The Objective2 (O2) Headphone Amp DIY Project - Page 104 - diyAudio
Go Back   Home > Forums > Amplifiers > Headphone Systems

Headphone Systems Everything to do with Headphones

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 1st December 2011, 06:16 PM   #1031
timpert is offline timpert  Netherlands
diyAudio Member
 
timpert's Avatar
 
Join Date: Nov 2004
Location: Netherlands
Quote:
Set the DMM for AC voltage and measure again. Each should be under 1 volt.


A strong caution is in place here, if a cheap multimeter is being used for this. Some of the ultra cheap meters have a problem measuring AC superimposed on DC. I have several meters, of which one is such a dinky little meter. It works fine, but when I set the meter to AC and feed it with DC, it displays the DOUBLE DC value, instead of zero. So when you measure the voltage on the input caps of the regulators in AC setting, don't be surprised that some meters from the bargain basket display alarmingly high voltages when attempting to measure ripple voltage. It's the meter's fault.

I never bothered to find out why this meter behaves like this, but I noticed several other cheapies do this as well. I always keep this in mind when I am condemned to using it. My other meters don't have this issue. So before attempting this measurement, check if it displays 0 when set to AC and connected to a 9V battery.
  Reply With Quote
Old 1st December 2011, 06:23 PM   #1032
qusp is offline qusp  Australia
diyAudio Member
 
qusp's Avatar
 
Join Date: Oct 2009
Location: Brisbane, Australia
its because its not a true RMS meter, but rather uses averaging to display AC and the average of AC has a DC value. basically cheap meters are useless for AC power, or measuring AC riding on a DC offset unless the AC is a pure sinewave; even then i wouldnt trust it

you have to spend some pretty serious cash on a DMM to get good quality VAC RMS metering; even those that claim to do it are mostly pretty useless until you get away from the hobbiest models. thats why i have an old benchtop HP3400A voltmeter with a modified input circuit around the workshop, because even my fluke is a bit dinky with offset AC

Last edited by qusp; 1st December 2011 at 06:31 PM.
  Reply With Quote
Old 1st December 2011, 06:31 PM   #1033
diyAudio Member
 
RocketScientist's Avatar
 
Join Date: Mar 2008
Location: Northwest
Good points, thanks. I've never seen that but I could certainly imagine it happening. I just checked a seriously cheesy $5 "pen style" DMM that I got free from somewhere (with one probe built into the pen shaped body of the DMM) and even it reads close to zero on the primary side of the O2 when set to AC.

If anyone does measure a high AC voltage with a questionable meter they can desolder one side of D3 and D4 and use the resistance range to test them in both polarities for hopefully a low resistance in one direction and infinite in the other (some meters may read infinite in both directions).
__________________
http://nwavguy.com - Personal non-commercial audio blog
  Reply With Quote
Old 1st December 2011, 06:31 PM   #1034
timpert is offline timpert  Netherlands
diyAudio Member
 
timpert's Avatar
 
Join Date: Nov 2004
Location: Netherlands
I have another (better) meter which is also not a true RMS meter, but it doesn't have this problem. I think that the manufacturer of the cheap meter has left out the input capacitor on the AC ranges. Instead, it just rectifies the input voltage and measures the average resulting voltage and displays it with a correction factor. However, that doesn't explain the observed factor of exactly two...

Edit: mine was even less than € 5, and it is a handheld meter with "normal" test leads. Imagine how cheesy that one must be inside.

Last edited by timpert; 1st December 2011 at 06:33 PM.
  Reply With Quote
Old 1st December 2011, 06:37 PM   #1035
orpheus is offline orpheus  United States
diyAudio Member
 
Join Date: Jul 2004
Location: NYC
Quote:
Originally Posted by RocketScientist View Post
Unlike amps with virtual grounds, the O2 has all the grounds at the same "true ground" potential (assuming you're not using a bench DC supply to run it). So there shouldn't be any DC risk with ground loops--i.e. coupling caps may not help. And you can safely put up to +/- 7 volts of DC into the input of the O2 without harm and none of it will make it to the 4556 op amps or output. So DC on the input is not a problem.
@rocketscientist : Does this mean that I can use the output from a voltage output DAC chip without coupling capacitors? Could I use a source with 2-3 volts of DC without any consequences? I have a modded iPod with the output taken directly from the Wolfson chip to film capacitors to block DC. If the O2 is not effected by the DC at it's input, then I could just connect the DAC output directly to the O2. That would be very interesting for the whole DIYmod/imod crowd, since fitting great caps into the iPod case is a major issue, or for anyone using a voltage output DAC, since they usually don't have more than a few volts of DC.

If this is absurd, I apologize. I hadn't thought that putting DC into the input of an amplifier was reasonable, but since you wrote the statement above, I thought I would ask. Thank you for your consideration.

Aaron.
  Reply With Quote
Old 1st December 2011, 06:48 PM   #1036
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
Keep in mind that while DC of a few volts will not damage the O2, it can still drive the input amp into clipping since it's amplified with the same gain as the signal there.

The ODA will hopefully have an extra coupling capacitor (bipolar 'lytic should do) and input pull-down resistor. I guess space was too tight in the O2.

Last edited by sgrossklass; 1st December 2011 at 06:56 PM.
  Reply With Quote
Old 1st December 2011, 06:55 PM   #1037
diyAudio Member
 
RocketScientist's Avatar
 
Join Date: Mar 2008
Location: Northwest
@orpheus, I was talking about things that might damage the ICs in the O2. DC up to 7 volts won't hurt the O2 but if the DC is high enough it will force the first stage into clipping. At 2.5X gain, 2 volts of DC would shift the operating point of the first stage to +5 volts leaving only about 5 volts left for "swing". That would limit the max output to about 3.5 Vrms. 3 volts would severely limit the maximum undistorted output.

If you can live with 1X gain, you could easily get away with it on AC power. So it depends on what headphones you're trying to drive. There would likely be a potentially nasty turn on/off transient if you powered up/down the iPod with the O2 on and the headphones connected.

There's room to "hack" some high quality film input caps into the O2 if you want. Two more of same highly transparent 2.2 uF caps already used can probably sit behind the input jack and gain switch laying on their sides. You could desolder the "input" end of R3 and R7 and wire from those resistors and the pads they were just desoldered from. This would put the caps in series with the resistors and solve the issue with no external hardware. Just keep the wires really short (wire wrap wire works well). The caps could be secured with a dab of hot glue or similar.
__________________
http://nwavguy.com - Personal non-commercial audio blog
  Reply With Quote
Old 1st December 2011, 07:20 PM   #1038
jtktam is offline jtktam  Canada
diyAudio Member
 
Join Date: Apr 2009
Location: Toronto, Ontario, Canada
Blog Entries: 17
I apologize if I sound condescending in anyway, I am just saying is that sometimes some people don't' take time and care to build it, check clearance etc..

I agree that any help should be short and to the point.. leave the politics out of the solutions

-joe
  Reply With Quote
Old 1st December 2011, 07:23 PM   #1039
orpheus is offline orpheus  United States
diyAudio Member
 
Join Date: Jul 2004
Location: NYC
Quote:
Originally Posted by RocketScientist View Post
@orpheus, I was talking about things that might damage the ICs in the O2. DC up to 7 volts won't hurt the O2 but if the DC is high enough it will force the first stage into clipping. At 2.5X gain, 2 volts of DC would shift the operating point of the first stage to +5 volts leaving only about 5 volts left for "swing". That would limit the max output to about 3.5 Vrms. 3 volts would severely limit the maximum undistorted output.

If you can live with 1X gain, you could easily get away with it on AC power. So it depends on what headphones you're trying to drive. There would likely be a potentially nasty turn on/off transient if you powered up/down the iPod with the O2 on and the headphones connected.

There's room to "hack" some high quality film input caps into the O2 if you want. Two more of same highly transparent 2.2 uF caps already used can probably sit behind the input jack and gain switch laying on their sides. You could desolder the "input" end of R3 and R7 and wire from those resistors and the pads they were just desoldered from. This would put the caps in series with the resistors and solve the issue with no external hardware. Just keep the wires really short (wire wrap wire works well). The caps could be secured with a dab of hot glue or similar.

Thank you for taking the time to respond. I might try adding some input caps to my O2, or just keep the caps in the iPod as I have them now. I appreciate the consistent thoroughness of your responses. Thank you again for all of your work and for sharing this amplifier with all of us.

Aaron.
  Reply With Quote
Old 1st December 2011, 09:12 PM   #1040
qusp is offline qusp  Australia
diyAudio Member
 
qusp's Avatar
 
Join Date: Oct 2009
Location: Brisbane, Australia
Quote:
Originally Posted by timpert View Post
I have another (better) meter which is also not a true RMS meter, but it doesn't have this problem. I think that the manufacturer of the cheap meter has left out the input capacitor on the AC ranges. Instead, it just rectifies the input voltage and measures the average resulting voltage and displays it with a correction factor. However, that doesn't explain the observed factor of exactly two...

Edit: mine was even less than € 5, and it is a handheld meter with "normal" test leads. Imagine how cheesy that one must be inside.
as i said it depends on the type of AC, if using it on a music signal +DC, rather than plain regular sinewave based AC power, you will have this problem on anything but a true RMS meter. for example square wave causes big issues. if you cheapie works on that its a miracle

i'm not saying it will display double, since that is just straight up weird, but the AC or DC readings you get will not be accurate (as in...you know the mV it claims to show is meaningful). without one that is true RMS to check it against you dont have a point of reference.

i dunno if the above explanation makes sense, since i only just opened my eyes after 5hrs sleep and there is no caffeine coursing through my veins yet

Last edited by qusp; 1st December 2011 at 09:28 PM.
  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
Headphone amp project grahamfocal Headphone Systems 7 21st June 2011 05:28 PM
My latest project - DAC + headphone amp gmarsh Digital Line Level 105 13th December 2010 02:31 PM
My headphone amp project trifidmaster Headphone Systems 2 22nd December 2008 03:52 PM
Mini Ipod headphone amp DIY project Nordic Headphone Systems 14 11th March 2007 02:35 PM
Need help for a battery headphone amp project dismalonyx Headphone Systems 1 15th April 2004 11:22 PM


New To Site? Need Help?

All times are GMT. The time now is 12:31 PM.


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