Access-312 Build Thread

[frazzman]

So my millmax sockets arrived today.... in a burst of excitement I decided to jump in and start assembling my preamps. Calibrated by Fivefish 2448 PSU no problems - I have 16V rails and 48V phantom power all working well.

I hooked everything up and I for the life of me cannot get any sound out of the pre's

I've tried two of the boards I have stuffed (identical),

Both are configured as follows:

- Using Cinemag Trafos
- Using 10k rev log pot from Smallbearelec
- Using Whistlerock ML2520 opamps

Board is stuffed as follows:

R1, R2 & R11: 6k8
R3, R10, R9: 470R
R5: 10K
R4: 100R
R6/R7: 150K
R8: 120R
RC: 150K
C7: 47pf (using 10k rev log pot)
C8: Jumper

I tested voltage at opamp sockets and they are receiving correct voltage so this isn't an issue. I assembled 4x opamps and have tried swapping them but it hasn't made a difference.

The only things missing from my build so far are ground wires and the pushbuttons but they shouldn't matter at this stage. I was hoping I should still be able to pass some audio.

I don't think that I have the pot wired properly as I am not sure which is pin 1 and which is pin 3, either way - I have tried multiple configurations and it hasn't got the pre to pass any audio - and besides it should pass audio even without a gain pot attached right???

Also - I have jumpered the ratio pins so I know the issue isn't here. I have tried moving the jumper - no difference.

I have attached some pics of my volume pot (3nity - I know you used these smallbear pots before, can you verify their configuration) and a stuffed board, if anyone can see anything wrong or has any ideas based on the above I would love some feedback....

Do I need to change a resistor value perhaps? quite confused as this build looks easy on paper and I have kept it quite clean and uncomplicated....

thanks a lot

 

[frazzman]

Sorry about the crappy photo, anyone able to spot any obvious errors?

 

[Harpo]

The only things missing from my build so far are ground wires and the pushbuttons but they shouldn't matter at this stage. I was hoping I should still be able to pass some audio.

Wrong. You'd at least need to jumper the polarity switch to get audio passing. The series resistors at the not fitted pad switch would slightly degrade, but pass audio.

I don't think that I have the pot wired properly as I am not sure which is pin 1 and which is pin 3, either way - I have tried multiple configurations and it hasn't got the pre to pass any audio - and besides it should pass audio even without a gain pot attached right???

From your pic (looking at the shaft, pins facing down), one wire to the rightmost pin 3, the other wire to the center pin 2. You could tie pin 1 to pin 2 in order to prevent a open connection if the pot wiper fails one day. Essentially, pot dialed CW for max.gain->lowest resistance, pot dialed CCW for min.gain->highest resistance.

 

[frazzman]

Thank you Harpo,

Helpful as always,

I guess I was to eager... will have to sit tight and wait for my pushbutton switches to arrive in that case.

I am guessing all my resistor values are right, I wasn't 100% sure on value for RC as it is missing in different revisions of the board

The only things missing from my build so far are ground wires and the pushbuttons but they shouldn't matter at this stage. I was hoping I should still be able to pass some audio.

Wrong. You'd at least need to jumper the polarity switch to get audio passing. The series resistors at the not fitted pad switch would slightly degrade, but pass audio.

I don't think that I have the pot wired properly as I am not sure which is pin 1 and which is pin 3, either way - I have tried multiple configurations and it hasn't got the pre to pass any audio - and besides it should pass audio even without a gain pot attached right???

From your pic (looking at the shaft, pins facing down), one wire to the rightmost pin 3, the other wire to the center pin 2. You could tie pin 1 to pin 2 in order to prevent a open connection if the pot wiper fails one day. Essentially, pot dialed CW for max.gain->lowest resistance, pot dialed CCW for min.gain->highest resistance.

 

[3nity]

Harpo beat me to it!
Good looking project btw.
Thanks

 

[frazzman]

Thanks 3nity,

My pushbutton switches finally arrived and I dropped them in today... the pre's worked almost straight off the cuff.

I had some issues with the gain pots, I wired them backwards first time around so it was hard to tell if the pre's were working properly. I've got 2 out of 4 pre's wired up, I noticed one strange thing on 1 of the 2 pre's - in the last turn of gain I get a heap of hiss and noise, perfectly quiet until then.

I thought it was the gain pot/wiring, but since I used molex headers I plugged the pot into the other working pre and it worked fine there - no noise at all so I am not too sure what it could be.

I am not too concerned at this stage since I can't expect ever needing to put these on full gain since they have tonnes as it is....

For those of you using smallbearelec pots, referring to my pic in the previous post, wire pins 1+2 together - these goto Gain pin 2 on PCB, pin 3 goes to Gain pin 1 on PCB

 

[buildafriend]

I  got these freekin overkill $12 each reverse log pots from digikey

Thanks for the wiring info, I was wondering about it.   

 

[frazzman]

I  got these freekin overkill $12 each reverse log pots from digikey

Thanks for the wiring info, I was wondering about it. 

Yeah total rip off! I remember shopping around for pots for a while and there was no way I could bring myself to spend $50 on 4 pots + postage - so thanks to 3nity for the tip off on smallbearelec. They are just alpha lots so you could probably find them else where but they are sure cheap at smallbear

 

[buildafriend]

Yeah well.. Whatever It's spilled milk to me now. Plus they are nice big heavy pots. I can't dislike more robust parts.

 

[atticmike]

What is the rack-depth you've used for this unit?

looking at the board (with the transformer to the back)

R O
Y G
B P

The switch should have the following

looking at the back of the switch
P B
G Y
O R

And you can connect your LED for the phantom power + to Purple - to Blue



This is before i cleaned up the insides (after i got my transformers)

The pre is towards the bottom just below the green Joe meek in this shot (just showing what it looks like with the switches and added 5fish VU meters.

 

[sr1200]

not a clue, maybe 11"

 

[buildafriend]

I think I could have fit mine in an 8" deep chassis. But I used an 11 deep chassis and had plenly of room for 4 units. If I could go back I would also have built 6-8 pres and mounted them sideways in a 2u.

 

[buildafriend]

Are people running these transformerless or something? If yes, how?

 

[3nity]

Are people running these transformerless or something? If yes, how?

I tried using THAT balancers and it works..just wire the IC to +/-.
you can run the output balanced with a few resistors and caps.
Thanks.

 

[sr1200]

if you're referring to my pic, that was before I got the cinemags in the mail. But thats interesting to know that theycan work without tx;s.... hmmmmmmmm

 

[MicDaddy]

hmmmm indeed  ???

 

[tfletchii]

I'd like to wire a power on led  and an led for 48v on.

For the power led I'm guessing I could get a 120v from radioshack.com, and connect one side to the neutral AC socket and the other to the "out" side of the power switch.
or I have one of these from mouser I could use for the power light (30ma) but I'm not sure how I would wire it in. I'm assuming I'll need to put a resistor between it and the power supply.
http://www.mouser.com/Search/ProductDetail.aspx?R=SSI-LXH8080IDvirtualkey69600000virtualkey696-SSI-LXH8080ID

I bought these for the 48v light
http://www.mouser.com/ProductDetail/Lumex/SSI-LXH312SID-150/?qs=iLtNwxw1bwKzhRWCyyO9JA%3d%3d

 

[buildafriend]

I am thinking about getting a lamp for mine. A nice vintage fender guitar amp looking one. For now I will not be using an indicator. I was hesitant to even add an on switch. I see no point when I have a rack mount surge protector.

I am not interested in running these transformerless, but I bet one of my broke friends would want me to build it and test it since that sounds really cheap.

I am almost done, I will post picks soon.

 

[3nity]

Some pix of my build.

 

[3nity]

another one