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Old 19th May 2013, 09:35 PM   #61
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Quote:
Originally Posted by bolserst View Post
Looking at your impedance measurements, there seems to be a considerable shift in resonance frequency between curves for 300pF and 460pF load. Is it possible that the 460pF measurements were taken with two transformers but the 300pF was taken with a single transformer?

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Originally Posted by esltransformer View Post
No.
When driving a capacitive load, transformers have two main resonances:
1) HFres = series resonance between capacitive load and leakage inductance resulting in impedance minimum.
2) LFres = parallel resonance between capacitive load and primary inductance resulting in impedance maximum.

In either case, the resonance frequency is proportional to 1/sqrt(LC).
So, if C is increased by a factor of 2 with L remaining constant, resonance frequency decreases by sqrt(2) = 1.414.
Note that the total C used for this calculation would be the sum of the transformer winding capacitance and the externally connected capacitive load.

In the case of your measurements for capacitive loads of 300pF & 460pF, it would be expected that the resonance frequencies would decrease by 10% - 20% depending on the transformer winding capacitance. However your measurements show decreases of 60% - 100%.

Any thoughts on what is causing this discrepancy between theory and measurement?
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Old 19th May 2013, 09:59 PM   #62
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Wait a minute.....

if those curves are developing peaks as a result of changes in load (as well as quite peaked input-impedance curves and marked resonances right in the treble region), how can the panel output be flat? Hard to say what load our panels are presenting isn't it? Aren't we looking for flat (or at least, linear) voltage to the ESLs?

Ben
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Last edited by bentoronto; 19th May 2013 at 10:03 PM.
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Old 19th May 2013, 10:30 PM   #63
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Originally Posted by bentoronto View Post
Aren't we looking for flat (or at least, linear) voltage to the ESLs?
In general yes, we are looking for linear voltage output(ie frequency independent amplitude) at the secondary of the step-up transformer. The peaks and dips in the impedance won't affect the output voltage if the primary is driven by a low impedance source, which fortunately most amplifiers are. However, the peaks and dips in the impedance have a direct affect on the current drawn from the amplifier at different frequencies when it is producing the linear voltage response. Frequencies where the impedance is low require more current from the amplifier than frequencies where the impedance is high.
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Old 20th May 2013, 09:17 AM   #64
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Maybe my amplifier is one of the reasons. It has a -3dB point of 70kHz so the high frequencies are not very reliable.

Quote:
Originally Posted by bolserst View Post
In general yes, we are looking for linear voltage output(ie frequency independent amplitude) at the secondary of the step-up transformer. The peaks and dips in the impedance won't affect the output voltage if the primary is driven by a low impedance source, which fortunately most amplifiers are. However, the peaks and dips in the impedance have a direct affect on the current drawn from the amplifier at different frequencies when it is producing the linear voltage response. Frequencies where the impedance is low require more current from the amplifier than frequencies where the impedance is high.
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