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Post by DavidR on Aug 14, 2021 9:05:46 GMT -5
Below is an attachment of the crossover from the speakers I normally use with my ST-120 tube amp. It has no trouble driving them.
Good Grief! What do I see? 5 inductors and a handful of caps? I wouldn't know where to begin. but for sure? needs to be measured. DId stereophile ever measure this speaker? You missed the autoformer/transformer |
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Post by leonski on Aug 14, 2021 16:46:04 GMT -5
No I Didn't......I IGNORED it....!!!
Schematicall, multiple windings / input-output so NOT an autoformer which is a single winding.
This might be an impedance matching device.
Did they get any competent professional reviews?
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Post by leonski on Aug 14, 2021 16:49:42 GMT -5
I hear ya.....but must say that the ST70 SHOULD have enough juice for a 96db sensitie speaker. Certainly another 3db output in the ST120 MUST be enough......
I think the BUILD would be somewhat easier for the monos. A little more chassis space and only a single channel. I'd apply the diode mod immediately and think about the SS recitifier.
Bob urges uilding ONE-AT-A-Time.......which after refelction is a terrific idea.
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Post by leonski on Aug 14, 2021 16:50:06 GMT -5
I hear ya.....but must say that the ST70 SHOULD have enough juice for a 96db sensitie speaker. Certainly another 3db output in the ST120 MUST be enough......
I think the BUILD would be somewhat easier for the monos. A little more chassis space and only a single channel. I'd apply the diode mod immediately and think about the SS recitifier.
Bob urges uilding ONE-AT-A-Time.......which after refelction is a terrific idea.
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Post by audiobill on Aug 14, 2021 17:00:06 GMT -5
Leonski, the ST 120 is the “best value” for your needs. Way plenty for 96db speakers. Have you read Boomzilla’s review in “Secrets of high fidelity? The M125 too.
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Post by leonski on Aug 14, 2021 18:21:50 GMT -5
In no particular order? YES and AGREED. And yes, again!
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Post by DavidR on Aug 15, 2021 8:55:09 GMT -5
No I Didn't......I IGNORED it....!!! Schematicall, multiple windings / input-output so NOT an autoformer which is a single winding. This might be an impedance matching device. Did they get any competent professional reviews? It was VERY innovative for the time (1977). The FIRST attempt at room correction. Yes, it changes the impedance (but not for matching equipment). AR calls it an autoformer/autotransformer and since most of them went to MIT and I only went to Northeastern (across the river) I will not argue the point. I don't know about reviews from back then. It's a much sought after speaker and is hard to find. |
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KeithL
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Post by KeithL on Aug 16, 2021 10:50:27 GMT -5
That is a VERY complex crossover... and modern crossovers rarely have transformers in them. Incidentally, even though they describe it as "an autotransformer", what is shown in the schematic is actually an ordinary transformer, with separate primary and secondary windings. (The definition of an "autotransformer" is that the primary and secondary windings are not isolated... so it's possible that the connection shown between the two windings is actually internal...)
Either way, that is a very complex crossover, and its impedance will actually change significantly depending on where the various switches or jumpers are set.
(Basically you're going to have to re-draw the schematic and redo your calculations for each combination of settings.)
Such unusual load impedances CAN cause all sorts of odd interactions with the output transformers on some tube amps... However, that may not necessarily be the case, so as long as it's working well, then don't worry about it. 
Below is an attachment of the crossover from the speakers I normally use with my ST-120 tube amp. It has no trouble driving them. |
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Post by DavidR on Aug 16, 2021 11:20:20 GMT -5
That is a VERY complex crossover... and modern crossovers rarely have transformers in them. Incidentally, even though they describe it as "an autotransformer", what is shown in the schematic is actually an ordinary transformer, with separate primary and secondary windings. (The definition of an "autotransformer" is that the primary and secondary windings are not isolated... so it's possible that the connection shown between the two windings is actually internal...)
Either way, that is a very complex crossover, and its impedance will actually change significantly depending on where the various switches or jumpers are set.
(Basically you're going to have to re-draw the schematic and redo your calculations for each combination of settings.)
Such unusual load impedances CAN cause all sorts of odd interactions with the output transformers on some tube amps... However, that may not necessarily be the case, so as long as it's working well, then don't worry about it.
Below is an attachment of the crossover from the speakers I normally use with my ST-120 tube amp. It has no trouble driving them. The speakers can vary from 4 to 16 ohm depending on the 3 switch positions. Because my tube amp is internally wired for a 4 ohm load I leave the switches in the position for 4 ohm performance. Come to think of it I've never hooked them up to my SA250. Hmmm, something to do....................... |
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KeithL
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Post by KeithL on Aug 16, 2021 11:37:37 GMT -5
You are quiet right about speakers and damping.
Tube amplifiers with output transformers have a relatively high output impedance - and so a relatively low damping factor. A typical tube amp, with an output transformer, has a damping factor of around 4 (8 is considered high for a tube amp).
Most solid state amps have a damping factor in the high hundreds. Because of this, in order to sound good, and avoid boomy bass, speakers intended to work with tube amps must provide their own (mechanical) damping.
Most modern speakers have a powerful magnet, a powerful motor structure, and very little internal damping... This offers the amplifier the opportunity to control the motion of the cone very tightly... but only if the amplifier has a very high damping factor.
So this sort of speaker works best with amplifiers that offer a lot of damping.
(When used with an amplifier with insufficient damping many modern speakers produce bass that sounds smooth, but "loose", or poorly controlled.)
Toroid transformers are rarely seen as output transformers... There are various reasons for this, including their electrical and magnetic characteristics, although it certainly can be done. (Note that their "sensitivity to DC offset" is NOT among those reasons... this is a virtue because the DC plate current from the output tubes is intended to pass through the output transformer unimpeded.)
SMPS power supplies have in fact been used in tube amps recently - but most commonly for small tube amps (several of those 5 watt/channel mini-amps you see use a small SMPS).... There is certainly no technical reason why this wouldn't make sense...
However, one of the biggest benefits of SMPS is efficiency, and efficiency is usually not a major consideration with big tube amps...
Most "common" SMPS are designed for delivering a lot of current at relatively low voltage.
Because of this, designing an SMPS for the voltages required by a tube amp is "outside the comfort zone" of both many tube amp designers, as well as many SMPS designers...
And, similarly, running a tube amplifier from a complex solid state power supply is outside the "comfort zone" of many tube amplifier aficionados... (It's really mostly a matter of how comfortable many folks are with "mixing dissimilar technologies"...)
As for that "10 kHz square wave response"... The ability of an amplifier to pass "a clean 10 kHz square wave" serves to demonstrate that it can pass frequencies well above the limit of audibility with little distortion or phase shift. As such, especially with early designs, "being able to pass a clean 10 kHz square wave" was considered an indication that a circuit would also work well inside the audible range of frequencies. However, this is not specifically true, and many modern designs actually limit their response outside the audible band, for various reasons.
In point of fact, as far as audibility is concerned, a perfect 10 kHz square wave sounds exactly the same as a perfect 10 kHz sine wave. (A perfect square wave consists of the full sequence of odd order harmonics... so 10 kHz, 30 kHz, 50 kHz... and the only one of those we humans can hear is the 10 kHz fundamental.) (Therefore, while being able to produce a clean 10 kHz square wave may IMPLY other useful and audible virtues... it is in and of itself meaningless.)
Similarly, with tube amplifiers, "being able to produce a clean 20 Hz square wave whose top doesn't tilt" was considered an indication of good low frequency response.
(Back when good response down to 20 Hz was considered both desirable and difficult to achieve.)
A tube amp when heavily overdriven clips just like a SS amp. Often with worse distortion due to hysteresis in the output transformer. Because of the nature of their circuit gain stages, a tube amp will usually "round" the signal more as it approaches clipping compared to a SS amp. This has more to do with the amount of feedback used in a SS amp compared to tube circuits -- SS amps usually have much higher internal (loop) gain, which allows greater feedback, which provides lower distortion, lower output impedance, higher input impedance, and so forth. SS designs also tend toward differential internal stages, which suppress even harmonics, while single-ended tube designs do not. Tube designs usually have higher distortion and it begins with the second harmonic, which generally leads to more "benign" sounding distortion than the third harmonic that dominates SS designs. The catch is that the SS designs' distortion is usually much lower than that of the tube design. An endless debate... IME/IMO, most of the "tube sound" is not intrinsic to the tubes themselves, but rather a consequence of other design choices, like single-ended topology and transformer coupling. No reason a tube design should not sound the same as SS or vice-versa, but the market demands "tube sound". It is not too hard to change a SS amp to sound like a tube amp, and a properly designed tube amp will sound similar to a SS amp with similar performance (low distortion and all that jazz). Doing blind tests with tube vs. SS amps can be humbling as often they sound indistinguishable. When I was doing such things, I found the biggest factor was from tube amps' generally much higher output impedance, which meant frequency variations into speakers that was picked up in blind AB or ABX testing. Simply sticking a resistor in series with the output of the SS amp was enough to make them indistinguishable most of the time. As I said, humbling, to find out my nice ARC tube amp could be emulated by a fairly cheap little SS amp, and least sonically. Interesting factoid: The mathematical series expansion of a tube's distortion series is factorial, while for a bipolar transistor it is exponential. That means distortion rises faster in a transistor than in a tube. In the real world, other factors than intrinsic device characteristics dominate the overall performance, natch. FWIWFM - Don What I HEARD at shows was that the BEST of SS and Tube sounded remarkablly similar. I wouldn't know if I could tell 'em apart in blind listening. As long as you didn't try to cause hearing damage to the listeners. But one thing that DOES seem real is that the speakers for tube / SS have differences. Damping is one. Tubes work well with speakers that have high internal damping. SS likes to be the damping.....Thus the high damping factor. Might be other differences as well. I'm not convinced that tubes handle highly reactive loads as well as SS. But ONE thing which Does seem correct is that the BEST designs of transformers DO provide wide bandwidth. I'd be curious to know why I see nearly ZERO Toroids in Tube Gear. Either power or output. And Why Not a switcher in a tube amp? Could THAT be made to work? To any advantage? And while lots of people are hung-up on distortion and such, maybe we want to at least ALSO look at 10khz Square Wave response? |
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KeithL
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Post by KeithL on Aug 16, 2021 11:58:03 GMT -5
You must also remember that impedance is NOT simply a single number...
And this gets especially complicated when transformers are involved...
For example... An electrostatic speaker is almost a pure capacitor. And, if you connect an electrostatic speaker directly to an amplifier, it will see that capacitive load.
However, if you connect that same amplifier to that same speaker, through a transformer, that load may appear to the amplifier as capacitive or inductive... (And it may appear capacitive at certain frequencies, and inductive at other frequencies, depending on things like the phase shift in the transformer at various frequencies.)
We tend to use a simplified model that models an output transformer as "a simple impedance matching device".
However, in reality, that is only true over a certain range of frequencies, and with a specific load. And note that the speaker drivers themselves are very complex loads to begin with.
Once you start adding complications, like loads which can be varied, then the reality becomes much more complicated...
In order to accurately model "what the amplifier sees" with that crossover you would have to start by having an accurate electrical model of each driver connected to it...
You would then have to choose one specific combination of switch positions and model the entire circuit... You would then have to repeat that with each and every other combination of switch settings (because the result for each will be different)... And don't forget to include things like the fact that the values of the inductors will change with different signal levels due to saturation... (Luckily these days we have computer programs which will do most of the more complicated calculations for us.)
To be quite honest I wouldn't expect the SA-250 to have trouble with any of them... But, with a tube amp, with an output transformer, you MIGHT find that certain settings work well, while others may not... (You may even find that the amplifier oscillates or is unstable with certain specific combinations of settings...) (Or it might work fine with all of them.)
That is a VERY complex crossover... and modern crossovers rarely have transformers in them. Incidentally, even though they describe it as "an autotransformer", what is shown in the schematic is actually an ordinary transformer, with separate primary and secondary windings. (The definition of an "autotransformer" is that the primary and secondary windings are not isolated... so it's possible that the connection shown between the two windings is actually internal...) Either way, that is a very complex crossover, and its impedance will actually change significantly depending on where the various switches or jumpers are set.
(Basically you're going to have to re-draw the schematic and redo your calculations for each combination of settings.)
Such unusual load impedances CAN cause all sorts of odd interactions with the output transformers on some tube amps... However, that may not necessarily be the case, so as long as it's working well, then don't worry about it. The speakers can vary from 4 to 16 ohm depending on the 3 switch positions. Because my tube amp is internally wired for a 4 ohm load I leave the switches in the position for 4 ohm performance. Come to think of it I've never hooked them up to my SA250. Hmmm, something to do....................... |
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Post by DavidR on Aug 16, 2021 13:41:10 GMT -5
Yes, I'm aware that the impedance can change with frequency. My regular laptop went to the shop this morning so I can not post any before/after pictures of the xover resto.
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Post by leonski on Aug 16, 2021 16:35:18 GMT -5
You must also remember that impedance is NOT simply a single number...
And this gets especially complicated when transformers are involved...
For example... An electrostatic speaker is almost a pure capacitor. And, if you connect an electrostatic speaker directly to an amplifier, it will see that capacitive load.
However, if you connect that same amplifier to that same speaker, through a transformer, that load may appear to the amplifier as capacitive or inductive... (And it may appear capacitive at certain frequencies, and inductive at other frequencies, depending on things like the phase shift in the transformer at various frequencies.)
We tend to use a simplified model that models an output transformer as "a simple impedance matching device".
However, in reality, that is only true over a certain range of frequencies, and with a specific load. And note that the speaker drivers themselves are very complex loads to begin with.
Once you start adding complications, like loads which can be varied, then the reality becomes much more complicated...
In order to accurately model "what the amplifier sees" with that crossover you would have to start by having an accurate electrical model of each driver connected to it...
You would then have to choose one specific combination of switch positions and model the entire circuit... You would then have to repeat that with each and every other combination of switch settings (because the result for each will be different)... And don't forget to include things like the fact that the values of the inductors will change with different signal levels due to saturation... (Luckily these days we have computer programs which will do most of the more complicated calculations for us.)
To be quite honest I wouldn't expect the SA-250 to have trouble with any of them... But, with a tube amp, with an output transformer, you MIGHT find that certain settings work well, while others may not... (You may even find that the amplifier oscillates or is unstable with certain specific combinations of settings...) (Or it might work fine with all of them.)
The speakers can vary from 4 to 16 ohm depending on the 3 switch positions. Because my tube amp is internally wired for a 4 ohm load I leave the switches in the position for 4 ohm performance. Come to think of it I've never hooked them up to my SA250. Hmmm, something to do....................... We've discussed this before. Your post is exactly why I 'stump' for a GOOD speaker-simulating, dummy load to be used as an amplifier test. Nothing wacky to intentionally induce failure, but still must have sufficient reactance to 'test' the amps ability to work into what is a real load. To start? I'd GUESS at 2.8 ohms MINIMUM impedance and +-45 degrees....... Roger that, about 'Stats Especially above 10khz which is OK, since the power requried at those frequencies is maybe 15% of the total required. For the CURIOUS, who are vanishingly few? Investigate 'Power Factor'. This is commonly applied to the power grid / load structure, BUT also applies to your amplifier / speaker combination. As phase angle of voltage / current approaches 90 degrees, Power delivered drops to Zero.... |
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