Access-312 Build Thread

[3nity]

not normal for a pre to work that way.
Assembly mistake thats all.
RC Network is not labelled on PCB??? what you tell me of R6-7 and C8?
I dont know the values for those on the CAPI transformers.

Whats your potentiometer value 10K rev log? whats the value for R5 and R4?? and C7?
R5 should be 10K in this case and R4 100R..

in any case as i told you send it to Montreal it shouldn be that long of a fix.

 

[Guitaratomxbest]

Oups! My bad! RC is label on the pcb and i stuffed it with the good value!

150k on RC
10k at R6-7. (It's just 1 resistor, it's normal?)
220pf on C8
10k rev log pot
100 on r4
10k on r5
100pf on c7
1500uf/6,3v on c6

Tell me if there's a mistake? I'll check another round and will send it to you if I can't find anything! Thanks!

 

[3nity]

R6 Or R7.
R6 was to designate 150K if using Cinemag
R7 was to X value if using OEP but obviously no one got it..

 

[3nity]

Make sure to clean the board with friction alcohol and a toothbrush.
then make sure the wires to pot are not short circuiting anything.
Then can you remove R5 and see what happens?, by the same measure R5 to make sure its 10K.
Thanks.

 

[dirty1_1garry]

Hi guys!

Yesterday I made a first start of my access 312 pre. And i have couple problems...

Preamp sound too dark, no top end.
Preamp gives a high frequency hum  at about 8-14kHz.

So top end problem could be solve with input transformer zobel but I used Cinemag  one and in BOM it's marked that it's unnecessary to use zobel (R7 and C8).
Hum problem could be solved with ground loops may be.
Here is layout with my components (attached)

Any ideas?)

Yes, if I will read all pages if this thread may be I will find but may be one you will be kind to me and save some time=)

And what about ground? How it's ore correctly to ground 312 preamp?

 

[dirty1_1garry]

Hi guys!

Yesterday I made a first start of my access 312 pre. And i have couple problems...

Preamp sound too dark, no top end.
Preamp gives a high frequency hum  at about 8-14kHz.

So top end problem could be solve with input transformer zobel but I used Cinemag  one and in BOM it's marked that it's unnecessary to use zobel (R7 and C8).
Hum problem could be solved with ground loops may be.
Here is layout with my components (attached)

Any ideas?)

Yes, if I will read all pages if this thread may be I will find some tips, but may be one of you will be kind to me and save my time?=)

And what about ground? How it's ore correctly to ground 312 preamp?

 

[3nity]

go to page 22 for grounding scheme.
Cinemag demands a 150K between secondary + and ground...what else is it to it?

 

[dirty1_1garry]

Looks like I solve problems described in Reply #465. Thank you 3nity!!
All what I've done - connect all gnds as ground star. No buzz, no hum. And looks like it sound much more open on top end... or I start to hear top frequencies that was failed by hum before))

My 312:
Cinemag CM-75101APC input trans (150Ohm input = 1+2 and 3+4)
Cinemag CMOQ-2S output trans
Scott Liebers 2520 red dot DOA
Rc = 150K
R5 = 220R
C7 = 120pF styro
I used cheap pushbuttons that gives rustles from first start, so I delete them and you simple jumpers at this moment.
22K log pot that should be replaced with anti log.

 

[dirty1_1garry]

I  made a little comparison test of access 312 preamp and Neve V3 console preamp.
Could any one of you listen and say your opinion?
https://yadi.sk/d/tCXAAYYCdGXTx
https://yadi.sk/d/p7m1jUh1dGXUq

How it’s made:
Martin 12 strings acoustic guitar > U67 tube mic > radial js3 mic splitter > 312 and V3 preamps at same time > prism sound ADA converters (+16dBu settings)

I never heard real api preamps. I heard from guys who know api sound that they sounds pretty fast and midrange then another preamps. Hmmm... looks like I hear the same result))

I was a little confused about gain of 312. Pot was really close to zero but signal was really hot, up to +12dBu in peaks on acoustic guitar! Please confirm if it’s Ok or not! (trans was in 150Ohm input)

My questions:
- What about primary of input trans. Series or parallel? Any opinion about how it sound better? As I know it  always wired in series in typical 312.
- Is it possible to open a top end a little bit?

 

[Harpo]

I was a little confused about gain of 312. Pot was really close to zero but signal was really hot, up to +12dBu in peaks on acoustic guitar! Please confirm if it’s Ok or not! (trans was in 150Ohm input)

Why confused, with input transformer hooked up for a 1:10 ratio (=+20dB), DOA min.gain setting 2 (=+6dB) and output transformer probably hooked up for a 1:2 ratio (=+6dB), giving an overall +32dB min.gain setting (+12dB with pad engaged).
Series connected primary windings of your input transformer for a 1:5 ratio (=+14dB, overall 6dB less) might be more useful.

My questions:
- What about primary of input trans. Series or parallel? Any opinion about how it sound better? As I know it  always wired in series in typical 312.

see previous sentence. Opinion on sound might be better asked at name_drop.com for an often offensive and/or unrelated reply.

- Is it possible to open a top end a little bit?

With your parts values feedback resistor R5 increased to 20K (assuming your typo 220R value belongs to R4 for real, and in context with a 20K rev.log pot for gain setting) and C7 increased to 120pF, these are forming a 66kHz LPF. Decreasing C7 to 47pF for a 169kHz or 68pF for a 117kHz LPF response should remove the towel and keep the DOA within its comfort zone. (Just for comparison, Jorges parts values 10K for R5 and 100pF compensation cap set this LPF at 159kHz.)

 

[3nity]

Harpo thanks for your wise input at all times.
really apreciated.

 

[dirty1_1garry]

Any real difference between combinations of
20K rev log pot, R4= 220R, R5 = 20K
and
10K rev log pot, R4= 100R, R5 = 10K
?
Does they making same job and gives same result or not?

 

[Harpo]

Any real difference between combinations of
20K rev log pot, R4= 220R, R5 = 20K
and
10K rev log pot, R4= 100R, R5 = 10K
?
Does they making same job and gives same result or not?

Missing context of cap values and result will be different. If this difference matters to you will depend on application, ie. you might want to use HPF and/or LPF filters anyway when operating microphones. Things might change when you want to use this amp as a makeup amp for a passive equalizer or passive summing device.

For DC conditions and your parts values, result will be 0.82dB different, ie. LOG(1+R5/R4)*20. (you fill in the numbers and check yourself...)
Assuming same cap value across the feedback resistor, the -3dB LPF cutoff point will be one octave down, ie. 1/(2PI()*R5*cap value in F). At 20kHz, the difference with your 20K resistor value is still additionally 0.26dB less than using the 10K resistor value and phase response angle is 7.16° worse.
Assuming same cap value in series to the shunt arm resistor with gain pot set for max.gain (=zero ohm, shorted out), the -3dB HPF cutoff point will drop by more than one octave (your R4 value 220R is more than double of 100R), ie. 1/(2PI()*R4*cap value in F). At 20Hz the difference with your 220R resistor value is additionally 0.42dB more than using the 100R resistor value and phase response angle is 10.55° better.

 

[Deepdark]

Hi

I would like to know if there is some board to sell somewhere? Thanks

 

[3nity]

As i told you contact Dan at Collectivecases.com
He has them in store!
Good luck!

 

[Deepdark]

Quick one for you guys. Will the mil-Max #0344 fit the #3103 pin from classicapi? How do you manage to install them?

 

[Deepdark]

Last ones. Am I ok to go with 5mm stand off for the board? Also, i was looking at the psu section. I plan using the 2448 from fivefish and their power xfmr. My concern is about mounting it in the case. The M5 screw's head will arise under the case. I dont know the height of the bolt head, maybe I'm stressing uselessly. I'm planning using the collective case. By the way, using an external psu isn't an option for my client. Am i ok with 1A fuse sloblo? Thanks.

 

[straypacket]

You can use a flathead screw and countersink the screw hole, or you can use rubber feet - screw-mounted or adhesive mounted.  I usually use adhesive feet.

I am also using Dan Deurloo's case, and would be interested in finding the correct stand off height.

 

[Deepdark]

Is it the right push botton switch??

http://ca.mouser.com/ProductDetail/CK-Components/51281/?qs=sGAEpiMZZMsqIr59i2oRcspXnj7fTNyJ6Mpu7raK3Xg%3d

The actuator looks to be round has said on the description, but it looks to just have de pc pin. All switch I see here in this thread have pc pic and solder lugs. Can you confirm that it's the right switch. Thanks

 

[straypacket]

The switch in your link is correct.

This is the part number for the round cap for the end of the switch:
611-F0202