MC and MM Resistor and Capacitor Loading
Hi Guys and Girls,
Just want to get an idea of the most popular loading for MC and MM carts.
Want to use two 1-pole, 12 step switches: one for resistance and one for capacitance.
I know this topic can cause some controversy, so please, let's keep it friendly. If my plans are over-kill, and I'm worrying too much, feel free to let me know. Seems that this could be overly done, but there are lots of opinions on loading and tons of carts out there, so I'd like to make my phono pre as flexible as possible. It's Jfet input phono pre.
Skimming over the internet, I have found some of these values and some I have used to fill in the gaps. I would like to use low resistance in case I decide to use a step-up transformer with a MC, like a .25mV output, of which I own. Also soon to be owner of 1.6mV MC.
Some of what I read indicates no capactive loading is necessary for MC carts.
What's your opinion?
Just copy my list and edit it in a new post. I'm pretty sure I'm not the only one with this question, so you'll be helping others as well.
MC Resistance Loading
MM Resistance Loading
MC Capacitance Load
500pF to 2000pF
MM Capacitance Loading
50pF to 470pF
You will need 2-pole switches, unless you want separate ones for each channel. 12 different capacitance options seems like overkill. MC may need none at all; MM can be handled by 3 or 4 at the most. Ortofon MMs will probably need the highest capacitance but even then only 2-300pF as the cable will add some. How about 0-100-220-330pF?
Be careful about wiring, as that will add capacitance and hum/interference pickup. Personally, I would use a small DIP switch on the PCB to keep connections short or even a few wire tags and change it with soldered links. The last thing you want is intermittent contacts.
I have a nice Grayhill for the job. It's small but feels like a mains breaker.
I plan on always shutting down before changing a cart and it will not happen often.
Thanks for the reply!
Unless your pre-amplifier is designed for direct use with LOMC cartridges most of the MC values you list will not be needed as you will not have the required gain or noise performance to connect them directly. Loading in this instance would either be handled by a head amp or SUT most of which seem to like secondary load resistances of 10K - 47K IMVLE.. HOMC generally are designed for use directly into 47K although you may empirically determine that they perform better into a lower resistance..
I misunderstood this chart. The low loads are for the SUT impedence, not the preamp that comes after the SUT.
Discussion on MC Cartridge Loading
Maybe 80 ohm would be the lowest?
Can I make a wild guess that this is the first phono preamp you (the OP) have built? That would explain the attempted over-engineering in one tiny area. Keep things simple. One option is to add a parallel input socket so you can add loading via a plug.
I'm new to analog altogether, so I probably am over doing it because I don't understand it all very well. I just noticed that the range of cartridges is huge, even inside the MC genre of carts.
Maybe some sort of a DIP off the board will do, then I can just buy resistors or caps as I need them.
A friend has an AR phono and we were listening to different resistor and cap loads to determine what he liked best. He was able to change settings at-will and I liked this function. I don't mind it being more manual when it comes to changing. I just don't like permanent where change could take place, e.i., unscrew the board, unsolder resistors, etc. Know what I mean?
So, I should concentrate more on what values are important to me now. I get it.
Maybe just bracket 150 ohms and 47k ohms?
I just built the Pearl II, if I didn't mention that bit.
If you add a second phono socket in parallel to the input phono socket you can add the loading by inserting a loaded plug into the second socket. The main advantage of this method is that you can accomodate most any cartridge and also "fine tune" to get the result you are after.
Interesting to see your thread pop up.
I am trying to work out a similar thing at present.
I have started on a new phono-pre.
The 2 previous ones were both for MM only, & had 47K Ohms & a few pF's on the board.
The new phono-pre, will mainly be for a Denon DL-110.
But since I have 2 other TT's and several different MM cartridges...
...some flexibility would seem an advantage.
Also am building this phono-pre for a friend as well ( 2 pre's )...
...he will run a Denon DL-110 also, but could want MM compatibility as well.
3 solutions come to mind as already said.
1. Dip switches
2. Rotary switch.
I have to say that I like the idea of a 'convenient' rotary-switch as your friends AR 'hot swapping' seems a must for A-B listening.
I have just got 2 double-deck 5 way rotarys to experiment with ( 12 options seemed to much to me; but what's TOO much ! dunno...why not 12 ? ).
A couple of R + C decade-boxes connected to the TT ( just kiddin' ).
No signal path connections here, as such.
I had considered 'doubling-up' rotary-poles for 1/2 the contact resistance ( although this is obviously a fraction of a potential load.
The other thing is...
...to perhaps have a 47K across the input, always...
...and then just switch your extra load R\'s in across it.
47K in parallel with 100R, is virtualy 100R anyhow.
Can\'t decide on the values at present.
Will be interesting to hear more of what people think about this.
It seems you want \'pre-installed\' loading, as the AR and other pre\'s have.
They don\'t know what cart you gonna run, so give a choice.
Some commercial offerings just have a 2 way option.
What did the AR have ?
Maybe do the same ?
Bear in mind that adjusting the loading until it \'sounds right\' is essentially using the cartridge and load as a tone control, so it might be better to just add tone controls to the line stage. Cartridge manufacturers and the better reviews often suggest optimum loading for a flat response.
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