Access-312 Build Thread

[3nity]

http://www.smallbearelec.com/Detail.bok?no=116
this one...

 

[tfletchii]

which capacitor for C6? One BOM says 1500uf/6v and the other says 100-470uf/10-16v.

how does a 220uf/16v sound? I have some laying around.

thanks everyone

 

[3nity]

Wich one the updated  says BOM?
It will work ok.
Thanks

 

[tfletchii]

thanks for the help

 

[andia]

hi all! I'm posting here in the hope, that someone can help me out with my problem. I posted this also on the access 312 whitemarket-thread, where jorge aka. 3nity really helped me out a lot. thanks again!

there is still one problem left with my access 312 build: I use (or want to use) a stepped gain switch ( http://classicapi.com/catalog/product_info.php?cPath=51_133&products_id=237 ) for input gain control, but I can't get the switch to work, no matter what I try.

everything else on the pre works as expected (pad, phase reverse, +48V thanks 3nity).

input xformer: 2622
output xformer: 2503
doa: gar2520

the preamp is putting signal through from input to output. when a 1k signaltone is applied the amplification from in to out is 22dB.

unfortunately, when i turn the switch absolutely nothing happens...

does anyone have had a similar problem or have an idea where the problem could be??

pics here: http://www.groupdiy.com/index.php?topic=43484.msg587674#msg587674

thanks,

andi

 

[mikefatom]

Andia,

Disconnect the gain switch and measure the resistance across at each step.  Do they change? Are they correct?

If they are changing and the values are correct, then you must have something going on on the main board.  Check for shorts across the gain connections.

Good luck,

Mike

 

[3nity]

I suggest you bypass the gain connection 1 to 2....if you have lots of gain then the switch is not good.
About your op-amps you said they all do the same however you have a signal...

Thanks.

 

[andia]

I bypassed the gain connection 1-2 (by bypassing you mean connecting 1 - 2 with a wire?) but there is no change in gain at all...

so it have to be the opamps??

I put all four opamps (all gar2520) I have into this channel and they all behave the same: signal goes through, approx. 22dB gain from in to out, when bypassing gain connection nothing happens.

 

[andia]

another odd thing I just found: R1 and R2 now only show half the resistance, instead of 6k8 both only show 3k4. are they gone??

or is this because they are in parallel somehow?

 

[andia]

now that the correct opamp sockets came in I tested a second pcb...exact same behaviour.

I'm  :'(

 

[3nity]

can you test the voltages at op-amp sockets + 0v and -?
YOu dont have any other op-amp than the ones you  built?
Im looking for a way to test the preamp with a solid op-amp.
?? if you only had some perfboard..pins and a NE5534.

 

[andia]

I have some NE5534 here and I can get some perfboard tomorrow. can you send me a schematic of how to build the "testdevice"??

 

[Harpo]

andia, reason for misbehaviour might be that you used the BOM2 instead of the schematic.
At least exchange R4 with R5 when using a 10K rev.log pot.

Using the gain switch kit with Jeffs parts (these series string of resistors add up to about 25K), use 20K for the feedback resistor R5 and 1/100 of this parts value = 200R for R4, giving a more linear increasing 40dB gain range (+fixed transformer gain).

 

[3nity]

he did change those resistors.
I asked before.

Hope its somethng else..

 

[andia]

hey harpo! you are right, I had 25k as R4 and the 250R as R5. I exchanged them and voila...switch works!!!!!!!! yippieee

At first i had some noise issues, but I think that is because of unpropper grounding.

I now have a noisefloor with no mic connected of -80dB. is this ok, or does it go lower with proper grounding?

thank you all for your help, you are awesome!!!!!!

 

[tfletchii]

I just got my power transformer in from hairball and it doesn't fit in a 1u rack. The measurements on the spec sheet says it should fit fine but it is not. I guess I'm going to use it for another project.

Which one are you 120 folks using that fits in a 1u rack

 

[sr1200]

Mine didnt really "fit" into a 1u either, i just insulated it (top and bottom) and MADE it fit.  The case looks a little bowed out at the back but works fine.

 

[3nity]

Guys i recommend using Antek transformers they are cheap and ship fast and cheap!
i bought 3 over the past month and cant complaint...plus they fit into 1U racks

 

[atticmike]

what's the 312's main application?

in which areas does it shine?

 

[atticmike]

Also, is there an alternative for the EA2503?

classic api is out of stock.