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

[khstudio]

Here it is...
my 8 channel API 312 style pre made from Yamaha parts from stripping a Yamaha M1516 board (input & output trannys, Discrete Op-amps - NE80200, switches, XLR I/O jacks, hand/home made PCB's from scratch, Par-Metal chassis, JLM ACDC Power Supply (built into an external scrap midi chassis, Omeg 22k pots (Thanks Joel), 1/4 watt metal film, ceramic caps (pulled from M1516), Nichicon PW caps = 1000uf/35v on Gain pot & 220uf/25v on PS Filtering + .01 mono Bypass, ZOBEL, Phantom circuit & Feedback network copied from a Yamaha PM2000 which uses these EXACT input & output trannys & Discrete Pre-amp Op-amp NE80200.

Special Thanks to Matt Moogus from JLM, Greg, Dave, Raffafred, CJ & everyone eles for helping me thru this project & helping me learn.

This project opened my eyes to several things I never knew... for one, you can't make an API pre out of Yamaha parts but you can follow the basic layout/schematic & adjust the circuit to fit your parts - Trannys = Zobel & Op-amp = Feedback

The original thread starts below...
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OK, I asked this question before but the other topic title gave minimal responces. I want to keep it simple & use what I have so I thought an API "LIKE" pre would be cool. If anyone has a better idea I'm all for it. BTW, I will be ordering a pair of JLM 99v when I can afford it but wanted to have some fun NOW :wink:

API 312 - I gathered all my parts last nite but got stuck on the output tranny. I know the API is gapped & noticed there's NO output coupling cap. I'm not sure if the Yamaha/Tamura 600:600 is or not... Can I check if an output tranny is gapped or not without opening it up & HOW?
OR
Has anyone tried NOT using a DC gapped OT with the API circuit?
What does the gap do?
My guess is it enables you to NOT use an output coupling cap but I'm not sure.

Part's I've got -

Input tranny's:
(8) UTC - 150/600:60k
(16) Yamaha/Tamura - 300:2.7k
(4) Yamaha/Tamura - 1k:3k

Output tranny's:
(12) Yamaha/Tamura 600:600
There are 3 pins per side but the center ones don't seem to connect to anything or give a reading:?

Yamaha discrete op-amps:
NE-80200's & a few 80100's
BTW, These 2 op-amps are NOT the same... I had a bad 80100 & opened it up (you can see thru the bottom, filler is clear) & the 80100 has a few different components than the 80200. For 1, it looks like it uses 2 regulators next to the 4 pin side.

Kevin

 

[khstudio]

O, I forgot to ask. Could I just add an output coupling cap to the API circuit to use with my OT or would this be bad?

This question might not apply because I still don't understand what the gap does
core Size = aprox 1.5 in square
Weight of tranny = aprox 1 lb (feels a little heavier than a new 1lb solder roll)
ohm res pri = 20 ohms
ohm res pri = 20 ohms

3 pins on each side but the center doesn't seem to connect to anything :?

I've got one of these hooked up to a single PM2000 channel strip & it & the strip kick ass, more output than you'd ever need & clean... so they must be able to handle some dB.

 

[Greg]

EDIT: Bad info !

 

[soundguy]

that 4804 is a gapped transformer? I had no idea about that...

312 has no cap, the 325 does both sound the same enough to me.

dave

 

[edanderson]

the api output transformer is not gapped. the 2520 does not have a large DC offset on the output, and the steel core of the 2503 output transformer can handle the few mA of DC current without being gapped. i would bet that a steel cored tamura output would also work fine. it wont hurt anything to add a DC blocking cap between the opamp output and the transformer, however.

ed

 

[khstudio]

OK getting closer...

After I hook this thing up (no cap) how much DC is TOO MUCH?
At what point should I add a cap?

This is totaly experimental at this point.

 

[khstudio]

I made some measurements of these op-amps in my PM2000 channel strip... I'm working on racking it up but it's still open on the table.
Here's what I got.

DC measured @ the output pin (in the PM2000 circuit)
NE-80200 (pre-amp, feedback resistor of 12k & input gain knob wide open)
= 2.5mv DC

NE-80100 (Final output Line-amp, feedback resistor of 4.7k)
= 73.6mv DC

Does this help:?:
I can give more info if needed.

Thanks,
Kevin

 

[khstudio]

I'm going to keep moving ahead with this but I'm unclear how to hook up the GAIN TRIM POT?...should I tie the wiper to one side (if so, which one?)

 

[CJ]

Plug it in, it should work. The core is probably 50 EI instead of 625 EI, which is smaller than the stock API.

But, if the turns are there, you can get away with it. Output will be 1/2 that of stock 2503 due to running 1:1 instead of 1:2, like the stock setup, 75 ohm pri. , 600 ohm sec.

Core area will tell you the difference in Bmax between 50 EI and 625 EI. BTW, the 50 in 50 EI means the tongue is 1/2 inch.
The 625 means 0.625 in, and 75 EI means the tongue is 3/4, get it?

Metric is different, but screw the europeans. Faraday wa a Brit.

area of 625 EI stock API is 5/8 times 5/8 = 0.39

area of Tamura 50 EI is 1/2 times 1/2 = .25.

So you will have 1.56 times as much flux with the Tamura.
This means you need to decrease level by 0.64 of stock to run Tamura at same flux density.




Your offset voltages look ok.

Worst one is 76 mv, so 0.076/20 = about 4 ma dc, which the core should handle, I do not know if the dc will screw up the circuit from which it came from.

I would try a cap feed first, check the sound, then just put a jumper across the cap for a second and look for smoke.

 

[mattmoogus]

Just use a cap like Yamaha do, then theres noooo worries. Otherwise you can use our gapped 1:1:1 output xformer, but that would be more for getting the extra 6db of gain if youre just using one opamp in your 312 like pre.

BTW the 99V MB PCB is perfect for this job...



M

 

[Greg]

[quote author="mattmoogus"]BTW the 99V MB PCB is perfect for this job...[/quote]
Plug plug plug... what do you work for JLM or something... hehe :green: !

 

[Mike Keith]

If it was me I would put a 470uf cap to be safe.
Thats what I use on my JM 130 pre and everyone loves it so far...
People seem to get crazy abouts caps, use a good one and it will sound fine...

 

[khstudio]

OK Thanks, I'm on it.

Yamaha uses a 100uf NP cap.

Will I need a pull down resistor before or after the cap for ground ref, or just let it float?

 

[khstudio]

Also, along with the last Q, Should I use the Zobel network from the original API schematic or copy the one that Yamaha used scince I'm using those tranny's?

I have 3 choices -

300:2.7k (is used in the PM2000 mic input)
1k:3k (is used in the PM1000 mic input)
UTC 150/600:60k


I'm etching some boards now :green:

Kevin

 

[mattmoogus]

Id use a bigger cap, at least 470, preferably 1000uF.

Use the zobel from the PM2000 if youre using those transformers.


M

 

[khstudio]

OK, The Yamaha show:
input channel (with some res. gain switching before the tranny)
150k to GND, then 2.7k - 470p ZOBEL, Then 33K TO gnd, 4.7uf ELECTRO Cap then an RC net. to GND = 22k/470p then a ferrite bead just before the input... Do I need all this, especially the input cap :?:

Thanks for helping :thumb:

O, Will I need a pull down resistor just after the output coupling cap?

 

[mattmoogus]

I really should ask Joe about this, I may be wrong but...

Id say all you need to load the xformer like they do is the 150k and 2K7 + 470pF zobel across the secondary.

Id also try it without the zobel, loading it with 2k7 and 10k and seeing what it looks/sounds like. If you find it sounds good with other loads, you might include a load switch for two different tones.

In the PM2000 it sounds pretty well damped, the circuit from mic pre in to insert out is about 0.7db down at 20k with a nice smooth gentle roll off.


M

 

[mattmoogus]

Ive just noticed there seems to be a mistake on my PM2000 schematic - the input transformer is labeled 300:2.7k in the schematic and 300:27k in the block diagram.

300:27k makes more sense so Im assuming thats what it really is.

So instead of trying to load it hard with 2k7, try 27k or 33k and then perhaps 75k and 150k.

Otherwise stick to their loading of 150k, then the zobel, then 33k.


M

 

[khstudio]

I noticed the same thing... I thought it was a mistake. I'm going to check the part #'s on the actual trannys.

 

[mattmoogus]

Measure the DC resistance of the primary and secondary. That will give us an idea of the ratio.


M