[Spencerleehorton]

Hi,

Just thought I'd document my api-312 500 series build.
Single sided pcb which you can make through the iron on method using laser printer, winding my own txfs, 48v, -10db pad, polarity switch, di input, gnd lift, output attenuator pot.
Gonna use a couple of 2520 op amps I had left over.

Have design and printed pcbs and going to put them in ferric chloride then drill.
Will detail parts etc once I get to that stage.

[ruffrecords]

A 10dB pad is hard to do whilst maintaining the right loads for both the microphone and the transformer. 15dB is about the minimum you can do and meet these criteria, 20dB is more normal.

Cheers

Ian

[Spencerleehorton]

I wondered that Ian, I might just do the pad the normal way rather than this capacitor way.
Normal way I do it is with 3 resistors, 120R and 2 x 470R.

[ruffrecords]

I wondered that Ian, I might just do the pad the normal way rather than this capacitor way.
Normal way I do it is with 3 resistors, 120R and 2 x 470R.

That should be OK. I use 150R and 2 x 680R

Cheers

Ian

[Spencerleehorton]

oh right, so i could have, 120R, 470R or 150R, 680R or 160R, 750R.
what would be the differnce between them?

i actually find that with my api-312 preamps at the moment my -20db pad is a little bit too much, as in, when i pad for instance the snare without it padded its a little bit too much and when its padded i have to crank the gain nearly all the way!!! think im using 120R and 470R.

[ruffrecords]

oh right, so i could have, 120R, 470R or 150R, 680R or 160R, 750R.
what would be the differnce between them?

Typical mic pre designs are arranged so their input impedance is about 10 times the expected source impedance. Most people assume the source impedance is typically 150 ohms so the input is designed to look like 1500 ohms. To maintain this with a pad in circuit you need to do two things. First arrange the output impedance of the pad to be about 150 ohms - which is the value of the padding resistor I use, and also for the input impedance to still look like 1500 ohms which means the sum of the two input resistors needs to be at least 1500 - 150 =1350 ohms. A pair of 680 ohm resistors will be 1360 ohms.

Of course it is a little more complex than that, but there is quite a bit of leeway, so lots of combinations in this region will do the job.

i actually find that with my api-312 preamps at the moment my -20db pad is a little bit too much, as in, when i pad for instance the snare without it padded its a little bit too much and when its padded i have to crank the gain nearly all the way!!! think im using 120R and 470R.

Depends on the gain range of your preamps but it should not be an issue in practice. If your gain range extends from 20dB to 60dB it should not be a problem unless your pad is a lot more than 20dB.

Cheers

Ian

[Spencerleehorton]

Ah ok I'll stick with your values, 150R and 680R and see how that works.

[Whoops]

good luck for your build, please keep us informed and post some pictures of the process

best regards

[Spencerleehorton]

No probs, I'll post some pics of the pcbs later once drilled.
Then I need to get the components together and get winding!!

[Spencerleehorton]

Thought i'd share the schematic im working of to see what you guys thought, im not going to have the -10db pad on the instrument input, going to have -20db on the mic input and polarity and 48v added.
Will leave out the gnd lift probably and add the 300R resistors for the 1k log pot output off the secondarys of the output txf.
Just interested in whats going of with this arrangement if anyones got any thoughts?

[Harpo]

Spencer, please allow some remarks,
your "22kOhm log pot" will be a rev log taper pot,
the +/- orientation of C3 is depending on type of DOA used,
125uF is a last century parts value for C1 and C2 (btw with original parts value 12.5uF) and fitting a 35VDC capacitor rating will allow the use of +/-24VDC supply rails as well at assumed same cost,
zobel parts values of R3 and C5, if needed at all, is depending on type and series or parallel hookup of T1,
J2 pin1 connects to shield, not ground (different symbol/node), that is not neccessarily the same in this spot,
C6 and C7 will be placed close to XLR-in, maybe not on pcb, caps common side connected to shield, not ground (in order to more effectively block/reduce rf coming in from the outside world, fi no mobile phones back then...)
best regards

[Spencerleehorton]

Harpo,

Just to qiery your term "shield" do you mean 0v/audio GND rather than chassis GND?
also i will use 470uf/35v for C1/C2.
I'll update the schematic top what i think you've described.

[Spencerleehorton]

here is the updated version

[Spencerleehorton]

Ive changed it again, bit happier with this one.

what do you think?

[Harpo]

Just to qiery your term "shield" do you mean 0v/audio GND rather than chassis GND?

Shield meaning chassis GND. This node, as well as phantom current return, is connected to 0V/audio GND in a single spot, probably at the psu.

also i will use 470uf/35v for C1/C2.

A heavy rise of capacitance (factor 37.6). Depending on channel count, is your psu able to charge these?

[Spencerleehorton]

Yah my psu is pretty hefty and it's only going to be two channels.

[Spencerleehorton]

So I'd be better with 125uf/35v for c1/C2?

[My3gger]

Iirc API used 1k pot at the output without 300R resistors, it works well for other trafo coupled line outs too. Haven't compared both and wonder if there is any change in sound between them.

[Spencerleehorton]

It's a good talking point defo the use 300R with the 1k, if anyone could explain?

[boji]

fwiw, smaller psu caps are typical. I imagine the Forsell 990 has the additional two 3u's for sake of transients.



Also your 220 output cap could likely be raised to 16v without issue.

the use 300R with the 1k, if anyone could explain?

Someone correct me but I think the reason for the 300r's is for sake of connecting to older gear. Also at full CCW, the 2ndary would otherwise reflect an unloaded short (not sure how much it matters).

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