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| [#1] 問 DAC的 OpAmp gain 调校方案? 各位師兄， 小弟出于好奇試試的心態，從其中淘宝店， 買佐一對HDMA Dream opjl 1973 (代49720HA)和 OPJL 1541一對 (代49710HA) ，共4粒，每粒大概700元(人仔，共2800元人仔)。 測試结果: (先針對技术問題)想請教下： 1) 放系 一部 雙解碼的 ，用雙電源(十/一 13v)的DAC機 ： a)其中的一對1973(代49720HA)，出來的聲係失真的!有 問題! (但店家話出貨時，有試過無問題的)。 b)另外一對1541 (代49710)的，就Ok。。。(先暫不討論音質，效果)。 2)於是，再將其中那對在正負13v機有失真的1973, 試放系第二台用單電源(即20v，0v)的DAC機，發覺出來的聲相對正常，但效果是不如普通十零元的opamp! 3)於是再將這對 1973 ，放系相對低用電壓的sound blaster AE9 試(官網寫 AE9 用正負 10.8v)，就一切正常， 這對 1973，為什麽會係雙電源(十/一13v)DAC機出失真聲，和在單電源(20／0v)相對正常，但效果差，但只能在 ＋/一 10.8v的 sound blaster work 呢？ 個人估計，失真問題除了同電壓有關之外，可能會吾會同解碼機的 放大( gain )，circuit 設計有關連。。。 個人想請教有 關 雙解 DAC機的 第一级opamp 增益可以怎樣才能調底的，或有冇相關DAC 的線路圖可提供？ 備注： 1) 1541代 49710係work 的，(即配 49720HA 一對 和 1541一對 是無問題)的。 2)主要問题在 係 49720度 ，先針對正負13V DAC來講，試過用 1973，或Opa2111km 代 49720就失真, (而配用返 497120HA 和1541(或49710)就work. 不能用代用的(例如1973或opa2111或其他)，如果用其他opamp 代 49720，出來的聲就都會失真的！ 因此個人猜 此機( 正負13V的) DAC ，只是可能 在 49720放大 增益(gain )線路設計上，可能需要再改细小小，就可能匹配到 1973，2111或其他代opamp了。 因此想請教下有冇師兄可以提供DAC参考線路圖，或是 有其他方法調低49720级的gain的方法。 吾該曬 |
jamesckm 正式會員125.xxx.xxx.178 |
2022-01-18 18:31 | |
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| [#2] 問 DAC的 OpAmp gain 调校方案? Ref below, cHanging the feedback resistor of the I/v stage will help. but the opamp’s different gain bandwidth may cause parasitic oscillation distortion。  |
LaLaLam 會員 49.xxx.xxx.204 |
2022-01-18 20:07 |
| [#3] 問 DAC的 OpAmp gain 调校方案? Hi LaLaLam Ching, appreciate the ref. diagram and the advice... these really help ... By the way , as DAC Opamp often uses inversed mode as the signal input, do you think it would be better to ulitize the R1(i.e.measuring resistor) by increase the R1 value , while decreasing the R2 value at the same time , so to achieve even lesser gain for the I/V stage? thanks a lot again. 最後修改時間: 2022-01-18 22:58:41 |
jamesckm 正式會員 125.xxx.xxx.178 |
2022-01-18 22:56 |
| [#4] 問 DAC的 OpAmp gain 调校方案? Referring the conceptual schematic at #2, the 1st stage opamp after DAC is called transconductance or transimpedance amplifier with the assumption of the DAC in current output mode. The R1 is used for measuring DAC output current at design stages only. To increase value of R1 may overload and affect the linearity of DAC. In real application, the R1 is a short link. The basic principal of this I/V stage is that when DAC output is maximum say in 2.5mA, the OP1 will output a negative voltage with -2.5 mA across R2, so as to maintain null input current in OP1. This feedback path together with C1 formed a so called compensated feedback factor (ref below curve 1/beta). The meet point of GBW and Feedback factor will have a peaking or resonance effect causing unstable oscillating distortion in poor design. Similar unwant circuit behaviour will also happen by just changing the op amp without any circuit alignment. Therefore, it is not appropriate to change value of R1. You can change R2 to alter the current/voltage ratio, but you need to adjust the compensation via C1 as well. Another way is to adjust the peak current from DAC by change the DAC's reference voltage.  |
LaLaLam 會員 219.xxx.xxx.171 |
2022-01-19 07:51 |
| [#5] 問 DAC的 OpAmp gain 调校方案? Hi! LaLaLam Ching, really appreciate your technical information given below, it is definetly very helpful and professional, as well as simple & easy to understand ... as always... just curious , recap fr below: 1)what is " GBW "stand for? 2) " Another way is to adjust the peak current from DAC by change the DAC's ref voltage" Do you think I can seek your further advice about how this can be done ? e.g. which pin of DAC that I can try to change the ref volt ? many thanks again.  |
jamesckm 正式會員 125.xxx.xxx.178 |
2022-01-19 18:19 |
| [#6] 問 DAC的 OpAmp gain 调校方案? 1. GBW: The gain–bandwidth product (designated as GBWP, GBW, GBP, or GB) for an op. amplifier is the product of the amplifier's bandwidth and the gain at which the bandwidth is measured. 2. PCM1794 pin20 Iref is designed to control the maximum mirrored current to the DAC, factory claimed that the best value is 10K, ref. below link. However, some Diy'er found that the best value is 6K which means a lower gain effect. You can test with other values for best result. https://e2e.ti.com/support/audio-group/audio/f/audio-forum/469717/what-s-the-best-way-to-adjust-volume-using-pcm1794a |
LaLaLam 會員 219.xxx.xxx.171 |
2022-01-19 21:05 |
| [#7] 問 DAC的 OpAmp gain 调校方案? 有個建議，opamp用正負15V,多數會好咗。 |
peter827 正式會員 182.xxx.xxx.54 |
2022-01-20 12:17 |
| [#8] 問 DAC的 OpAmp gain 调校方案? what cause op amp osc and how to fix it tinyurl.com/mwbw9wst ; 估 有機會係 叫機 "". try these things, in this order: Add a small resistor to the op-amp's output, either inside or outside the feedback loop. ... Do the same as in the previous step, except use a ferrite bead or chip ferrite instead of the resistor. ... Raise the amp's gain a bit.."" |
cornercube 正式會員 14.xxx.xxx.81 |
2022-01-20 13:23 |
| [#9] 問 DAC的 OpAmp gain 调校方案? Re #6 LaLaLam Ching , greatly appreciated the concise and useful technical information given again... espically the way by changing the DAC's ref volt at pin 20 only, which this seemed comparably quite simpler than before... many thanks. |
jamesckm 正式會員 125.xxx.xxx.178 |
2022-01-20 15:28 |
| [#10] 問 DAC的 OpAmp gain 调校方案? Re #7 , peter 師兄，多謝建議。 之前没想過去調高電壓，主要因為 將失真1973 opamp 放咕咗係 更低電壓(只有正負10.8v)的soundblaster AE9 ，就 正常返。。。! 但都會再安排，抽返1973出來, 試下放係電壓調到+/一 15v的DAC，睇下效果的。 不過 係 正負13v，開耐咗 那對 49720HA(有放heatsink)都己經好盪手的，估計升到15V 會更盪手! 無論點，試下無壞。。。 thanks again. |
jamesckm 正式會員 125.xxx.xxx.178 |
2022-01-20 15:33 |
| [#11] 問 DAC的 OpAmp gain 调校方案? Re #8 多謝 cornercube 師兄的資信和建議的。。。 "add small resister to the op-amp output...", 安排試時，是否建議可先從100R 電阻 開始呢？ thanks again... 最後修改時間: 2022-01-20 15:41:25 |
jamesckm 正式會員 125.xxx.xxx.178 |
2022-01-20 15:36 |
| [#12] 問 DAC的 OpAmp gain 调校方案? 前級 輸 入/出端 串電阻 阻值 由100Ω 到1000Ω 都見過. |
cornercube 正式會員 14.xxx.xxx.81 |
2022-01-20 15:57 |
| [#13] 問 DAC的 OpAmp gain 调校方案? ＞＞#10..正負13v...開耐咗 那對 49720HA(有放heatsink)都..好盪手 ------------------------------------------ 9x9 係 叫機 |
cornercube 正式會員 14.xxx.xxx.81 |
2022-01-20 17:01 |
| [#14] 問 DAC的 OpAmp gain 调校方案? 如果試咗opamp係正常，出問題就係線路問題，如果線路已係大路跟data sheet嘅話，基本好難再搞，用番work嘅opamp好過，有時間有技術有儀器當然可以玩一大輪搵問題。 我都有用49720HA好多年，已經係我用過最靚opamp,影響聲音嘅其他每個環節更重要。 最後修改時間: 2022-01-20 19:12:57 |
peter827 正式會員 182.xxx.xxx.54 |
2022-01-20 19:10 |
| [#15] 問 DAC的 OpAmp gain 调校方案? Re #12／＃13 好多謝 cornercube 師兄 的建議方案的，會安排試做下實驗的。 備注：開耐咗，那對 "49720HA”都會好盪手，係原機的正常無失真的。 many thanks. |
jamesckm 正式會員 125.xxx.xxx.178 |
2022-01-21 19:32 |
| [#16] 問 DAC的 OpAmp gain 调校方案? ＃14 很多謝Peter 師兄寶貴的經驗分享。。。 能夠在這平台，可以跟多位師兄學到寶貴發燒技術野，添人生一樂也。。。 最後修改時間: 2022-01-21 19:36:26 |
jamesckm 正式會員 125.xxx.xxx.178 |
2022-01-21 19:35 |
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