Bare en liten input hva for eksempel Audio Note sier om motkobling :
Audio Note
The design practices most obviously eschewed in the development of the Audio Note Real Audio amplifiers (using direct heated power triodes) is the use of negative or local feedback. Negative feedback, quite simply, is the application of an inverted portion of an amplifier's output signal to its input terminals. This "extra" signal is subtracted from the input and serves to reduce the effective amplifier gain (as the input signal is then smaller). In addition, steady state distortion is thought to be reduced as the out-of-phase distortion components contained in the feedback signal cancels out some of the errors created by the amplifier circuitry.
This scheme presents two very obvious problems. Firstly, all amplifiers introduce some delay to passing a signal from its input, to its output and then back to its input. During this delay period, a feedback amplifier is operating at its natural (referred to as "open-loop") gain. It is not until this initial delay period is over, that the circuit begins to exhibit its intended operating ("closed loop") gain characteristics. There must be, by the very definition of a feedback system, some change in the gain factor G, during the transition from open to closed loop operation. This gain modulation would probably not be audible by itself, as the propagation delays of most good amplifiers are quite small, except that the increased gain of the amplifier during the initialization period results in a decreased maximum input capability before overload. Simply put, an amplifier which utilizes 20 dB of feedback (a relatively modest amount by modern standards) and requires an input of two volts to clip during closed loop operation, would overload with only two tenths of a volt input during the forward delay period. Once the amplifier is overdriven, it may take many times its delay period to become fully restored to normal operation. The distortion created by this condition has been commonly referred to as Transient Intermodulation Distortion (TIM), Dynamic Intermodulation Distortion (DIM), and Slew Induced Distortion (SID).
In addition to this obvious form of feedback induced distortion, there exists another more subtle effect of signal regeneration. Because all amplifiers have some forward propagation delay, the fed back portion of the output signal will always lag behind the input. There is therefore a constant introduction of "out of date" information into the amplifier. Under transient conditions (which is what music is; transients), this results in the presentation of an error correction signal intended to reduce the distortion of an input signal which has already passed through the amplifier and is either already out of the circuit or well on the way out of the circuit. The signal present at the input by the time the feedback has arrived may bear no relation to the previous signal and thus will not be properly acted upon by the regenerated information. The current input signal is then distorted once, through the subtraction of an erroneous feedback waveform, and again by the amplifier. Additionally, the error signal present in feedback is passed through the amplifier and again fed back, with all of the newly created distortions, to make yet another trip through the circuit, until it is allowed to decay through successive attenuation. Thus, a distortion signal which originally may have lasted only a few microseconds, can pass through the amplifier enough times for its effective duration to have exceeded the threshold of human audibility. The mechanism originally designed to reduce audible distortion, actually, under transient conditions, serves to regenerate, emphasize and, in fact, create distortion.
Er ikke ute etter å ta Hypex,liker det jeg har hørt som sagt veldig godt,men jeg synes det er feil å gi andre inntrykk av at Hypex er "perfekt" .Der er mange kompromisser,og en god rørforsterker har muligens enda flere utfordringer.Alle både legger til og trekker fra ganske så solidt.
Jeg er igrunnen enig med Audio Note,en av de beste måtene å fastslå nøytralitet og mest mulig ufarget gjengivelse er sammenligning av kontraster.De komponenter som har minst egenkarakter er de som vil forandre seg mest fra innspilling til innspilling.Det råeste jeg har hørt på det punktet var et dyrere Audio Note oppsett jeg hadde med deres egne høytalere,300B forsterker med sølvtrafoer,osv. Innspillinger kunne gå fra å høres ut som de var innspillt via et telefonrør,men likefullt musikalsk innsiktsfullt,til å låte så mektig,stort og fett at man kunne tro gulv,tak og vegger var forsvunnet.Mine billige Hypex UcD400 har aldri vist like store kontraster,selv om jeg regner med nCore er enda et hakk vassere etter å ha lest folks kommentarer på de.Men som det var nemnt tidligere,motkobling skal øke båndbredden,og slik høres det ut på alle innspillinger,et eget preg i bassen,et eget preg i toppen,som alltid ligger der.Dette preget har jeg hørt forsvinne i stor grad på de rørforsterkere der jeg har kuttet motkoblings-sløyfen.Plutselig blir bassen mindre bass som i en egen lyd og sound,men heller mer bassgitar og basstromme,og større forskjeler på de forskjellige bassgitarer og trommer.Og generelt større forskjel på innspillingene,noen får feitere bass,andre mindre,osv.Jeg mener jeg har hørt en fellesnevner,et eget preg,på alle variantene jeg har hørt av klasse D,ICE,Midgard,Hypex,Channel Island Audio og Spectron Musician.
Dette er ikke ment som noen kritikk av Hypex for min del,som sagt så plukker jeg fram min egen Hypex sak fra tid til annen,og har ingen plan om selge den,fordi jeg koser meg med musikk med den.Men jeg synes det er feil å dra ufarget kortet om de.
Og som sagt,de beste rørforsterkere har også masse kompromisser og egenkarakter