نگاهی به گذشته ساخت آمپلی فایر

نگاهی به گذشته ساخت آمپلی فایر

یکشنبه 26 سپتامبر 2010
/ / /

برخلاف تصور رایج که اینطور فرض شد مشکلات صدای آمپلی فایر ها با ظهور Solidstate شروع شد باید بگم مشکل خرابی صدا از همون زمان لامپ ها شروع شد (http://en.wikipedia.org/wiki/Valve_audio_amplifier)  و بعد ها با اومدن ترانزیستور تکمیل شد. دلیل این موضوع اینه قبل از اینکه اصلا ترانزیستور ها پا به عرصه وجود بگذارند کمپانی های سازنده آمپلی فایر که همشون لامپی میساختند شروع کردند به طراحی آمپلی فایرهای پوش پول لامپی پرقدرت تو مد پنتود با فیدبک منفی بالا. میدونید که بهترین و خطی ترین پاسخ رو مد ترایود سینگل اندد لامپ داره اما طراحان آمپلی فایرهای لامپی به سمت گرفتن توان بیشتر رفتند و هم از حالت سینگل به سمت پوش پول روی آوردند و هم مد ترایود رو به پنتود که توان بیشتری میداد تغییر دادند و کار رو بجایی رسوندند که از یک آمپلی فایر لامپی توانی نزدیک به 600 وات گرفتند (مثل Audio Research) اما این توان زیاد به لطف فیدبک منفی زیاد و موازی کردن تعداد زیادی لامپ و استفاده از پوش پول و مد پنتود بدست اومد که همه این شاخص ها دقیقا باعث کاهش کیفیت صدا بودند.

این طراحان با اومدن ترانزیستور دیدند همین مسیر اشتباه رو میتونند به شکل بهتری با ترانزیستور انجام بدهند (هم حرارت تولیدی کمتر میشد و هم جای کمتری میگرفت و هم میشد براحتی تو تیراژ بالا تولیدش کرد)  و شروع کردند به ساخت آمپلی فایرهای پرقدرت ترانزیستوری بالای 100 وات و هنوز که هنوزه این روند اشتباه هم تو حوزه لامپ و هم تو حوزه ترانزیستور ادامه داره و مجلات هم از این حرکت پشتیبانی میکنند.

من بارها گفتم یک طراح غیر حرفه ای هم از لامپ و هم از ترانزیستور صدای بدی میگیره و یک طراح خوب از هر دو تکنولوژی پاسخ خوب میگیره (مثل نلسون و پیتر) و رومی هم همیشه گفته باید ببینید خود طراح چی در ذهنش داره و نمیشه به صرف تکنولوژی شما چیزی رو قضاوت کنید.

در شکل بالا که از سایت یکی از طراحان آمپلی فایر لامپی بنام Lynn Olson هست (این آدم تو فستیوالهای ترایود هم حضور داره) نشون داده شروع این مسیر اشتباه با لامپ بوده (تو شکل بالا ترانزیستوری ها با آبی و لامپی ها با زرد مشخص شدند) و البته از نگاه من فرقی نمیکنه با کدوم شروع بشه و چیزی که مهمه اینه که این  طراح هست که میتونه با ایده هاش به پاسخ مطلوب برسه و تکنولوژی فقط ابزار هست.

آقای Lynn Olson دقیقا آنچه گفتم رو بشکل دقیقتری شرح داده و گفته چطور طراحان آمپلی فایر های لامپی به مسیر اشتباهی وارد شدند، این لینک ها حتما رو بخونید :

http://www.nutshellhifi.com/library/tinyamps.html

http://www.nutshellhifi.com/library/tinyhistory1.html

http://www.nutshellhifi.com/library/tinyhistory1.html

http://www.nutshellhifi.com/library/FindingCG.html

http://www.audioxpress.com/resource/audioclass/

من همیشه گفتم از Audio Research لامپی همونقدر بدم میاد که از Soulution ترانزیستوری و از Audio Note لامپی همونقدر خوشم میاد که از Vitus ترانزیستوری. متاسفانه الانا با مد شدن لامپ ما شاهد تصحیح مسیر نیستیم بلکه مورون بازی همونه اما فقط شکلش عوض شده و ما باید بجای آمپلی فایر های پرقدرت ترانزیستوری باید نوع مورونی لامپی اش یعنی Audio Research و Conrad Johnson رو بخریم.

The Effects of Feedback on Harmonic Structure

The Williamson amplifier of 1947 was the design that did the most to popularise the “feedback cures all ills” philosophy. It is interesting during the period from 1948 to 1956, almost all commercial hi-fi amplifiers were Williamson topologies (with minor exceptions for Quad II, McIntosh, and EV Circlotron). During this formative period the mantra of “more power, lower THD” became the driving force in the industry. By 1960, ultra-wide bandwidth, heavy feedback, and Class AB EL34 and 6550 UL circuits ruled the industry.

In the span of twelve years, the traditional audio-engineering prejudice against high-distortion devices faded, opening the door to high-power pentodes and Class AB operation. Each “improvement” was characterized by an increase in device distortion, which was then “corrected” by more and more feedback. Transistors circuits with even higher feedback ratios were the next obvious step – after all, they had more power, lower THD, more bandwidth, and most important of all, cost less to build.

Norman Crowhurst wrote a fascinating analysis of feedback multiplying the order of harmonics, which has been reprinted in “Glass Audio,” Vol 7-6, pp. 20 through 30. He starts with one tube generating only 2nd harmonic, adds a second tube in series (resulting in 2nd, 3rd, and 4th), and then makes the whole thing push-pull (resulting in 3rd, 5th, 7th, and 9th), and last but not least, adds feedback to the circuit, which creates a series of harmonics out to the 81st. All of this complexity from “ideal” tubes that only create 2nd harmonic!

With real devices there are even more harmonics. In terms of IM, actual amplifiers have complex and dynamic noise floors thanks to the hundreds of sum-and-difference IM terms. That’s not even counting the effects of reactive loads, which adds a frequency dependency to the harmonic structure! (With reactive loads, additional harmonics appear due to the elliptical loadline seen by the power tubes. The elliptical load-line dips into the very nonlinear low-current region, resulting in an instantaneous increase in upper harmonics. This spectral “roughening” is most audible with strong low frequency program material and hard-to-drive horn or vented bass drivers.)

As Crowhurst noted, feedback mostly reduces the 2nd and 3rd harmonics, leaving the upper ones more or less alone, or sometimes even greater than before. Feedback fools the simple THD meter, but the spectrum analyzer sees through the shell game. Too bad raw power and almost useless THD measurements became the end-all and be-all for more than 50 years. If more engineers and reviewers had access to spectrum analyzers, the misleading nature of raw THD measurements would have been discovered earlier, and amplifier design might have taken a different course.

If device-level linearity and absence of high-order harmonics become your goal, then direct-heated triodes are the only way to go – they have about 1/3 the distortion of triode-connected pentodes and beam tetrodes. The spectral distribution is better as well. Seen in this context the Sakuma amps with their 300B direct-heated driver tubes start to make sense – with a 300B swinging 65V rms into a high-impedance transformer load, the driver will have very low distortion, maybe as low as 0.1% for push-pull drivers.

For once, the driver tube would be out of the picture as far as distortion is concerned — and this is much more rare than you would think. Very few amplifiers have driver sections with distortion 1/3 of the output stage and 5 dB or more of headroom — this is true for triode, pentode, or transistor amps as well! Much more typical is 1/2 or more distortion compared to the output, and 1 to 2 dB of headroom. As a result, 2A3 and 300B amps all sound different, depending on the linearity and current delivery of the driver circuit.

بی ربط به بحث بالا، نوشته رومی رو آوردم در مورد ساخت یک سینگل و نقد Lamm ML3 :

http://www.goodsoundclub.com/Forums/ShowPost.aspx?PostID=1461

What is important is to undusted that my view  less targets the Lamm ML3 but rather it targets the ML3 as a representative of entire class of amplifiers. When I told that Lamm ML3 is “fundamentally improper design” I did not mean to critique the ML3 specifically but rather the entire family of expensive Single-Ended amplifiers where the “cost became to be a major design impediment”. I will try to explain it further. People who have brain understand that everything in audio is learning experience and I would like to describe why I feel that “expensive SET” dose not comply with my vision of a “correct direction”. It is juts my opinion and it based on my own experience. Your mileage might wary but where did you see me to care about your mileage? Anyhow, I have my opineon and I agree with it, so it comes…

For beginning I would like to observe a SET amplifier as a progression of quality and price, in this case prices will be very much an applied tool. We all know that SET is very simple amplifier: usually 2-3 stages, few parts, short paths etc… Let pretend that price DOES have direct relationship to the quality of amplifier (or sound), in fact why shouldn’t it?  In today world a simple OK-performing SET made around “simple” 2A3, 45, 6C33C or 300B cost under $1000. There a number of Chinese companies that do chips SETs (under $500), AN (Audio Note) does their 300B set for under $600 I believe, there are a number of American companies who do very inexpensive sets (Bottlehead, AudioMirror and many others). The all sound better or worth but they in fact closer to “better sound” (with an adequate load) then many $20.000-$60.000  solid-state monsters build by Morons with diplomas of electricians (a long list of the companies goes here).

Do these $1000 SETs sound as good as SET could? Of course not – there is a quite a room for Sound from here. Now we begin to add price/expense and we will have a more or less proportional growth of quality (if the designer knows what he does). The specials case when people employ phony “expensive solution”, like use of 0.001 precision resistirs, gold transforms, or the chassis made form Agarwood with platinum bumper around the amplifier I will leave out of scope, as those actions have no relation to Audio.  So, regardless of the topology if your SET has more properly used chokes, better transformers, better power supplies, better amps design, better assembling techniques, better drivers (very critical), better protection and control, better tubes and many other factors then the amps could add quality and price. Still, any SET is basically is 2-3 dozens of parts and soon or later, regardless what you do you reach a “price threshold” where better parts, better supplies, better assembling techniques, better tubes do not add anything anymore within any MEANINGFUL scale. My estimate, based upon my experiences, that a self-cost of a pair of SET monoblocks at hits the “price threshold” level would be around $4000-$5000 for the amps that use low voltage output tubes and $7000-$8000 for amps that use high voltage transmission tubes. From here there is only one way to add quality and consequentially price to your SET – to improve further the only thing that maters in SETs: the output transformer (OPT).

The OPT is the main bitch of any SET and in the same time it is a subject of glory of any SET.  Something very positive happens to Sound when goes through transformer, the key is that the transformer should be good and it arguably should be the only transformer in a unit. So, the OPT: the frequency response, articulation, dynamic, tone, inner-tonal connectively, balance between “resolution” and “space” and  whatever else you might value in Sound came in it’s majority from the quality of OPT of SET. A person whose amps is at the “thresholds prices level” from here might jump into all imaginable OPT exotica, chasing in Sound whatever he fells needs to be chaise. A good OPT might cost a few thousand dollars it is all depends of the budget and the intentions/expertise/experiences of the person who design and who makes and who build the transformer. However, regardless how fantastic SET transformer would be it still hit its own topological limitations. To get bass you need inductance. I’m not necessary taking about better number of bass but rather about bass as quality. The OPT do not just need inductance to do bass but the inductance as “something else”. You can increase the core size as much as you wish to pay, building up inductance but inductance kills HF. It would be simple if the inductance kills juts HF numbers (there are ways to fight it) but the inductance kills HF “quietly”. Higher frequencies are opposite: they heed fewer turns and lower inductance…. So, people got into many different more or less sophisticating techniques (complicated core materials, tricky sectioning, intricate winding techniques and many others) to get out of SET’s OPT proper reproduction of boundaries. When people go into the high voltage tubes the situation become even more complicated as high voltage requires more isolation between the turns, which increases space within winding that builds up parasitic capacitance. Those capacitances dehydrate HF’s transients. When people listen all of those high voltage amplifiers with the “big tubes” they report “Big Sound”. (I call it the “Elephant Sound” – search my site I have written about it before). However, a nature or this “Big Sound” is dehydration of HF. Take any speaker, increase its tweeter output for a 3db and then place in front of the tweeter a soft hairy fabrics that would eat this 3db up. Now you will have a mimic of that  “Elephant Sound” – that in fact is not the “Big Sound” but rather a sound with unevenness of subjective transients across the range (MF are “faster” then HF). So, retuning back to the transformer – it is very complicated to do the OPT transformer for any more or less mindful full range, that would presumably also sound properly. Very few people out there know how it might be done. Very view can actually implement it if they even told how it might be done and very few, if any, go for a recursive subjective assessment using PROPER LISTENING TECHNIQUES of achieved results when they build transformers. Still, making even a theoretically perfect transformer for a given SET any person hits the dead wall of the fundamental bandwidth limitations for OPT and it is imposable to fight them while keeping the aim to the exoteric properties of sound in the same time. Russians have a good old fairytale about a village person who caught a fox and decided to make a soup with it. He put the fox onto a pot but the tail was sticking out. He pushed the tail into the pot but the fox’s head moved out of the pot. He pushed the head in the pot but the tail went out…. The very same with OPT and people cook their foxes disregarding the fact that heads and tails in OPT are improperly cooked….

I have heard many SETs and all of them had issues with wide bandwidth, it is not that they did not have bass or did not have HF but when they try to get it something else turned to go wrong. The only one wide-bandwidth-interesting amp that I’m familiar with was Lamm ML2 (old production not the ML2.1). It is not perfect SET from the perspective of “as good as it could be” but it is “better then anything else that I heard”. The ML2 was the only amp that was trying to do HF and LF (still with it limitations as I learned eventually) and do not go apart in anything else at the same time. So, a cogent reader would ask: “Romy, if Lamm was capable to design the ML2’s OPT and get the result the he got with that amps then why you feel that he might not do the same good job with ML3?” It is an excellent questing and I have two answers: first is the Lamm’s attitude and second is the “fundamental design flaw of ML3”.

خیلی جالبه اگر نگاه رومی رو بخونید :

is one tube better then other: it is like a car driving: the horsepower in the engine is hardly meaningful if a driver is Morons. Many years ago a friend of mine, a professional auto racer of very high caliber, when we were driving together demonstrated me how to drive. He was driving no more the 40km/h… but no one was able to pass him. It was extremely fun to watch how a multi-lane road was practically blocked behind our car but at the same time – the friend of mine did virtually nothing that I or anyone else would intermit as “actions”. The very same about the art of playback building: juts dumping a good amplifier and good speaker into the room create a random and not tailored sound. The Real high-end does not stats form equipment but with the different level of seriousness… Retuning back to ML3: this amp is not implementation of seriousness to me. It would be if the ML3 were a DSET for LF and HF. Only then it would be possible to say something objectively about the amp. A single “expensive” SET if juts a paradise for a reviewer but not a tool for demanding users.

The Cat

Post a Comment

نشانی ایمیل شما منتشر نخواهد شد. بخش‌های موردنیاز علامت‌گذاری شده‌اند *

*