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Au-919 mc question

ghamilton

ghamilton

Super Member
I am trying to get a 47K-ohm load in my MC section. I am using a low output (.5mv) grado wood-body. I have changed r-33 & r-34 (33ohm) to 47.5 K-ohm resistors. This made a big difference. However, when compaired to my Shure M97xE running through the MM section, it's easy to hear some roll-off. Is there some way to figure out what resistors should be in there to get a proper 47K-ohm load?
The Grado is not affected by load capacitance.
Thanks for your help!
 
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ghamilton said:
The Grado is not affected by load capacitance.
Thanks for your help!
Click to expand...
Not even a 1000pf cap? When you're talking about a 47K load it makes no sense to say that capacitance won't have an effect (which I believe is what you're saying).

Remove C1 and C2. It might be too bright, but you can work it from there (many MC input stages have no input caps at all, so it might just sound fine...as long as you don't start receiving radio stations when listening to the phono).

Tip: Next time, ID the cart. Grado makes lots of different ones. :)
 
Thanks EW, I changed those 1000pf to 120pf a long time ago. No effect on sound. The only thing that made any difference were the 33ohm to 47.5kohm caps. That almost made the top end right. Not quite though.
Correction on my first post. I ment that I changed R-39 & R-40. I never changed R-33 & R-34. Perhaps thats the problem. Maybe all 4 should be changed. What do you think?
G
 
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I think you're still confused...yes, R39 & R40 are the caps that provide the load for the cart. R33 & R34 don't have anything to do with loading. Maybe you mean R41~R43, the 1 ohm resistors? No reason to fool with them either.

Possible your cable capacitance is swamping the input capacitance? Either that, or that cart simply has a rolled-off top end.
 
Perhaps changing r39 & r40 to 47k does not mean that the end result is 47kohms due to other factors. Is there a way to calculate what resistors would equal 47Kohms for that circuit?
 
The input impedance (in this case, and ignoring capacitance) is the impedance of the input FET's (probably around 2 or 3 megaohms each) in parallel with R39. Even if we assume that the input impedance of each FET (there are six in parallel for each signal half) is a middling 1Mohm, then you'd have 1M / 6 = 167K. Put 167K in parallel with 47K and you have 36.66K. If we stay with our middling figure of 1M for the FET's, then a 68K resistor at the input would get you close to 47K as an input impedance.

Few carts would be so sensitive as to have any real sound deviation between 47K and our half-assed calculation of 36.7K. And input caps & cable capacitance has lots more to do with the behavior of carts on the high end when we're talking about high impedance.
 
EchoWars said:
The input impedance (in this case, and ignoring capacitance) is the impedance of the input FET's (probably around 2 or 3 megaohms each) in parallel with R39. Even if we assume that the input impedance of each FET (there are six in parallel for each signal half) is a middling 1Mohm, then you'd have 1M / 6 = 167K. Put 167K in parallel with 47K and you have 36.66K. If we stay with our middling figure of 1M for the FET's, then a 68K resistor at the input would get you close to 47K as an input impedance.

Few carts would be so sensitive as to have any real sound deviation between 47K and our half-assed calculation of 36.7K. And input caps & cable capacitance has lots more to do with the behavior of carts on the high end when we're talking about high impedance.
Click to expand...

Are you saying 68kohm resistors should go in r39 & r40 to get 47kohms?:saywhat:
 
IF the input impedance of the FET's was about 1M, then yes.
EchoWars said:
The [effective] input impedance (in this case, and ignoring capacitance) is the impedance of the input FET's (probably around 2 or 3 megaohms each) in parallel with R39.
Click to expand...
 
Because it's a lot of trouble to keep going in there to change those resistors,
I have ordered two 100Kohm 25 turn bourns trimers with side adjustments. This way I can leave the covers off and do small adjustments until it sounds right. After looking at the location of those resistors on that board this should work.:banana:
 
It turns out that 62Kohms works. I dialed in the correct sound for the grado & measured the trimmers after I took them out. Thats very close!
Thanks EW!
Greg:music:
 
I've been VERY lazy with this,...but finally!:yippy:
I tried the 68kohm resistors on the MC board and I now have NO roll-off on the top-end. It's not too bright either. It's just right.:music:
EW. You were on the money and I was too lazy to go back in there to change those resistors. The Statement/Reference ($1200.) is hardly broke in and I have many hours of rediscovering my records.
Thanks
 
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