Resistor value to swith Phono stage from MC to MM?

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I got this nice Famco Nuance preamp. It is currently set to use MC phono cartriges, and has a switch inside that you can use to switch between MC and MM. However, just switching to MM and using an MM cartridge give a dull tone which is lacking the hi frequencies. I've noticed there are two 1K resistors put in sockets (each for each channel) that you should, or may replace to support either MC or MM, but I'm not sure. Should the resistors be replaced so MM sounds right, and to which resistor values?
 

Attachments

  • 20170802_122721.jpg
    20170802_122721.jpg
    521.2 KB · Views: 403
Last edited:
If those resistors are connected across the inputs, the correct value for moving magnet is probably somewhere between 47 kohm and infinity. That is, moving-magnet cartridges are normally designed for a load resistance of 47 kohm, but I don't know what the input resistance of the remainder of the amplifier is. A much too low resistance will indeed cause a loss of high frequencies.

Are the resistors connected straight across the inputs? Do you have a schematic?
 
Took a while to find information on Famco ,it seems the pre-amp is UK designed -French built by a Franco-American company.



I have got to thank the thread starter as it lead me to an audio website where a long (and friendly ) discussion was going on as to actually where most audio equipment is made ---VERY interesting !


Replies were from all over the world ,it seems the audio posters are more honest than some audio equipment manufacturers and it depends how you rate the actual design as being made as opposed to the actual manufacturing base.


Still looking for a circuit diagram as MarcelvdG comments on.
 
Thanks everyone, will try this out.

One thing I was wondering about is why they used the orange Simens caps in some of the circuit, while cheap chinese/tiwanese for the rest of the circuit. Are the orange ones in the actual audio path while the others aren't?

I'm recapping the pre and replaced all cheap ones with Jamicons that I had available, but left the Simense intact until I have audio-grade replacement ones.
 
I am trying to understand why an upmarket company like Famco would load the MM input with 1k ?


Thats why I would appreciate looking at a circuit diagram.


In any case prove it Gilwe measure the resistance value of the MM input --one leg to the input socket outer cover and the other to the signal input and check its not already 47k or there about.


Are you sure its not series-ed .


The MC input selector will have a much lower value paralleled with the 1k resistor but are you also sure the selector switch isn't faulty ?
 
Last edited:
The 1k was for MC loading. There's only one phono input, with an internal switch for gain.
Then why permanently connect a 1k resistor across the input

and then say --"its designed for MM" , I find it hard to believe a French company employing audio design engineers ,who I presume have university degrees to design a circuit not in any sense meant for MM but MC .


Was this a 3pm Friday thought ?


What MM cartridge needs a 1k input impedance ?


I could understand if the 1k resistor is regulated by the switch but permanently across the only phono input ?



Could you explain the engineering logic ?---obviously I am missing some vital point but what it is I do not know .


I have the hardbook of Hi-Fi Choice audio amps of circa 1970-1985 not one of the audio amps vary much from 47k input impedance,
some 50k.


No design by JLH/D.Self /etc varies much from the correct impedance of a MM cartridge but if you know better ??
 
I would permanently install the 47k on the board, with the 1k as a parallel option in sockets.
Perhaps that is done here. Some MC users prefer to use a lower value than 1k, so sockets
are a reasonable option, without resorting to a bank of DIP switches.
 
Last edited:
Maybe you missed the fifth sentence of the opening post: "I've noticed there are two 1K resistors put in sockets (each for each channel) that you should, or may replace to support either MC or MM, but I'm not sure."
Thanks for that Marcel but "put in sockets " by whom --
the company or owner ?
Its a bit obtuse/vague, if its added by the owner then removing them re-instates the logical design of control being carried out by the internal switch.


If by the original owner he must have left the switch in the MC position as he was only using a MC cartridge but in that case the 1k resistors served no real purpose if the value of the MC resistance was very low it would only increase the noise factor.
Much better changing the value of the MC resistors on the board to the correct impedance of his MC cartridge.
 
Thanks guys. The sockets were there initially, i.e they are a part of the design. The resistance across each socket legs is 100K when no resistor is in place, and with no cartridge connected to the phono input. This is with the switch set to either MM or MC.
 
Last edited:
That narrows down the range of possible correct values for MM to 88.7 kohm...infinity.

One of the reasons why there is still a range rather than a value is that a DC resistance measurement doesn't tell you what happens at the other end of a DC blocking capacitor, if any. For example, if there is 100 kohm straight across the input, then a DC blocking capacitor, and then 88.7 kohm to put a bias voltage on the other side of the capacitor, you won't need to place any resistor at all.
 
And what about the input capacitance.
Is there also a separate socket to place a cap, or should this cap be combined with the input resistance ?
Hopefuly there is no fixed cap inststalled for an MC Cart, because these are quite often in the nF range, much too high for an MM Cart, also causing a dull sound.

Hans
 
Perhaps you read the following info? Effet bizarre entre preampli et ampli

In an other thread it mentions that Famco became YBA, so you might be able to contact them Contactez-nous-YBA-haut de gamme HiFi

Would it be possible that it's both 47K for MM and MC? My Perreaux SM-3 has a low/high switch which boosts the signal, and uses 47K + load plugs to get for example 100ohm. You could perhaps buy y-cables to experiment with rca plugs with a load of your choice. Some MC cartridges can be used with 47K and others might sound off.
 
@ gilwe
The easiest option may in fact be to add at least one set of RCAs on the back panel, so that loading plugs can be made up. Just a suggestion. If possible take a few close-up photos of the RIAA area on the board and post them here.
I dropped an e-mail to YBA yesterday, asking them if there is a user manual available. Maybe they'll answer.
Kevin
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.