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DIY Workshop » M³ headphone amplifier (part 2) amb  Headphone Council Joined: Apr. 1, 2004 |  Message [#260] posted on: 02-26-2009 04:03 PM CST (US).   |  |  OK. What are you using as the source (your player)? Is it AC powered? Do you still get the electric shock (either touching the amp or the player) if it's disconnected from the amp? |
| habba Member Joined: Feb. 14, 2009 | Message [#261] posted on: 02-27-2009 01:26 AM CST (US). | | I'm using my ipod touch. It does happens even if the source is disconnected from the amp. |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#262] posted on: 02-27-2009 01:32 AM CST (US). | | Can you describe your external AC-DC adapter? Does it have a 3-prong AC plug? Is it totally isolated? (i.e., the AC plug's ground pin should not be internally connected to either of its + and - DC outputs). I wonder if some AC is being "leaked" into your amp's signal ground through the AC-DC adapter. Do you have another power supply to test with? |
| habba Member Joined: Feb. 14, 2009 | Message [#263] posted on: 02-27-2009 02:02 AM CST (US). | | I'll look for another power supply to test with. thx. |
| jackhsu Member Joined: Dec. 9, 2008 | Message [#264] posted on: 03-07-2009 10:35 AM CST (US). | | I just finished building the M3 and now going through the initial setup. However, no matter how I turn the trimpot, there seems no BIAS current going through the MOSFETs. They also stay cold after one hour. I then tried connecting my iPod and a headphone, and the music came out with a lot of cracking sound. What shall I do to make it work properly? |
| jackhsu Member Joined: Dec. 9, 2008 | Message [#265] posted on: 03-07-2009 10:11 PM CST (US). | | Apart from the previous problem, I also found the DC offset at OR is 4.7mV. It's below the 5mV suggested but a lot higher than that of OL and OG. Will it be a problem? And how may I bring down the DC offset at OR to be inline with OG and OL? Thank you very much. |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#266] posted on: 03-08-2009 05:12 AM CST (US). | | jackhsu, having no bias through the output stage would definitely cause it to sound bad. Also, the DC offset measurement is not very meaningful until the output stage has bias. Is the bias problem only on one channel or all channels? Did you verify that you have the correct parts installed everywhere, in their proper orientations? Were you able to adjust the BIASOP trimpot to schieve a reasonable voltage drop across all the R5s? |
| jackhsu Member Joined: Dec. 9, 2008 | Message [#267] posted on: 03-08-2009 06:19 AM CST (US). | | AMB, thank you for your reply. The no-BIAS problem exists on all channels. The three BISAOP trimpots work properly and they are all set to 4.5mA BIAS. I have checked all the parts and they are installed properly, but I will check it again anyway. I have also checked the three BIAS trimpots and they all work fine. The resistance varies with each turn. |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#268] posted on: 03-08-2009 03:02 PM CST (US). | | jackhsu, did you by any chance install the wrong value trimpots for BIAS? They should be 5K ohms. If you had swapped the BIAS and BIASOP trimpots, then you would have 1K ohm trimpots in the BIAS locations which won't have enough adjustment range. [Edited by amb on 03-08-2009 at 03:03 PM.] |
| jackhsu Member Joined: Dec. 9, 2008 | Message [#269] posted on: 03-09-2009 03:53 AM CST (US). | | AMB, the BIAS trimpots are correct, 5K. Is there any check poit I can take measure at, so I can identify the problem easier? |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#270] posted on: 03-09-2009 04:01 AM CST (US). | |
With the amp power off and all capacitors discharged, make actual resistance measurements across the outside pins of the trimpots while they are set to fully-clockwise position. What do you get? Make sure to set them back to fully-counterclowise position before applying power again. Check for cold solder joints or wrong parts values for R6, R7, R8+, R8-, R9+ and R9- on all channels. Also make sure you did not mix up the IRFZ24N (Q1+) and IRF9Z34N (Q1-) MOSFETs. Read the labels carefully. |
| jackhsu Member Joined: Dec. 9, 2008 | Message [#271] posted on: 03-10-2009 10:24 AM CST (US). | | AMB, The measures of the trimpots are 5.06K, 5.22K, and 5.26K. I am sure all the parts are in the right position and orientation. Shall I desolder Q1s and Q2s and have them tested? |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#272] posted on: 03-10-2009 12:43 PM CST (US). | |
Starting with the BIAS trimpots in the fully-counterclockwise position, power up the amp. Measure the voltage across C2 as you slowly turn the trimpot clockwise. You should see the voltage increase. Don't go over 7V while you do this exercise to avoid over-biasing the MOSFETs. If the voltage across C2 does not increase, then your Q2s might be bad (although it's hard to imagine that all-three of them would be bad). Did you happen to have installed the Fairchild 2N3904C transistors which have the wrong pin-out? If the C2 voltage does increase as you turn the trimpot, but the voltage across R9+ and R9- does not increase, then power off amd measure the R9s' resistance to make sure they are not blown open. |
| jackhsu Member Joined: Dec. 9, 2008 | Message [#273] posted on: 03-11-2009 10:23 PM CST (US). | | AMB, after testing the C2 and cunsulting the transistor supplier, the Q2 I've got is confirmed to be 2N3904CTA. I will buy new 2N3904 and hope it will work fine then. Thank you so much. You've been very helpful. |
| a1m80t Member Joined: Feb. 26, 2009 | Message [#274] posted on: 03-12-2009 05:21 PM CST (US). | | I got my M3 rocking on top of a cardboard box, but I do not know how to properly wire a single switch with a separate power supply. I know you should “put the switch inline with the power input jack in the amp.” I thought you were supposed to put a single switch on the input of the power supply but that doesn't sound like what the quote is saying. Please be specific on how I should wire this. Thanks so much [Edited by a1m80t on 03-12-2009 at 05:24 PM.] |
Mister X HeadWize Fanatic Joined: Jul. 23, 2003 | Message [#275] posted on: 03-12-2009 05:59 PM CST (US). | | http://www.amb.o...ma11/wiring.png I hope you have good fire insurance because cardboard and mains voltage do not mix well. |
| a1m80t Member Joined: Feb. 26, 2009 | Message [#276] posted on: 03-12-2009 06:14 PM CST (US). | | I'm running a freebie DC wall wart w/ in an enclosed treads so my AC handling is minimal. Only the m3 is set on a asus mobo box. I plan to build an o11 really soon. Anyway, thanks for the valuable information :-) |
| ShinyFalcon Member Joined: Mar. 5, 2009 | Message [#277] posted on: 03-13-2009 01:12 PM CST (US). | | I put mines inside a priority mail box. :p There's something that I'm concerned about. When I biased my M³ my ground channel offset was around 2V, and I initially thought that the difference with L/R's quiescent current was too high (±10mA), so I attempted to stabilize them near 80mA as best as I could. After tinkering around I checked the DC offsets for the three and they all were zero (except a channel which has 0.3mV). Three days later while I was listening I noticed that the bass disappeared along a span of ten seconds and I knew that something was wrong with the ground channel. Removing the headphone (and stupidly trying to insert it back in, tip of headphone touching a 1/4" adapter) resulted in rather loud pops. I don't know if 2.7V is damaging to headphones, but nothing bad happened to my headphones. I can't really check and see if the diaphragm is dislocated. What exactly happened here? Would a loose opamp cause this problem? |
| a1m80t Member Joined: Feb. 26, 2009 | Message [#278] posted on: 03-13-2009 04:04 PM CST (US). | | If I’m not mistaken, it’s normal to have small pops while plugging in the headphones while the amp is live. Setting the volume to min when plugging in/out headphones reduces the pops but ideally the amp should be off first. |
| a1m80t Member Joined: Feb. 26, 2009 | Message [#279] posted on: 03-13-2009 06:43 PM CST (US). | | *ignore* i asked a question and found the answer in this forum [Edited by a1m80t on 03-13-2009 at 09:08 PM.] |
| jackhsu Member Joined: Dec. 9, 2008 | Message [#280] posted on: 03-18-2009 09:21 AM CST (US). | | AMB, Finally, after installing the new and correct 2N3904, I got my BIAS working fine. Thank you so much for your help. One little thing bothering me is the DC offset between IG and OR, which is 4mV, much higher than that of the other two channels. Is this a problem? Any way I can do to bring down the DC offset inline with the other two channels? |
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