or Connect
Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › How much DC offset would you consider harmful to HP and after which point would you throw the commercial amp out?
New Posts  All Forums:Forum Nav:

# How much DC offset would you consider harmful to HP and after which point would you throw the commercial amp out? - Page 3

ipad O-scope  The only problem with ipad O-scope is it cannot go beyond the audio range, but were testing out audio.  You guys know of anything for PC?

Edited by High_Q - 4/30/11 at 3:00pm

We should be able to use the microphone input on the PC as a O-scope to check source's output right?

Just need a software for it.

Quote:
Originally Posted by High_Q

We should be able to use the microphone input on the PC as a O-scope to check source's output right?

Just need a software for it.

or we could get one of these and not use an i-poop (plus, considering the quality of built-in codec inputs, I doubt the mic in is a measurement tool ) : http://cgi.ebay.com.au/DSO5200-200MS-s-PC-USB-Digital-Storage-Oscilloscope-2CH-/280575179679?pt=BI_Oscilloscopes&hash=item4153957b9f
http://cgi.ebay.com.au/2x100MHz-PC-USB-Oscilloscope-Spectrum-Analyzer-Meter-/120673812295?pt=LH_DefaultDomain_0&hash=item1c18b83f47
http://cgi.ebay.com.au/DSO2090-USB-PC-Virtual-Storage-Oscilloscope-40MHz-100MS-/280456846465?pt=BI_Oscilloscopes&hash=item414c87dc81
http://cgi.ebay.com.au/Hantek-DSO5200A-Virtual-USB-Oscilloscope-200Mhz-250MS-s-/120586423894?pt=AU_B_I_Electrical_Test_Equipment&hash=item1c1382ce56
http://cgi.ebay.com.au/5-Channel-Portable-PC-USB-2-0-Oscilloscope-Probe-CD-/260760514721?pt=BI_Oscilloscopes&hash=item3cb689b0a1

also, what's with the copy pastes from wiki ? just use links to the appropriate paragraphs and at least annotate the links.

>most research i've done says any DC offset upto 50mv is considered normal & 'safe' under practical real world conditions. alot of amps deviate upwards to a few hundred mv with no apparent detriment to speakers (atleast structurely). 0mv is best ofcos!

I would've thought speakers accept a lot higher signal voltage, so absolute DC offset voltages of a 1-few hundred mV wouldn't be of much concern as opposed to HP outs, where that same 100mv would be 10-20-30% of the total input...

>I've tried measuring audio signal out of my DMM one time, and thought occurred to me: Wait a minute, I'm measuring a audio signal(it was fluxuating), maybe I should use a O-Scope!!! One thing I don't understand is, how is offset of 10mV(thats 100th of a volt) dangerous if the signal swings way beyond that if you raise the gain?

When you raise the gain e.g. 1000x for 60db stax amps the 100mv becomes 0.1x1000=100v (although that's at the max vol setting, about 70% on the listenable). 100v is pretty audible on the stax headphones (a hum, at least it was on the SRM-1/mk2, I don't think you'd want to put an extra 50-100v DC through your amp output stages anyway...)

>Output voltage(without load) is total output from the amp, and the lower value is when loaded with headphone(voltage drop accross it) depends on how much output impedance there is. But offset or DC voltage drop is real as it deals with Real impedance, which also depends on real part of output impdeance, and heaphonephone impedance. If output voltage is much greater than headphone impdance, there will not be significant offset on the headphones.

I'm still interested why D100 had a large drop to near 0dc offset when I connected a 25 oh load, and MD11 stayed at around 120/140V.
Edited by svyr - 4/30/11 at 6:48pm

i dont know if full range home speakers have a much higher DC tolerance but even assuming that they do, dont forget that the ave home amplifier has a gain of 30+db & output wattages of 50w rms while the ave gain of dedicated headamps is round 6 db at 150mv. so the normal headphone has only a fraction of the amplified DC to deal with as opposed to speakers.

personally i think its a wash as to which particular transducer has more DC tolerance in actual real world application. the standard home amp has as much as 100x more power than the ave headamp so deviations of a few hundred mv's isnt rare & built in trim pots are there to solve this.

OTOH an 100x less powerful amp with similar DC offset issues signifies a much bigger problem & is prolly inherently a bad design. however its still relatively easy to solve it with a simple mod-a-trim-pot at the appropriate transistor output to adjust DC or cap coupled it at the output.

its still my opinion that its a storm in a teacup to worry about DC offsets destroying cans esp when dealing with the micro voltages inherent in dedicated headamps & headfi.

Quote:
Originally Posted by svyr

I would've thought speakers accept a lot higher signal voltage, so absolute DC offset voltages of a 1-few hundred mV wouldn't be of much concern as opposed to HP outs, where that same 100mv would be 10-20-30% of the total input...
>i dont know if full range home speakers have a much higher DC tolerance but even assuming that they do, dont forget that the ave home amplifier has a gain of 30+db & output wattages of 50w rms while the ave gain of dedicated headamps is round 6 db at 150mv. so the normal headphone has only a fraction of the amplified DC to deal with as opposed to speakers.

I though we were talking about DC out on the actual voltages post gain - i.e. speaker amp outs, not your source going into the speaker amp. If we're talking about source->amp DC offset, I think it's more of a relative thing anyway. I.e. Even if it's a 50w rms system, the proportion of the DC offset to the signal would hold post gain, so as long as we aren't talking about hundreds of mv DC (from the 2v line out signal max) it should be fine despite the higher gain. (i.e. it'll be within the speakers max rated power, if DC + signal amplified is less than the rated voltage)

Although I have confused people by providing the figures for MD11 line out along with the HP out and then talking about the 1000x gain for the stax amps connected the it.
Edited by svyr - 5/1/11 at 12:05am

I had an inclination Mic input O-scope can be done..  mic input is analog in to pc.  It takes in voltage, and there is a ADC in the sound card on the pc, and you need to find the right software to analyze the data.  Here is a blog of someone attempted that.

http://my.opera.com/ceez/blog/pc-oscilloscope

And also, this:

http://my.opera.com/ceez/blog/pc-oscilloscope

Edited by High_Q - 5/1/11 at 6:23am
Ahhh ic what u mean. Folks here's been getting all OCD'd bout even as little as 10-20mv of DC offset & I naturally assumed tat it was at the source since offset readings are done with both source unhook'd & unloaded.

What I meant was tat it's fairly common for home amps to have as high as a coupla hundred mv of DC offset considering the Voltage/wattage involved but it's darn ridiculous to have similar amts of offset from microamps ie. headamps! Fo sho there's something rotten in the state of Denmark & it's a faulty design or a faulty circuit atleast.

Since most headamps don't have DC (or Bias current for tat matter) trimpots, I think a lot of them are output cap coupled - I know both of my ex iBasso porty amps are. Regardless I wouldn't lose any sleep as long as DC is <50mv - at output tat is. If it does destroy your cans, I'd be hoofing it to the nearest lotto outlet....pronto;)
Edited by scottiebabie - 5/1/11 at 11:54am
It's magical musilol time:
Quote:
Originally Posted by svyr
just to clarify what I want support tell me about MD11:

a) Will the replacement unit have a working power button? (currently you can turn the unit on, and sometimes it shows a white screen and nothing happens)
b/c)Just to clarify what I mean by DC offset on HP and line out.

Take a multimeter. Connect the terminals to the Headphone out in DC Voltage measurement mode:

For HP out you get 145mV DC (nearly 0.15v)

For line out you get 71mV DC (nearly 0.08v)

THESE SHOULD NOT HAPPEN. Both of those measurements should be near 0mv DC.

Please consult support with those questions for me.

Quote:
Originally Posted by Customer support
Back upstairs, this is normal.
OCL output, then there will be a DC offset. Line out the current, to a small point. Headphone output as current, to a large point.

Back upstairs, this is normal.
OCL output words (no additional capacitive coupling), there will be a DC offset. Line out the current, to a small point. Headphone output current is high due ...

Quote:
Originally Posted by svyr
You're supposed to trim DC offset on both line out and HP out. (it remains constant at any volume level). 150mv DC offset on HP out is not acceptable. Neither is 70mv on line out.
150mv DC is nearly 1/3 of what I listen to (ac out of 450mv on some of my IEMs). That reduces dynamic range for headphones.

For line out, when you connect a directly coupled amp (e.g. stax driver units) the high DC offset translates to extraneous audible noise.

IF this is normal it's evidence of very poor design. At the very least I'd expect the offsets to be trimmed to <10mv for line out, or <20mv for headphone out.
Quote:
Originally Posted by Customer support
Line output is less than 10mv, headphone output is less than 50mv, no capacitive coupling for the circuit is difficult to do. Need to do a lot of auxiliary circuits, such equipment is astronomical, do not appear in MD11, MD30 price of such devices.
Op-amp output will be more than 50mv DC offset, not to mention the power transistor output. Unless it is coupled with a capacitor.

Quote:
Originally Posted by svyr
>Line output is less than 10mv, headphone output is less than 50mv, no capacitive coupling for the circuit is difficult to do.

yes, but MD11 is 7 times that on line out and 3 times that on hp out. Surely Musiland could have at least made an effort to get closer to 10 and 50?

Let me put it this way :

For line out - I can get a cable with capacitors in it. Yes it will be inconvenient, but I did it for my Stax SRM-1/mk2 while using the MD11....However, for an average user, if they have a similar problem, they will blame it on the MD11, and won't even think about doing that. In fact, I didn't know what the problem was, until a person who helped me restore my SRM-1/mk2 suggested to measure the DC out.

Moreover, what are you supposed to do to fix it for headphone out ? It's not like you can get a cable with caps in line. Oh wait, you sort of can... 6.3 adapter extension cable.

Look, either way, the fact that it's not coupled on either output should be clearly indicated on the marketing material and product page, so the users know about before buying. Instead of discovering it may be an issue when they get it home or after a few months of use... Or even worse, find out while trying to fix the device connected to MD11 that's misbehaving because of the DC offset.

So um, musiland basically said - too hard, this is normal (by design for both HP/line out), trimming DC offset is hard for them/expensive (woot?), cptn obvious reports the line out an hp out are not capped, to get capped outputs you need to buy a \$600 MD30 lol ... uuuum 'cool story?'
again, it doesn't mention anywhere that the line out and the hp out are not capped, and wtf would normal people do, buy cables with caps for hp and line lol ? Yea, those are really sold in your corner shops...My DIY line out one looked like something out of horror films
Edited by svyr - 5/30/11 at 7:54am
Tried to measure DC offset on XLR out for another source the other day... Got

IF I remember correctly I had about 300mv DC offset between 2 and 3, but not between 1/3 or 2/1

The pins are:
1 Chassis ground (cable shield)
2 Positive polarity terminal (hot)
3 Return terminal[4] (cold)

Crickey, bin it? or supposed to measure to ground?

Quote:
IF I remember correctly I had about 300mv DC offset between 2 and 3, but not between 1/3 or 2/1

The DC offset is between pins 2 & 3.

300mV is excessive.

Quote:
Originally Posted by wink

The DC offset is between pins 2 & 3.

300mV is excessive.

thanks!
eeeh my bad, memory did not serve me right. It doesn't have a positive to neutral offset, it has the 300mv offset when I measure either neutral or positive to ground... nooo idea what that means though.
Edited by svyr - 6/3/11 at 3:23am
Quote:
Originally Posted by svyr

Quote:
Originally Posted by wink

The DC offset is between pins 2 & 3.

300mV is excessive.

thanks!
eeeh my bad, memory did not serve me right. It doesn't have a positive to neutral offset, it has the 300mv offset when I measure either neutral or positive to ground... nooo idea what that means though.

For headphones the output could be quite a few volts above ground, as long as pins 2&3 measure ~0V between each other.

For a source to an amp there are cases where having the output a bit off from ground does nothing, and others where it screws everything up. Best practice is to use floating outputs but very few people want to pay extra for the circuitry or transformers transformers to do that. Mega kudos to the people who do. Second best is to reference the output to ground. A LARGE majority of consumer balanced gear has ground-referenced balanced outputs and very few people have problems.

Why I stated to use O-scope?  The Digital Mulitmeter (DMM) only measures RMS.  Explain to me how the dmm measure the offset.

Quote:
Originally Posted by qusp

need a scope for measuring dc offset? er.......no, a dmm is usually fine depending on the signal you are using. if you wanna get crazy you can use a voltmeter and play a known voltage sine wave, whatever the heating value over what is expected after gain and converted to dc is your offset. of course as scope can be used to good effect, but isnt needed imo.

as far as i know, you dont need to go over max rated input to cause issues, that is not what its about except in the extreme case where you will overheat the coils, its more about excursion of the driver and therefor dynamic range being effected. your 110mv amount is pretty high in this regard

Edited by High_Q - 6/3/11 at 7:16pm

Quote:
Originally Posted by High_Q

Why I stated to use O-scope?  The Digital Mulitmeter (DMM) only measures RMS.  Explain to me how the dmm measure the offset.

I have no idea why you said to use an oscilloscope to do something that is easily accomplished with a very affordable and accessible tool - a simple volt meter. Re-read your first post in this thread.

As an engineer is it morally acceptable to you to recommend a tool that is harder to use and generally more expensive when a tool which is much easier to use and FAR less expensive to achieve the same end result?

Now, for how to measure voltage offset with a DMM we must start with the assumption (going FAR out on a limb here because it could be anything with you) that the DMM you are reffering to is a voltage, current, and resistance type - perhaps with a few other features thrown in (such as AC volts) but I think this is a reasonable assumption. People with LCR meters , digital oscilloscopes, or other more specialized meters typically refer to them as such despite the fact that they are equally or more digital compared to most voltage/current/resistance devices.

Now, assuming your DMM has a DC VOLTAGE setting one would set the DMM for "DC volts" on the appropriate range (or if it autoranges just let it do its own thing) and measure the offset from output to ground in an SE amp or inphase output to outofphase output in a balanced or bridged amp.

You left the units off of RMS in this sentience:

"DMMs measure only RMS"

RMS what? My DMM Measures RMS current amongst other things.

Edited by nikongod - 6/3/11 at 7:54pm

No, we recommend for accuracy and precision(simply for the right values) as an EE.  So, we don't say measure offset using dmm if it's not measuring offset.  We recommend a tool that measures offset.

Wrong, DC measurments, when input is DC.  Audio signal is AC+ offset(there will be some offset).  explain to me how offset is being measure by the dmm with the DC setting.

RMS what? Duhh. Voltage.. what is this thread about again, you should know by now..

FYI

The RMS value of a set of values (or a continuous-time waveform) is the square root of the arithmetic mean (average) of the squares of the original values (or the square of the function that defines the continuous waveform).

In the case of a set of n values $\{x_1,x_2,\dots,x_n\}$, the RMS value is given by:

$x_{\mathrm{rms}} = \sqrt {{{x_1}^2 + {x_2}^2 + \cdots + {x_n}^2} \over n}.$

The corresponding formula for a continuous function (or waveform) f(t) defined over the interval $T_1 \le t \le T_2$ is

$f_{\mathrm{rms}} = \sqrt {{1 \over {T_2-T_1}} {\int_{T_1}^{T_2} {[f(t)]}^2\, dt}},$

and the RMS for a function over all time is

$f_\mathrm{rms} = \lim_{T\rightarrow \infty} \sqrt {{1 \over {2T}} {\int_{-T}^{T} {[f(t)]}^2\, dt}}.$

The RMS over all time of a periodic function is equal to the RMS of one period of the function. The RMS value of a continuous function or signal can be approximated by taking the RMS of a series of equally spaced samples. Additionally, the RMS value of various waveforms can also be determined without calculus, as shown by Cartwright.[1]

In the case of the RMS statistic of a random process, the expected value is used instead of the mean.

Quote:
Originally Posted by nikongod

I have no idea why you said to use an oscilloscope to do something that is easily accomplished with a very affordable and accessible tool - a simple volt meter. Re-read your first post in this thread.

As an engineer is it morally acceptable to you to recommend a tool that is harder to use and generally more expensive when a tool which is much easier to use and FAR less expensive to achieve the same end result?

Now, for how to measure voltage offset with a DMM we must start with the assumption (going FAR out on a limb here because it could be anything with you) that the DMM you are reffering to is a voltage, current, and resistance type - perhaps with a few other features thrown in (such as AC volts) but I think this is a reasonable assumption. People with LCR meters , digital oscilloscopes, or other more specialized meters typically refer to them as such despite the fact that they are equally or more digital compared to most voltage/current/resistance devices.

Now, assuming your DMM has a DC VOLTAGE setting one would set the DMM for "DC volts" on the appropriate range (or if it autoranges just let it do its own thing) and measure the offset from output to ground in an SE amp or inphase output to outofphase output in a balanced or bridged amp.

You left the units off of RMS in this sentience:

"DMMs measure only RMS"

RMS what? My DMM Measures RMS current amongst other things.

Edited by High_Q - 6/3/11 at 8:32pm
New Posts  All Forums:Forum Nav:
Return Home
• How much DC offset would you consider harmful to HP and after which point would you throw the commercial amp out?
Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › How much DC offset would you consider harmful to HP and after which point would you throw the commercial amp out?