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About jwzhan

  • Birthday 03/02/1993

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  • Biography
  • Occupation
    DFA Chiller Salesperson
  • Headphones
    DT880, K702, W1000X, ESP950, SR-009
  • Headphone Amps
    iBasso D4, CKKIII, KGSSHV
  • Sources
    Parasound D/AC 1600HD, Gamma 2, IPC

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  1. jwzhan


    http://www.tempoelectric.com/azl_sale.htm this guy has some interesting things (mostly audio related) for sale at fairly low price... definitely worth checking out.
  2. well... I'll get back to you on the actual value in a bit when I get home.. Yes, I did measure with the input shorted to ground.
  3. Measuring across the 2*20ohm resistors, at stable temp, they are all around 900mV. This is the lowest value I can get without changing 500ohm to a higher value.
  4. I tried to get it as low as possible but it drifts quite a bit... by quite a bit.. I mean 50~100mV at stable temp... Balance between the channels would drift by about 10mV as well... so achieving 1~2mV without opamp is very difficult on the balanced version to say the least.. Yeah 4 pots. 2 off voltage rails controlling dc offset and bias voltage somewhat..For every turn, the bias voltage would change by about 2mV or so.... and at 0.9V, I'm already very close to the max resistance of the pot.. 2 after JFETs. These controll the balance between the channels and the distribution of the over all bias voltage... both of my amp boards have around 1.8V, so 0.9V/0.9V is about the best I can do... I'm using the default 500ohm with the pots. 0.9V seems to be fine for me... the off board heatsinks are capable of handling the heat.. runs about 45C at stable temp. irrc, using a DMM, the voltage ramps up to something like 1.2V at shut down for both boards.. Also.. I've been using single ended headphones on the amp... haven't tested with balanced headphones yet..
  5. I just powered up my dynahi and used cheap apple buds for testing... The result is... weird... There is a somewhat loud weird inconsistent crackling/humming noise for a brief 1 or 2 seconds immediatly following turn on when the earbud is plugged in.. and there is a sharp pitched noise about 1 second after turning off (not immediately, but occurs after 1 sec. as DC offset ramps up assumably.) There is also a consistent light humming noise in the background which is likely due to how the amp is setup and wired at the moment. Well... I dared to plug my DT880 250ohm (which is probably not a smart move) in there given the above result since the apple earbuds didn't blow up on me, and did play music when the amp was on. The initial crackling/humming noise is still there with similar volume. But the turn-off pitch is not there or lessened by a significant amount. I have adjusted the amps so that DC offsets are under 100mV without the OPAMP in warm state. With OPAMP, the DC Offset is ±10mV. The voltages across the 2*20ohm resistors in all channels are 0.9V, which doesn't really matter.... I know that it takes time for the OPAMP (445) to start working, could it be the cause? It occurs in both amp boards. I'll have more detail tonight if I have more time to work on the amp.
  6. Is the DIY Audio person not selling it anymore? I recall he was selling the GR grade 10 for $15... Or is GR grade hfe not high enough? (didn't read the entire thread.)
  7. jwzhan

    AKG K812

    I'm sure there is a Chinese buyer out there who will beg you for it if you are good at BSing. The Chinese HiFi community is still raving about rudi and ultrasone after all, not to mention Lehmann, Graham Slee, hedgehog SAC KK and the sorts. Many even still think D7100 is a worthy successor of D7K... All thanks to some douchebag bloggers...
  8. that's no joke at all.... I've seen some chinese meet where people pull lines straight out of the transformer box to the equipments... they claim it produce the cleanest sound...
  9. I agree that the filtered outlet is mostly maketing, but at least it has some effect.... unlike the furutech rare metal plated stuff... I believe a well designed PSU should be able to get rid of a reasonable amount of noise from the AC line, but the sometimes.... a reasonbale amount is not good enough... I have experienced it first hand when I was living in the college dorm where a whole bunch of microwave, air conditioner and refrigerators are plugged into the same AC line and there was a shit ton of noise... I remember being able to hear them through ESP950 and CKKIII... a $50 APC surge suppressor that did -45dB on EMI/RFI help a lot but still didn't get rid of the noise entirely... I doubt that any household AC line will get that bad.. but filtering noise from AC line never hurts... and as I stated.. the $400 price tag is on the high side. I'll never buy it at that price, but I might buy it if it were $100, which will never happen...
  10. These might make a difference. I remember looking at them awhile back and saw someone testing these using a AC line noise sniffer thingy. They did find that the AC line noise is reduced through these. I've always wanted to try the filtered AC outlet.. but the price is just way too high to justify any possible improvement... EDIT: here is the video...
  11. the new hifiman electrostatic prototype
  12. finally got off my butt and fixed the PSU. It's making music now, and I hope it will continue do so... I'm now using the trafo Palchiu gave me... loaded, the voltage drops to 460VAC, so I guess it's fine. That trafo weighes about 0.3lb heavier than the SumR one while with the same output current at 150mA. eggil, that sucks... I'm sure RIchard will replace yours. He did replace mine, but took around 1 month. I still have the SumR raplcement on hand, if you need it, I can send it to you now while you wait for your replacement. Just PM me. That's at least the 4th one that KGSSHV has killed... maybe 150mA is not enough...
  13. yup... either that or they have really bad eyes and as long as they send me the wrong set that they are looking at which doesn't have problems, I might still be fine... That thing came up a few times during some conversations... don't exactly know what it is but the description claims "its design is as good as 717/727 and uses better material, so it should beat them with ease..." which is the description every taobao crap amp uses... I can't find any internal photo which is not surprising given that the person hasn't sold any...
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