Cinemag CMMI-8-PCA vs. EA2622 (vs. AP2622)

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mitsos said:
...So, I started wondering about trafo loading and zobels... The EA2622 is a 1:7 trafo, yet the recommended RL (150K) is the same as for the Cinemag 75101, which is 1:10...  This would give you an input impedance of 3K vs 1K5 with the Cinemag. Does this not make a difference in the sound? I'm assuming Ed (who knows much more than I do about trafos) was the one to suggest this value and I'm curious to know why/how.

Of course, I did not consider the zobel.  I know you select zobels by passing a square wave through the trafo while the trafo is in the circuit, right?  But how does the zobel resistor affect the overall load on the trafo?  Meaning if you use the values Jeff suggests, (RL=150K, zobel = 10K + 220pF) what is the effective load on the EA2622 and what is the input impedance? 

Thanks!
Hi mitsos,

For the values I mentioned, they were recommended by the old API crew as that is how they populated my input channels.  ;D  My channels have original AP2622's. I figured I would use their original Zobel and load R as a starting point for my VP26 pre's with Ed's 2622's and tune with the ear as suggested by CJ. I asked Ed about this early on. He had sent me this in an email.

"for the zobel, i've also seen 22k in series with 100pf, in parallel
with a 100k loading resistor -- that is what was on the card that i
removed the 2622 i used for research from.  any of these values will
work, but the frequency response will be slightly different."


I also have a 1K series R between the Zobel and a 33uf coupling cap, then the 150K shunt R and a 27pf decoupling cap.

Depending upon the pad configuration, I assume that will contribute to the imput impedance calculation as well?

Cheers, Jeff  :)
 
Hi guys thanks for the replies. Still trying to get my head around transformers.
CJ,
so put in the right value resistor, then try caps
do you mean RL?  If so, what is the best way to calculate this? Do you start with desired input impedance and multiply by the impedance ratio?  That's where the 150K for the 1:10 Cinemag input comes from, right?  how much can/should you vary from that? (I am wondering because Mike says he used 17K with that 1:10 Cinemag.?)

Jeff,
Have you had any feedback from people using your boards with other trafos? Are they keeping everything the same besides RL and the zobel? Would that be your recommendation in that case? 
 
mitsos said:
...Jeff,
Have you had any feedback from people using your boards with other trafos? Are they keeping everything the same besides RL and the zobel? Would that be your recommendation in that case? 
mitsos,

I have not had any feedback on boards purchased with no trafo's. Only on boards with EA2622's. I am not sure what iron people are using or what they are doing for any of the Zobel/RL situations.

Best, Jeff
 
two reasons why there is no exact "ideal" value for the load resistor and zobel resistor plus cap:

1) transformer impedance VARIES as frequency AND level change

2) because there are so many different types of mics, with widely varying output impedances

with that said, for the 2622, the values that jeff posted give a good compromise for most situations. 

the load resistor value can vary a lot and not make much difference in most situations... 100k is fine but you aren't going to see a big difference with 150k or 200k or 50k.  going as low as 15k will probably drop the level a bit but still work and may sound very good on some dynamics that like to see a lower load, or condensers at lower levels where the output amp is loafing.  but ribbons and some dynamics may be rolled off with a too low load resistor.  and condenser mic head amps may run out of juice at high levels and distort.

the zobel values given do a good job damping the ring without rolling off the high end, but in some cases an overdamped or underdamped response would sound good too.  one man's ring or rolloff could be another man's zing or vintaj tone.

if you want to decide on your own values, i'd say start with the ones given, hook up a scope and start tweaking, taking a break to listen every now and again.  there is a good paper posted here somewhere from one of the jensen folks about determining zobel, and i think steve hogan (ex-jensen) also posted on the topic.

ed
 
Thanks for clearing that up, Ed. I just thought it strange that, while there are standard RL and zobel values for this or that trafo, sometimes (like in Mike's case) you see people using something completely different and it seems to work.

I did read the Jensen paper (I believe it's an email) but am without a sig generator so I've been experimenting by ear on a "problem" trafo (in Neve circuit, but it relates I guess).

I can see how the series R has an affect on RL, but does the Zobel R also affect it?

thanks again Ed!
 
if you really wanna get neurotic about it,  you could be the first kid on the block with the "new and improved roto-zobel!"

iqzdbb.jpg


 
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