Hybrid 312 finished PCBs by Bob Schwenkler

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Hi! today i finished a pair of pres which i started 3 years ago, but for many reasons i couldnt finish before

My 2 channels are working, no noise, incredible amount of gain(which i think it is a bit extreme)

Here is the link from the project

http://www.schwenkleraudio.com/hybrid312/

Some photos of 2 days ago

I wanted to ask a few questions anyway,

1: Relays Bob used on the BOM which are the one i bought
Panasonic TQ2-12V and in the PCB is marked as 15v supply, i am using a 16v supply actually(five fish audio 2448 set to 16v)
Isnt using 15v on a 12v relay a little dangerous for it? if it is accepted, would the 16v positive work too?(i will have to make a 6 voltages power supply later anyway)

2: Grayhill Switches:
I am using them, but... i cant find anywhere the pin to make the rotary stop, but i dont know and i cant find information about this. can i use a resistor leg and cut it? and, i should place it  on position 12 ?

3. I am making more these months since i really like them and want to try making one with 990s and other one with some older 2520 or melchor DOA and some different transformer(to get another type of sound), for 990, i wanted to know which output transformer i could use which would fit in a 2503 place? i was thinking in some jensen, but dimensions arent right, they are a little too big for this PCB. For input i will use some CMMI2c from cinemag, or Jensen JT 16 B i guess it was the model(have to check it out before buying anything).

4. I am getting quite an extreme Gain, i already get a good lvl for microphones with minimum set of gain from grayhills, maybe i have to trim the trimpots a little(havent dont that yet), converters used are SSL Alpha Link MX, set ut 16dbu. I hope i didnt confuse in any resistor on the chain, will have to check it later.

Thank you and i really like these pres on the little things i could try, some vocals of mine and with a guitar and DI box, which i am no guitarist. I got them working just 4 hours ago so i am all fired up.

 

[bobschwenkler]

Santiago, congrats on the build!! Looks amazing!

1. Look up the relay specs. They have a pretty significant tolerance for +/- voltages. A couple volts should be just fine, but look anyways to be sure.

2. The stop holes are normally located on the front of the switch, you should see a series of small holes surrounding the shaft.

3. I'll let someone more knowledgeable on output iron answer this one!

4. You can tailor the overall gain in a few ways. First place to look is to re-jumper the input iron to change the ratio for lower gain there.  You can also play with the R6 or R7 values to adjust the overall negative feedback. If you change these values too much you'll end up changing the gain between steps on the Grayhill switch, but that might not matter too much in practice.

 

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Thank you bob Again!

1. Max Applied Voltage says 150% of he nominal, so it would be a max voltage of 18v i guess.. it wouldnt harm them. i could connect 16v with 15v(datasheet from mouser)

2. Yeah, but the pin to put inside i didnt found along with the metallic sheet covering it. A normal resistor leg would do the trick? I guess the one i should limit it would be number 12 in the image, right?

4. R7 its just a jumper and R6 is 7.5k(as on BOM)Transformer, i soldered in 2 pairs,

Should i change the wiring of the transformer? adjust the 5k trimpot for that reason?(i havent touched it yet) Or change R7 with a resistor? Sorry its my first experience with grayhill switches

 

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Well.. i limited the position of the grayhill without a problem, only had to check where i wanted it to stop in checking with a multimeter.

Anyways... for:
1: Relays worked fine with 16v

I changed R7 with a 6k8ohm resistance, i realized the 10k front panel trim was 5k.. so i added the resistor i had near that value.. was 6.8k, the only problem, is that i can only set the gain with a margin of 15dBs.. i can still vary the trimpot to have lower resistance and more gain i suppose(R35). Changed back R7 to 0 ohms, working better now

Anyways, i have the output still hot, but, i can use it without a problem while i am not recording nothing loud. Well.. my U87 clone which needs tons of gain, needs a bit more, while other microphones at minimum i got the level i need for me speaking at 10cms.

Anything to recommend to fix this?

Thanks!!

 

[bobschwenkler]

Santiago, did you ever get this resolved?