps that are equally well considered and ac coupled.

Thanks a lot again.
Kind regards,

beppe

Indeed. The whole point is competent design. That can be done with or without coupling caps. Some times trying to avoid coupling caps requires additional active devices. Would that be better?

Jan Didden
 
That particular review is mostly marketing blahblah.

The Vibe signal path consists of first a gain stage comprising of a dual opamp, with one half in the feedback loop of the forward amp ("error-correction circuitry and the application of psychoacoustic principles " or rather just application note AN-107 from Analog Devices), with a DC servo arround it. That DC servo uses (cheap) capacitors. Then an attenuator, then a current feedback opamp with unity gain.

As for phase itself ... you can be sure that from the mouth of the signer to the output of the loudspeakers in your room the signal's phase has been mangled bigtime, with or without a DC-coupled line stage. As long as the accumulated phase shift is equal in both channels this is not even too bad.
 
beppe61 said:
With caps in the signal path, the edges of notes and the spaces between them are missing.


Knowing it's a Tom Evans design puts this statement in a specific context. He loves using the cheapest and nastiest tantalums. I won't be surprised if whole notes go missing through them :)

As most radical statements in audio this is pure, unmitigated BS. Especially if you consider its repercussions on tube amplification.

Fwiw, i have had a system, which between moving coil input and speakers had neither series caps, nor servos. It didn't play music in any particularly remarkable way.
 
Thank you very much again Sirs for your very helpful comments.
I can reply just today because during the working days I have no access to the web.
Thinking more about the whole subject I remember I saw some lab reports of cap coupled units indeed passing a square wave test in a remarkable way.
At least in the normal audio bandwidth.
So I think that really it could be more a question of good selection of caps maybe, but in general they can be used in an audio amplification device with good results.

Thank you very much again and kindest regards,

beppe
 

tedr

Member
2006-07-14 1:29 am
the original question was about DC coupling. here are some basic concepts to get us on the same map.

By definition opamps operate with DC on the inputs and outputs, that is the purpose of an opamp to amplify DC and they do it very well, the DC output is very predictable. For example at the input is a 100mV DC signal, the gain is 100x so the output is 10V DC. When the input goes from +100mV to 0V DC the output also goes to 0VDC that is called a linear amplifier.

The general case of the DC circuit is no input capacitor and no output capacitor. The DC supply to the opamp must be greater than the maximum output voltage required and less than the maximum allowed for the IC. The normal type of DC supply for the opamp is with a single DC supply, there are two connections, the positive DC supply and the ground of the DC supply. The input signal and output signal use the DC ground as the input to output common connection.

Some opamps can also be used to amplify audio, which is a form of AC signal. But to begin you must add a DC bias voltage to the input signal equal to half the DC supply voltage. You must also arrange to eliminate DC gain while keeping AC gain. So if the DC supply is 20V, the input must be at 10V DC. This allows the output to swing between 0V DC and +20V DC. For example there is 100mV sine wave 1kHz at the input. The gain is 100x so the output is 10V sine wave. The output of the opamp is now two signals combined, it has a positive DC voltage equal to half the DC supply voltage and the 10V sine wave is superimposed on it. The DC voltage can be got rid of with a blocking capacitor.

Audio is not the general DC situation, it is a special case. It is usual to operate the opamp with two DC supplies, one the symmetrical mirror of the other, and there is a third connection the midpoint of the two DC supplies at 0V DC. For example +15V DC and -15V DC, the center point is also used as the input and output common. Now signals can pass which have positive amplitude and negative amplitude, which is what audio is. In this arrangement capacitors are not needed at the input or output, with one exception. There are small DC voltages on the opamp input and output terminals, a few millivolts. Often these are not a problem but they can cause noise on pots (volume control) and switches so even though the opamp does not need capacitors at the input and output they are sometimes included to stop noise from pots and switches.

When you look for opamps that are intended for audio use chose the ones that show operation from dual DC supplies, the symmetrical +/-15V DC arrangement is the most common in professional audio, +/- 12V or +/-10V in hifi sometimes. Avoid the opamps that are intended for use with single DC supplies, (it says so in the technical description)

There is whole new generation of opamps specially for audio with logic chips that use single DC supply +3 or +5V DC. For these applications capacitors are sometimes needed to block DC.

Hope this helps

Ted
 
tedr said:
the original question was about DC coupling.
here are some basic concepts to get us on the same map
tedr said:
Thank you very much Mr Ted.
This is indeed what I need, basic concepts.
I am everything but an educated diyer.

By definition opamps operate with DC on the inputs and outputs, that is the purpose of an opamp to amplify DC and they do it very well, the DC output is very predictable. For example at the input is a 100mV DC signal, the gain is 100x so the output is 10V DC.
When the input goes from +100mV to 0V DC the output also goes to 0VDC that is called a linear amplifier.


So if I wire the opamp as a buffer (no voltage gain) the offset at the input is the same offset at the output.
I use sources (dvd players) that usually have no output offset, being cap coupled at the output.
By the way i would like to confirm that the problem of caps in series with the signal has been "re-sized"
I have seen lab reports of cap coupled units passing quite well square wave. So caps indeed are not that bad for signals.
Anyway ,,

The general case of the DC circuit is no input capacitor and no output capacitor.
The DC supply to the opamp must be greater than the maximum output voltage required and less than the maximum allowed for the IC


I understand. OK.

The normal type of DC supply for the opamp is with a single DC supply, there are two connections, the positive DC supply and the ground of the DC supply.
The input signal and output signal use the DC ground as the input to output common connection.
Some opamps can also be used to amplify audio, which is a form of AC signal. But to begin you must add a DC bias voltage to the input signal equal to half the DC supply voltage. You must also arrange to eliminate DC gain while keeping AC gain. So if the DC supply is 20V, the input must be at 10V DC. This allows the output to swing between 0V DC and +20V DC. For example there is 100mV sine wave 1kHz at the input. The gain is 100x so the output is 10V sine wave. The output of the opamp is now two signals combined, it has a positive DC voltage equal to half the DC supply voltage and the 10V sine wave is superimposed on it. The DC voltage can be got rid of with a blocking capacitor.


This is not as I thought.
I believed dual supply was more common actually.
Nevertheless I am very interested in a single supply solution.
Maybe with one 12V SLA battery, or two for a 24V total.


Audio is not the general DC situation, it is a special case.
It is usual to operate the opamp with two DC supplies, one the symmetrical mirror of the other, and there is a third connection the midpoint of the two DC supplies at 0V DC.
For example +15V DC and -15V DC, the center point is also used as the input and output common.
Now signals can pass which have positive amplitude and negative amplitude, which is what audio is.


Ok. I had to read this one. Actually dual supplies are much more common for audio.

In this arrangement capacitors are not needed at the input or output, with one exception.
There are small DC voltages on the opamp input and output terminals, a few millivolts.
Often these are not a problem but they can cause noise on pots (volume control) and switches so even though the opamp does not need capacitors at the input and output they are sometimes included to stop noise from pots and switches.


Very interesting . Thank you for the advice.
So, as the input impedance of an opamp is usually quite high it would be better to put a small high quality film cap after the pot before the opamp.
Then I can measure the output offset to decide if I have to place another cap at the output.
By the way after all these very kind and useful advice I see the caps as more user friendly.
They are better than what I thought at the beginning,

When you look for opamps that are intended for audio use chose the ones that show operation from dual DC supplies, the symmetrical +/-15V DC arrangement is the most common in professional audio, +/- 12V or +/-10V in hifi sometimes.
Avoid the opamps that are intended for use with single DC supplies, (it says so in the technical description)


OK. Only dual supply intended OAs then. All right.


There is whole new generation of opamps specially for audio with logic chips that use single DC supply +3 or +5V DC.
For these applications capacitors are sometimes needed to block DC


well, I think this is too complex for me.
To be clear I was thinking to start with a nice NE5534 used as unity gain buffer with a nice attenuator in front of it to drive properly my power amp.
The only thing "high-endish" would be a dual supply built around two 12V sla rechargeable batteries.
I select this opamp on the basis of these consideration:
1) it can be found even in series at the signal in very well reviewed units, like dacs and preamps. So it must be good for audio
2) it draws very little current so the batteries' charge should last a bit
3) last but not least is quite cheap. Actually unbelievably cheap.

Hope this helps
Ted


Dear Mr Ted, your very kind advice has helped me a lot indeed.
It prevents me to do the wrong things at the start.
Sometimes I read about the limitation of opamps, but I am not that convinced.
I see more and more units (dvd players, dacs, preamps, etc.) that used them and that have very well performance during listening tests.

Thank you very much indeed.
Kindest regards,

beppe

P.S. as i am about to leave i will be able to reply only by friday evening.
I have no access to the web during the working days.
Thanks to everyone.
 
AndrewT said:
Hi,
the 5534 does not make a nice unity gain buffer.
It is not unity gain stable.
See the datasheet for the extra capacitor that must be added to make the opamp unity gain stable and after fitting this compensation cap (~22pF) the opamp behaves completely differently from it's higher gain specification.


Oops ! Thank you very much Mr Andrew.
I did not know.
What is the minimum gain to make it stable ? 2 is enough ?
I do not want to much voltage gain
My amp is very sensitive. It needs only 0,5V at the input to give its maximum power.
Instead it has a very low 5K input impedance.
A though load I understand.
This for instance causes problems with tube line stage, of which I like the sound unfortunately.
Well I have pairs of the following ones at hand:
AD 797, 811, BUF04, 826
LM 6181, 7171
BB OPA132PA, OPA604AP
TL 081CN
Which would you pick for a unity gain buffer ?


Thanks again and kind regards,

beppe
 
Gordy said:
But only if using a single supply voltage!
I think beppe61 is looking to use bi-polar + / - supplies.


I guess Mr Ted wanted to give a complete view of all the voltage supply options.
By the way only nowI understand clearly that for audio purpose the one way to go, in order to limit any further problems, is the dual supply configuration.
That is indeed what I would use.
But at first I did not know this exactly,

Thanks again to everyone,
Kindest regards,

beppe
 
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