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Old 7th July 2006, 02:36 PM
Ianm2 Ianm2 is offline
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Default excellent cartridge info.

I don't know how many are aware of this, its a gem and really needs highlighting and learning.

http://www.hagtech.com/loading.html

essentially

1/ find the resonant frequency

note, your cartridge inductance due to the coil is largely fixed, so you can only vary the loading cap within limits.

the cap is a combination of your tags, arm lead, next lead, plugs and input cap., too!!

2/ from this, calculate your loading resistance for a FLAT response.

note, there is something called Q factor, this equation assumes that of 1, which is underdamped, a butterworth is 0.707 which is optimum, its the same as other filters that feature in speaker alignments!!

check out Q factor full equation from a book if you need to know more...

3/ and thus calculate your bandwidth.

note that higher R's and lower inductances give bigger bandwidth.

hence low inducance moving coils give wider bandwidth...

hope that helps enhance a few points of this brill article.


A little discourse on transformer matching...

Its been a while since I studied this and I have gotten very rusty...I may add as I rediscover my prior notes, but...

A typical moving coil output is 0.5 millivolts, and

moving magnet is 5 millivolts....

Your RIAA stage essentially will multiply this voltage by whatever its gain is at 1000 hz, (due to the riaa it will amplify it at differing amounts)

so, lets say the typical value is x100, which is 40 decibels.

your cartridge voltage is made to be 500 millivolts, or half of one volt.

to get the moving coil to that, we need another x 10, which will be a total of 100 x10, or a gain of 1000, which is 60 decibels.

if you transformer has a ratio of 1 to 10, it does the trick, however, what about loading?

well your transformer will do some odd things...

to get the loading resistance presented to the cartridge, you

1/ times the ratio by itself (square it)

thus 10x10=100

2/ you divide your loading resistor at the riaa stage by that, say 47k

so you get 47k/100

and the cartridge is presented a load of 470 ohms.

typical step ups are 1-10, 1-15, 1-20, 1-40.

so 1-20 will give 47,000/ 20 x 20, which is 47,000/ 400, near to 100 ohms.

1-40....work it out, so you can now work out the loading to you cartrige if you have the mm stage input, and also amount of volts your coil will put out.

how does internal resistance of the coil come into it?

well your coil may have a resistance of 3 ohms, so 3 ohms into 100 ohms ( for maximum voltage we need around 10x more and I am getting very rusty here, anything less that 30 ohms is not going to be good)

how does amplifier sensitivity come into it?...

sensitivity means volts in to give full power out ( which you can work out in volts)

say your power amp. needs 2 volts for full power,

and your phonostage puts out ??? well your 5mv MM cart x 100 equals half a volt, it wont' give full power, so you need at least

half to 2 volts equals times 4, you need an active preamp with a gain of at least 4.


Summary:

you need to know

1/ coil inducance
2/lead capacitance
to calculate loading resistor, bandwidth

also

gain of your phonostage
output volts of your cartridge

input sensitivity of your power amplifier.

to see if you need an active pre with enough gain.




If I haven't lost anyone yet, I may now, power equals (volts) squared divided by load resistance

so if you have 100 watts, 8 ohms, you get square root of 800 which is about 28 volts, this is RMS volts, peak is 1.4 times RMS, peak to peak, 2.82 times RMS.

peak to peak is top to bottom of your waveform, peak is just to the top (crest)

so if your amp has 28 volts RMS out and 2 volts in, it has a gain of low and behold times 14

now clipping is where the voltage rails can't take aneemoore captain!!, so they clip at the peak value, which is 28RMS x 1.4, around 40 volts, so we need 40 volt rails ( and a little more...!!) for 28 volts RMS, 100 watts power into 8 ohms. ( these are the beginnings of amplifier design)


What is quite incredible is that there are only a few factors....

ohms law, V=IR in its various forms
Watts law P=VI again....

that's all there is to DC basically, for most purposes.

AC adds capacitance and inductance, both essentially frequency variable resistors ( called reactances). Impedance is a combination of resistance and reactance.

and phase.

so you have volts, current, resistance and its AC forms, (caps and Ls), power, and phase, boil it all down, start from there and put it all together....
I can't emphasise how important these basics are for understanding what is going on, learn them well!! There is LOTS more...

If anyone wishses to correct errors or add, please feel free....If anyone is interested in tuners, please ask, I have a simplified explanation in a plain style....

Last edited by Ianm2; 7th July 2006 at 03:24 PM.
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Old 8th July 2006, 08:44 AM
Globe112a
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Default Re: excellent cartridge info.

"how does internal resistance of the coil come into it?

well your coil may have a resistance of 3 ohms, so 3 ohms into 100 ohms ( for maximum voltage we need around 10x more and I am getting very rusty here, anything less that 30 ohms is not going to be good)"

You must be talking about the cartridge coil here because the step up transformer has a higher resistance.

Firstly you must not put an ohm meter on your cartridge it might blow the fine coil. Look it up in manufacturers data.

But in terms of what you are trying to convey, the impedance ratio from stage to stage for minimal losses is as you say, 1:20 is preferable, but it's based on ac theory not dc, you aren't comparing the ratio properly if you compute using the dc resistance.

Secondly, the most efficient vehicle for transfer of power by no means sound best or we would all be listening to T amps.
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Old 8th July 2006, 03:08 PM
Ianm2 Ianm2 is offline
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Default Re: excellent cartridge info.

just as an addenum to the above, my 1042 recommends 200pf and a 47 k input, based upon the stuff above, I am getting an input of around 16k, with a 1.0 q factor, this rises to 23k with a 0.707 butterworth type, so I am wondering if this accounts for the hugely wide variations, 0.5, critically damped, would again raise this substantially, but by then I am sure you are pulling the treble down.

This would explain partly my prior dislike of mm's as the top is so veiled. A pis poor rega deck didn't help in that respect.

Its gonna take some working out with capacitance and resistance to find best matching, coupled with a scope praps...

no wonder vinyl is fiddly, things get FAR FAR worse with transformers....

Its important to remember you can't avoid capacitance, as of the leads adding at least 100pf.

To conclude for now, some peoples mm's may well be pulled down due to excessive arm leads, and input capacitance and resistance.

remember the coil inducance is absolutely fixed by your cartridge, largely the lead inducance, resistance and capacitance are, too, unless you change them,

so your only variable you can realistically alter is loading resistance. And a very short low capacitance lead.

Research and devel. may be required further...

Last edited by Ianm2; 8th July 2006 at 03:18 PM.
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