The Objective2 (O2) Headphone Amp DIY Project
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 7th January 2014, 11:48 AM #3701 CyberKiller   diyAudio Member   Join Date: Jan 2014 Ok I have the amp playing music through my creative aurvana lives and it all sounds good. It seems make things sound better by reducing some of the 'muddyness' in the bass and making things clearer sounding (had my headphones plugged into the lineout of my X-FI before). I guess this is due to better output impedance. Just one last question with the amp at 1x gain it still amplifies the sound by about 20% with the pot turned all the way up, is this normal? Last edited by CyberKiller; 7th January 2014 at 12:00 PM.
 7th January 2014, 12:00 PM #3702 Mooly   diyAudio Moderator     Join Date: Sep 2007 Yes. The voltage gain of the first stage is approx the value of R16 plus whichever selectable resistor is in circuit and then divide that by the selected resistor. Lets say you have 1k selected. So you have (1500+1000)/1000 which is 2.5 Did you want less gain ?
 7th January 2014, 12:02 PM #3703 CyberKiller   diyAudio Member   Join Date: Jan 2014 No, I already have no gain resistors in at all, I was just wondering how it can still be amplifying the sound by about 20%. I will add one of the pairs of gain resistors when I upgrade my headphones in the future. Last edited by CyberKiller; 7th January 2014 at 12:13 PM.
 7th January 2014, 12:27 PM #3704 Mooly   diyAudio Moderator     Join Date: Sep 2007 If none of the resistors R17,19,21 and 23 are in circuit then you have gain of actually slightly under 1 If you have any resistors from pins 2 and 6 of U1 to ground then you have gain. Look at the circuit The output stage is unity gain (1) because there is 100% feedback wrapped around the opamps. The input stage has unity gain as long as there is no resistance from pins 2 and 6 to ground. The 1.5k feedback resistor doesn't have any effect in that situation, its still unity gain. The 100 to 274 ohm inout resistor and the 10k form an attenuator but the effect is negligible. So no gain set resistors and you have unity gain. Any resistance present and you have gain depending on the value of those resistors. Even if you have no test equipment other than a meter you can "prove" the gain by setting up a small DC voltage on the input (with a preset pot) and seeing how much that voltage gets amplified.
 7th January 2014, 01:08 PM #3705 Mooly   diyAudio Moderator     Join Date: Sep 2007 Try this and it will make sense, Op Amp Gain Calculator You want the second example, non inverting. Edit... how are you calculating or determining that the gain is "20%" etc. One effect you might be encountering is the "loading" of the O2 vs whatever you are comparing against. The O2 presents as a 10k load to the source. If your source impedance is "high" then the O2 may be loading it less than whatever else you are comparing against and thus seem as though it is "amplifying" when in fact it is not.
 7th January 2014, 01:31 PM #3706 Mooly   diyAudio Moderator     Join Date: Sep 2007 Read the edit to the above post
 7th January 2014, 03:34 PM #3707 CyberKiller   diyAudio Member   Join Date: Jan 2014 I am comparing it to: X-FI line out ---> Headphones (CALs) vs X-FI line out ---> O2 Amp ---> Headphones (CALs) They sound louder with the O2, my guess would be that the X-FI has a high output impedance and what you said in your post is correct.
 8th January 2014, 04:56 PM #3708 CobelAli diyAudio Member   Join Date: Jan 2014 I want to get better quality 3.5 jacks for my amp. I tried searching for objective2 and jacks on the site and the result pointed to this thread, all 370 pages of it. I recently built my first O2 and there was only one thing damaged that I had to fix. I'm really enjoying the amp, as long as it is stationary. As soon as I try to take it with me, I get annoying scratching noises when the input cable is moved. I tried 2 different 3.5-3.5 cables, but they all fit loosely. Is there a high quality direct-fit replacement for these jacks? Maybe something with higher spring tension and gold plated contacts?
 9th January 2014, 05:00 PM #3709 Willakan   diyAudio Member   Join Date: Aug 2011 Pretty sure that you don't get much leeway for the jacks used on the PCB: if you want a better one, your only option is to wire a panel-mount jack into the header adjacent to the output.
 15th January 2014, 09:47 PM #3710 chickwhite   diyAudio Member   Join Date: Nov 2004 Location: Volcancito, Boquete, Panama I just finished building an O2 for myself. This is the second one. I went through the tests and finally ran it with headphones. Every thing seemed fine. I had it plugged into my pc and was listening to iTunes when the volume more than doubled without touching anything. It appears to happen to both channels. It doesn't matter which setting the gain switch is in. The O2 turns on and works for 30 secs to several minutes and then suddenly more than doubles the volume. I'm going to try swapping the 2068 opamp between the two units to see if the problem follows the opamp. I have access to an oscope and a cheap generator. I'll try getting some measurements in case the problem does not follow the 2068. Any suggestions for other troubleshooting steps?

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