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发表于 2012-1-21 22:45:45
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有些想上FIRST WATT B1，搞一个FIRST WATT的西装
KK的那个夹头不是问题，只要耳垫瘪掉就行了
K兄的那个无源前级到时候要带过来。我这边只有电视机机顶盒用的那个垃圾前级
FIRST WATT B1前级
见文一：
First Watt B1是一台无源前级，巧妙地采用了一个高输入阻抗的JFET电晶体作信号缓冲，对于线路它是没有提供任何的增益。这台 "缓冲前置放大器 "以极其简洁的线路和极高等级的元件、线路组成一台高质量的前级。 
firstwatt.jpg
 
见文二：牙尖嘴利音響頻道
玉作搔頭金步搖，高張苦調響連宵。欲知寫盡相思夢，渡水尋雲不用橋。
2009年1月3日星期六
新年新希望：First Watt B1 Buffer Preamp
2008 年夏天，Nelson Pass 在他旗下的 First Watt 網站上，發表了一個極簡的晶體前級線路：First Watt B1 Buffer Preamp。
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 發表聲明，指責 Pass 剽竊他的設計。
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/PassDIY 網站上下載到 PDF 格式的檔案。
World Premiere!
The First Watt B1 Buffer Preamplifier
Article By Nelson Pass Of First Watt
Difficulty Level
Side A
So here we are in the New Millennium,and thanks to Tom Holman and THX we've got lots of gain in our electronics.More gain than some of us need or want. At least 10dB more. Think of it thisway: If you are running your volume control down around 9 o'clock, you areactually throwing away signal level so that a subsequent gain stage can make itback up. Routinely DIYers opt to make themselves a "passive preamp" ?just an input selector and a volume control. What could be better? Hardly anynoise or distortion added by these simple passive parts. No feedback, noworrying about what type of capacitors - just musical perfection. And yet thereare guys out there who don't care for the result. "It sucks the life outof the music", is a commonly heard refrain (really - I'm being serioushere!). Maybe they are reacting psychologically to the need to turn the volumecontrol up compared to an active preamp.
I suppose if I had to floor theaccelerator to drive 55 mph, maybe I'd think the life was being sucked out ofmy driving. Then again, maybe I like 55. Nice and safe, good gas mileage... Isimpedance matching an issue? Passive volume controls do have to make atrade-off between input impedance and output impedance. If the input impedanceis 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 theinput impedance of the power amplifier. And vice versa: If your amplifierprefers low source impedance, then your signal source might have to look at lowimpedance in the volume control. This suggests the possibility of using a highquality buffer in conjunction with a volume control. A buffer is still anactive circuit using tubes or transistors, but it has no voltage gain - it onlyinterposes itself to make a low impedance into a high impedance, or vice versa.
If you put a buffer in front of avolume control, the control's low impedance looks like high impedance. If youput 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 thinghere 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 noiseand very wide bandwidth, all without negative feedback. There are lots ofdifferent possibilities for buffers, but we are going to pick my favorite:
Side B
Figure 1 above shows the full schematicof the B1 buffered passive preamp. You can see a full-sized PDF by clickinghere. There are two channels shown with a common power supply. Supply parts incommon 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 resistorsare 0.25 watt ? I used RN55D types, but you can use whatever you like. C1 andC2 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 thesevalues 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, C100AND C200 are 1 uFmetallized polypropylene film capacitors (Digikey BC2076-ND)C101 and C201 are 10 uF polyester film capacitors (I used Axon 10 uFmetallizedpolypropylene from Orca Design). Feel free to use any comparable types.
D1 is a generic 1N914 type diode, andD2 is a generic LED for indicating power the board. All of the transistors areN channel JFETs. The stock parts are 2SK170's, LSK170's or 2SK370's, and youcan use substitutes having Idss between than 5 and 10 milli-Amps andtransconductance numbers from 5 to 30 milli-Siemens. The potentiometers arelinear taper at 25 Kohm, but again you can easily use higher or lower values asyou like. The buffer uses an external power supply from 18 to 24 Volts DC. Youcan power it with batteries, but most convenient is an external regulatedsupply 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 thecircuit is pretty good at ignoring noise on the supply and minor fluctuations.The design uses RCA input and output connectors, and a DPDT switch forselecting one of two inputs. You are, of course, free to use a switch with manymore inputs.
For a fact this circuit can be easilyconstructed with perf-board and point-to-point wiring. I know, however, thatmany people won't start a project like this without a circuit board. The Gerberartwork is posted at www.passdiy.com and I have arranged to have finished PCboards available at cost around the time you read this.
Side C
Let's talk about what some of theseparts do. The input switch selects one of the two input signals, routing it tothe top (clockwise position) of potentiometers P100 or P200. A divided inputsignal appears on the wiper. This signal goes to the Gates of Q100 and Q200through a resistor R102, R202 and capacitor C100, C200. As a practical matter,the input impedance of this preamp is determined by this volume controlpotentiometer. A 25 K Ohm pot gives 25 K Ohm input impedance.
R102 and R202 are there to preventparasitic oscillation with the very wide bandwidth JFETs. C100 and C200 arethere because the Gate of the JFETs needs to be set at half the DC voltage ofthe power supply - a voltage delivered to the buffer inputs by R2, R3, and C2through R103 or R203.
D1 performs the service of drawing downthis DC voltage with the power supply when the power is turned off, otherwiseC2 may hold a charge for a long enough time to give you a turn-on thump whenpowered back on. By the way, the time constant of R2, R3, and C2 are longenough that it takes a minute or two for the circuit to reach normal operatingvalues, so don't get excited if there's no sound for a few seconds when youturn it on.
R1 and C1 filter noise coming from theexternal supply.
Q100 and Q200 are JFETs operated asfollower transistors. The Source pins of these transistors follow the voltageat the Gates. The input impedance of the Gate is exceedingly high - manymillions of ohms, and the output impedance at the Source pin is about 50 ohms.
Q101 and Q201 are constant currentsources formed by simply attaching the Gate pins of the JFETs to the Drainpins. They provide without loading them down or creating significantdistortion.
The best performance generally comesfrom matching the Idss of Q100 and Q101, and also Q200 and Q201. The Idss issimply the current that flows through the JFET when the Gate and Source aregrounded and +10 volts or so is applied to the Drain. Often when you buy JFETsyou can get them in Idss grades. I use GR or BL grades for this project.
The Source DC voltage of the JFETs Q100and Q200 is about the same as the Gate DC voltage (1/2 the supply voltage), andthe output from the Source needs to have the DC voltage removed by C101 orC201, leaving the AC output signal, which passes through another safetyresistor R104 or R204.
Resistors R100, R200, R105, R205 arethere to prevent the various potential thumps from switching inputs and turn-ontransients.
Side D
The circuit of Figure 1 has quite goodperformance specifications. Upon measuring, I noted that distortion at 1kHz wasextremely low and flat at all frequencies. Below a volt, the distortion comesin at about .0007%, and about the time your amp is clipping, it measures about.003%
Figure 3 above shows the .0007%distortion waveform at 1 Volt and 1 kHz. Careful measurements show me that thedistortion versus frequency is extremely low and does not alter over the audioband. Figure 5 below shows a 100 kHz square wave at 1 volt at the output. Thebandwidth of the preamp is -3dB at about 700 kHz. On the bottom end, the lowfrequency roll-off is about 1.5 Hz into a 10 Kohm load and about 0.3 Hz into 47Kohm.
Side E
So how does it sound? Well, no suckagehere. I've noticed that simple no-feedback circuits have tremendous clarity ifthe circuit has wide bandwidth and really low distortion. At the moment I'mdriving a pair of Lowther DX55's with some passive baffle-step correction (6 dBloss there) and an F3 amp with only 12.5 dB voltage gain. The preamp is fed byanXono phono stage with a low output cartridge (Grado Statement). The sound isextremely neutral without being clinical - just about what you were looking forwhen you were thinking about a passive preamp. A teensy bit of second harmonicand no noise at all.
There's just enough gain. If you wereusing any other power amp, you'd get 8 to 18 dB more gain, and would be able tobreak your lease or the speaker, or both. Do I feel like the pedal's to the metaland I'm only doing 55? No, as I'm listening as loud as I want and sleep soundlyat night, knowing that I'm not throwing away signal with my volume control.
Side F
Have you noticed that they're puttingout some LP's with not just 2, not just 4, but with 6 sides holding the contentof a CD? It's sweeeet. Two cuts to a side, and when you look at the grooves youcan see the land area between them. Right now I'm listening to High FidelityLounge, vol 4, Side F.
NOTE: You can see Dick Olsher's review ofthis unit by clicking here.
Manufacturer
First Watt (assembled unit) or Pass DIY(kit version)
Phone (530) 367-3690
Website: www.passdiy.com
Price: $1000 assembled, kit versionprice not quite set at this time.
[ 本帖最后由 davidxtb 于 2012-1-31 23:27 编辑 ] |
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发表于 2012-1-21 20:37:05
