先要科普一文才能真正理解什么是Buffer Preamp 主动、被动?谈前级扩大机的设计形式Side A I suppose if I had to floor the accelerator to drive 55 mph, maybe I'd think the life was being sucked out of my driving. Then again, maybe I like 55. Nice and safe, good gas mileage... Is impedance matching an issue? Passive volume controls do have to make a trade-off between input impedance and output impedance. If the input impedance is high, making the input to the volume control easy for the source to drive, then the output impedance is also high, possibly creating difficulty with the input impedance of the power amplifier. And vice versa: If your amplifier prefers low source impedance, then your signal source might have to look at low impedance in the volume control. This suggests the possibility of using a high quality buffer in conjunction with a volume control. A buffer is still an active circuit using tubes or transistors, but it has no voltage gain - it only interposes itself to make a low impedance into a high impedance, or vice versa. If you put a buffer in front of a volume control, the control's low impedance looks like high impedance. If you put a buffer after a volume control, it makes the output impedance much lower. You can put buffers before and after a volume control if you want. The thing here is to try to make a buffer that is very neutral. Given the simple task, it's pretty easy to construct simple buffers with very low distortion and noise and very wide bandwidth, all without negative feedback. There are lots of different possibilities for buffers, but we are going to pick my favorite: Side B Figure 1 above shows the full schematic of the B1 buffered passive preamp. You can see a full-sized PDF by clicking here. There are two channels shown with a common power supply. Supply parts in common are numbered from 1 to 99. Parts in the right channel are 100 to 199, and the left channel is 200 to 299. With the exception of R1, all the resistors are 0.25 watt ? I used RN55D types, but you can use whatever you like. C1 and C2 are big electrolytic types, with a nominal rating of 15000 uF at 25 Volts. C100, 200, 101 and 201 are high quality film capacitors. You can use these values or substitute in your favorites. For C1 and C2 I used Digikey P6890-ND. The value is not critical, and you can use as low as 1,000 uF at 25V. C3, C100 AND C200 are 1 uFmetallized polypropylene film capacitors (Digikey BC2076-ND) C101 and C201 are 10 uF polyester film capacitors (I used Axon 10 uFmetallized polypropylene from Orca Design). Feel free to use any comparable types. D1 is a generic 1N914 type diode, and D2 is a generic LED for indicating power the board. All of the transistors are N channel JFETs. The stock parts are 2SK170's, LSK170's or 2SK370's, and you can use substitutes having Idss between than 5 and 10 milli-Amps and transconductance numbers from 5 to 30 milli-Siemens. The potentiometers are linear taper at 25 Kohm, but again you can easily use higher or lower values as you like. The buffer uses an external power supply from 18 to 24 Volts DC. You can power it with batteries, but most convenient is an external regulated supply running off the wall. The preamp typically draws fewer that 0.02 Amps, so current is not much of an issue. A regulated supply is better, but the circuit is pretty good at ignoring noise on the supply and minor fluctuations. The design uses RCA input and output connectors, and a DPDT switch for selecting one of two inputs. You are, of course, free to use a switch with many more inputs. For a fact this circuit can be easily constructed with perf-board and point-to-point wiring. I know, however, that many people won't start a project like this without a circuit board. The Gerber artwork is posted at www. and I have arranged to have finished PC boards available at cost around the time you read this. Side C R102 and R202 are there to prevent parasitic oscillation with the very wide bandwidth JFETs. C100 and C200 are there because the Gate of the JFETs needs to be set at half the DC voltage of the power supply - a voltage delivered to the buffer inputs by R2, R3, and C2 through R103 or R203. D1 performs the service of drawing down this DC voltage with the power supply when the power is turned off, otherwise C2 may hold a charge for a long enough time to give you a turn-on thump when powered back on. By the way, the time constant of R2, R3, and C2 are long enough that it takes a minute or two for the circuit to reach normal operating values, so don't get excited if there's no sound for a few seconds when you turn it on. R1 and C1 filter noise coming from the external supply. Q100 and Q200 are JFETs operated as follower transistors. The Source pins of these transistors follow the voltage at the Gates. The input impedance of the Gate is exceedingly high - many millions of ohms, and the output impedance at the Source pin is about 50 ohms. Q101 and Q201 are constant current sources formed by simply attaching the Gate pins of the JFETs to the Drain pins. They provide without loading them down or creating significant distortion. The best performance generally comes from matching the Idss of Q100 and Q101, and also Q200 and Q201. The Idss is simply the current that flows through the JFET when the Gate and Source are grounded and +10 volts or so is applied to the Drain. Often when you buy JFETs you can get them in Idss grades. I use GR or BL grades for this project. The Source DC voltage of the JFETs Q100 and Q200 is about the same as the Gate DC voltage (1/2 the supply voltage), and the output from the Source needs to have the DC voltage removed by C101 or C201, leaving the AC output signal, which passes through another safety resistor R104 or R204. Resistors R100, R200, R105, R205 are there to prevent the various potential thumps from switching inputs and turn-on transients. Side D Side E There's just enough gain. If you were using any other power amp, you'd get 8 to 18 dB more gain, and would be able to break your lease or the speaker, or both. Do I feel like the pedal's to the metal and I'm only doing 55? Side F NOTE: You can see Dick Olsher's review of this unit by clicking here. Manufacturer Price: $1000 assembled, kit version price not quite set at this time. 聆听美国First Watt B1前级放大器 搭配SIT 2立体声后级放大器的绝妙组合
用心设计和精良制作
结语 新年新希望:First Watt B1 Buffer Preamp 2008 年夏天,Nelson Pass 在他旗下的 這個線路非常簡單,一言以蔽就是以一級 JFET 做源極隨耦器 (source follower),並以另一顆 JFET 做恆流源(constant current source),前後各一顆交連電容,如此而已。 這個電路不但簡單,其實還很基本,之前塞爾維亞的 DIY 名人 Rogic Pedja 就曾在網站上提出類似的架構。也因此 Pedja 不論如何,Nelson Pass 總之是寫了一篇清楚明瞭的好文章,詳細地告訴你 B1 的工作原理、測試數據、零件的選擇、製作要點。照著 Pass 的解說來做,從選料、搭棚、裝機,都會非常愉快順利。 奇怪的是,曾有一段時間,國內的 DIYer 很熱衷 clone Pass 的機器。這次在幾個主要的國內 DIY 網站上,卻都沒有看到相關的討論。連用 Google 去搜尋 "Pass B1 Buffer” 都只有一篇繁體中文的結果,還是香港的。不知是國內的 DIY 資訊落後,還是 DIY 風氣冷清。 自從 Gainclone 和 TDA1541A DAC 漸漸穩定,又購入了 CEC TL51X 轉盤,我的興趣漸漸移到了前級上。本來已經有一件 Audio Note Japan M7 Line Stage 的半成品,但一直找不到適當的機殼,所以遲遲沒有完成。這次卻被 Pass 的 B1 Buffer 所吸引,因為他強調現在的前級,其實不太需要多餘的增益,用 single end JFET 作簡單的電流放大即可得到自然中性的聲音。Pass 對於 DIY 有很多貢獻,他推崇的極簡線路,single end 放大,也都與我的偏好不謀而合。但之前一直沒有興趣製作他的線路,因為他的後級總是非常難搞,即使號稱小功率、極簡,也需要笨重的變壓器和龐大的散熱片,累煞人也。相比之下,前級簡單多了,線路極簡,製作起來就真的不會太麻煩。所以今年看到這個線路,就立刻開始準備製作這台前級。 可能太久沒有製作新機器,我花了很多時間才把心情沉澱,並把相關的零件和機箱準備好。結果拖到了年底,這才一切就緒,於是趁著新年假期把第一版試作機裝了起來。 製作筆記: 1. B1 Buffer 每聲道要兩顆 2SK170-BL,而且要配對,Pass 更強調這是好聲的關鍵所在。我手邊剛好有將近 20 顆 2SK170-BL,用洞洞板搭了一個簡單的線路,2SK170 的 source 和 gate 接地,以 9V 電池,透過 1k 電阻送給 drain。最後選了四顆 Idss 在 9.01 - 9.03 mA 之間的晶體。 2. Pass 本來採用外部的 switching power 作為 18V 的輸入。這樣當然非常簡單省事,而且 Pass 說這個線路對電源並不敏感,所以用 switching power 就很不錯了。但我一直覺得好的 switching power 很難找,如果隨便用上一個不熟悉的電源,將來在試聽和調音時會增加不確的因素。所以我還是搭了一個簡單的 18V 線性供電。用 Schottky Diode 11DQ06 整流,LT317A 作簡單的穩壓。電容有 Panasonic FM、Elna Silmic、BCC 013、UCC SPP 等,是從庫存裡湊出來的。 3. 主放大電路,Pass 用了兩顆 15000uf 電容供電,我手邊沒有那麼大的電容,也不喜歡那麼大的電容,加上 Pass 說幾千到幾萬 uf 都可以,所以我改用 Panasonic FM 2200uf 代替。 4. 每聲道有兩顆交連電容,輸入交連是 1uf,輸出交連是 10uf。Pass 建議用 metalized film 電容,但我不喜歡 metalized film 電容的聲音。然而,我手邊雖有 1uf 的 MultiCap RTX,非常好聽,但體積實在太大,裝在這個小小電路板上未免費事。現在既然只是試做機,我一律用 Black Gate N 代替,這已經是非常好的交連電容了,我覺得 Black Gate N 甚至勝過許多 film 電容,體積又小。 5. 本來電路上有訊源切換的設計,但我的訊源只有 DAC,所以直接捨棄,訊號進來就直入 VR。 6. VR 採用手邊東京光音很便宜的一款產品(價位在800塊上下),這款 VR 聲音偏軟,清爽甜美。 7. 我把電路板鎖在一大片一公分厚的 MDF 上,再置入唐竹賣的厚鋁黑色機箱,搭配銀色霧面旋鈕,非常美觀穩重。 8. 所有的電阻都是 Philips MPR-24 0.1%,除了連接 LED 指示燈的改用便宜的 MRS-25。 一開機就很順利,電源的電壓、輸出直流等測試都一切正常。接上我的系統試聽,背景很乾淨,聽不到噪訊。一開始就發出非常生活有朝氣的聲音,與原本的被動式前級比起來,B1 雄渾有力。但聲也有一點衝,有一點硬,不如被動式前級柔軟純淨。Break-in 半天之後,B1 試作機聲音柔順許多,但還是有一股「晶體味」,但還算耐聽,至少我聽了一天下來,覺得相當愉快。目前試過古典樂、Jazz、人聲,音色的確如 Pass 所言,非常中性,頻帶很寬。加上 B1 之後,我的系統聲底變得陽剛許多,不像之前那麼陰柔。 First Watt B1 Buffer Preamp 的確是簡單好玩的機器,試作機只是開始,可以調的東西很多,例如電源、零件、layout 等等。我對晶體線路沒什麼經驗,還有很多很多地方要實驗和了解,接下來 2009 年可以好好的改機調音了。 線路圖和 Nelson Pass 的解說,可以在 First Watt B1 Buffer Preamp 升級自從一月多完成 Nelson Pass 設計的 三月中在 由於之前已有一部試作機,機箱、變壓器、電源、VR 等雜七雜八的部份都已經就緒,如今我只要裝好這張 PCB,再替換掉之前搭棚的板子即可。所以,晚上花了點時間,就完成了新版的 B1 buffer。 為了先了解 PCB 與原先自行 layout 搭棚的差異,零件的廠牌系列幾乎都沒有更動,儘量按照原先的搭棚版作。電阻幾乎全用熟悉的 Philips MPR24 0.1% metal film 電阻,濾波電容是 Panasonic FM 系列,輸出入交連電容也仍是 Black Gate N/NX 系列。比較大的差異只有兩處,一是把輸出串連電阻改為解析力高而音色甘甜的 Caddock MK132,二是把所有訊號路徑上的配線,由 Mogami 多芯銅線換為 Cardas Litz 機內線。當然,還有 一個額外的差別,就是原先使用的 2SK170BL 現在換成了隨 PCB 一起寄來的 2SK370。 有了 PCB,裝機就成了 trivial 的事情。主板很快就完成了,倒是花了很多時間在處理麻煩的 Cardas Litz 線。完成之後,上電測試都非常順利,就拿到房間裡,接上主系統試聽。雖然新的 Black Gate 需要長時間 break-in,但現在的聲音仍然非常好,比原本清澈許多,樂器的分離度更好。當然這有可能是新的 PCB 所帶來的影響,有可能是 Caddock 電阻的好處,有可能是 JFET 的差別,也有可能是 Cardas 線所帶來的提升。音響 DIY 的困難和有趣之處就在這:即使線路都一樣,只要製作上的環節稍有變化,幾乎都能反應在聲音上。無論如何,現在得到了更好的聲音。 PassDIY 出品的 PCB 動輒搶購一空,不過品質的確優秀。新版的 First Watt B1 buffer preamp 令人滿意,配上新的喇叭,整體的聲音彷彿又升級了。接下來就是繼續調整,換換電容的規格和材質,試試不同的電源,改用規格更好的 JFET LSK170。這樣一組小巧簡單的前級,消耗功率不到1W,不熱不燙不耗電,聲音又無可批評之處,真是相當好用。如此一來,我又更沒有動力把做到一半 5687 單管前級裝起來了…… |
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来自: only4dragon > 《audio》