API Mic pre (with Yamaha parts) ***COMPLETED***

[rafafredd]

I re-read about the PAD's & YES the formulas are pretty easy but I don't understand how they got the 10:1 ratio out of 20dB?

From the site:

Convert 20dB to a ratio. The formula is: k = 10^(db/20). That's 10 to the power (db/20). The calculator says 10. 20 dB is a ratio of 10:1.

sorry, but I can´t see how it could be clearer than this. The formula leads you to k=10^(20/20) = k=10^1=10. So, k=10

Let´s say you want 30db now... Your formula becomes k (ratio) = 10^(30/20). Now, you have k=10^1.5 = 31.62. Now your ratio is 31.62, for a 30dB pad.

Got it? isn´t it really easy?

I would personally use a shunt of 300R, because your yamaha transformer specs syas 300Z primary. BUT, you could try many different combinations and actually HEAR wht you like most. It´s just three resistors after all.

 

[Greg]

Thanks for the info on the Omeg kruz... hey khstudio, aren't you building like 8 or 10 channels of this? If so, you can just order from Omeg. :thumb: IIRC, they're not carbon, so they should last longer.

 

[khstudio]

Killer :thumb:

Thank you... just what I was looking for.

YES, I'm building 1x 8 ch & 1x stereo = 10.

 

[khstudio]

I would personally use a shunt of 300R, because your yamaha transformer specs syas 300Z primary. BUT, you could try many different combinations and actually HEAR what you like most. It´s just three resistors after all.

Exactly what I was thinking... since I don't think I'll need a PAD too much anyway, on the 8 channel I'm doing I'm going to set up a few different pairs, so I can listen to what happens myself. Ultimately, this is how I check ALL my gear but it doesn't hurt to dig around a little first, to see what works for others.

Just to get this straight, your saying to match the input of the tranny? The reason I ask is that othere pre's, like the JLM stuff, use a much lower value resistor than the impedence of their input trannys... kind of confussing to me. When I see differences like this from one pre or brand to another it makes me wonder why & want to learn why.

You are right, the only way to know is to hear it myself. :wink:

Kevin

 

[mattmoogus]

Kevin,

I dont get why there is so much confusion about the pad and gain pot.

Now dont get me wrong, I dont care where you buy your pots or anything else from - I sure dont benefit in any way if you get them from us...just so thats clear. But reading your posts has confused me - what do you mean by "I also don't like the idea of having to series the pots to get 20k."

Is one resistor ok but two in series not?

Anyway you dont have to use a 20k pot - just change the 22k resistor to 10k and you can use a 10k pot. Please correct me if Im wrong on this point!

If you can get the pots from Omeg, great. And if you want plastic conductance you might have no other choice. But Id be very (plesantly) surprised if theyre cheaper than AU$10. Let us know what they quote you for 10.


Re the pad:
like I said before, Yamaha used those values because the pad was in a mixing console with only one main input per channel. So they optimised it to provide the right load for the kind of gutless effects processors that might be plugged into it when its set to low gain. Once the gain is up enough that the input will be being fed by a mic, the load drops back to 600ohms, which loads the mic correctly.

Because youre making mic pres, not a mixing console, you should use a pad which loads mics correctly. Anywhere from about 500ohms to 1k should be fine. The values I suggested will work well.

As far as the amount of attenuation you want, Yamaha actually attenuates by 24db to give it a clipping point of +24db. So if you want to do it like they did, you need a 24db pad. However 20db is well and truely close enough if youre not running line level into it.


Reading this stuff was just weirding me out too much, I had to post. Sorry for being blunt about it but these are my thoughts and I hope they help.


M

 

[khstudio]

Don't be sorry... thanks for helping.

To be honest, I did realize how expensive the pots were. I should have just got them from you guys when I placed my last order... would have saved me from this headache. The 20k feedback is ending up to sound the best so a 20k pot will be perfect.

About you being weireded out, just remember, I'm a musician 90% & technician %10 of the time. :wink:

I'm lucky enough to get the shit I build to work but understanding exactly how... I'm still workin on that one :green:

Kevin

 

[khstudio]

I found an old Rolls phantom power box & hooked it up last nite so I could see what my U87 would sound like... it was great :green: Then I started wondering if the phase was correct & to my surprise, it was WRONG - out of phase with ALL my other pre's. ummmm... OK. It still sounded great but wasn't as phat & in my face. Then I noticed myother pre's sounded pretty cool with the phased revered too.

I usually looked at the phase switch as a corrective device but now I feel I might have been missing another creative tool. Don't get me wrong, I always knew I could use it that way... just never did.

Anyway, after correcting the phase it sounded much more in line with the other pre's & honestly was a little harder to tell but it was a nano-nut better, with a little more headroom & fidelity.

I was reading about Lynn Fuston & comparing mic pre's -
I gathered (& could hear for myself) that, in their linear range, with an even gain/vol it's much harder to tell a difference.

Kevin

 

[mattmoogus]

:shock:

Are you hearing these differences by talking in to a mic and listening on headphones??



M

 

[khstudio]

Well NO... I only listened to the phase change with a 57 & my studio monitors @ a decent volume.

 

[mattmoogus]

But you were listening to your voice right?

Male voices make asymetrical waveforms when recorded, so they will sometimes sound different when you flip the phase. However this is really to do with your particular speakers and room so you cant rely on the effect to be the same everywhere else. If all else is equal, flipped phase should sound the same.

Transformer coupled pres will also deal with asymetrical signals differently to solid state gear so your presonus gear will sound different to anything with transformers.


M

 

[3nity]

wow you really taught me something today..mattmogus... :thumb:

 

[mattmoogus]

[quote author="khstudio"]Well NO... I only listened to the phase change with a 57 & my studio monitors @ a decent volume.[/quote]

Oh, wait, you mean you were talking into the 57 and listening to it with your monitors at the same time? Not recording it and listening back?

If you do this and flip the phase you will hear a massive change in sound determined by the position of your head, the mic and your speakers in relation to eachother and the room youre in..

.....

:shock:



M

 

[mattmoogus]

[quote author="khstudio"]I was reading about Lynn Fuston & comparing mic pre's -
I gathered (& could hear for myself) that, in their linear range, with an even gain/vol it's much harder to tell a difference.

Kevin[/quote]


I couldnt dissagree with this more!

 

[khstudio]

I took a break from it for a while, now I'm back to it.

After more listening it DOES sound better.

I tried a kick & snare & it definitly beat the other pres using the same amount of gain & mic placement.:green:

BTW, My Presonus pre's have Jensen input tranny's.
Maybe I just liked my voice with it flipped thru the speakers :?

TOROIDAL HELP :!:
I know some of you showed me what to buy but I need to calculate myself.

Am I doing this right:

I have 8 pre's @ about 70ma each = 560ma
Then I'll need some Phantom, lets just say 1000ma

1.56 amps X 24volts = 37.44 VA

If this is not right please help me understand.

Thanks,
Kevin

[quote author="mattmoogus"][quote author="khstudio"]Well NO... I only listened to the phase change with a 57 & my studio monitors @ a decent volume.[/quote]

Oh, wait, you mean you were talking into the 57 and listening to it with your monitors at the same time? Not recording it and listening back?

If you do this and flip the phase you will hear a massive change in sound determined by the position of your head, the mic and your speakers in relation to eachother and the room youre in..

.....

:shock:

M[/quote]

I see/hear what your saying & it makes perfect sense.
I don't usually test my gear this way or this much. It's my first DIY mic pre & I just want to run it thru all the test to see how it performs.

I'm currently done with all this testng shit & I'm just gonna build the damn thing... it sounds good. When I'm done I'll test & tweek :green:

 

[fallout]

Are you sure you got your phantom current draw correct? I'd figure around 10mA per mic needing phantom...

-Jay

 

[khstudio]

I guess not from what your saying... maybe I misunderstood my friend but I thought he said mics can sometimes pull up to 250ma. I'll check my mic manuals for their current draw.


Did I calulculate the Voltage Amps right :?:

Kevin

 

[fallout]

[quote author="khstudio"]
Did I calulculate the Voltage Amps right :?: [/quote]

yes

-Jay

 

[khstudio]

Thanks Jay :thumb:

So, 50VA would be plenty for 8 channels + Phantom... cool.


I have a few other circuits I need to find the current draw for...
When you guys test for current draw, where do you put the meter?
I tried placing my meter in series with the AC in, before the power tranny & it didn't seem to work

What am I doing wrong?

I'm using an RMS MultiMeter, set to mA (on the 400mA jacks)

Bear with me... I almost got it :green:

Kevin

 

[mattmoogus]

Yep definitely take a 0 off that phantom current figure. Most mics use 1-3mA max.

Joe reckons the 80100s wont be drawing that much, theyd need a heatsink if they did. If you allow 40mA that should be enough.

As for the power transformer, Ill let someone who knows for sure comment on that.

M

 

[khstudio]

Matt,
Thanks man :thumb:

Do you think 50VA is too much?

Also I'm using the NE80200 if that makes a difference.

I'm also building that PM2000 2ch rack & need one for that... I was thinking about 1amp for both strips + Phantom = 24VA.


Can anyone help me with how/where to test for current draw? I'm really just estemating what I need & haven't REALLY measured...
Does it matter if it's @ the AC input or between the supply & circuit (DC).
:?
I'm guessing that if my math is correct it doesn't matter :?:

Please help, once I get past these little hang-ups of ignorance I can move on :green:

Thanks,
Kevin