DC coupled = worst dynamics - is it true? - 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 22nd August 2014, 03:21 PM   #1
PauloPT is offline PauloPT  Portugal
diyAudio Member
 
Join Date: Jan 2010
Location: Lisboa, Portugal
Send a message via MSN to PauloPT
Default DC coupled = worst dynamics - is it true?

Hi,

I have a couple of questions regarding DC/AC coupled amplifiers:

1) Is it true that removing the NFB capacitor results in a less dynamic sound, or is it a mith? Will an AC coupled amp have more energy due to the caps and therefore more dynamics?

2) Is it possible to make the NFB capacitor 'invisible' by choosing the right time constants for input and NFB RC filters? I tried this with input RC = 90 ms and NFB RC = 150 ms but I think I still can hear the cap! It's not like distortion but lesser resolution with the cap in place. DC coupled I believe I can hear more details and a more precise sound. Am I hearing imaginary things?

Thank you.
__________________
Kind Regards,
Paulo.
  Reply With Quote
Old 22nd August 2014, 03:27 PM   #2
RNMarsh is offline RNMarsh  United States
diyAudio Member
 
RNMarsh's Avatar
 
Join Date: May 2012
Location: 2457 Cascade Trail; Cool, CA. 95614
Hi,

No you are not hearing things in your imagination. Less resolution is what everyone hears about caps in the signal path. But you might not be able to just remove it because it is eliminating a dc offset on your output. You would have to add a dc servo to keep the output having no dc.

-RNM
  Reply With Quote
Old 22nd August 2014, 03:44 PM   #3
diyAudio Member
 
Join Date: Jan 2008
... and DC servo will likely "slow down" the speed and transient response.
  Reply With Quote
Old 22nd August 2014, 03:49 PM   #4
Bigun is offline Bigun  Canada
diyAudio Member
 
Bigun's Avatar
 
Join Date: Jan 2009
Blog Entries: 2
A servo is another place for caps to hide, so is the power supply.

It's like dreaming of a world with no bacteria, all clean. It turns out that it isn't possible nor desirable (we wouldn't survive without them).

I've generally found no issue having caps in my amplifiers regarding the sound. Only once did I 'hear' a cap sound 'bad' and that was after putting a cheapo electro on the output of a dual-rail amplifier just to find out.
__________________
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
  Reply With Quote
Old 22nd August 2014, 05:01 PM   #5
PauloPT is offline PauloPT  Portugal
diyAudio Member
 
Join Date: Jan 2010
Location: Lisboa, Portugal
Send a message via MSN to PauloPT
I realise a servo has its issues but I suppose a properly design one has less influence in the sound than a capacitor. My preamplifier has servos and sounds very good.

I also realise we always have a great deal of caps in a power amp and they are good thing and unavoidable. I love PSU with lots of them!

What I want to find out is if it's possible to have a NFB capacitor with close no none influence in the sound. I'm disappointed that adjusting the RC constants didn't make the capacitor disappear. Maybe if I use a really big value?

Other thing I'd like to find out is if DC coupling the amplifier kills the dynamics? A trade off between dynamics (AC coupled) and fine detail (DC coupled)? My listening tests weren't conclusive about this...
__________________
Kind Regards,
Paulo.
  Reply With Quote
Old 22nd August 2014, 06:24 PM   #6
jcx is offline jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
signal coupling Electrolytics do have measurable distortion - but still low order, low levels compared to most dynamic loudspeakers

Quote:
Originally Posted by jcx View Post
Doug Self #8 Capacitor Distortion - on his list for decades, with measurements
Distortion In Power Amplifiers

Cyril Bateman "Capacitor Sound" articles in Electronics/Wireless World when it still had valuable content
Capacitor Sounds, Speaker Cables and Crossover Inductors. Archive.org retrieval - not active

but I am skeptical of too simple a subjective correlation - particularly if not demonstrated double blind with level matching, ability for fast switching - not by "memory" with a soldering iron in your hand - such uncontrolled "just listen" "tests" are simply useless for informing engineering except in the grossest errors of % high distortion or dB frequency response changes

read Sy's Testing article at linear audio http://www.linearaudio.net/index.php...d=39&Itemid=64

Last edited by jcx; 22nd August 2014 at 06:30 PM.
  Reply With Quote
Old 22nd August 2014, 07:15 PM   #7
PauloPT is offline PauloPT  Portugal
diyAudio Member
 
Join Date: Jan 2010
Location: Lisboa, Portugal
Send a message via MSN to PauloPT
That's interesting reading, thanks for the links jcx

I'm aware of the importance of blind tests, that's why I come to the forum asking if I'm imagining things. And if I'm not, then I'd like to know the objective explication for the differences in sound.

I believe the difference between cap and no cap is real even without a proper blind test.

Dynamics, I'm not so sure. It's here I should do a proper blind test. Is there any objective reason for a fully DC coupled amp have lesser dynamics than it's AC coupled version?
__________________
Kind Regards,
Paulo.
  Reply With Quote
Old 22nd August 2014, 08:17 PM   #8
Bare is offline Bare  Canada
diyAudio Member
 
Join Date: Dec 2001
Location: vancouver
Humans pretty well All harbor 'beliefs', often derived only from anecdotes and or teachings...typically by those with an agenda.
Faith indeed moves mountains.
Factual or even credible is rarely a requirement .
Witness the world around us
  Reply With Quote
Old 22nd August 2014, 08:22 PM   #9
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
Quote:
Originally Posted by PauloPT View Post
I believe the difference between cap and no cap is real even without a proper blind test.
Don't be fooled into thinking that because a cap is inside the signal path or feedback loop rather than beside, it is more important than the one(s) that shapes your DC servo: in order to eliminate DC or VLF, you have to use a number of poles, and lousy pole components have more or less the same kind of effect, wherever they are located.
The ones in the DC servo will come with a vengeance: some of their undesirable features will be amplified, and the nasties from the servo will also be coupled in some way, even if they are attenuated (by regular passives, of course).

There is no magic bullet: DC servoes are convenient but they do nothing for the quality of reproduction, quite the opposite in fact. Which is probably why they are sonically preferred to simple passives: it is similar to the tube sound effect. The result is more colourful, less boring...

If you want DC-servo advantages without the opamp and its "features", you can use multiple constants feedbacks.

In this example (using opamp rather than PA for convenience), the settling time would be governed by the time constant of R1 and C1, in this case 100 seconds, which would be absolutely unbearable for practical purposes.

A second, much faster loop is added, but it is normally isolated from the regular FB path by back to back diodes.
Here, the time constant is only 470ms, which allows for a shortened stabilizing time.
The sim shows the situation towards the end of the stabilizing process: the current through the diodes is progressively decreasing to ~0. The signal is a maximum amplitude 10Hz waveform, normally near the maximum possible stress.

This is just an example, the constants can be multiple and adapted, etc.
At least, here you see clearly where the non-linearities are susceptible to come from.

For the rest, I can only recommend you make proper A-B-X tests: you will probably find that most of the folk lore is unnecessary, and simplify your life as a consequence...
Attached Images
File Type: png Noservo.png (76.0 KB, 238 views)
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪

Last edited by Elvee; 22nd August 2014 at 08:24 PM.
  Reply With Quote
Old 23rd August 2014, 12:32 AM   #10
expert in tautology
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
Hang on a second.

The OP asked about DC coupled vs. AC coupled in the title and then went on to talk about 1) "NFB capacitor" and 2) making this cap "invisible" via other means.

Presumably he is talking about the (usually electrolytic) cap at the inverting input to the typical diff pair at the input of the amp? The purpose of which is to raise the NFB at very low freqs so that at DC the amp has unity gain, thus preventing excess gain causing DC offset...

There is the "other" NFB cap that might be there to drop the HF gain down to unity at some frequency, aka "stability cap".

So which one are we talking about?

Or are we talking about a cap at the input to ground, or a cap in series with the input (DC blocking)?

How about a cap in series with the output?

Also there are and have been amps designed and built that are DC stable without the use of a servo or a DC blocking cap at the input.

Thought it might be good to clear up which caps are being adjusted to do what?

Imho, "dynamics" are a subjective perception that can't be related directly to any one or combination of factors. It's more of a system wide subjective impression, also known as "jump factor" and "slam". Otoh there are all sorts of things that you can do to a given amp that may alter the perceived subjective impression that may or may not be difficult to measure, but even having been measured may not be correlated back to be useful to alter the sound of another amp at another time, or even be clear as to why or how that particular amp appeared to change character.
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  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
DC coupled preamp? fmjunkie Solid State 13 28th September 2013 08:50 PM
DC coupled 6P15P mr2racer Tubes / Valves 14 20th May 2013 03:03 AM
2a3 DC coupled andyjevans Tubes / Valves 25 1st November 2012 09:01 PM
True direct-coupled PP amp JesseG Tubes / Valves 20 15th January 2010 04:17 PM
Opamp that can be dc coupled ? beppe61 Solid State 37 20th January 2008 08:21 AM


New To Site? Need Help?

All times are GMT. The time now is 05:33 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