api 312 input transformer ratio discussion

Hi there,

So I have completed some api 312 clones...I did a 4ch unit

ch 1 Cinemag 75101 on the input and cmoq2s for the output and a melcor 1731
ch 2 Cinemag 75101 on the input and EA2503 output transformer and a melcor 1731
ch 3 EA input transformer Cinemag cmoq2s output and an api 2520
ch4 EA in and EA2503 out and api 2520

So I have given myself a variety of choices to play aorund with (and 4 more by swapping around the opamps)

The one thing I have noticed right off the bat is these pres are HOT!!  So much so that when doing vocals the input pad is on for sure and when tracking guitars...both pads need to be on.

Now I currently have th eprimary windings in parallel

so for the EA give a 1:7 ratio
and the cinemag is a 1:10 i believe

Now what are the effects of putting the coils in series and reducing the step up ratio

Now the EA would be 1:3.5
and the cinemag 75101 would be 1:5

There will be a lower impedance on the secondary side going into the opamp?  Would this not be a better idea to reduce a loading effect?

Why do the original API's use such a high step up ratio and using the 990's you need a much lower ratio?

I only used a zobel network on the EA input transformer, since it seems that the cinemag ones do not need one?  I stuck with the standard recomended values for that, what does changing those values affect?  If I input a step it would control the amount of damping? wether ts over, under, or critically damped?

Also with the pots supplied they measure at about 19k so I put in an Rf resistor of 22k....I could reduce that to 18K or even 15k to reduce the overall gain...minimum gain will be 1+Rf/Ri for the opamp...is there a problem with going to low on the feedback resistor?

My other question is with the output transformer there are 3 secondary windings of different impedances...to make a balanced signal I understand that 2 winding need to be used so that we can get + sig, - sig and common?

Can anything be done there to reduce overall gain?

At the end of the day I am trying to get a pre that I am not alwyas having to pad for everything....doesnt make a whole lot of sense to me to have to pad the pre.

Any thoughts or comments?

the relatively higher input transformer stepup ratio was chosen because of the noise performance of the 2520.  the 990 opamp is lower noise at a lower impedance, so a smaller stepup is used.

if you reduce the input transformer ratio, you will get worse input noise performance... but you may not really notice or care about a few dB higher noise -- could be swamped by noise from other sources.  the transformers will also respond slightly differently with a lower ratio configuration.  in short, it will still work, but whether it works for you or not depends on what you care about.

you can change the output transformer configuration to get less overall gain.  wire two windings in parallel as the primary and the other two in parallel as the secondary, giving you a 1:1 ratio.  this will give 6dB less gain than the usual 1:2 arrangement.  you don't need a connection for common on the output -- part of the benefit of transformer output is that it is floating, with no reference to ground.  sometimes you see a center tap tied to ground on the output, but that is not needed for most studio configurations.

yes, the zobel network controls the damping.  if you have the correct network you should see an almost perfect square wave, with little or no undershoot on the rising edge.

you can reduce gain in the opamp if you want.  many api users like the "tone" of the opamp running at high gain, so they add a pad at the output to attenuate the signal to the desired level.  see the classicapi store.  using a pad all the time may seem odd, but it is common enough that jeff built the feature into his preamp designs.

ed

Using an attenuator (~20db) on the output should net a slightly better signal to noise performance as well.

Given the use of hot output mics and the wide dynamic range of studio sources there's really no way around an input pad or step attenuator.

Pads are standard fare on 312 preamps. They are pretty much necessary in most configurations.  The most gain in the minimum comes from the input trafo so this is where I'd make changes if you feel they are too hot. Put the primaries in series and see if you like it. This is definitely possible with the cinemag, not sure if the 2520 will work well with a 2622 as 1:3.5. You'll need to play with the load R in any case, something like 15K will give you 600R on the primary, if you want to go lower you'll need a lower RL. Or don't load it at all and see how it reacts.  It's all been tried before I'm sure.

This will take out 6dB from your minimum gain, if it's still too hot, rewire the output as Ed says, that will be another 6dB less. 

And while you might be able to get the opamp down to unity, it may not be stable so YMMV, but I'd leave it as is and rewire the trafos. Use a feedback R = to your pot max R, and a shunt resistor 1/100 of that to give you 40dB max gain (so, for 20K use 200R).  Also, if you change that, you may need to up the cap value to give you the same low pass filter.  22K/100pF gives you ~73K, 19K/100pF gives you ~83K, and 15K/100pF gives you ~106K. Just something to keep in mind.

anyway with the trafo changes, you'll go from a minimum gain of about
20dB + 6dB + 6dB = 32dB
to:
14+6+0=20dB

Hope that helps!

I dont know if it would be bad for signal qualitty, but considered putting in some switched to change input and output transformer primary winding in series and parallel....just thought I would see what people thoughts would be on implementing something like that for experimentation

You might have problems doing that on the low level signal on the front end, but might be OK on the output. If it were me, I'd just rewire one channel and see how it works, you can always change them back.

okay thanks for the advice

Finished two original layout 312's...everything seems to be working ok.
However, when presented with "modern" level signals (near  0dB ) I get distortion, even with the 20dB Pad after the output transformer engaged.
Was this to be expected? I thought the 312 was able to handle hot input levels.
I've wired both in and output tranny like the original..that is: 2622 primaries in series and Cinemag cmoq2s secondary 1 and 2 in series as well.

I built six 312's with original parts...2622, 2503, etc. Don't have any problems with too much/too little gain...they seem about right with regular acoustic recording levels. The only thing i would have to use pad on is drums.. (snare/kick)

desol said:

...they seem about right with regular acoustic recording levels.

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I have no problem with regular acoustic levels. It's when I insert the 312 on the 2 bus that I get distortion.

desol said:

The only thing i would have to use pad on is drums.. (snare/kick)

Click to expand...

Is your pad before the input transformer? Mine is behind the output transformer so I can run the 312 kinda hot for some colour.

Distortion in your DAW? or in the 312? Can you give more info on what you are doing?

mitsos said:

Distortion in your DAW? or in the 312? Can you give more info on what you are doing?

Click to expand...

Distortion in the 312...
I have the 312 inserted on the master bus and it gets a signal around -5dB ...

If I present it with a say -20dB signal it's all good. I can boost it till my converters clip..no prob there.
It just seems it can't handle high input signals.
My 20dB pad is AFTER  the output transformer BTW.

Unless you have wired up line inputs on your 312, you are sending a line level signal into a mic level input. That would easily distort any mic pre.

Insomniaclown said:

Unless you have wired up line inputs on your 312, you are sending a line level signal into a mic level input. That would easily distort any mic pre.

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Yes, but I thought the 312 could handle such levels....hm...I'll trace the signal with my scope tomorrow and also try putting the 20dB pad in front of the input transformer instead after the output transformer. That definitely should do it...
Thanks all!

Insomniaclown said:

Unless you have wired up line inputs on your 312, you are sending a line level signal into a mic level input. That would easily distort any mic pre.

Click to expand...

I was going to ask the same thing. 312 is a mic pre, not line amp...but i know you know that..  
Your probably giving too much to the opamp. My input pad is before the input...but for no particular reason.
I used go betweens. Why don't you use a couple 325's for that job? Or add a couple discrete DI's to the 312's?

desol said:

I was going to ask the same thing. 312 is a mic pre, not line amp...but i know you know that..  
Your probably giving too much to the opamp. My input pad is before the input...but for no particular reason.
I used go betweens. Why don't you use a couple 325's for that job? Or add a  ple discrete DI's to the 312's?

Click to expand...

Yeah, I'll start by moving the pad up front. Might do the Bo Hansen mod as well.
Then there's those universal input boards I've laying around as well...but they want 24V...I guess that's NEve stuff.

The reason for all this was this line from the API application notes

Because of the high input acceptance level, the unit is also ideal as a line booster amplifier

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and from the spec's

INPUT LEVEL:Signal amplitude as high as 0dBm will produce no more than 0,5% THD, 30Hz to 20KHz, within the output ratings of the ampllefier.

Click to expand...

edanderson said:

yes, the zobel network controls the damping.  if you have the correct network you should see an almost perfect square wave, with little or no undershoot on the rising edge.

Click to expand...

Please note that tuning of Zobel network is source impedance dependant. You must do this measurement with 150-200R impedance generator.

I will agree that the 312 has huge headroom. I put output trimmers on both my channels. First day I got it working, I turned the output trim all the way down, and the gain all the way up (of course), and it took a big scream from me to get some colour using an SM 7b. Even then, it wasn't ridiculous distortion, but I liked it. I have my inputs wired cinemag 75101 1:5 and EA 2622 wired 1:7. After reading this, I am going to experiment with running DI bass tracks through the pre from my Ensemble. See if that works out.

dbm is one that I am not familiar with. I don't know how it relates to dbu or dbfs. I have read that 0dbu can be anywhere from -20 to -16 dbfs depending on whose standard you go by.