Modifying the input impedance of the Bugle 2?

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tlainhart

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Modifying the input impedance of the Bugle 2?
« on: 1 Nov 2014, 04:10 pm »
Hi Jim -

I was wondering if my (kit-built) B2 allows for modifying the input impedance?

I've got an Ortofon 2M Black on input.  The internal inductance of that cartridge is 630mH, according to Ortofon's website.  The capacitance of the tonearm cable is a measured 140pf.  I think you told me that the capacitance of the B2/C3 was negligible, so I'll make up that it's 10pf, for a system total of 150pf.

If I put these numbers in your loading calculator (http://www.hagtech.com/loading.html) I see a resonant peak of 16.4kHz.  Your calc recommends an optimal load resistance of 64.8k to dampen this.

My system is bright as it is, and I'd like to tame this cartridge.  Is it possible for me to change some of the board values to bump the 47kHz load to 65kHz?

Thanks -- Todd

hagtech

Re: Modifying the input impedance of the Bugle 2?
« Reply #1 on: 4 Nov 2014, 07:43 am »
Yes, you can change the 47.5k input resistors to virtually any value you would like.  The 65k would make the system even more bright, so I tend to think the 47k is just fine.

I would suggest finding out what causes your system to be bright, and address that.  Don't compensate by overdamping the cartridge. 

jh

tlainhart

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Re: Modifying the input impedance of the Bugle 2?
« Reply #2 on: 5 Nov 2014, 01:36 am »
The 65k would make the system even more bright, so I tend to think the 47k is just fine.

Oh - I guess I misinterpreted your loading/calc page.  I thought that the calculated resistance load was for bringing down that 16.4k resonant peak.

How should I interpret the "optimal load resistance" of 64.8k that the calculator shows for an cartridge inductance of 630mH, and a system capacitance load of 150pf?  What would I do with that number?

Davey

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Re: Modifying the input impedance of the Bugle 2?
« Reply #3 on: 5 Nov 2014, 02:36 am »
Todd,

I think you might be confusing yourself and, as you say, misinterpreting the loading/calc page.  (It is confusing.)  :)
For a 630mH cartridge with 150pF/47k loading, you have a fairly ideal setup with no peaking at all.  (150pf-300pf is the recommended range for the Ortofon Black.)  Go for it.

To yield appreciable peaking you'd need to use a cartridge with much lower inductance.

Cheers,

Dave.

tlainhart

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Re: Modifying the input impedance of the Bugle 2?
« Reply #4 on: 6 Nov 2014, 01:37 pm »
For a 630mH cartridge with 150pF/47k loading, you have a fairly ideal setup with no peaking at all.  (150pf-300pf is the recommended range for the Ortofon Black.)  Go for it.

Hi Dave - thanks for clearing that up.  Guess I'll have to look elsewhere to figure out where the brightness/fatigue is coming from.

  -- Todd

acmn

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Re: Modifying the input impedance of the Bugle 2?
« Reply #5 on: 12 Dec 2014, 04:44 pm »
For a 630mH cartridge with 150pF/47k loading, you have a fairly ideal setup with no peaking at all.

Sorry, I must disagree. Of course there are different views about optimal loading but if the
goal is to achieve critically damped system having no peaking in the frequency domain and no
ringing in the time domain, this is the formula for the optimal input resistance:

R(opt) = L/(2sqrt(LC) + RC)
R = Series resistance of cartridge coil
L = Inductance of cartridge coil
C = Cable capacitance.

If L = 630mH, C = 150pF and R is not known but assumed to be negligible, we get 32k.
And of course if R is taken into account this value is even lower. So, if 47k is used the
system is clearly under damped with brightness and restlessness in the sound...