彙整 | 一月, 2007

Q factor 的聯想 ( 1 ) — DC 放大器

26 一月

朋友有一部直交的 71A胆機,低頻很好,很煙韌,朋友說這是直交機的緣故。旺記說毫無關係,因為要講直交,石機一早在70年代已有。

講到 DC 放大器,勾起旺記不少回憶。想起一九七幾年的時候,旺記剛剛學人揸棘雞,那時時興做套件,通用,美聲、南記的 lo 至 mid fi;精密金工、英光電藝社和鸭寮街東京的 mid 至 hi end,應有盡有。

當年的放大器,一律係DC 放大器或近DC,統統 spec 响 0 – 100KHz-3dB,彷彿不及這個响應便是罪過。而且因為用深度負回輸,所以輸出阻抗低,DF幾百閒閒地。

近年胆機熱重現,直交機古老當時興,其實四、五十年代的 analog computer 用胆砌運算放大器,已經係DC放大器,唔係點做 DC summation 。

剛剛今期 MJ 翻炒舊嘢,旺記也贈興貼上兩個 page。安井章和金田明彦,依家仲有寫嘢,佢地D文章當年港台電聲雜誌翻譯唔少,乃我等衆生精神食糧也。

廣告

德國全音篇之 葵花寶典 (九)

25 一月

今天朋友問旺記工行值不值得買。

旺記說若非手上的 1398 是 IPO 認購回來的話大概一早已經賣掉了。

為甚麼?先說曾讓旺記輸大錢的 941。

中移動終於回到二千年的價位。

反過來說,中移動在二千年的價位,已反映了它七年後的價值。

那麼,現價的工商銀行,反映了它幾多年後的價值?

長揸?Value investment 的所謂長揸,是揸價值未被市價反映的股票。已反映的,揸住就是為了股息,投資意義等同債券,不過隨時可走人而已。

旺記不賣工行另一原因,是號碼正斗。1398,夠晒醒神。That is.

續  葵花寶典 (八)

Starting your search(1)

 

This is the question everybody wanted to be answered during my last years: What brand name should I look for? With questions like that you’re fooling yourself, definitely. There are things you should pay attention to instead because they are much more crucial.

 

First of all you need speakers of good condition. I never found NOS, the best you can get is near mint or very good. To check this look for the paper of the cone. Any damage?. Watch the multipleats using a bright light, preferable sunshine. Do they look regular? If the paper shows a brown to yellowish coloring the speaker was damaged by the heat of the tubes, you can’t use it any longer. This happens especial1y in smaller radios, and in bigger ones to the supertweeters which sometimes are placed above the power tubes causing burnout after some decades. Would be wise to go for not too small radios which come (in Germany !) mostly with 8 inchers. Yes, I’ve seen a Philco big box carrying a most interesting 10 or 12 incher inside, but this was an ultra rarity. And a 12 inch wide range unit running up to 12 kHz? Would be better to go for 8 inchers.

Furthermore you should move the cone a little and take care there is no scratching. These old timers had forty years of time to pick up metal dust or small pieces of glue, and it’s impossible to remove these because the air gap is too small. If you find lots of dust on the chassis frame the speakers were exposed to humidity, always troubling the paper cone.

 

 在最著魔的時候,旺記就是一部一部收音機從德國運過來,拆去單元,丢掉機肉。

 

極不環保,罪過 罪過。

德國全音篇之 葵花寶典 (八)

20 一月

這個星期在公司忙瘋了,差連 wordpress 的 password 都忘記了 !

前幾天往深圳工商銀行攪轉匯,人並不太多,竟然等了 一個半鐘 !甚麼五行三保樣樣好前景, 這是甚麼效率 !  天,我的 1398 !

續 葵花寶典 (七)

"

Till today it’s impossible to build a wide or full range speaker whose cone is moving precisely like a piston. If the material of the cone is capable of high frequencies it’s flexible too, and high frequencies are radiated by the center part of the cone solely. That’s why the tweeter is built the same way; the highest frequencies should be generated as it is done by the main driver to guarantee the best match. The tweeters are about 4 inch and in any other parameter the smaller brothers of the 8 inchers – same paper, alnico magnets etc. – except one. Coming in above 8 kHz. they’re not allowed to move like a piston, and this is reached by fixing the cones with a center screw. The main driver covers about seven and a half octaves of music, the tweeter barely one. Looking at frequency ranges of instruments you’ll see the 8 incher handles the biggest part of the business, acting alone on most signals, only occasionally the tweeter adds a shot of clarity and life. Switching over to our modem HiFi parlance this is a conception without crossover, combining one wide range unit with a supertweeter protected by one cap of 2.5 uf. This is the last time in HiFi history (as far as I know) where tweeters and main drivers are designed and built to a non-compromise matching. These supertweeters aren’t intended to work with any other main speaker than their own!

 "

呢句勁 :  This is the last time in HiFi history (as far as I know) where tweeters and main drivers are designed and built to a non-compromise matching.

頗同意。時興 ribbon tweeters。Ribbon tweeter 的反應很快,在 0.1-0.2mSec 內 反應,賣 ribbon tweeter 的 商人通常會 plot 個 impulse response 出來講 ribbon 有幾快幾好,但在三路分音裡,中音在 1ms 起動,低音在 1-2ms後 起動,ribbon在 1ms內已做晒,勘接呢 ?不過 ribbon輕,resolution 很好,音不自然不勘接成其次。

HiFi,detail ,刺激,大動態 便是現代揚器的方向。

自然,articulate 衹有一小撮人 追求。

德國全音篇之 葵花寶典 (七)

15 一月

" Why they are, how they are

­

You don’t believe those tiny old speakers are outperforming newer ones? Take a break, have a chat with some experienced player of an electrical guitar. Just ask him if he couldn’t obtain two old style 12 inchers, say Jensen P 12 Q out of the fifties, to you. They are built exactly the aforementioned way. The answer might surprise you. Whoever owns one thinks of dying first before leaving them out of his hands. The sound they produce can’t be reached with newer units. A German mag for professional musicians just checked it and got the same result. And guess what? The speaker finishing the test next best to the Jensen was a replica, the only one of the modem crowd using the expensive alnico magnet. Trusting in old time speakers you’re not alone, guitar players, radio lovers and others did it before. By the way we’re talking same old story you already know. The original recordings of the fifties on old vinyl are the most wonderful, even their newer remakes stay behind. The Stradivarius violins can’t be rebuilt or beaten, they Ire simply the best. The tubes of the fifties… ok, ok, this you already know. The story goes on and on, I just opened the chapter “vintage radio speakers" for you.

To reach such a quality you have to optimize everything, not only the parameters you know or today’s wisdom notices, but those, too, you can hardly feel or guess. Frederick Forsyth got the heart of it: “In matter of technica1 skill there are four levels – competent, very good, brilliant, and a natural. The last category goes beyond mere skill and into an area where all technical knowledge is backed by an innate feel, a gut instinct, a sixth sense, an empathy with the subject and the machinery that cannot be taught in textbooks'". The more you see old radio drivers, the more you heard them, the more you’ll be convinced they’re designed by men who in terms of speaker building were naturals.


In Germany 8 inchers in old radios aren’t full range units, -they’re wide range only. I’ve seen quite a lot but only two twin-cones for full range application. Amplitude frequency response shows those with one cone are running up to 10 kHz, maybe 12 kHz as the WE 755 is supposed to do. But it isn’t enough for a quality reproduction of FM broadcasting, for High Fidelity they wanted 15 kHz at least. That’s why there are tweeters of the same kind, usually two, mounted on each side of the radio. They’re protected against electrical burnout by a capacitor, ensuring they don’t get too much energy at lower frequencies. “

 

 

旺記最認同呢句 : The more you see old radio drivers, the more you heard them, the more you’ll be convinced they’re designed by men who in terms of speaker building were naturals.

 

Amen.

好好玩的 sonic impact soundpad 全音

10 一月

06年1月號的 audio xpress 刊登了 sonic impact sound pax speaker 的 review。

令旺記 impressed 的,是它的 squarewave response。正如 旺記一直在這裡強調的,出得像樣的方波,声音不會差到那裡。

隔了一年,终於買了幾個 Soundpad。Soundpax speaker 已經停產,取而代之给用家貼上其他振板的 Soundpad。更好,正合吾意。

買回來,發觉是 NXT transducer。

有点失望,因 旺記在八年前已玩過NXT,當年是 cheapie 的 PC speaker。效果不好,歸咎於 NXT transducer 不好。

上網取經,才知道 NXT transducer 有兩款。竟然已出了 moving coil type 的 transducer,可出更大音壓。

旺記起初認為應貼在 high stiffness ,高密度的板上,因此一開盒,接上 power amp, 立刻往玻璃窗上貼。

噢,错了。

声音很少。明顯是 mass 的問题。

立刻反轉思想,要 low mass 但仍是 stiff 的東西,因為 bended wave device 要傳声也。

剛好家中有一塊發泡膠板 一吋厚 3呎 x 2呎。

立刻貼上。

Bingo ! Tonal Balance 非常好,效率也頗高,家中常用的全音由 88dB 至 96dB 不等。由 Soundpad + 發泡膠板出的音量,應有 90dB左右。

問题來了,transducer 太少,振板太大,明顯不同位置音壓不同。

好,用夠兩個!因為是 moving coil,所以並聯或串聯都可以,Soundpad 官方 impedance 是 6 ohm,一於並聯。都不管兩個 transducer會否有分別而出 beat frequency 了,總之貼上再算。

掂!音壓增加,声音更 fullbody,efficiency ? 起碼有 92dB!

除了高频有點 roll off 外,声音很 balance ,relax,是典型 open baffle dipole speaker 的声音。更美的是,bass 不错。一般兩呎的 open baffle,會有甚麼低音呢,除非把單元放在離地不足一呎的位置。

Next step 是找其他 low mass 又 stiff 的 振版。

Postoffice 的萬用箱的纸質好像不錯 ….

德國全音篇之 葵花寶典 (六)

8 一月

"

Some new “audiophile” speakers use big magnets promising big driving power. They’re overlooking the most Crucial part of the business. It’s not the destination to optimize one single parameter. it’s the goal to optimize the speaker as a whole. If the car with the biggest engine would give the most acceleration, racing drivers would have use trucks only, right? Within a racing car everything has to be as small, light-weighted and dainty as possible. Same with a winning speaker. The more mass you stuff in the magnet the more mechanical vibration it will store, that’s what the laws of physics say. And it’s measured already by at least one of today’s speaker constructors, Renaud de Vergnette (of Triangle) confirmed: The magnet indeed is accumulating parasitic vibrations which make him resonate, and this is a big construction problem. The transformation of electrical waves to acoustical ones of course has to avoid any resonance as much as possible. And a dainty and light-weighted construction of a speaker stores less vibrating and resonating energy, that’s why the old engineers did it that way. They wanted to create racing cars, not trucks."

 

 

關於 alnico 與粉磁之爭,大細磁孰優孰劣的問题,真喺十日十夜都講唔完。不過大家會發覺,早期的德国古董單元磁体很大,並且是環狀,發燒友當然認為越大越好。但 Briggs 在 Loudspeakers 一書中又另有說法。按 Brigg 的资料, 1955年前的 alnico磁力較弱,55-58年後 alnicomax才出现,所以磁体較少,也改為在中心,另磁漏更少。而英國要到60年後 alnicomax才有材料入口,早期的衹能在德或美品中使用。

 

旺記對磁石實在外行,不過小 alnico 磁的 德国單元好声過大環 alnico 磁卻是明顯的事實。

葵花寶典 外典之 airgap

6 一月

Post 上兩張相,上為 Coral Beta 10 的 motor air gap 。下為 Saba 8 吋 Greencone。

Coral Beta 10  air gap 寬約 1.5mm, Saba 約 0.8-0.9mm。

日本人把生產穩定性放第一位, airgap 定得很寬。

Coral Beta 系列最勁的是紙盤,10 吋的 cone 連 whizzer 祗約 10 gram 重,在巨磁 motor帶動下飛快地活動,因此 資訊 特別 detail ,efficiency 也很高 ( 96dB),如果 airgap 像 Lowther 一像 aggressive ,成就無法想像。