Dx Blame MKIII-Hx - Builder's thread - Page 116

canonnica · 20th April 2012, 06:10 PM

Quote:

Originally Posted by Junie

Mart I did open my amp again

Negative connected to each other.

Hahaha, sorry for that Junie

Thanks for your comments Andrew. Being in the process of learning, I think I started from the "empirical" side driving down to the more theorical and by-the-book side right now. Your technical comments help.

Mart.

canonnica · 20th April 2012, 08:11 PM

Ok... I did some calculations and even simulations and here's my findings. I only worked on the low-pass thing.

Refer to the image below for nomenclature.

With R3=220r and C4=220pF, the f0 is 3.3Mhz and RC is 48ns, which is somewhere on unknown roads (or is it know to be wrong roads?)...

Setting value of R3=2.7K and C4=470pF, f0 goes down to 125Khz and RC goes to 1.3us which would put us back on known pastures.
It will have an impact on the high-pass filtering as well because R3 and R1 are in series to the ground, but it will be somewhat negligible.
The sensitivity of the input will be decreased by about 5% (thus 5% less "global amplifier" gain).

Am I right with all of this?

And not the question that kills... why would one do this mod? ... Will it even alter the sound? Will it alter the stability of it?

Mart.

a.wayne · 20th April 2012, 09:12 PM

You said you simulated it ? and pics of such ....?

AndrewT · 21st April 2012, 10:56 AM

Amps are quite sensitive to what appears at their inputs.
Interference for instance can be at VHF. It is usually better to attenuate that VHF interference before it gets into any amplifier.

Amps are also susceptible to misbehaving if the input is fed with very fast changing signals.
Filtering attenuates the very fast signals and thus gives the amp the chance to not misbehave.

For these reasons the low pass filter, that I believe should never be omitted, should be set as low as possible.
That "low as possible" must be compatible with good audio quality. Ideally it must not remove any audio information. Ideally it must not allow the amplifier to misbehave.

Getting both of those conditions satisfied is not easy. Hopefully the amp designer has got the misbehaviour part already sorted. Then it comes down to the user getting all the audio passing the filter as he/she requires.

a.wayne · 21st April 2012, 04:43 PM

By reducing overall gain, adjusting Fb for stability reduces the chance of VHF oscillations, getting rid of the dreaded inline cap will make for better sonics.
Granted with low resolution input signal and speakers , most may not hear the difference .

Capacitors are evil ..........

canonnica · 21st April 2012, 08:29 PM

I did some tests of bypassing the input cap with a jumper on my HX.

Note: My source has a DC offset of 0.3mV, which is nothing.

Soundwise; I didn't notice anything. Sound will change more if you just move your head. Lows, Mids, Highs are the same, i didn't notice any difference.

Technicalwise; With the input cap jumped, there's about -460mV offset at the speakers... woups, that's bad. That's enough to make my woofer recess a bit in its enclosure.

It is probably due to the -375mV bias voltage at the signal input of the differential pair. Watch-out for that voltage to leak into your source with cap removed/jumped!!

So I won't be seen jumping that cap.

Martin.

a.wayne · 21st April 2012, 08:42 PM

Again it depends on your speaker and source resolution, impossible to believe that input cap does not have a "sound ", all caps add color
Cant see why this should be a problem at the output, could this be an gnd loop issue..?

AndrewT · 09:04 AM

Quote:

Originally Posted by a.wayne

............ impossible to believe that input cap does not have a "sound ", all caps add color .............

I will not agree to this all encompassing blanket statement.

He reported what he heard.
He reported how he did it.
He reported the downside of unpredictable output offset.

I would much rather believe what he presents than believe you.

I would also never recommend DC coupling nor mixed AC & DC coupling without taking the other precautions that I believe become a necessity and have detailed in previous posts. That does not detract from Canon's post.

Quote:

Capacitors are evil ..........

what evidence have you obtained to support this conclusion?
Are you prepared to offer up that evidence?
Or do we just ignore your unsubstantiated comment/s?

AndrewT · 22nd April 2012, 09:07 AM

Quote:

Am I right with all of this?

the input filters are passive. They are also outside the feedback loops.
The changing of the R3 to R1 ratio changes the sensitivity of the input. It does not change the amplifier gain.

a.wayne · 22nd April 2012, 10:34 AM

Are you asking me to proved proof that capacitors "have a sound" ..

I'm also not denying his reported results , actually applaud the effort, just surprised by it , that's all ..

Now what about my question about GL issues ?

What about the increased DC offset any suggestions there, Martin did not state the value with no input, that may tell us something ..

21st April 2012, 08:29 PM	#1156
canonnica diyAudio Member Join Date: May 2011 Location: Quebec	I jumped the input cap and.... I did some tests of bypassing the input cap with a jumper on my HX. Note: My source has a DC offset of 0.3mV, which is nothing. Soundwise; I didn't notice anything. Sound will change more if you just move your head. Lows, Mids, Highs are the same, i didn't notice any difference. Technicalwise; With the input cap jumped, there's about -460mV offset at the speakers... woups, that's bad. That's enough to make my woofer recess a bit in its enclosure. It is probably due to the -375mV bias voltage at the signal input of the differential pair. Watch-out for that voltage to leak into your source with cap removed/jumped!! So I won't be seen jumping that cap. Martin.

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20th April 2012, 09:12 PM	#1153
a.wayne diyAudio Member Join Date: May 2008 Location: Front Row Center	You said you simulated it ? and pics of such ....?

21st April 2012, 10:56 AM	#1154
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Amps are quite sensitive to what appears at their inputs. Interference for instance can be at VHF. It is usually better to attenuate that VHF interference before it gets into any amplifier. Amps are also susceptible to misbehaving if the input is fed with very fast changing signals. Filtering attenuates the very fast signals and thus gives the amp the chance to not misbehave. For these reasons the low pass filter, that I believe should never be omitted, should be set as low as possible. That "low as possible" must be compatible with good audio quality. Ideally it must not remove any audio information. Ideally it must not allow the amplifier to misbehave. Getting both of those conditions satisfied is not easy. Hopefully the amp designer has got the misbehaviour part already sorted. Then it comes down to the user getting all the audio passing the filter as he/she requires.

21st April 2012, 04:43 PM	#1155
a.wayne diyAudio Member Join Date: May 2008 Location: Front Row Center	By reducing overall gain, adjusting Fb for stability reduces the chance of VHF oscillations, getting rid of the dreaded inline cap will make for better sonics. Granted with low resolution input signal and speakers , most may not hear the difference . Capacitors are evil ..........

21st April 2012, 08:42 PM	#1157
a.wayne diyAudio Member Join Date: May 2008 Location: Front Row Center	Again it depends on your speaker and source resolution, impossible to believe that input cap does not have a "sound ", all caps add color Cant see why this should be a problem at the output, could this be an gnd loop issue..?

22nd April 2012, 10:34 AM	#1160
a.wayne diyAudio Member Join Date: May 2008 Location: Front Row Center	Are you asking me to proved proof that capacitors "have a sound" .. I'm also not denying his reported results , actually applaud the effort, just surprised by it , that's all .. Now what about my question about GL issues ? What about the increased DC offset any suggestions there, Martin did not state the value with no input, that may tell us something ..