Access-312 Build Thread

[Rellister]

Finishing up this project, but something is strange;
I wired the cinemag outputs per the silkscreen, but I get continuity between XLR out 2 and 3.
Is something wrong?

EDIT: 2 and 3 of course.

 

[buildafriend]

the top and bottom of the same transformer winding should be the same wire that are hitting pins 2 and 3.. I would guess that what you are saying makes sense because in that formation you might even be able to look at the configuration as a inductor when the other side of your output transformer is not in use.

I see pin 2 aka the hot or positive pin with a wire connected to it that is then wrapped around a magnet (representing the secondary hi), then the other end of the wire coming from the magnet (representing the secondary lo) is hitting pin 3 also considered your negative or cold pin.

I can't test to tell you right now because I'm not near my stuff but yeah I think your good as long as everything else is okay  ;D

 

[Rellister]

Ok, makes sense, thanks.
I guess my DMM shows continuity below a certain resistance also.
I measured 56ohm between 2+3.

Crappy phone pic;

Visually good at least!
Norwegian beer in background for size comparison.
Dandeurloo case, cinemags, 2xLaz25, 2xgar2520.

Looking forward to testing tomorrow!

 

[3nity]

Looks good.
Beer should be good too!

 

[Jermbob]

Every one of these preamps pass audio and, in my opinion, sound better than my 3124. Also they were surprisingly clean, no buzz, or goofiness. There is only one problem. The gain pots don't work...oops!

I tested the pots, and they work great.

I tried swapping R4 and R5. The pot worked, but it introduced some crazy noise, when I turned it up. Also it caused my tpad to make a similar noise, when I would attenuate.

I tried swapping the opamp with a 2520, with no affect on the problem.

I tried lifting one leg of RC...nope! Not it.

I tried changing my jp2 config, from 2->3 to two separate jumpers. One from 1->2, and one from 3->4.

The Stuff:
R1,2,11: 6.8k
R3,10,9: 4700f
R5: 100r
R4: 10k
R6,7: 150k
R8: 1200f
RC: 150k
C1,2,5: 220uf 35v
C6: 1500uf 6.3v
C7: .1uf 5%
C8: jumper
Input transformer: JT-115k-e
Output transformer: JT-123-DJCF
Push buttons: f2uee
Pots: 10k rev log
Pad attenuator: Hairball bourns 600ohm tpad attenuator, after output transformer.
jp2: jumper 2->3
Power: 16.04v
4ch.

Any ideas?

 

[Jermbob]

Anyone have schematics that include jp2 and rc?

 

[3nity]

Jumper 1-2 and 3-4 or 2-3 doesnt matter just the ratios that will change.
What transformers are you using??

 

[buildafriend]

Ok, makes sense, thanks.
I guess my DMM shows continuity below a certain resistance also.
I measured 56ohm between 2+3.

Crappy phone pic;

Visually good at least!
Norwegian beer in background for size comparison.
Dandeurloo case, cinemags, 2xLaz25, 2xgar2520.

Looking forward to testing tomorrow!

So I hope you drank the beer.

But did you get it working yet?

 

[3nity]

Sorry theres a mistake on the BOM that i posted.
R5 should be 10K and r4 should be 100R...swap those and you should be good.
Thanks.

 

[MicDaddy]

Jumper 1-2 and 3-4 or 2-3 doesnt matter just the ratios that will change.
What transformers are you using??

It's a Jensen JT-115K-E

I don't think it likes the jumper 

 

[Harpo]

Jermbob, your list looks like a lot of hopefully only typos, so
The edited Stuff:
R1,2,11: 6.8k / rating >= 0.5W
R3,10,9: 4700f 470 R
R5: 100r 10 K
R4: 10k 100 R
R6,7: omit for your JT-115K-E ([R7/R6] on silkscreen is R7 from schematic)
R8: 1200f 120 R
RC: 150k (for your JT-115K-E) ([RC] on silkscreen, missing in the layout file from pg.1, is R6 from schematic)
C1,2,5: 220uF / 35V
C5: 220uF / 63V
C6: 1500uF / 6.3V for HPF set at 1Hz with R4 being 100R and gainpot cranked fully CW for 0R
      using 1000uF for HPF at 1.6Hz, 680uF for HPF at 2.3Hz or 470uF for HPF at 3.4Hz will probably do it as well
      (probably any avail.cap voltage rating will be sufficient, but lower capacitance/rating will come with smaller outer dimensions
      and a cap with the correct spacing might be an easier fit)
C7: .1uf 5% 100pF c0g/NP0 with R5 being 10K for LPF set at 159kHz
      use 47pF from BOM for LPF at 339kHz if your DOA doubtfully can handle this
C8: omit for JT-115K-E
Input transformer: JT-115k-e; red-1, brown-4; yellow-5, orange-8
Output transformer: JT-123-DJCF
Push buttons: f2uee
Pots: 10k rev log (connecting to the fully CW pin (your black wire) and the center pin (red wire). Order of wires doesn't matter.
        Center pin connected to the pots fully CCW pin as well to prevent an open circuit if the pots wiper ever looses contact/fails)
Pad attenuator: Hairball bourns 600ohm tpad attenuator, after output transformer.
jp2: omit (for JT-115K-E doing nothing with both pins 2/3 not making a connection to your transformer)
Power: +16V/-16V (depending on type of DOA used, these rail voltages might be increased up to +/-24V)

 

[atticmike]

Power: +16V/-16V (depending on type of DOA used, these rail voltages might be increased up to +/-24V)

yup, you can run 2520 at 17.5 like the original as 3nity's suggested.

Mike

 

[Rellister]

So I hope you drank the beer.

But did you get it working yet?

Beer=Good, Pre=silent as the grave, all channels.
I expected it at least to pass audio. Will digg deeper into Harpos list up the page and see whats going on.
Noticed silkscreen differs from layout regarding XLR in/outs, hopefully something simple.

 

[Rellister]

Still not passing audio, not one of the four channels.
Tried to swap the 150K at RC to R6/R7.
Here´s my list if anyone can spot anything;

R1,2=6K8
R3=470R
R4=100R
R5=10K
R6/7=Jumper
RC=150K
R8=120R
R9,10=470R
R11=6K8
C1,2=220uF/35V
C3,4=100n 63V
C5=220uF/63V
C6=220uF/16V
C7=47pf Ceramic
C8=Jumper
Input:  75101APC
Output: CMOQ-2S

JMP2=2-3 connected
+/-16V

DOAs: 2xLaz25, 2xgar2520

Voltages good at psu output.
Voltages good at opamp sockets.

All help appreciated!

 

[Harpo]

Still not passing audio, not one of the four channels.
Tried to swap the 150K at RC to R6/R7.
R6/7=Jumper
RC=150K
C8=Jumper

Get one of these jumpers out to interrupt the short between transformers secondary hot/cold.

C7 with 47pF looks a little optimistic, YMMV. Maybe your DOAs don't oscillate with pole set that high.
C6 with 220uF set a HPF at 7.2Hz with gain pot in full CW position, giving a phase response of -19.9° tested at 20Hz.

 

[3nity]

Really dont know what to say.
a member here sent me 1 of his channels that wasnt working...plugged it in my lab...it worked...

Check all solder joints see if you aint got any cold solder.

 

[buildafriend]

DOAs: 2xLaz25, 2xgar2520

Voltages good at psu output.
Voltages good at opamp sockets.

All help appreciated!

Youre at the point where I would plug a sine wave generator into my input at 0.776V at 1k and I would start following the audio with an oscilloscope through the audio path by following the schematic to try to find where it stops.

Do you have access to a freq generator and a scope? If not you could end up sitting there checking everything for hours and hours.

Out of desperation you could try everything, see if there is voltage coming in and out of the pots, see if there is voltage before your output transformer, see if there is voltage at your input. You can make a cheepo sound source out of a lot of things with a little impedence correction in between. Come out of your DAW at .775v.. there has to be a free test tone generator computer application out there... 

 

[Rellister]

Still not passing audio, not one of the four channels.
Tried to swap the 150K at RC to R6/R7.
R6/7=Jumper
RC=150K
C8=Jumper

Get one of these jumpers out to interrupt the short between transformers secondary hot/cold.

C7 with 47pF looks a little optimistic, YMMV. Maybe your DOAs don't oscillate with pole set that high.
C6 with 220uF set a HPF at 7.2Hz with gain pot in full CW position, giving a phase response of -19.9° tested at 20Hz.

you can jumper as you like!

I did. Maybe not the smartest thing to say to newbies like me.

Harpo, I think you point me in the right direction here as usual.
Lets hope my transformers survived. There are no components burned or anything.
I understand that C7 forms a LPF in conjunction with R5 into the DOA, lets say I want it set at 30-40Khz or so.
Which calculation do I use to get the C7 value?
One of my DOAs  are oscillating a bit - thats the only sound I´ve been getting!

Regarding C6 - Is this phase misalignment ok?

Again, thanks!

 

[Harpo]

Lets hope my transformers survived. There are no components burned or anything.

Mic input transformers will be fine. Don't hook up a freq.oscillator in front until you get the shorted zobel provision across your input transformers secondary (R6/7 in series to C8) out. The oscillator probably will not like it to drive a zero ohm load.

I understand that C7 forms a LPF in conjunction with R5 into the DOA, lets say I want it set at 30-40Khz or so.
Which calculation do I use to get the C7 value?
One of my DOAs  are oscillating a bit - thats the only sound I´ve been getting!

3-dB cutoff frequency for a 1st.order (-6dB/oct.; -20dB/dec.) HPF or LPF is 1/(2*PI()*R*C) with R in ohm and C in Farad.
With R5 10K and C7 47pF this is set at 1/(6.283 * 10000 * 47E-12)=~339kHz.
Cap value for your wanted 30kHz cutoff would be 1/(30000/(1/(6.283*10000))), giving 5.308E-10=530.8pF. Next avail.standard parts value would be 470pF. Your DOAs phase margin probably will do better than 30 or 40kHz for the given task, but who knows by what amount for a non-industrial wide tolerance self made assembly that might vary a lot between different builds. I'd try a 100pF for 159kHz and watch the scope. Parallel another 22...47pF cap to this 100pF if the DOA misbehaves. YMMV.

Regarding C6 - Is this phase misalignment ok?

Assuming the audible range of interest between 20Hz and 20kHz and maybe using this mic-preamp as a makeup gain device for a passive summer (with a mic in front you probably don't want this 20Hz rumble only -3dB attenuated), you most often try to keep phase response within +/-5° for the passband. Have a look at your opamps/DOAs GBW/noisegain to see if this is the right part for the job.
Phase response for LPF is ARCTAN(Testfreq./LPFfreq.)*180/PI() with Testfreq.maybe set for 20kHz.
Phase response for HPF is -ARCTAN(HPFfreq./Testfreq.)*180/PI() with Testfreq.maybe set for 20Hz.

 

[Rellister]

I removed the jumpers - and there was sound
A pleasant one too.
I should have studied the schematic a bit more before inserting these.
Two channels are good as it is, (the ones with GAR2520) but the last two (different) opamps are misbehaving a bit. One oscillates a little at higher gain settings - but works.
The last one just makes hiss and crackle.
This is probably directly related to C6/7, as explained by Harpo earlier, or the last one may be defect.
I´ll try a 470uF for C6 - and a 100pF for C7 before assuming it is.

Thanks everybody for help, especially Harpo for your in-depth explanations!