AudioCircle
Audio/Video Gear and Systems => The Lab => Topic started by: andyr on 21 Feb 2017, 10:08 pm
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I was wondering whether anyone has experienced a severe rolling off of high frequencies - say, 15Khz and above - with a mosfet amp?
I did some measurements on my mosfet amp which drives 2 ohm tweeters (in an active setup). To my surprise, the output at 16Khz was only 80% of the output at 6Khz (for a constant input) ... and the output at 20Khz was down to 40%. :( (This is with the 2 ohm tweeter connected.)
I have read some people - who perhaps are biased towards BJT amps :) - call this the "mosfet power envelope" problem. Has anyone else experienced this kind of problem?
Thanks,
Andy
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You used music or sinusoidal signal of that freq?
If test was w/music is rare a musical piece has any fundamental freq above 5kHz only hamonics unless its church organ, over 10kHz even presence of harmonics is rare.
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You used music or sinusoidal signal of that freq?
If test was w/music is rare a musical piece has any fundamental freq above 5kHz only harmonics unless its church organ, over 10kHz even presence of harmonics is rare.
I used sine waves from my sig-gen. (Sorry, should have noted that.)
Andy
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I was wondering whether anyone has experienced a severe rolling off of high frequencies - say, 15Khz and above - with a mosfet amp?
I did some measurements on my mosfet amp which drives 2 ohm tweeters (in an active setup). To my surprise, the output at 16Khz was only 80% of the output at 6Khz (for a constant input) ... and the output at 20Khz was down to 40%. :( (This is with the 2 ohm tweeter connected.)
I have read some people - who perhaps are biased towards BJT amps :) - call this the "mosfet power envelope" problem. Has anyone else experienced this kind of problem?
Thanks,
Andy
roll off has nothing to do with impedance of load,mosfets have high input capacitance that causes this,never the less if mosfet amps are driven hard there is no problem there.
cheers :green:
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roll off has nothing to do with impedance of load,mosfets have high input capacitance that causes this,never the less if mosfet amps are driven hard there is no problem there.
cheers :green:
Sorry, I don't understand what you are trying to say.
When my mosfet amp is plugged into an 8 ohms load ... it has no problem delivering 30Khz.
However, when it is plugged into my 2 ohm ribbons ... its output at 30Khz is only 40% of its output at 6Khz. To my way of thinking, that makes it pretty useless for a tweeter amp! :D
Andy
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Sorry, I don't understand what you are trying to say.
When my mosfet amp is plugged into an 8 ohms load ... it has no problem delivering 30Khz.
However, when it is plugged into my 2 ohm ribbons ... its output at 30Khz is only 40% of its output at 6Khz. To my way of thinking, that makes it pretty useless for a tweeter amp! :D
Andy
sounds like a lousy tweeter mate... :green:
mosfets bw goes up to 100khz when designed right
sorry i can't say more (proprietary technology)
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mosfets bw goes up to 100khz when designed right
Good to hear, GG! :D
Andy
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Is the amp actually rated to drive a 2 ohm load?
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Is the amp actually rated to drive a 2 ohm load?
Supposedly, yes, John. When I upgraded from my previous ribbon amp, the designer said that it would be 'able to handle' 2 ohms ... but would get warm.
Andy
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Is the amp actually rated to drive a 2 ohm load?
Hi John
Mr MOSFET NP look at last feature of this link:
https://passlabs.com/products/amplifiers/integrated-series
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Were you measuring at the amplifier terminals or at the loudspeaker terminals?
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Andy,
Are you positive your measuring scheme is without issue?
Your generator is not rolling off at high frequencies? Your voltmeter is not rolling off at high frequencies? Etc, etc, etc.
Do you have 2 ohm and 8 ohm resistors you can measure with vice a loudspeaker load?
You could also try making a measurement with no load connected......to establish a reference.
I the roll-off is indeed real, then you have a either a poorly designed and/or defective amplifier.
Dave.
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You could also try making a measurement with no load connected......to establish a reference.
I would add making a measurement with a 8 ohms load.
2 ohms is more a short circuit than a load.
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2 ohms is more a short circuit than a load.
Not really.
2 ohms is much greater than the output resistance of this amplifier so a simple, low-power, 2 ohm measurement at high frequencies should not cause any decent amplifier a problem. If Andy's significant roll-off is indeed real it implies there would be a significant roll-off with a 4 ohm load as well.
That said, I suspect this is a measurement issue.
Dave.
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Not really.
2 ohms is much greater than the output resistance of this amplifier so a simple, low-power, 2 ohm measurement at high frequencies should not cause any decent amplifier a problem. If Andy's significant roll-off is indeed real it implies there would be a significant roll-off with a 4 ohm load as well.
That said, I suspect this is a measurement issue.
Dave.
2 ohms for audio is suicidal, I had a diode fried in a Hartke HA3200 basshead w/a 4 ohms pro FR speaker. I alert that operating at 2 ohms can lead to great emotions.
(http://www.audiocircle.com/image.php?id=22372)
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The suicidal rhetoric is unnecessary.
I'm talking about a quick simple low-power test to confirm (or not) this HF roll-off of Andy's amp. I'm NOT talking about the long-term normal power operational characteristics when dealing with a real load year in and year out.
Let's stay focused please.
Dave.
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Were you measuring at the amplifier terminals or at the loudspeaker terminals?
I was measuring AC voltage with a multimeter on the driver input terminals - I also had my CRO connected to the amp's input terminals, so I could adjust the sig-gen output to make sure it was outputting the same signal level at all frequencies.
Andy
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Andy,
Are you positive your measuring scheme is without issue?
Your generator is not rolling off at high frequencies? Your voltmeter is not rolling off at high frequencies? Etc, etc, etc.
Dave.
Hi Dave,
I have my CRO on the amp's input terminal, to make sure the sig-gen inputs the same voltage (60mV) to the amp, at all frequencies tested.
Not sure how the multimeter (set to the 2v scale) could be "rolling off at high frequencies" ... all it's doing is measuring the voltage at the ribbon input terminals - so shirley it doesn't care what frequency the input signal is?
Do you have 2 ohm and 8 ohm resistors you can measure with vice a loudspeaker load?
You could also try making a measurement with no load connected......to establish a reference.
I do have suitable 2 & 8 ohm reses ... but my thinking was that it is better to use the real (driver) load ... as the ribbon may have a capacitative element which means it's a harder load to drive than a straight resistor?
I the roll-off is indeed real, then you have a either a poorly designed and/or defective amplifier.
Poorly designed! :cry: :cry:
Andy
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First using an old fashioned CD player, double check your DMM at the output of the player using a test tone CD.
Then measure at the output terminals of the amp with no load then 8 Ohm and 2 Ohm loads. Finally the loudspeaker but keep the probes on the amps terminals. Found that out decades ago with an industrial amp that drove a shaker table.
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Andy,
Many AC voltmeters don't have flat response to 20khz. Many don't have response above approximately 400Hz.
You can check yours as Speedskater suggested by attaching directly to your signal source to verify it.
If your speaker load is not resistive then you should set it aside for the time being.
Check your amplifier only with load resistors and/or with no load, so you can characterize it first before you create a complex load by attaching a speaker.
Dave.
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Andy,
Many AC voltmeters don't have flat response to 20khz. Many don't have response above approximately 400Hz.
You can check yours as Speedskater suggested by attaching directly to your signal source to verify it.
Dave.
Aah, OK, dave. That is disturbing (and shows that my measurement process may well have been suss!!) :(
What if I measure the voltage at the ribbon terminals with my CRO?
Shirley that should be flat to beyond 30Khz?
Andy
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Yes, your scope should work okay.
BUT, be VERY careful with grounding since your scope is AC powered and probably not floating like your voltmeter.
I would recommend using your voltmeter to be on the safe side......but (obviously) only if it has response to 20khz. (You need to characterize it for future usage anyway.)
Dave.
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Yes, your scope should work okay.
BUT, be VERY careful with grounding since your scope is AC powered and probably not floating like your voltmeter.
I would recommend using your voltmeter to be on the safe side......but (obviously) only if it has response to 20khz. (You need to characterize it for future usage anyway.)
Dave.
Thanks, Dave.
Will re-measure at the w/e.
Andy
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Yes, your scope should work okay.
BUT, be VERY careful with grounding since your scope is AC powered and probably not floating like your voltmeter.
Dave.
Thank you Dave for your suggestion that the multimeter was not reading true. :thumb:
Did the same measurements with the CRO this evening - and got an identical voltage reading from 6khz to 30khz. So that is a relief - it would seem my amp does not 'have a problem' with 2 ohms ... it can pump out the same signal voltage from 6Khz to 30Khz.
So this leads on to ... if the amp is not at fault ... why does REW show a steep drop-off at 16Khz when:
* the mic is a calibrated mic (so has a calibration file which is fed into REW)
* the amp is delivering flat to 30Khz?
Andy
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What is your CRO?
Cathode Ray Oscilloscope?
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* the mic is a calibrated mic (so has a calibration file which is fed into REW)
I am totally confused. You have not mentioned anything about a microphone before in this thread?
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What tweeters?
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Thank you Dave for your suggestion that the multimeter was not reading true. :thumb:
Did the same measurements with the CRO this evening - and got an identical voltage reading from 6khz to 30khz. So that is a relief - it would seem my amp does not 'have a problem' with 2 ohms ... it can pump out the same signal voltage from 6Khz to 30Khz.
So this leads on to ... if the amp is not at fault ... why does REW show a steep drop-off at 16Khz when:
* the mic is a calibrated mic (so has a calibration file which is fed into REW)
* the amp is delivering flat to 30Khz?
Andy
Dunno. :)
Either your speakers are indeed rolling off above 16khz and/or it's something in your measuring setup/scheme.
Dave.
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andyr, where is your mic located when the measurement is taken? It is common to see a roll off in high frequencies at the listening position due to the rooms absorption characteristics. If the mic is maybe 3 inches away from the tweeter on axis and time gated eliminate the sound from the other drivers then you have a problem. You haven't quantified the magnitude of the HF roll off. How many dB down is it at 20kHz.?
Scotty
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A microphone measuring system is a poor choice for testing amplifiers.
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A microphone measuring system is a poor choice for testing amplifiers.
I believe that you can't use REW without using a mic?
andyr, where is your mic located when the measurement is taken? It is common to see a roll off in high frequencies at the listening position due to the rooms absorption characteristics. If the mic is maybe 3 inches away from the tweeter on axis and time gated eliminate the sound from the other drivers then you have a problem. You haven't quantified the magnitude of the HF roll off. How many dB down is it at 20kHz.?
Scotty
Aaah, that is a very interesting piece of information, Scotty. :thumb:
My mike was at the listening position, pointing to the middle between the Maggies - so I will try re-measuring just the ribbon, with the mic 3" away from the ribbon, on axis.
But if a room typically absorbs HFs, why do speaker designers (AIUI) aim for slightly downwards-tilted FR (downwards towards the HFs) ... if the room itself is absorbing HFs? Surely they would aim for 'flat', knowing the room will provide the absorption which makes the speaker sound pleasant to the ear?
The REW curve shows a flat response from about 6Khz to 16Khz, then a sharp roll-off. The graph is 4.5dB down at 20Khz.
Dunno. :)
Either your speakers are indeed rolling off above 16khz and/or it's something in your measuring setup/scheme.
Dave.
Absolutely, Dave.
So apart from doing the measurement I mentioned above, I will be borrowing an ME 75C amp in a couple of weeks time and using that on my ribbons in place of my current amp ... and then doing the same "listening position" measurement. This amp is known to be able to drive 2 ohm loads with no problem.
If the roll-off starts higher ... then my current amp is inadequate. If it shows the same 16Khz roll-off point ... then "that's just how it is"! :D
Regards,
Andy
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I believe that you can't use REW without using a mic?
....................................... ........
Regards,
Andy
Then it's the wrong tool for measuring an amplifier.
Only use microphones for measuring rooms and loudspeakers.
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Then it's the wrong tool for measuring an amplifier.
Only use microphones for measuring rooms and loudspeakers.
Perhaps you should read my thread, Kevin - so as to get the full story?
I'll summarise for you, if you don't 'get it'.
Andy
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Perhaps you should read my thread, Kevin - so as to get the full story?
I'll summarise for you, if you don't 'get it'.
Andy
A different thread than this one?
In this thread, post #1 you are measuring an amplifier.
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Now that we know you own Maggies, have you looked at Stereophile's measurements of the MG 3.6R? Might be useful:
http://www.stereophile.com/content/magnepan-magneplanar-mg36r-loudspeaker-measurements#E5xw1jfjkmOsWYVL.97
Here is the 1.6R:
http://www.stereophile.com/content/magnepan-magneplanar-mg16qr-loudspeaker-measurements#chlkD3PakaaMY1ER.97
Here is Sound & Visions for the MMGW:
http://www.soundandvision.com/content/magnepan-mmg-speaker-system-measurements#iIz408mSaUPCezrO.97
My guess is that many of the Magnepan models in the upper midrange and treble measure similarly and they are challenging to measure accurately due to the dipoles nature. And off axis, the HF can take a nose dive. Your listening position (where your current mic measurements are) is very likely to be off axis.
Another thing. If you were measuring at the listening position, and making measurements greater than 300Hz, you will have some erroneous data if measuring both speakers at the same time due to comb filtering. Best to do either measurements outside (groundplane would be best) or have the mic within a few inches of the ribbon (as Scotty suggested) in order to minimize room effects. And measure one speaker at a time.
Regarding your amplifier, I agree with everything Davey stated earlier. And if you are going to make any conclusions about the amplifier, please have it competently measured. Don't derive conclusions based on in room measurements. This can give you undue concern about whether or not the amplifier is misbehaving.
Best,
Anand.
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A different thread than this one?
In this thread, post #1 you are measuring an amplifier.
My apologies, Kevin. Going back to post #1, I see I forgot to say why I was measuring my amplifier. :oops:
The reason was that REW sweeps showed a steep drop at 16Khz - 4.5dB down at 20Khz ... yet the Maggie true-ribbon is specced to 40Khz. Another person with true-ribbon Maggies that he is driving 3-way actively had found a similar roll-off and when he substituted a different amp for his ribbons, he said the roll-off went away. So his first amp was unable to deal with the low ribbon resistance - whereas the replacement amp could.
So I was wondering whether my own ribbon amp was causing my roll-off. Hence my proceeding to measure my amp - unfortunately, with the wrong tool, first up.
Now I know there isn't anything wrong with my amp (and the roll-off might just be caused by room absorption), I will do some more measurements with the mic in front of a ribbon ... and test out a different amp on the ribbons in a couple of weeks time.
Now that we know you own Maggies, have you looked at Stereophile's measurements of the MG 3.6R? Might be useful:
http://www.stereophile.com/content/magnepan-magneplanar-mg36r-loudspeaker-measurements#E5xw1jfjkmOsWYVL.97
Best,
Anand.
That graph is for a complete 3.6 - which consists of a bass panel, a mid panel, a ribbon tweeter plus a 3-way passive XO, each side. I drive my Maggies 3-way active - so I am just putting sound through - and measuring - the ribbon, by itself.
Another thing. If you were measuring at the listening position, and making measurements greater than 300Hz, you will have some erroneous data if measuring both speakers at the same time due to comb filtering. Best to do either measurements outside (groundplane would be best) or have the mic within a few inches of the ribbon (as Scotty suggested) in order to minimize room effects. And measure one speaker at a time.
Sure, I then went on to do an REW sweep on just one speaker,
Regards,
Andy
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Andy,
At this point I'm not sure what your objective is.
You've pretty much confirmed your amplifier is not rolling off drive at high frequencies. Unless you have some other mystery device between the tweeter and your amplifier, there's nothing left to test.
If both left and right tweeters are measuring the same, then you must assume they're working correctly. Any measurement anomaly from the expected flat response to 40khz MUST be the result of your measuring scheme and not a problem with the tweeters themselves.
I hope you're not going to apply equalization to the tweeters to boost their outputs. :)
My recommendation would be to declare victory on this and move on to the next issue.
Dave.
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Andy,
At this point I'm not sure what your objective is.
You've pretty much confirmed your amplifier is not rolling off drive at high frequencies. Unless you have some other mystery device between the tweeter and your amplifier, there's nothing left to test.
If both left and right tweeters are measuring the same, then you must assume they're working correctly. Any measurement anomaly from the expected flat response to 40khz MUST be the result of your measuring scheme and not a problem with the tweeters themselves.
I hope you're not going to apply equalization to the tweeters to boost their outputs. :)
My recommendation would be to declare victory on this and move on to the next issue.
Dave.
All very true, Dave. :thumb:
The amp measures flat (when using the CRO to measure).
So it looks very like the roll-off I see on the REW graph is due to room HF-absorption.
So I will re-measure with the mic 3" in front of the ribbon - and see what I get. Obviously, the lower frequencies on the REW graph will be completely different as the mic is now pointing directly at the ribbon, instead of being at the listening position, pointing at a speaker - but the HFs should now be flat to 20Khz (the limit of REW).
However, given the other guy with Maggies said his roll-off disappeared when he used a different amp, I am going to try a different amp in a couple of weeks time. (Of course, he may not be telling the truth! :D )
Andy
PS: Many thanks to you and others on this thread for pointing me in the right direction.
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He's either not telling the truth or doesn't have a clue what he's doing. (I'd bank on the latter.)
When "other guys" experience and post about issues like this, I always make the assumption there's much more to the story that they don't understand.
I would certainly not base any of your own premise/investigations on what some "other guy" is doing. You're most likely guaranteed to run off on some incorrect tangent......like you did here.
If you really want to know what's going on, just post a query to the other forum. That "other guy" Satie will reply and set you straight. He's got an answer for everything. :)
Dave.
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He's either not telling the truth or doesn't have a clue what he's doing. (I'd bank on the latter.)
Dave.
Yes, the other guy is a flake and probably knows even less than I do! :D He's newly arrived at MUG - although that's not the Forum I was referring to - have a read of some of his posts and see what you think. I'll email you his moniker.
Trying out another amp will be an educational instance AFAIAC. :D
If you really want to know what's going on, just post a query to the other forum. That "other guy" Satie will reply and set you straight. He's got an answer for everything. :)
Dave.
No, it's not 'Mr S'! :)
Regards,
Andy
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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.
Cheers
Steve
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wow ........for a minute there I was worried, but hell, I can't hear anything above 15khz anyways !
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wow ........for a minute there I was worried, but hell, I can't hear anything above 15khz anyways !
How did you come up with 15khz? :green:
Olson's work identified that the higher the harmonic the easier to perceive changes in the amplitude.
Thus instruments and voices are critically affected by very small changes.
Rane also performed some research pertaining to FR perception. Pretty interesting stuff.
Cheers
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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.
Cheers
Steve
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. :lol: 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! :o
Regards,
Andy
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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. :lol: 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).
I understand. Looking forward to your experiments Andy.
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! :o
Regards,
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.
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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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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?
No, S - I didn't measure IM.
I was only interested in (solving) the frequency roll-off.
Now that I have removed the cap which was providing a LP filter @ 120Khz, my audio band FR is pretty 'flat' (taking into account the RIAA correction).
Andy
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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?
Time was back in the 80s, will attempt to remember what I can. As you know, the frequency response changes (FR) become very small as one goes towards mid band. As such, using a scope on a sinewave with very small FR changes is rather difficult. I would rather use a rectangular waveform, especially with a higher fundamental frequency as one can check rise and fall times, ringing etc easier.
Never checked IM, but the HD distortion spec on the pre was at least -78db down. Changing the FR, Rise and fall times would be affected, which would be perceived according to several studies, including Medical inclusion in studies and Jneutron's reference which I do not remember.
(Jneutron is an avid audiophile who teaches science classes to other scientists, has worked at FermiLab, CERN, and last time I heard at Brookhaven National Laboratory.)
With -1db down at 150khz, 75khz would be very approximately -0,4db down. At 37khz response would be down approximately -0,15db. From going lower would require my computer simulation, but that computer has recently acquired video problems. These are general approximate figures when halving frequency; except as the frequency approaches mid band the minus db figures tends to really "flatten out" of course.
With -1db down at 200khz, 100khz would again be very approximately -0.4db down, and etc etc.
The roll off with the circuit I used would be 1st order, so approx. 6db per octave.
Another recent experiment has been my test speakers. For the past 4 years I have been adjusting the zobel and xover networks.
As such, one one experiment involves adjusting a parallel resistor across the full range driver. Over and over, adjusting the 9,000 ohm (9K ohm) resistor by very easily less than 0,1 ohms is clearly perceived by individuals. That is not only 1 part in 90,000, but is approaching 1 part in 180,000, equating to a frequency response change so small, in the neighborhood of -105db. (This is not an spl change across the entire audio band.)
Previously, over several years, in another experiment, bass response has been altered by a parallel adjustment on the back of my preamplifier. This adjustment compensates for the differences in amplifier input impedance. Over and over, with a single cello, we could perceive differences in the 32udb (32 micro db range) at 20hz. Yet a cello's lowest fundamental is in the range around 60 or 65 hz.
Each of the different methods I have used and still use, and by other studies, resulted in nearly the same results, that extremely small changes in frequency response, at both ends of the audio spectrum, are perceived. Of course, venue, musical selections, component quality etc would make for different results.
Olson's work included the effects of perception as the harmonic number rises (2nd harmonic, 3rd harmonic, ...... 9th harmonic etc). The higher the harmonic number, the more perceptible it is when that harmonic is altered in relation to the fundamental. It is incredible the sensitivity of the "ear" when it comes to tonal balance.
Cheers
Steve