JohnRoberts said:Once again the ancients are stealing our ideas...
JR
who woulda thought time travel would be invented in the past....
i guess they need it the most.
JohnRoberts said:Once again the ancients are stealing our ideas...
JR
LdSpkrPro said:The input impedance is 604 ohms, (or in special cases, 2.4k) and the typical dynamic 200 ohm nominal microphone is usually about 120 ohms, so the input transistor sees the parallel impedance of that 120 ohms across its 604 ohm resistor. This is important, because this input circuit gets quieter as the input impedance gets lower.
This configuration is noticeably quieter than most other inputs ... and the difference isn't subtle. There are several other things that contribute to the low noise floor, but that configuration was chosen specifically for its low noise characteristics.
I would say that it HAS to be 1:1. The circuit's input impedance is governed by the emitter resistor. The transistor's Re (25/Ic), which is in parallels with it is in fact almost completely compensated by NFB, so the resulting impedance is probably about 2k. It is in accordance with the accepted standard of 10x the mic's impedance. Using a step-up xfmr would improperly load the microphone.LdSpkrPro said:The reason the emitter input is used is several fold. First of all, it is inherently low-impedance. That means, when an input transformer is used, it can be a 1 to 1 ratio.
The actual source impedance seen by the transistor is dominated by the microphone, at about 200 ohms, and the 100r res to ground, so the transistor's operating point must be tailored for such a low impedance; that's why it runs at about 500uA.Secondly, the circuit is inherently quieter than the common emitter designs used in op-amps. For nearly all the op-amp designs, the lowest noise at the input will be with about a 10 k input impedance... so if operated from a microphone, without a matching transformer, you are not operating at the lowest point of the noise curve.
There is a law of diminishing returns there, because the loss introduced by the emitter resistor increases more rapidly than the noise decrease. When the input impedance is equal to the source impedance, the noise advantage is a tad above 3dB (3dB because of the Johnson formula, and a tad because the contribution of noise current is slightly smaller, but noise voltage is constant and dominant), but the signal loss is 6dB.With the emitter input, whatever source impedance you have is in parallel with the emitter resistor, and therefore the input transistor, which determines the noise floor, sees an even lower impedance. The input impedance is 604 ohms, (or in special cases, 2.4k) and the typical dynamic 200 ohm nominal microphone is usually about 120 ohms, so the input transistor sees the parallel impedance of that 120 ohms across its 604 ohm resistor. This is important, because this input circuit gets quieter as the input impedance gets lower.
I must say I'm not convinced. My left brain says it cannot be better in terms of noise than a common-emitter stage. My right brain says if it was indeed better, why hasn't it become the de facto standard?This configuration is noticeably quieter than most other inputs ... and the difference isn't subtle. There are several other things that contribute to the low noise floor, but that configuration was chosen specifically for its low noise characteristics.
I haven't seen this paper; maybe I'm missing something, but I can't help thinking that the designer has found that a finely-tuned common-base stage performs better than an out-of-whack common-emitter.The original AES paper on the 101 actually spells out the derivation of this circuitry, and the whys and hows that it came to be.
?? You paint an amusing picture, but I can't say I recognize that behavior in any peers, let alone an entire industry.LdSpkrPro said:Abbey road d enfer ...
In circuit, measured, you'll find it to be just about 600 ohms or bit higher.
The fact that it has not become a de facto standard probably has something to do with the fact that the entire audio business is like a parade of circus elephants, every one of them holding onto the tail of the one in front of it, *****ing about the view, but afraid to let go and do any independent thinking.
You need new friends, or at least different acquaintances. Your rose colored glasses are a little dark.There is so little original thinking, instead, I see a lot of outright copying in the audio business. That and the fact that people seem content with Band-aids for their problems, instead of getting rid of them in the first place.
I recall some interesting disagreements (opinions?) about how exactly to specify ein, but I think Buff's article in RE/P back when there was an RE/P, was decent. Paul even sold a noise measurement rig for a while. I always preferred to think in terms of NF, for low noise design, since it is harder to game with BW or weighting curves.. This isn't rocket science but admittedly the rocket scientists were busy making rockets.It has been my experience that people who want to argue about the input noise have usually had very little in-field experience measuring it. I used to install SS consoles, and the last part of the installation was to demonstrate the guaranteed performance to the purchaser ... and that included the 80 dB signal to noise ratio as shown on an HP 400GL. We were always able to demonstrate that ... while I know of other manufacturers who had to make cash rebates because consoles didn't meet their advertised specs.
Indeed, the schemo published by the OP indicates a 2.43k res; it may be a special version. This is the one I'm referring to. Who am I to know?LdSpkrPro said:Abbey road d enfer ...
In circuit, measured, you'll find it to be just about 600 ohms or bit higher.
Not me , sir... I have designed mixers since 1976, achieving -128dBu EIN at 200 ohms/20k BW, which is 1.7dB NF with off-the-shelf transformers and commonly available discrete transistors. Once this not particularly difficult issue was addressed, I devoted most of my time solving the much more complex problem(s) of longitudinal noise.It has been my experience that people who want to argue about the input noise have usually had very little in-field experience measuring it.
In fact, designing an input amp with -125dBu is nothing to write home about, but it would measure at 85dB S/N with a-40dBu input signal.and that included the 80 dB signal to noise ratio as shown on an HP 400GL.
JohnRoberts said:I recall some interesting disagreements (opinions?) about how exactly to specify ein, but I think Buff's article in RE/P back when there was an RE/P, was decent.
JR
Certainly the latter.LdSpkrPro said:It seems that we are talking about apples and oranges ... or a least significantly different measurement conditions.
This is only one measurement, not much related to the typical operation of a mixer. Who wants a mixer with just one input used?When we demonstrated guaranteed performance after the installation of a Spectra Sonics audio mixing console, these were the measurement conditions: a - 50 signal was input to a microphone preamp, and the gain thru the console adjusted to produce +4 dBm at the output. All other faders down. The signal was removed and replaced by a 200 ohm resistor. Then the noise on the console output was measured over a 20 to 20 kHz bandwidth.
That is less than the Johnson noise of the 200r source. So these mixers had negative Noise Factor? I understand your desire to past the paint with rosy colours, but it must stand the test of scrutiny.It was guaranteed to be 80 dB below the + 4 dBm output. This was thru the entire console, installed. That computes to - 130 dBu EIN (-76 dBu output noise, 54 dB gain)
At what frequency? At 1 khz, most xfmrs have less than 10 ppm THD.The other interesting spec was the harmonic distortion, again, thru the entire console. It was spec-ed and delivered at less than 0.01 %, 20 to 20 kHz, at output level. That was also demonstrated. And this was using 'off the shelf' transformers ... well.
You seem to have a very pessimistic opinion of the state of the industry; it may be realistic for some of the cheaper brands, but there are a number of respectable manufacturers who devote themselves to the avancemnt of the audio art. I think Millenia, Forsell, RND, and many others do not deserve your despise.Today, people like to quote theoretical limits on noise as gospel... or excuses for mediocre performance.
gridcurrent said:JohnRoberts said:I recall some interesting disagreements (opinions?) about how exactly to specify ein, but I think Buff's article in RE/P back when there was an RE/P, was decent.
JR
Paul's measurements did not agree with anyone's. A theoritical limit of EIN of about -124 dBme, whatever that was supposed to be.
Maniacal? I recall when the TL07x was so new that the process wasn't very stable (I had to test them 100% for first few years), but it was a remarkable value for the modest price back in the '70s. Even today it is not a very weak link when properly applied. Of course we have a lot of better choices (for more money) today to use in more demanding sockets. I don't see much value in debating the hammers or nails without specific context of exactly how used, and not sure I want to go down that rabbit hole very far.LdSpkrPro said:Yes, that is only one measurement. However, you could pick any of the inputs and get the same measurement! And as to the point of you don't use only one input ... that is quite correct. If you start with a substantially lower noise floor on each individual input, then you wind up with a lower noise floor in the mix ... that's the whole point.
It's hard to call it painting the past with rosy colors, that's what I actually measured ... without any games, with HP 400GL. As I pointed out in an earlier post, the input circuit delivers better than expected noise floor because of the configuration. Everyone seems to miss that point. It delivers better than the theoretical.
But the real significant point here is, that was the measurement for a fully terminated console, with all 24 tracks and peripheral gear connected. And if you think that doesn't contribute to noise floor because of grounding considerations, they I would suggest you've not terminated very many large consoles!
I remember one console termination where we had to disconnect the clients ground rod which they had carefully, and with much difficulty driven 12 feet into the earth, right thru the trough beneath the console. It was a suberb RF antenna. Apparently, it didn't occur to them that the ground would be quite dry under the 100 year old building.
The harmonic distortion specification was for any frequency, 20 to 20 kHz.
I agree there is some interesting work being done ... but there's also a maniacal hanging on to variations of the TL0-72. And I seen copies of other manufacturer's printed circuit boards copied, without even removing their logos ... I think my characterization of the audio business as a parade of circus elephants is apt.
This ends the discussion, doesn't it? You have the right to believe that the company you worked for had magic powers that allowed them to dispense with the laws of physics, you have the right to believe that you are so much above us mere mortals, I have the right to be uninterested in sterile divagations.LdSpkrPro said:It delivers better than the theoretical.
You don't know me, you don't know what my professional history is. I'm not going to start a pissing contest with you.And if you think that doesn't contribute to noise floor because of grounding considerations, they I would suggest you've not terminated very many large consoles!
Lets keep this about things not people (or elephants).LdSpkrPro said:I guess you think that referring to me as believing in magical powers somehow diminishes what I reported. The simple facts of the matter are the discussion was about the 'strange' input circuitry, and the whys it existed. I attempted to explain some of the whys of the circuit, and what the advantages were, and that the configuration allowed for a significant improvement noise-wise. I also reported what I measured, and the conditions under which it was measured. That you don't believe it possible, that is measures better than the so-called theoretical limits for a given impedance, only demonstrates that you've not personally measured it. I'm neither a fool nor a liar, nor do I believe in 'magic.' I do know what I measured, and I reported it accurately.
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