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wow ........for a minute there I was worried, but hell, I can't hear anything above 15khz anyways !
Several points I consider for perception of slight loss of mid/highs, explained for ease of understanding.1) In my experience, I have found that even minute drops in high frequency response (FR) is consistently perceptible. Over the years, I have performed many tests with individuals/auditioners perceiving changes with alterations of 1 part in 180,000 to a resistor in a zobel network across the highs. That is approximately -105db or so from the reference. (We are talking FR change, not SPL change over the entire audio band.) (Perception will depend upon the amount of masking from setup, venue, components etc.2) The FET output devices (if I assume the wrong type of output devices, please forgive me) operated as source followers; there are several considerations. A. The junction capacitances of output FETs are large, but is much lower in Source follower circuits by definition. Now parallel FETs and the capacitance is much larger (add capacitances of each that is paralleled). The voltages across the FET also affect the junction capacitances. B. As the load impedance drops, the junction capacitance increases due to lower voltage gain (which is less than 1 anyway). This is by definition. C. The input Miller capacitance is also affected by decrease load due to less gain, thus the previous stage's load is affected. As one can see, altering the load causes several alterations in the performance. As such, I can understand the perception of slight alteration of the mid/highs. Whether this answers your question AndyR is another matter.CheersSteve
Thanks for posting, Steve. It actually wasn't that I was conscious of some HF roll-off - in fact, I always thought my Maggie ribbons extended a long way and sounded pretty damn good! As did others who came to listen to my system. It was simply that I started to do measurements in REW with a calibrated mic - and saw a steep roll-off after 16Khz, which I didn't think should be there. Now I understand that this kind of roll-off might be standard and due to room absorption ... so I will try out 2 further experiments, to see whether I get different results:1. try a different amp on my ribbons - one that is known to handle a 2ohm load with ease.2. measure with the mic 3" from the ribbon (instead of at the listening position).
And re. your experiments with minute drops in high frequency response, you might be interested in the following experiment I did a few years ago, during the early development of my 'Muse' JFET-based phono stage. The initial circuit (which someone sent me with the exhortation "You have to build this - it's the best phono stage I've ever heard.") was judged by several people - when I'd built it - to be slightly rolled off in the HFs. A friend of mine simulated the circuit in LTspice and found there was a cap between the Gate and the Drain of the 1st gain stage JFET, which was causing a roll-off at 120Khz. Removing it didn't have any negative affect on the circuit (in LTspice, anyway) - but it meant the circuit now didn't have that 120Khz roll-off.Sure enough, when people listened to v2 ... they no longer though it was rolled-off in the HFs! Regards,Andy
I understand. Looking forward to your experiments Andy.Your conclusion reinforces all my past work, and more, Andy. Back in the late 80s, I began research in the high frequency arena by performing an experiment where I altered the -1db point at 200khz to -1db at 150khz and visa versa, which alters the rise time and fall time. That change was clearly perceived over and over by all who have participated. Our work also agrees with research performed by several scientists, the medical community etc. (By the way, the small capacitor that was removed was to prevent any possible RF oscillations under unusual conditions.)Cheers and thanks for your information Andy.Steve ps. Please forgive me for the updates to this post.
Were there any visible differences in your high frequency O-scope inspection?Any differences in your high frequency IM tests?How about audio band frequency response?
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