Opamp that can be dc coupled ? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 6th January 2008, 03:10 PM   #1
beppe61 is offline beppe61  Italy
diyAudio Member
 
Join Date: Feb 2005
Location: torino
Default Opamp that can be dc coupled ?

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
  Reply With Quote
Old 6th January 2008, 03:20 PM   #2
diyAudio Member
 
Jan Dupont's Avatar
 
Join Date: Apr 2003
Location: Copenhagen, Denmark
Send a message via MSN to Jan Dupont
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.
__________________
Free Schematic and Service Manual downloads www.audio-circuit.dk, Company: www.dupont-audio.com, Joint venture: www.DupontMantra.com
  Reply With Quote
Old 6th January 2008, 03:36 PM   #3
beppe61 is offline beppe61  Italy
diyAudio Member
 
Join Date: Feb 2005
Location: 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
  Reply With Quote
Old 6th January 2008, 03:45 PM   #4
diyAudio Member
 
Jan Dupont's Avatar
 
Join Date: Apr 2003
Location: Copenhagen, Denmark
Send a message via MSN to Jan Dupont
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
__________________
Free Schematic and Service Manual downloads www.audio-circuit.dk, Company: www.dupont-audio.com, Joint venture: www.DupontMantra.com
  Reply With Quote
Old 6th January 2008, 04:15 PM   #5
beppe61 is offline beppe61  Italy
diyAudio Member
 
Join Date: Feb 2005
Location: torino
[b] 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

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
  Reply With Quote
Old 6th January 2008, 04:56 PM   #6
diyAudio Member
 
Jan Dupont's Avatar
 
Join Date: Apr 2003
Location: Copenhagen, Denmark
Send a message via MSN to Jan Dupont
Quote:
Excuse me may I ask which is your current choice of line preamp ?
A homemade DC coupled preamp with output buffer
__________________
Free Schematic and Service Manual downloads www.audio-circuit.dk, Company: www.dupont-audio.com, Joint venture: www.DupontMantra.com
  Reply With Quote
Old 6th January 2008, 05:08 PM   #7
Gordy is offline Gordy  United Kingdom
diyAudio Member
 
Join Date: Nov 2006
Default Re: Opamp that can be dc coupled ?

Quote:
Originally posted by beppe61
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.
  Reply With Quote
Old 6th January 2008, 05:36 PM   #8
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Default Re: Re: Opamp that can be dc coupled ?

Quote:
Originally posted by Gordy

[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
__________________
I won't make the tactical error to try to dislodge with rational arguments a conviction that is beyond reason - Daniel Dennett
Check out Linear Audio Vol 7!
  Reply With Quote
Old 6th January 2008, 05:44 PM   #9
diyAudio Member
 
Jan Dupont's Avatar
 
Join Date: Apr 2003
Location: Copenhagen, Denmark
Send a message via MSN to Jan Dupont
Nice and correct explaination, Jan
__________________
Free Schematic and Service Manual downloads www.audio-circuit.dk, Company: www.dupont-audio.com, Joint venture: www.DupontMantra.com
  Reply With Quote
Old 6th January 2008, 05:53 PM   #10
beppe61 is offline beppe61  Italy
diyAudio Member
 
Join Date: Feb 2005
Location: torino
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
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
capacitor coupled vs. direct coupled ouput hugobross Solid State 11 21st January 2011 01:56 AM
Bipolar PS plus AC coupled output from opamp? leadbelly Solid State 6 28th July 2009 11:12 AM
6sn7 direct coupled and transformer coupled neazoi Tubes / Valves 3 23rd May 2008 03:30 PM
Can someone explain parafeed & direct coupled & a.c. coupled? mwmitchell Tubes / Valves 2 11th September 2002 01:10 PM


New To Site? Need Help?

All times are GMT. The time now is 06:30 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2