Newbie 312 questions (output atten., DI, mic/line in)

Hi,

I have been trying to research on the 312 as I have a couple of questions regarding my new 312 kits. However, there is so much information (63 pages on the one thread!), I am getting a little overwhelmed and figured maybe someone can give me some clues of sorts...

I have been looking at Bo's page and have some questions regarding it, as well.

I see that the kits have 50k reverse log pots. Bo's schem and my memory indicate 22k was used in the 312 (I see 47k used with the 325). What difference does using 50k have? I may incorporate a rotary here and not sure which values to go with...

What's involved with wiring up an output attenuator? Does this get wired to the output secondary?

I was hoping to incorporate DI and mic and line ins with this 312. Is the pad essentially a mic/line in switch (schem reference)? Bo's schem doesn't seem to have an option for line in (instead he has mic/DI), but of course I am probably not getting it...

I was really reluctant to post, but as my boss used to say: "The only stupid question is the one that doesn't get asked."

Chris

Have you looked at the schematic for Fabio's boards yet? There's a link in the big 312 thread somewhere...

Here, I found it:
http://www.thediypill.phx.com.br/forumfiles/R312sch_rev3.pdf

There's a mic/line switch, resistor values for the rotary gain switch, DI with switching input jack. There's a pad too, but it's on the input, not the output.

Should at least give you some ideas...

the original output attenuator was a 1K pot between the + and - out of the transformer, use the - of the transformer for the - out and the pot wiper for the +
DI in goes straight to the + input of the op amp
gain pot is 25k reverse log
for a line in you would have to stomp way down on the mic in, say a 40dB pad

Fazeka,

You should have a look at the following links :

http://www.eisenaudio.com/diy500/
http://www.soundskulptor.com/uk/tech-mp12.html
http://www.seventhcircleaudio.com/A12/A12R16/a12_about.htm

There is also the DI kit from JLM : http://www.jlmaudio.com/JLMDI.htm

And of course the 312 Thread !! (63 pages is not that much BTW :wink: )

eD))

There's also something in the meta about making long threads searchable...

OK!

Thanks for the links. I will study those and hopefully if I have more questions I can come back here...

You're right, I guess 63 pages are not so much to go through, but I'm anxoius to get this build going. :green: Maybe last night at 2am wasn't the best time to be trying to make sense of all my questions... :shock:

Oh, I will check the meta for searching tips as well.

Thanks again! :thumb:

first of all 10k, 22k or 50k doesnt really matter! ask Bo!..i did it!
2nd with a little bit of thinking you could easily breadboard a small passif DI into the Access-312. if you want an active DI go see JLM.

If using a 12 step 22k Rotary switch use this resistor values for 3dB steps:
20k, 10k8, 6k5, 4k12, 2k68, 1k75, 1k15, 730R, 450R, 260R, 120R and a jumper for last. i would use a output trimmer (10k) and i would wire it in series with a 10K resistor in place of R1, see your boards.


Line in: use 2x 4k7 resistors and 1x 470R !

thanks.

Chris,

As 3nity say, it is ok to use other ohmic value for the gain potentiometer, and still have with same gain structure in both the 312 and 325 card.
But it is important to change the other components around the pot, and of course the pot. shall be a reverse logarithmic type (XX-K.rev.log or XX-KC)

You can use a normal logarithmic (audio taper) pot. as reverse log, but you most connect this pot. awkwardly, so the max gain are in minimum end.

If we use the 22k pot. as a reference, the the other components are:
22k and 100pF in the feedback loop, and 220ohm and 220uF in the max gain end. (22k.rev, 22k/100pF, 220ohm/220uF)

The alternative are:

4,7k.rev, 4,7k/470pF, 47ohm/1.000uF

10k.rev, 10k/220pF, 100ohm/470uF

22k.rev, 22k/100pF, 220ohm/220uF

47k.rev, 47k/47pF, 470ohm/100uF

(100k.rev, 100k/22pF, 1k/47uF)

All this alternative have approx. 6 dB as minimum gain and 40 dB as maximum gain, and this gain are not include the "step up" gain in input/output transformers, it is only the gain from input to output on the op-amp.

--Bo

thanks Bo!

Hello Bo,

I've just read your answer... I'm a bit confused... :?

I thought the cap in the feedback loop acted like a low pass filter and was facultative. I thought that if its value was reduced, a lost of hi frequencies will occur.

I don't understand why you have to change the value of the components around if you change the "gain pot". But, obviously, I don't understand the circuit very well. :wink:
What would be the consequences if the values of the others components are not changed ?

Could you elaborate a bit ? Is there a formula for calculating the values of the other components ?

Thank you,

eD)))

I'm sure I'm gonna dream about this circuit tonight !!! I'm completely addicted ! :green:

[quote author="vertiges"]I thought the cap in the feedback loop acted like a low pass filter and was facultative. I thought that if its value was reduced, a lost of hi frequencies will occur.

I don't understand why you have to change the value of the components around if you change the "gain pot".[/quote]
Hi vertiges,

the voltage gain of this non-inverting amp is 1+(Rfb / Rshunt) where Rshunt is the resistor to gnd in series with your potentiometer wired as rheostat. For 4,7k feedback resistor, 47R at gnd and 4,7k pot your voltage gain is varying between 101 (pot value 0R=max.gain) and 1,99 (pot value 4,7k=min.gain*) or 40dB to 6dB.

The cap across the feedback resistor forms a lowpass filter.
The corner frequency where the signal is dropped by -3dB is calculated by 1 / (2xPi() x Rfb x Cfb) with R in Ohm and C in Farad.
If you fill in the alternative parts couple from Bo's quote (4,7k/470pF, 10k/220pF, 22k/100pF...) your formula looks like 1/(2*Pi()* 4700 * 0,000000000470) resulting in about 72kHz.
This 72kHz is the frequency at wich your signal has dropped -3dB (voltage gain of 0,707). You most probably wont hear the 72kHz, but this filter only has a 6dB/oct. slope, so your interest might by, what is the loss at 20kHz.
Voltage gain at 20kHz for this is 1 / (squareroot (1+ (20000Hz / Lpf-frequency)^2)), giving 0,963. This loss in dB is log10(0,963)*20, giving -0,32dB. The introduced phase error at 20kHz for this Lpf is at Arctan(20000Hz / Lpf-freqency) * 180 / Pi(), giving 15,5°. (phase at 72kHz is 45°).

Same goes for the cap between gnd and Rshunt, but this forms a highpass filter.
The -3dB cutoff frequency for 47R and 1000uF is (same formula as above 1/(2*Pi()*R*C) 3,4Hz. (The highpass filter asumes worst case condition (value of potentiometer = 0). If you add up the 4,7k from the pot to the 47R (giving 4747R), this hpf goes down to 0,03Hz)
Your interest may be the loss at 20Hz, giving 1 / (squareroot (1+ (Hpf-frequency / 20Hz)^2)), giving voltage gain of 0,986 or -0,12dB. Phase error for this Hpf at 20Hz is -Arctan(Hpf-freqency / 20Hz) * 180 / Pi(), giving -9,6°. (phase at 3,4Hz is -45°).

edit: corrected example pot value from 22k to 4,7k

:shock: WOW !!!!

That's exactely the kind of answer I was dreaming of !!!
Thank you SO much for having taken the time to post that !

eD)))

PS : I'm not able to hear the 72kHz :wink: My speakers don't go over 35 kHz anyway.

[quote author="nielsk"]the original output attenuator was a 1K pot between the + and - out of the transformer, use the - of the transformer for the - out and the pot wiper for the +
[/quote]

[quote author="3nity"]
i would use a output trimmer (10k) and i would wire it in series with a 10K resistor in place of R1, see your boards.
[/quote]

Hi,

Trying to digest all of this slowly... :?

I'm not sure, but initially I thought nielsk was saying I can attenuate after the OT ("out of the transformer"), whereas Jorge is suggesting before the OT.

Now I am thinking that they are saying the same thing (after the DOA/before the OT).

I guess what I am after is attenuating the output in case I want to drive the front-end harder for any reason.

Would it be possible to attenuate AFTER the OT?

If so, what would be the difference between attenuating before the OT or after?

Apologies in advance if this is not obvious.

Thanks,
Chris

[quote author="3nity"]
Line in: use 2x 4k7 resistors and 1x 470R !
[/quote]

Jorge,

Duh! I get it now after having looked at Fabio's schematic!

Thanks for putting up with me! Everyone!

[quote author="nielsk"]
DI in goes straight to the + input of the op amp
[/quote]

[quote author="3nity"]
2nd with a little bit of thinking you could easily breadboard a small passif DI into the Access-312
[/quote]

OK, DI straight to +IN. Makes sense. :grin:

I'm not sure, but would an example of the "passive DI" Jorge mentioned be something like Fabio's?



If so, would I even need to "breadboard" it? I ask because I think I could just "point-to-point" it with less hassle, but I am not sure if there is an advantage to the breadboard approach. :?

The only other thing I can think of regarding the DI means I would need to decouple the XLR input up to and including the input transformer when a DI is plugged in. Right? If so, could I do this with a transfer- or switching-circuit 1/4" jack? Like Switchcraft 13A?

Chris that's Fabios DI version wich is fine i'd choose Bo Hansen version over Fabio.

Bo protects the DOA with 2x 1N4148 from + in DOA to - In DOA

i would use a switching jack i would use Fig# 2...so i would go from Input transformer to DOA but when inserting a jack it would break into a DI directly to the DOA.!

Look at this: (sorry its big)

[quote author="3nity"]Chris that's Fabios DI version wich is fine i'd choose Bo Hansen version over Fabio.

Bo protects the DOA with 2x 1N4148 from + in DOA to - In DOA.
[/quote]

Ah, OK. Is the risk high of damaging the op-amp with DI?

[quote author="3nity"]i would use a switching jack i would use Fig# 2...so i would go from Input transformer to DOA but when inserting a jack it would break into a DI directly to the DOA.![/quote]

OK, Jorge. Makes sense, a simple closed tip jack will do it. :thumb: Switchcraft 13A (figure 3) is overkill...

Alrighty, then. Still learning here... bear with me.

An output gain "trim" pot of 10k (with 10k resistor) takes the place of R1 (used with a 22k rev. log pot or attenuator). This I understand.

Now, according to the SCA schem, I see what looks like the output attentuator ("gain") "SW1b".

And also there seems to be, what I assume, an input(?) attenuator "SW1a" (I assume "input attenuator" because it comes after the input tranny/before the DOA). It looks to be on the other pole of the Grayhill.

If so, why? Maybe to overload/distort the input for "color"/"character"? Is SW1a accentuating while SW1b is attenuating?

When I do the DI to the op-amp, do I go directly into IN+? Reason why I ask, because on the Bo and Fabio schems, the DI goes to one leg of a 1 Meg resistor. But there is no resistor like this in my kits. It seems the only resistor is labeled "RL" on Jorge's schem (yes, I know it is for reference only), but I take this is kind of a zobel to reduce ringing? So wondering if I take the DI out and put into one leg of a 1 Meg resistor before going to IN+ or just go directly into IN+?

Finally, what kind of diodes are those labeled "2x 12V" in Bo's DI modification pic? Zeners?

Chris the boards were never meant for DI but it can be done.
For the DI you Need 4 Diodes, 2 caps and 3 resistors (1M included)
Diodes are 12V zeners.!

Chris if you see my board. the RL 150K can be soldered at RC to CC and use RL to solder the 1M!

About the SC-A12 i supposed its an atenuator before the DOA.

A great explantion from Harpo, he can his homework.

So as you understand now, that the components in the "gain feedback patch" have a relationship between each other, so if you change one of this componets, and also want to keep same min/max gain and frequency response, you must change the other components.

Of course there are a disadvantage to go to low or to high with the gain potentiometer value, and this because the 2520´s behavior and also a components choice concern size vs quality.
To high, 100 kohm and up, can result in to much noice, because 2520 have bipolar transistor inputs.
To low, 4,7 kohm and down, need big coupling capacitor to ground, and can maybe result in scraping/thumbing gain pot.
So for the 2520, I think the 22 kohm are a good average value.

About the "72 kHz" low pass filter capacitor.
The orginal 120 pF parallel with 20 kohm on the 312 mike amp card, this choise have probably done with when the API constructor get the 312 card stable in all gain settings, and not a favourite high frequency roll of corner.

About the input protective diodes (anti latch-up diodes) I use in my API DI-mod. application.
Because we play with a input switch between mike transformer and DI-input, it feel save to have them there, if some strange things will happen.
Same with the two protective zener diodes after the DI-input jack, this will take care of to high voltage, for example if you of mistake connect a amp speaker output do the DI-input.

--Bo

Great Bo!

Thanks for sharing, and your schematic!

:green: