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Old 29th January 2013, 03:35 AM   #33811
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Yes, Richard. Reasons why we prefer good transformers for mikes and lines in professional area, despite the price of devices like Lundahl.
Reasons why, often, power amps works better with a low pass filter in their inputs.
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Last edited by Esperado; 29th January 2013 at 03:38 AM.
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Old 29th January 2013, 04:19 AM   #33812
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Quote:
Originally Posted by Esperado View Post
Yes, Richard. Reasons why we prefer good transformers for mikes and lines in professional area, despite the price of devices like Lundahl.
Reasons why, often, power amps works better with a low pass filter in their inputs.
The input signals to all products should not be greater than the BW of the amp. Notice that the frequency response tests on M-Levenson products had rolled off high freq just above audio range/ 20Khz. This was done at the input... Limiting the distorting affects of EMI/RFI and unwanted HF/RF from entering the amps/preamps. Thx -Richard

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Old 29th January 2013, 05:26 AM   #33813
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The input signals to all products should not be greater than the BW of the amp. Notice that the frequency response tests on M-Levenson products had rolled off high freq just above audio range/ 20Khz. This was done at the input...
..... at the cost of transparency and resulting in a "special" sound. Especially with hires sound sources, this way is a bit unhappy. Much better to work on high BW product without necessity to limit input below 100kHz.
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Old 29th January 2013, 05:36 AM   #33814
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..... at the cost of transparency and resulting in a "special" sound. Especially with hires sound sources, this way is a bit unhappy. Much better to work on high BW product without necessity to limit input below 100kHz.
No need to limit amplifiers internal BW. A simple rc roll off on the input will help. Just design best amp possible then simple filter on input wont be audible and amp characteristics still all there. i am not saying ML didnt limit BW best way or at too low freq. its just what they did to avoid HF/Rf issues.
Personally, I would probably agree with 100KHz as a more reasonable number. Internally it can be very, very fast. As fast as needed.... most designs end up with very wide BW in process of making very linear amp using best transistors. But they invite HF/Rf into them with unintended side-effects that are not pleasant. -Thx RNMarsh

Last edited by RNMarsh; 29th January 2013 at 05:39 AM.
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Old 29th January 2013, 05:45 AM   #33815
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I know you speak about input RC - it was easy to understand what you meant. Anyway, I am saying that 20kHz input roll off is to low (frequency) and it is audible as a lack of transparency. So, IMO, it is much better to design the power amplifier properly and then there is no need to tune input RC below 100kHz. Without increase of RFI sensitivity. They should just do it right and not to use supporting crutch of the 20kHz input RC.
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Old 29th January 2013, 10:19 AM   #33816
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You know i like very fast amps, current feedback, for this reason: to do not create inter-modulation products with fast transients (including HF). I usually compensate the amp internally for flat bandwidth (no ringing near 5-10MHz) then input filter for no overshoot on square waves. That is the minimum input filter. From that point, i try to lower the filter, just by listening.
It seems that the best sound is when the phase is still flat at 20kHz, ie low pass at ~200Khz.
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Old 29th January 2013, 11:46 AM   #33817
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I agree with 200kHz, and maybe even more, if the amplifier is fast enough and not sensitive to HF intermodulations.
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Old 29th January 2013, 12:08 PM   #33818
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..... at the cost of transparency and resulting in a "special" sound. … Much better to work on high BW product without necessity to limit input below 100kHz.
This gentleman attributes the audible effects of band limiting to steep phase change and not to amplitude roll off. Phase starts changing early on (at 1/10fc)

The importance of the phase response


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For audio practices ---- common-mode (CM) interference signals are usually HF or RF and are field-coupled into a shield and/or two wires. The polarity is therfore in-phase or Common (-mode) in all wires. When the currents induced in the wires are thru different impedances as in a differential input amplifier that does not have EXACT same Z (R and X) at each input (+ and -), a difference potential is created with respect to common/grnd.... the CM to Differential-Mode conversion. That differentail HF/RF signal tries to then be amplified by the input stage.
It is why the amps CMRR is so important... to reject and minimize those imbalanced effects. I am sure many here can compute the CMR affects on such imbalances by degree of imbalance. [PS The quad-star works at low frequencies best] Thx-RNMarsh
This seems to be the main drive for using instrumentation amps.
http://phobos.iet.unipi.it/~barilla/...ignerGuide.pdf


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Old 29th January 2013, 12:35 PM   #33819
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This gentleman attributes the audible effects of band limiting to steep phase change and not to amplitude roll off.
That is what i said. And i can correlate it with recording equalization. Wen you use an analog corrector, you introduce phases shifts. The changes you make to the instruments are obvious with little- or even no- changes in frequency response (equalization outside of the band). When you use digital equalizers, you can change a lot the frequency response with no change in the 'texture' of the source.
Result, digital equalization is a must to clean a sound (remove parasitic low frequencies etc...) while analog is irreplaceable to sculpt a sound.

By example, we have often, on movie dialogs, some parasitic frequencies, due to lighting equipment. When you use an analog notch to remove-it, the voices are deeply degraded. Using the same notch with a digital filter, the parasitic frequency is gone, and the voices not affected. But no way to create a wah-wah effect with digital filters.

An other example is the use of brick wall analog filters, comparing to digital oversampling.
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Last edited by Esperado; 29th January 2013 at 12:45 PM.
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Old 29th January 2013, 01:03 PM   #33820
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Yes.

Now here are two different schematics. Given the same values for the parts in both circuits which will have less power supply noise on the output?
Ed, maybe not exactly pertinent but with power transformers being discussed again maybe its time to answer your own question (post 33160)

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-Antonio
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