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Old 24th July 2005, 07:31 PM   #1
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Default Bass Boost Q

The spec sheet provided for the MCM 150 watt plate amps says that it has a 5db boost at 35Hz. Does anyone know the Q of the boost?
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Old 25th July 2005, 09:15 PM   #2
Zaph is offline Zaph  United States
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This might help you. I've modeled the transfer function in Jeff B's sub sim. Also shown here is a design using the Peerless XLS 8" sub.

http://www.zaphaudio.com/temp/XLS8-ampwithboost.gif
http://www.zaphaudio.com/temp/XLS8.pdf
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Old 25th July 2005, 09:49 PM   #3
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Thanks again John. I saw that info from a response you posted for me previously. I'm just confused because the two amps are not totally identical. The one from Parts Express has a 6dB @ 30Hz boost. But I do see from the instructions to alter the boost, that at 35Hz, the MCM amp might have a Q of 1.8.

http://www.partsexpress.com/pdf/300-792.pdf

After altering various Q's during modeling, it appears that it won't matter much what it actually is for the speaker I'm modeling. It would be nice to know for future reference however. I have a few of the MCM amps on hand.
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Old 25th July 2005, 11:36 PM   #4
Ron E is offline Ron E  United States
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dB Boost ~= 20*log (Q)
10^(dBBoost/20) ~=Q

dB at resonant frequency = 20* log(Q)
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Old 26th July 2005, 12:52 AM   #5
Zaph is offline Zaph  United States
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Oh yeah, I recognize your nickname I think.

I have a couple of both the MCM and the PE version. (Obviously, I kind of like these or I wouldn't have so many laying around)

I have response curves of both and they are exactly the same. Not just kinda close, I can lay the response curve of one on top of the other and they are identical.

I didn't pay too much attention to what the specs said the boost Q was, I simply tweaked the values until I got the transfer function to look like what I measured. Note that there is a high pass filter on this sub amp too. Just compare the "electrical circuit transfer function" on the image I posted to the response curve Darren created here: http://www.partsexpress.com/pdf/300-792f.pdf Darren's measurements look just like mine for both the PE and MCM amps.

In summary, I don't think it's a matter of knowing the boost Q, it's just a matter of knowing the transfer function. I'm a guy who would rather get real measurements than rely on specs that may not be accurate.
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Old 26th July 2005, 08:49 PM   #6
Ron E is offline Ron E  United States
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Recognition?

I was merely answering the question - perhaps too briefly, as I have a habit of doing. The peak boost usually occurs at a slightly different frequency than the resonant frequency of the filter - there is a relation but for most purposes it is not necessary.

Most interesting is that if you put in Q=0.707 or B2 alignment, you get -3.0103dB and if you use Q=0.5 you get -6.0206dB, which are the values of the Transfer Function at Fc for those particular cases, so the relation works for any 2nd order highpass function.
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