That's a falacy, mine sound great with my Krell. However, have heard Goertz cables do awful things with it, and perhaps it was oscillation. It certainly was distortion. Why don't you e-mail Krell about it, I'm sure they'll answer you. |
I don't know about Krell products, but Spectral recommends only MIT cables with their equipment to decrease the risk of oscillation. |
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I've never heard that or experienced any problem with 2 different sets of MIT speaker cables on Adcom 555 or my current Classe 200. And good question Bday0000. |
E-mail response from Krell is that there are no problems using Transparent cables with their amps. Awaiting response from Tansparent, too. Incidentally, I still do not know what is meant by "oscillation". |
Nelson Pass' 1980 article on speaker cables mentions oscillation among other things. Find it at http://www.passlabs.com/spkrcabl.htm Here is an except from page 3: "When a wave travelling down a length of cable reaches the end of the cable, it will do one of three things depending on the impedance of the load. If there is a high impedance load, so that ZL, > Zc, the load wil reflect energy positively back down the cable to reappear at the source Fig. 9). If the load impedance is less than the characteristic impedance of the cable, the wave is reflected back negatively; and if ZL = Zo, then the wave is fully absorbed and none is reflected." Good luck! |
Well, I don't know what it means in this case (common usage), but I can tell you what it means in layman-engineering terms. Take Rockvirgo's email, the part about the wave being reflected back negatively. If the power amp has a feedback loop, it'll see that reflection, and try to correct for it. The feedback loop is fast -- but not instantaneous. Depending on the signal, the reflection, and the speed of the feedback loop, the amp will just never get the output right. First it'll try to push the power in one direction, overshoot and go back the other way. If things get really bad (you get some resonance) the amp can diverge wildly from the intended signal. But, if you have a reasonably talented person designing the amp and feedback loop, this won't happen -- at least at audible frequencies. Maybe this is why rumor has it that high bandwidth amps can be more prone to this effect? It's been a long time since I've done any engineer stuff -- looking back is a bit fuzzy.... Perhaps someone else can improve on this explanation. |
It's my understanding that if a speaker cable's own charactersitic impedance is too low, that this is what allows the wave to reflect back into the feedback loop of the amplifier. |
Oscillation? Have you ever seen the footage of that large suspension bridge that started rocking with the wind until it self-destructed? That is oscillation. |
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There's so much here, I hardly know where to start. First, in my opinion, if an amplifier can oscillate just because of using a certain brand of speaker cable, either the amp, or the cables, or both, are poorly designed. Second, oscillation is a signal coming out with no signal going in. Oscillation occurs in an amp when specific conditions are met regarding the resistance, capacitance, and inductance values connected to the amps input or output. The analogy to the bridge is a good one, or think of holding a mike next to a speaker in a PA system- you get a howl or a whistle. An amp that oscillates because of the speaker connections will put out a signal even if the inputs are disconnected; this is usually called singing (it's like a dog whistle at high volumes.) The amp will have a bright, edgy sound, because of all the distortion caused by the oscillation signals, and the output transistors will get very hot. In a really bad case (think dog whistle at 100 watts), all this happens in a few seconds, and then you get silence, because the guts of the transistors are now little melted balls of metal. (The next sound is usually the owner crying.) This situation just shouldn't be able to happen with changing cables. Last, I have a lot of respect for Pass, and what he says is true, but irrelevant. The cables are too short, and frequencies are too low, for there to be a significant reflection. Reflections are a problem when the cables's amplifier end is at one voltage, while the speaker end is at another (the amp is already putting out note B while note A is just getting to the speaker.) Don't worry about it until your speaker cables are over a half mile long. My apologies for the long post. |
Apparently with the appearance of low inductance cables and wide bandwidth amplifiers what 'just shouldn't be able to happen' is exactly what did happen in cable lengths exceeding three feet. Apparently some designers recognized the problem at that time and took steps to correct it. The intent of the Pass excerpt was to direct the reader to the article itself. If you read further you may find more relevance. Here is another teaser from the same article: "Note that these effects exist with all cables. The fact that only the newer, low inductance cables appear to affect amplifier stability brings us to a point which justifies our examination of a cable's performance in regions which are simply not audible. The lower resonant frequencies of the cables having low Zo enter into the output bandwidth of the amplifier as it approaches its unity loop gain, and by altering its phase response cause oscillation at the resonant frequency." Enjoy. |
I believe the issue is not that some amps oscillate with network cables but that some amps need to used with network cables to prevent oscillation. Hence the Spectral requirement to use MIT. Spectral is very high bandwidth, of the DC-to-light school of design. I have heard that the warranty on Spectral amps is voided if you do not use MIT cables, which, if true, strikes me as a bit arrogant. Dan Rubin. |
As I recall of a semi-recent Harry Pearson review of a Spectral amplifier, he thought the specified use of MIT cables was total hogwash, and was hell bent on using Nordost Quattro Fil interconnect and SPM speaker cable instead. Said it was the only way to get the best performance out of the amp. Personally, I don't think Mr. Pearson can hear much above about 6 kHz anyway, so it's possible he'd want all the high frequency boost he could get. Just my rack'n'pinion... |
Wow, have we hit a sensitive area. In trying to answer the previous readers questions, I have tried to avoid the use of technobable, since this forum is intended for a general readership. However, since I wasn't the first to bring in jargon, I'll throw in a bit of my own. Stability: Any amps stability is governed by the Nyquist stability criteria: all of the circuit poles must lie in the left half of the complex impedance plane for stability; or, an amp is unstable if the closed-loop gain plot encloses the point -1 + j0. Since these calculations may be difficult, a graphical method can be used; ie, measure and plot the phase and gain of the amp. This gives the well known Bode plot. The gain (hopefully negative) where the phase is -180 deg is the aforementioned gain margin, and the phase shift where the amps gain is 1 is the phase margin. (Reference "Integrated Electronics", by Millman and Halkias, or "Network Analysis and Feedback Amplifier Design", by Bode, a seminal work on the subject of stability. The amp designer has complete control of the phase/gain response. If he wishes to sacrifice stability to achieve a slight improvement in high frequency response, I consider that poor design practice. Cables: Audio cables are almost never matched to the source or the load, therefore, they can always be considered resonant line structures. But why bother? The wavelength in my speaker cable at 500 kHz, which is a good unity gain point, is still 1800 feet. (I measured my cables this morning, the impedance is 4.3 ohms, and the eff dielectric const is 1.2.) For such a long wavelength, one can do better by using a lumped model (discrete resistors, inductors, caps) for the cable/speaker, as opposed to a distributed transmission line model. (Reference "Transmission Lines, Antennas, And Waveguides", by King, Mimno, and Wing; also "High Speed Digital Design", by Johnson and Graham; they recommend using distributed models at frequencies above 1/4 wavelength, here, that's about 10 megahertz.) We can connect a couple of parts to the amp output, and get the same stability result as the cable and speaker. There is only a fraction of a degree of the signal spread out over the cable between the amp and the speaker. Reflections just don't exist in the speaker/cable system. This stuff is well known in the industry; computers, telephone equipment, radios, TVs are designed by standard principles every day, and no one questions whether they work. I stand by my previous statement: If an amp can be made to oscillate by a certain brand of cable, either the amp, the cable, or both are poorly designed. No apologies today for the very long post. Rich |
I know what I hear, and the Goertz speaker cables were distorted in a way, and to a degree, that no other cables I've tried have exhibited. Since speaker cabling was the only thing I was changing, I don't see how the distortion was geing caused by anything else. I don't know whether the amp was beginning to oscillate or not, but I do know that the strained distortion I heard gave me the feeling that I was hearing the two poles of the cable in proximity much too close to each other (with very little dielectric functionality/pole separation) over the cable's entire length. |
Both Krell and Transparent have assured me that they use Transparent cables and have had no problems regarding oscillation |
Marlec, have you tried MIT cables? |
hi, i have krell fpb 600s and found the goertz cables to be a bad match,,,very crisp and annoyingly metallic/sharp. i use cardas goldercross now,,,they are wonderful with the krells if your speakers are high resolution models. |
Your experience with the Goertz is identical to my experience with it, with my Krell. I suspect you would love MIT cables. They kill the Cardas Neutral Reference cables I've been trying for a while. |
Carl I have not used MIT. Was cautioned not to do so with my system. |
By who, a dealer who happens not to carry MIT cables? Hmmmm....I have a Krell amplifier, and gurantee that MIT cables have caused zero damage to it. Rather, they have made it finally perform to its full potential, to the point where I feel genuinely sorry for Krell owners who don't use MIT. |
MIT cables are occasionally REQUIRED to PREVENT speaker damage. Wide bandwidth equipment (like Spectral) need MIT cables with many speekers. |
