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DIY Workshop » β22: A discrete, cascoded, fully complementary, pure class A amplifier (part 3) amb  Headphone Council Joined: Apr. 1, 2004 |  Message [#80] posted on: 09-23-2008 01:46 AM CST (US).   |  |  lcp, did you check to see if there are other voltage anomalies against the troubleshooting pdf file? When you "swapped" Q1, Q3 and Q5, did you replace them with new parts? Is CR1 installed in the correct orientation? Is there 12V across D1? What are the voltages across R9 through R12? |
| lcp Member Joined: Nov. 12, 2006 | Message [#81] posted on: 09-23-2008 03:22 AM CST (US). | | Voltage at R9/10=0.66V R11/12=0V D1=0.66V D2=0V Q3/5 is new while Q1 is supposed to be new as it is used for swapping and checking the DC offset before. The orientation of component is Ok as it is working before. i will try to figure out the voltage at test point according to pdf file. |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#82] posted on: 09-23-2008 03:58 AM CST (US). | | Those voltages are all way off. Check the voltage at the outer pins of R9 and R11 relative to ground. If you don't have the rail voltage minus about 1V or so, then either Q25 or Q26 is blown. |
| lcp Member Joined: Nov. 12, 2006 | Message [#83] posted on: 09-24-2008 01:22 AM CST (US). | | It is worked again after replacing Q25/26. Many thanks AMB. |
vixr HeadWize Fanatic Joined: Sep. 17, 2005 | Message [#84] posted on: 10-01-2008 10:03 PM CST (US). | | I had my b22 powered up last night, listening to my brand new itouch (coolest gadget ever) and I just reached around to all the heatsinks on the boards and noticed that certain sinks are alot hotter than others...this was after about an hour. All the settings are perfect and the amp sounds perfect, but some of those sinks are really hot and some aren't. When I say really hot I mean I can keep my finger on them, but just barely. Anybody else have this too? |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#85] posted on: 10-01-2008 11:21 PM CST (US). | | The outer heatsinks will be hotter than the inner ones because in this design, the cascode MOSFETs dissipate a bit more power than the amplification MOSFETs. |
| jazzist Member Joined: May 9, 2008 | Message [#86] posted on: 10-02-2008 04:07 PM CST (US). | | Dear Those in UK, You may find these links useful kustompcs.co.uk stocks techflex, feet, vandal switches (bulgin and lamptron) - loads of types of these, etc. http://www.kusto.../info_2383.html http://www.kusto.../info_2384.html http://www.kusto...itches_etc.html Rapid Electronics rapidonline.com is very cheap (cheaper than RS, Farnell, etc.) and stocks neutrik XLRs, EI30 transformers, LEDs (very cheap on normally expensive colours like I chose, 1/4 price of mouser). Also useful for soldering equipment as they have reasonable prices on xytronic, so need to import from USA. Shipping free over a certain amount (40-50£ IIRC). They also have the bulgin vandal proof switches, good selection of IEC sockets for your s22, etc. Probably more I can think of and I can post if anyone is interested. |
| lcp Member Joined: Nov. 12, 2006 | Message [#87] posted on: 10-08-2008 10:46 PM CST (US). | | Hi Amb, I have problem from another B22. One of the channel DC offset cannot be adjusted below 40mV even I have used 5.1kohm as R13. The voltage can meet the initial setup doc. In addition, I have heard some high freq sound from both channel when no audio signal is fed. |
| lcp Member Joined: Nov. 12, 2006 | Message [#88] posted on: 10-09-2008 12:47 AM CST (US). | | Hi Amb, I have problem from another B22. One of the channel DC offset cannot be adjusted below 40mV even I have used 5.1kohm as R13. The voltage can meet the initial setup doc. In addition, I have heard some high freq sound from both channel when no audio signal is fed |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#89] posted on: 10-09-2008 04:11 PM CST (US). | | lcp, power down and measure the resistance of those 0.47 ohm resistors. Your DMM probably won't be accurate at those low resistances, but if you short the two probes together you'll get a baseline "zero" ohms reading, and then add the actual resistor reading to get an idea whether the resistor is shorted (or open circuit). Fix the output resistor problem before trying to adjust the DC offset again. As for high frequency sound, did you short the inputs while testing? If not, it's most likely picking up interference. |
| lcp Member Joined: Nov. 12, 2006 | Message [#90] posted on: 10-10-2008 09:03 PM CST (US). | | Hello Amb, I have replaced the 0.47ohm but got no improvement from DC offset. I think the grounding had some problem as the offset was varied with volume changing. I will try to fix it. However, I found that the MOSFET from one of the board was "cold" at all after power up. Anything wrong? |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#91] posted on: 10-10-2008 10:15 PM CST (US). | |
Have you adjusted the bias (VR2), according to the initial setup procedure at the β22 website? If so, did you get the proper voltage drops across R34 and R35? |
| Mimesis Member Joined: Oct. 16, 2008 | Message [#92] posted on: 10-16-2008 05:27 AM CST (US). | | I'm sorry, since this has probably been asked or covered before. (I read over this thread but not the first two.) Is there a gain setting that will accommodate both a 25 OHM headphone and a 300 OHM headphone with minimal hum? Or would a gain changing circuit need to be introduced. (I'm thinking to use a denon headphone (25ohm) and also a Senn HD 650.) Thanks in advance. |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#93] posted on: 10-16-2008 07:05 AM CST (US). | | Mimesis, it depends on what source you'd be using (i.e., how strong is its line level output). I say that a gain of 5 is probably a good compromise in your situation. The gain of this amp cannot be easily switched because not only must you switch the two feedback resistors, but also the value of four compensation caps. This is per channel. You can see that this would be a wiring nightmare and will most likely lead to amplifier instability. An input attenuator is a viable option, however, and I had posted a diagram on how to do this before, so look for it. |
| kschong Member Joined: Oct. 16, 2008 | Message [#94] posted on: 10-16-2008 01:50 PM CST (US). | | Hi amb, The b22 amp is silent around most of the pot but there's noise when I turn it to the lowest end of the pot. The noise is emitted only on one side. Also at times when the pot is turned too low, there's a loud high pitch noise on both sides. This goes away after warming up 20mins or so. What could be done to solve these problems? |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#95] posted on: 10-16-2008 02:36 PM CST (US). | | kschongm can you describe your build configuration? Also, describe your input wiring. Better yet, post some clear pictures? |
| kschong Member Joined: Oct. 16, 2008 | Message [#96] posted on: 10-16-2008 10:34 PM CST (US). | | It's a 2 board configuration currently because the heatsinks for my ground channel is still on the way to me. Sender forgot to pack them in. I don't have a camera with me right now so the best I can do is the diagram. See here: I checked my wiring again and now the noise is even on both sides when on low levels. At listening levels though the background is silent so it doesn't really matter. Also, the high pitched noise is gone for some reason |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#97] posted on: 10-16-2008 10:43 PM CST (US). | | kschong, the diagram looks fine, although you did not show whether the input ground is connected to the case (or is the amp cased yet?). Also, how did you wire the outputs? And the connections to the power supply? What headphones are you using to test with? |
| kschong Member Joined: Oct. 16, 2008 | Message [#98] posted on: 10-16-2008 11:01 PM CST (US). | | Nope the amp is not cased yet. I'm still figuring out the casework. The outputs are connected to the L/R boards and the output ground is connected to the ground of the PSU. I'm using a pair of dt880 @ 250ohms I'm thinking the noise has to do with the pot. |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#99] posted on: 10-16-2008 11:05 PM CST (US). | | I wouldn't worry about any of this until you case up your amp. When all the grounds are properly wired you may no longer have a problem. |
| kschong Member Joined: Oct. 16, 2008 | Message [#100] posted on: 10-16-2008 11:12 PM CST (US). | | Alright, I'd get the casework quickly then. Thanks! And also, the b22 amp is great! Hope to see more of such DIY projects. |
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