SSLtech
Well-known member
Dave,
Bore da, Boyo...
The LA2a electro-luminescent panel in the T4 DEFINITELY responds to WELL above 2kHz... It simply HAS to, because if I put a (constant-amplitude) swept tone in, I get essentially-consistent LDR reduction ALL the way to the top of the audio band...
If the EFL panel was truly going 'dark' above 2kHz, there'd probably be some significant reduction in Gain Reduction above that frequency.
But, you touch on an interesting point.
IF the panel is (linearly) less efficient at a certain frequency, then the RATIO at that frequency would be reduced. If the ONSET of illumination was higher, then the THRESHOLD at that frequency would be different.
It's hard to measure inside an LA2a circuit, because it's inside a feedback loop. If -for a simple example- the light turn-on "threshold" was slightly higher, the resultant signal increase would -in turn- increase the drive AC signal to the EFL panel, which would eventually result in gain reduction... the effect doesn't actually DISAPPEAR, but it is 'concealed' by the feed-back nature of the circuit.
For example, if take the cover off a T4 on even a dimly-lit workbench, the GR caused by ambient light leaking onto the optos reduces the signal to the EFL panel so much that the EFL panel almost never lights up. (the drive signal to the EFL panel is sourced post-attenuation, so 20dB of GR caused by ambient light spill, means a 20dB reduction in voltage to the EFL panel... Unless you wind the "peak reduction" control all the way towards the end stops, the T4 will NEVER light up. -If you ever want to see the EFL panel glow in a T4, you essentially have to have the room DARK, DARK, DARK!
Now, taking the T4 OUT of a feedback operation to measure them (as I've done) by slapping a Crown amplifier across the EFL panel connections, and slapping two Fluke 87's across both sets of opto terminals (one DVM measuring each opto) and then feeding it with a constant amplitude swept tone, you can see that the EFL panel does indeed respond comfortably to the upper end of the audio spectrum.
In my measurement setup, I was using a Neutrik A2, which has a calibrated output control, so using a nominal resistance amount (say 1000Ω for example) it was an easy matter to stop the sweep at any given frequency of interest and see how much change in drive level (in dB) would be needed to 'correct' any deviation in opto resistance (which would of course correspond to any nonlinearity in EITHER the opto or the EFL end of things.
Yes there was some fluctuation, but nothing TRULY significant. All the values for opto resistance deviation for each of the T4s which we tested, (including two original Universal Audio T4s, one New-Old-Stock JBL T4, two 'BLOO' T4s and one T4 manufactured by CJ himself) were entered into an excel spreadsheet, for an amplitude sweep/resistance values table. A second test was started for frequency over dB change needed for same opto resistances, but was abandoned early on precisely because it was largely dismissed as not being significant...
So...
It would be silly of me to suggest that you're not seeing what you're seeing, but I do wonder WHY you might be seeing it.
My first question would be -are you certain that you're using a directly comparable material? -Please forgive my ignirance, but are there perhaps more than one type of fluorescent design? How are the electrodes and what effect might surface area/linear size have? -How about series impedance? -Also, have you considered that the series capacitance in a T4 might be there to equalise or compensate for a drooping light output with rising frequency?
While I've not investigated most of these last points, it's not that I have no interese, but the need was never really "pressing" if you know what I mean. -I measured flattish results in-circuit (don't forget that the LA2a also has response-shaping abilities in the peak reduction drive circuit...)
So there's some unfinished investigation results from Keith-land... the only other comment that I'd really like to add is that CJ did send me a beautiful T4 which he himself created, and he's definitely done PLENTY of measuring, though I don't know all of the exact tests he's run... He's published some on his site, and I've run other tests some of which I abandoned when they largely started to corroborate his earlier findings. (no point asking all the same questions twice, after all! :wink
You raise an interesting point however: -Maybe there is an inherent HF decline in most EFL materials which is 'corrected' for in the implementation... or maybe there's more than one type of EFL panel. -Maybe there's a DIFFERENT answer waiting to be proven... Only one way to find out...
Test, measure, publish!
Keith
PS: -Whereabouts in the Welsh hills are you? -And does your surname resemble a christian name following a 'P'?
Bore da, Boyo...
The LA2a electro-luminescent panel in the T4 DEFINITELY responds to WELL above 2kHz... It simply HAS to, because if I put a (constant-amplitude) swept tone in, I get essentially-consistent LDR reduction ALL the way to the top of the audio band...
If the EFL panel was truly going 'dark' above 2kHz, there'd probably be some significant reduction in Gain Reduction above that frequency.
But, you touch on an interesting point.
IF the panel is (linearly) less efficient at a certain frequency, then the RATIO at that frequency would be reduced. If the ONSET of illumination was higher, then the THRESHOLD at that frequency would be different.
It's hard to measure inside an LA2a circuit, because it's inside a feedback loop. If -for a simple example- the light turn-on "threshold" was slightly higher, the resultant signal increase would -in turn- increase the drive AC signal to the EFL panel, which would eventually result in gain reduction... the effect doesn't actually DISAPPEAR, but it is 'concealed' by the feed-back nature of the circuit.
For example, if take the cover off a T4 on even a dimly-lit workbench, the GR caused by ambient light leaking onto the optos reduces the signal to the EFL panel so much that the EFL panel almost never lights up. (the drive signal to the EFL panel is sourced post-attenuation, so 20dB of GR caused by ambient light spill, means a 20dB reduction in voltage to the EFL panel... Unless you wind the "peak reduction" control all the way towards the end stops, the T4 will NEVER light up. -If you ever want to see the EFL panel glow in a T4, you essentially have to have the room DARK, DARK, DARK!
Now, taking the T4 OUT of a feedback operation to measure them (as I've done) by slapping a Crown amplifier across the EFL panel connections, and slapping two Fluke 87's across both sets of opto terminals (one DVM measuring each opto) and then feeding it with a constant amplitude swept tone, you can see that the EFL panel does indeed respond comfortably to the upper end of the audio spectrum.
In my measurement setup, I was using a Neutrik A2, which has a calibrated output control, so using a nominal resistance amount (say 1000Ω for example) it was an easy matter to stop the sweep at any given frequency of interest and see how much change in drive level (in dB) would be needed to 'correct' any deviation in opto resistance (which would of course correspond to any nonlinearity in EITHER the opto or the EFL end of things.
Yes there was some fluctuation, but nothing TRULY significant. All the values for opto resistance deviation for each of the T4s which we tested, (including two original Universal Audio T4s, one New-Old-Stock JBL T4, two 'BLOO' T4s and one T4 manufactured by CJ himself) were entered into an excel spreadsheet, for an amplitude sweep/resistance values table. A second test was started for frequency over dB change needed for same opto resistances, but was abandoned early on precisely because it was largely dismissed as not being significant...
So...
It would be silly of me to suggest that you're not seeing what you're seeing, but I do wonder WHY you might be seeing it.
My first question would be -are you certain that you're using a directly comparable material? -Please forgive my ignirance, but are there perhaps more than one type of fluorescent design? How are the electrodes and what effect might surface area/linear size have? -How about series impedance? -Also, have you considered that the series capacitance in a T4 might be there to equalise or compensate for a drooping light output with rising frequency?
While I've not investigated most of these last points, it's not that I have no interese, but the need was never really "pressing" if you know what I mean. -I measured flattish results in-circuit (don't forget that the LA2a also has response-shaping abilities in the peak reduction drive circuit...)
So there's some unfinished investigation results from Keith-land... the only other comment that I'd really like to add is that CJ did send me a beautiful T4 which he himself created, and he's definitely done PLENTY of measuring, though I don't know all of the exact tests he's run... He's published some on his site, and I've run other tests some of which I abandoned when they largely started to corroborate his earlier findings. (no point asking all the same questions twice, after all! :wink
You raise an interesting point however: -Maybe there is an inherent HF decline in most EFL materials which is 'corrected' for in the implementation... or maybe there's more than one type of EFL panel. -Maybe there's a DIFFERENT answer waiting to be proven... Only one way to find out...
Test, measure, publish!
Keith
PS: -Whereabouts in the Welsh hills are you? -And does your surname resemble a christian name following a 'P'?
