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Old 3rd April 2020, 07:08 PM   #2391
AcoustatAnswerMan is offline AcoustatAnswerMan  United States
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Quote:
Originally Posted by hhhh View Post
Thanks for the plots bolserst. I think I expected more difference between the two circuit designs. The higher low frequency content of the MK-121 version I would guess is due to the direct non-capacitive path through the pot that doesn't exist in the C version.

I'm surprised at the Q when the pot is near the extreme high end. Looks like if you want flat as possible, around 3.3 ohms is the spot and you're still only about -3dB at 20kHz. With the pot at 5.5, you get a substantial "old man hearing" compensator, but if this chart is to be believed, it doesn't begin to compensate: Click the image to open in full size.

The intent of the "C Mod" was not to change the HF response, but to decrease over-saturation of the HF transformer, and hence reduce distortion. Therefore, one shouldn't expect a significant difference in HF response.


And I could be wrong about this, but I believe the frequency response plots posted previously are for the drive signal presented to the panels, and not the acoustic response of the speaker. All ESLs require considerable equalization, at both high and low frequencies, to achieve "flat" in-room frequency response.
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Old 4th April 2020, 03:28 AM   #2392
bolserst is offline bolserst  United States
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Quote:
Originally Posted by mraudioguru View Post
I did the "C" mod to my MK121 interfaces using a 10ohm resistor and keeping the variable pot. Where should that be setting? I have it at the noon (12 o'clock) position. The manual says use it at the 3 o'clock position, but that was with a stock MK121 .
If you were after the same HF level you had before the “C”mod, you would position the pot in the same position. But, as AAMan mentioned, you should feel free to adjust the HF pot to provide the HF level you like…that is what it is there for. The “best” position can depend on how much you have the panels toed in and how live/dead your room is.
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Old 4th April 2020, 03:34 AM   #2393
bolserst is offline bolserst  United States
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Quote:
Originally Posted by hhhh View Post
…The higher low frequency content of the MK-121 version I would guess is due to the direct non-capacitive path through the pot that doesn't exist in the C version.
The difference in midrange response between the two versions arises because of the interaction in the mixer portion of the circuit. The 1st order HP network(57uF/~15ohm) on the low end of the HF transformer on the MK-121C version comes with additional phase lag. So in the mixer, the output from the MK-121C HF transformer doesn’t contribute as much to the summation with the LF transformer in the midrange below 2kHz. This HP network is what keeps the MK121C HF transformer from saturating as easily and avoid distortion; as mentioned by AAMan, this was the purpose of the Cmod. By contrast the MK121 transformer is essentially running full range. The portion of the resistor in parallel with the 230uF capacitor provides damping of the resonance between the 230uF capacitance and the primary inductance of the transformer.

If desired, the electrical response of the MK-121C interface can be made to closely match the MK-121 interface by changing the mixer resistors from 50K to 35K.


Quote:
I'm surprised at the Q when the pot is near the extreme high end. Looks like if you want flat as possible, around 3.3 ohms is the spot and you're still only about -3dB at 20kHz.
Remember these are plots of the electrical response of the interface. Once the acoustic behavior of the ESL panel is accounted for, some amount of electrical response peaking is required to produce an overall flat electro-acoustic response.
See this post for more details. Help with esl simulator
In particular note:
- HF rolloff due to mass of the diaphragm shown in Attachment #4
- HF rolloff due to listening slightly away from directly on-axis shown in Attachment #3

Last edited by bolserst; 4th April 2020 at 03:41 AM.
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