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

[khstudio]

Just testing for headroom of the circuit.

I have my pins like you said but can only test 1 at a time with probe... Like I said 1 hot & 1 not.

XLR pin 1 should go to electronic ground and case ground via 10ohms.

Input or output?

This is pretty irrelivant at the moment because you havnt got a final pad value yet.

What does the pad value have to do with the circuit itself clipping... I'm not clipping the input... Am I off, if so I'm sorry.

This OT seems to be a different animal :?

BTW - The PAD I made is external, not permanently attached.

 

[mattmoogus]

Both input and output pin 1s.

Are you clipping the ground lead of the CRO to ground? Clip it to the bottom tag of the secondary (which shouldnt be grounded) and the probe to the top tag and see what happens.

No youre right it doesnt have anything to do with output headroom, as long as youre sure youre not clipping the input transformer at all.

I dont know what the centre tags on the transformers are...they dont seem to be centre taps from what you say.


M

 

[khstudio]

No youre right it doesnt have anything to do with output headroom, as long as youre sure youre not clipping the input transformer at all.

Not at all... or the scope would show it. I already tried that test :wink:

I dont know what the centre tags on the transformers are...they dont seem to be centre taps from what you say.

Strange huh?


Every pre I have Pins 1, 2 & 3 react different from each other.
I'm afraid to type any more because there's so much left undone.

 

[mattmoogus]

[quote author="khstudio"] Every pre I have Pins 1, 2 & 3 react different from each other.
I'm afraid to type any more because there's so much left undone.[/quote]


Not quite sure what you mean by this, lets take a break and keep going tomorrow eh?


M

 

[khstudio]

Presonus with Voltmeter - for reference.

12 volts across pins 2 & 3
6 volts from 1 to 2
6 volts from 1 to 3

Seems normal BUT this was ALSO the just before clipping point so there's a 4 volt difference in headroom of the circuit.
How does this measure out to Db or headroom... is 16 volts good?

Are you clipping the ground lead of the CRO to ground? Clip it to the bottom tag of the secondary (which shouldnt be grounded) and the probe to the top tag and see what happens.

NO... to the tag like you said

 

[khstudio]

[quote author="mattmoogus"][quote author="khstudio"] Every pre I have Pins 1, 2 & 3 react different from each other.
I'm afraid to type any more because there's so much left undone.[/quote]


Not quite sure what you mean by this, lets take a break and keep going tomorrow eh?


M[/quote]

Yea, I need a break... the sun is comming up :green:

but while it's fresh in my mind & to clear things up:

I Found the clipping point of my pre's using a scope
(with a clean unclipped 1k input signal to each pre)

Then set the gain control so they were just under the clipping point.

Then I tested the output of the pre's with a volt meter - set to AC volts.

Measuring pins 2 & 3
PM1000 = 13.6 volts
MP20 = 12.00 volts
API/Yamaha = 16.00 volts

Using this calculator:
http://www.sengpielaudio.com/calculator-db-volt.htm

I was able to see that I'm getting aprox.
26.30088715 dBu before clipping

Looks good to me.

It started out a simple tasks until I noticed different measurments
from Pin 1-3 & 1-2

The PM1000 & MP20 reacted the same,
reading about 6v each...Pin 1-3 & 1-2
& 12 volts across pins 2 & 3

But the new pre was different:
from Pin 1-2 I got 16 volts
from Pin 1-3 I got close to nothing...

So ALL the signal is just @ pin 2,
I don't get it?

*COULD DC ON THE OTHER SIDE CAUSE THIS?

I don't know but since it sounds good & I'm confussing everyone I guess it's fine. :green:

Thanks for helping,
Back to RMAA tomorrow
Kevin

 

[mattmoogus]

The output transformers are probably limiting the output somewhat, tho youd have to test without them connected to know how much. Yamaha spec the PM2000 as being about to put out +24db max and 20hz @+20 will have 0.5% THD. Not amazing specs by modern standards but pretty inline with most old transformer gear. And anyway it sounds great :grin:

If it was me Id be doing the testing with a 20hz sine wave and with a 680ohm resistor across the secondary. That way youre testing performance in the worst case not the best.

Also if you want a cleaner output, you dont have to use the transformer. Its just there for balancing the output and adding distortion.

As long as you get full signal between pins 2 and 3, everything is fine with the OT.

Correct me if im wrong guys!


M

 

[Greg]

Yes, measure across pins 2 and 3 on the output. If you're getting all the signal on pin 2, then something is screwy... either with the preamp or your test.

khstudio, are you familiar with how balanced gear operates? In a nutshell, pins 2 and 3 have the same voltage, but are 180 degrees out-of-phase. So, as you experienced, measuring across pin 1 and 2... and 1 and 3... are half of measuring across 2 and 3. This is the way it's suppossed to operate. Then a differential input uses these two signal to reject any type of "noise" common to both.

 

[soundguy]

nobody that ever ever ever listens to your music recorded with this stuff will ever ever ever care about the specs of the gear you used, let alone much else in regards to the recording. Id be more concerned with what it sounds like.

dave

 

[khstudio]

If it was me Id be doing the testing with a 20hz sine wave and with a 680ohm resistor across the secondary. That way youre testing performance in the worst case not the best.

Instead of the 150k?
Wait... I think you explained this earlier... I'll look - then I will try this.

Also if you want a cleaner output, you dont have to use the transformer. Its just there for balancing the output and adding distortion.

I'll at least give it a listen.

As long as you get full signal between pins 2 and 3, everything is fine with the OT.

It looks like I am... I explained this the best I could

but like Greg said...

measure across pins 2 and 3 on the output. If you're getting all the signal on pin 2, then something is screwy... either with the preamp or your test.

I tested it the same as the other pre's... This is what's backing me up. I'm trying to figure out what's going on.
I re-checked my work & connections several times & it's all good.
I'm going to try a different tranny & a coupling cap to rule out possible problems. BTW, I am familiar with how balanced works... That's why I'm still posting this crap. What I set out to do should have been cut & dry but Nooooooooooooooo... always something.

nobody that ever ever ever listens to your music recorded with this stuff will ever ever ever care about the specs of the gear you used, let alone much else in regards to the recording. Id be more concerned with what it sounds like.

I only care about the specs so much but I would like to make sure it's up to spec :wink:
I'm learning a lot & have never done SOME of this stuff before.

Thanks,
Kevin

 

[khstudio]

I disconnected the OT & tested it :!:

I have 13 of the same trannys -
With an unballanced +4 signal on the primarys
Using a volt meter on AC

USING BOTH SECONDARY PINS
I got = 1.45 volts

But when testing each pin seperatly I got random readings from tranny to tranny.
... some did matched each other.

This is where I STOP!
I don't understand trannys enough to know what's going on OR if testing each pin is even valid?

I DO know that my other pre's showed an EVEN SIGNAL on EACH pin (about .6 volt on pin 2 or 3) but my new pre did not... Pin 2 was MUCH hotter than pin 3. That's why I investigated further.

--------------------------------------------------
MATT,
What are the Output tranny's used in the PM2000?
What's the PART #?

These are GA81720.
-------------------------------------------------

Kevin

 

[khstudio]

Wow, the forum was down for a long time :?

I'm working on the PCB layout & wanted to post it along with pics of the freq measurements, etc...But my entire website is gone
You'd think the guy knew what a RAID Array was :roll: oh well.
Until I find a place to upload pics I can't show what I've got... never did it any other way than on my own space.

Can anyone recommend a POT or Switch for the GAIN Control:?:
I think I'll end up with 20k (R3) in the feedback & a 100 or 200 (for R2).

I was thinking cost effective as possible - Lorlin 12 position???
Or a 15k or 20k reverse LOG but they're pretty expensive :shock:

Any recommendation would be helpful. I don't want to commit to the PCB until I got a plan for the GAIN... every thing else is going well. I do like the stepped gain idea but might need help with the values.

PAD, I never liked 20dB pads & think 10dB would be great:
I want to ask what values to use but you guys will probably just tell me to search myself...


Thanks,
Kevin

BTW, I will be sharing the PCB Layout & all associated files if anyone's interested... It's turning out great.
3x6 board, 48v, PAD, Phase, Discrete & enough space to lay down the Yamaha input tranny because it's a little tall.

 

[rafafredd]

PAD, I never liked 20dB pads & think 10dB would be great:
I want to ask what values to use but you guys will probably just tell me to search myself...

Well, if you read and calculate for yourself, you won´t have to ask us everytime... :wink:

Use an U configuration as in the example, and calculate for 10dB and the impedance of your transformer (was it 300 ???)

http://www.uneeda-audio.com/pads/

 

[khstudio]

Yes, tranny input is 300. The PM2000 also shows a 300 ohm res acrossed.

I did check out the link but I'm still a little confussed. Sorry.
I've been working for over 16 hours straight today... & yestarday... & the day before.

BTW, your values worked great... Maybe a little too much but that's OK I'm starting to understand what's going on a LOT more everyday.
I ordered the Toroidal, AD/DC supplies from JLM, 2 space rack from ParMetal, have ALL the other parts & almost done the PCB - EXCEPT FOR THE GAIN CONTROL... don't know which way to go yet, Pot or Stepped gain switch?

When I'm done this thing I'm gonna get smashed.
:guinness: :sam: :sam: :guinness: :sam: :guinness: :sam: :sam: :guinness: :guinness: :guinness: :sam:

Kevin

 

[rafafredd]

OK, for -10dB into 300ohms, use two 330ohms resistors and a 300ohms shunt resistor. Althought, the more you attenuate, the less heavy of a load you impose to the mics. I think that this is one of the resons that everybody uses 20dB pad. You could try a little higher value for the series resistors, like 390R or 470R, and listen if there are any dramatically improve in sound, with the most usual mics you use. Like anything else, try to reach an equilibrium between ATTENUATION you want and LOADING of the mic. I wouldn´t use 10dB personally, at least 14dB would be better.

I hope it makes sense.

 

[khstudio]

I did not know that about the loading :shock:

I could see on the chart & from reading there was some issue but didn't understand it... it does make more sense why you see 20dB all over.

I'll go 20dB for the PAD. The PM2000 show a 300 ohm across ( I guess that's the shunt?) but where to go from here. I'm trying to learn


Kevin

 

[rafafredd]

Man, just use the formula on that page. Follow the example one. It can´t be dificult for you. It´s all in there, as easy as eating a piece of cake. Come on. Try it here, if it´s wrong we will come.

 

[khstudio]

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?

The most confussing thing to me is the mic loading & the interaction of...
I guess there's no perfect PAD & after reading back thru this thread I saw that MATT posted to just use 470/470 - 120 shunt & mentioned it would give better mic loading... I missed it before

Using Yamaha's shunt of 300 ohms changes things a great deal with much higher series values when compaired to other PAD's like the JLM & others...

To be honest, I don't think I'll need the pad too much but wanted to get it right.

Reverse LOG POT's 20k, 18k or 15k :?:

Could anyone recommend a brand or know of a good place to find them in the US?

I know that JLM has them & I didn't order them because I thought I could source them fairly easily but I'm havng trouble... I also don't like the idea of having to series the pots to get 20k.

The pre's are going well & I will be posting more pics & info but my motherboard took a poop a few days ago & right before that the server my site was on completely lost my whole site... I've had the worst luck lately on top of being super busy in the studio... oh well, at least I got work. :green:

Is anyone still with me?

Thanks,
Kevin

 

[Greg]

Have you considered using Omeg pots?
http://www.omeg.co.uk/

They make a 22K reverse log taper. I used these in the 312s I built; however, I scored 6 or 8 in The Black Market just by chance. I've never ordered direct from Omeg... I don't even know if you can. I had originally planned to use a rotary and resistors. Anyway, try getting in touch with Omeg and find out their minimum quantity. Hopefully it's around 100 or so, and maybe a group order can get you/us to that point.

 

[kruz]

omeg offers a great service for us diy-ers.. the quality is good too.. and the minimum order is just 10pcs