API 312 Thread!

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So I did some comparisons with my DIY312 (Melcor opamp, JT110K input, and 4804 output) and an API 512. I did guitar, bass, and vocals. In all three cases, the DIY312 was much richer in the midrange and had a more open, softer top end to it. The API512 had a much harsher and extended top end response, but still did sound ok. For the guitars and vocals, I felt it was just too harsh, but for the particular bass I was working with it sounded good.

Anyway, take it for what it's worth. I didn't do any comparisons on percussion.
 
thanks Greg, was just wondering today who else was testing these, and results. I remember you said you had that 3124... Did you use Hi-Z for instruments, or line in?

I subbed 360Rs for R2 on BC opamps, and tried vs 100R and 450R.
This is with Q7/8=BD135/136... 360R is the best, 450R slightly more 'transistor' sound (at least that's what i call it) and 100R craps out a little at top gain as i said.
I liked 450R, but 360R is nicer.

Hi-Z input is NICE too. (old Silvertone/DeArmond guitar and old P-bass).
now understand opamp 'sound' better by bypassing input trafos.

...also subbed 2N4036 and NTE128 (repl for 2N3053).
NTE128 is speced different, (especially for 'replacement') but it works!
at least as nice as BDs, maybe slightly less of that sound i'm calling 'transistor'. (Like the small radios of my childhood).

They're really sounding nice now, didn't think they could get better.
Real 2N3053s, and 2nd scope on the way. Hopefully non-DOA.

Anyone else, results?
 
[quote author="pmroz"]thanks Greg, was just wondering today who else was testing these, and results. I remember you said you had that 3124... Did you use Hi-Z for instruments, or line in?[/quote]

They were all on a miced source... haven't tried the Hi-Z input yet, but the line input sound great as well. Coming back off tape I put the pad on ran kick and snare back through the 312s... got super punchy. I'm starting a mix today, and I might do that. It just depends on how it all comes together. I would suggest the line input option to anyone building these.
 
Hi
To confirm :?
2520 need some exchanging:
R3 - 20K instead 82K
R4 and R5 can be too together 20K (or one resistor 20K :? )
BTW, why API used two resistors (trim) :?:
R2 - 360ohm instead 100ohm
C5 - 10p instead 47p.
What to do with C2 and R10 and C3 and R14.
In my circuit i used 2N3053 and 2N4036 and BC550C and BC560C.
Thanks
 
[quote author="dukasound"]Hi
To confirm :?
2520 need some exchanging:
R3 - 20K instead 82K
R4 and R5 can be too together 20K (or one resistor 20K :? )
BTW, why API used two resistors (trim) :?:
R2 - 360ohm instead 100ohm
C5 - 10p instead 47p.
What to do with C2 and R10 and C3 and R14.
In my circuit i used 2N3053 and 2N4036 and BC550C and BC560C.
Thanks[/quote]

Duka again :oops:
Please, does this modification is too for 2520 R1 :?: :?:
 
Hey Duka,

2520 need some exchanging:
Click to expand...
this is for 25.20R1, 25.20BC and PP25.20 boards...

R3 - 20K instead 82K
R4 and R5 can be too together 20K (or one resistor 20K )
Click to expand...
yes and yes :)

R2 - 360ohm instead 100ohm
C5 - 10p instead 47p.
Click to expand...
:thumb: :thumb:

What to do with C2 and R10 and C3 and R14.
Click to expand...
dunno yet if we can remove this as stated on the schemo. The opamps
work great with this parts on, I will test some without it soon, but not next week. I NEED TO RECORD NOW !!! :green:

In my circuit i used 2N3053 and 2N4036 and BC550C and BC560C.
Click to expand...

Nice. Doing this mods we know til now will make you happy I believe... :wink:


:guinness:
Fabio
 
In my circuit i used 2N3053 and 2N4036 and BC550C and BC560C.
Click to expand...
Duka, i have been following along and changing/listening.
Don't have A/B unit to compare.
2N3053 and 2N4036, plus the changes listed above, work best here for BC boards. Using Hi-Z input/4804 out
Also using 550C and 560C everywhere else.
Maybe Greg can say which Q7/Q8...
he has an A/B unit.
 
[quote author="pmroz"]
In my circuit i used 2N3053 and 2N4036 and BC550C and BC560C.
Click to expand...
Duka, i have been following along and changing/listening.
Don't have A/B unit to compare.
2N3053 and 2N4036, plus the changes listed above, work best here for BC boards. Using Hi-Z input/4804 out
Also using 550C and 560C everywhere else.
Maybe Greg can say which Q7/Q8...
he has an A/B unit.[/quote]
Thanks :thumb:
I made in board like original (thanks Fabio for some pics who help me to make board).
2520_board1.JPG

Still missing parts to modify :sad:
 
Hey Duka, looks nice!!


I just got back from a friends studio, we have run some guitar thru the 312s, Melcor and Forssell micpres. His console is a Ghost

F_sincopa.gif


First impressions are the same Greg had:

Melcor sounded rich on mid range, way more pleasant than 25.20 in guitars (clean, dirty, whatever). API is very good also, and Forssell, according to the studio guy, sounded ´sureal´. This amp is really
unbelievable. We tested the ones with JT-16-B and with Cinemag CMMI-10B as input transformer.

We have listened also some JLM99V, but did not record, some clients schedule so we got to run... I´ll do next time. It sounded very different from the others, some nice top end quality going one there, a kinda dry defined top. Sweet.
It sounds very different from 992, as expected since this is a completly different design. Very interesting!! I´d like to check back against 992
, mainly the bottom end with drum and other stuff. We have schedule this for next week.


I´ll post the mp3 clips so you can judge by yourself.

Just great gentleman!

:guinness:
Fabio[/img]
 
Hi ppl !
I just finished the first of my 4 api312...

Used Cinemag 75101APC (Configured as 150 Ohm impedance) and Profile 4804 irons and Melcows... feedback resistor 20K and 220pf cap.

Everything seems to work smooth, but i have a little problem with lower gain... after a while the pot is turned to lower gain setting, the melcor start a strange oscillation (<- dunno if it's the right name for this kind of behaviour)

I've scoped the output of the opamp, and i see something like a strange saw wave @ 1Mhz... i've tried also different feedback caps, 120pf, 470pf etc... but no way to solve this...

Anyone can help me ?

Neeeeeeeeeeeeeeeeeeno

P.S.: i've done the test with the preamp out of the box, now i'm racking it, so, floating ground, and a lot of floating cables around... while i was testing the unit my father used to call me spiderman... dunno why ?!?! :shock:
 
Hi
Same story :grin:
I finished two Fabio API. It works in first moment :grin:
DSC07926b.JPG

Used OEP ans Profile 4804
I make version without servo out, unbalanced out but with socket for NE5534.
I find two pots 22K Rev log in old broken H-H VS Musician guitar amplifier.
Have one API 2520 (Seth sold me), two BTI OA400 (gift from Seth) and mine 2520 and Melcor (unfinished)
I test 5534, 2520 and BTI. Sounds different.
Havent any hum if I test outside chassis.
DSC07928b.JPG

Because I make test on the bench I didnt put switches for +48, pad, phase, and line.
I used Fabio PSU and I will try (if we cant find better solution) to connect with 5V regulator after 18VDC or maybe before LM317. I didnt use 10ohm between 2200uF
Here are some voltages:
Before diodes ? 21,6VAC
After diodes ? 28VDC
Before LM317 for phantom ? 55,6VDC
I saw only 44,8VDC on +48V connector.
I must change resistors. Fabio put 240ohm and 8,2K. Do anyone have value of resistor (instead 8,2K) to see 48VDC.

Also I finished original board API with Jensen JT-110K-HPC and Profile. Used PSU from Dual Fabio API
DSC07929a.JPG

Works nice. I check with Sennheiser 441 and use 2520.

Any suggestions?

Thanks to Fabio, Goran for OEP (birthday gift), Joe (for relays), Seth for two BTI (gift), Frank for Mill-Max and to all who teach me about this.
Cheers
Duka
 
Looking great Duka!!!

About phantom, try 220R and 8k2 ....
Tell us more about the sound when you can :)

I still have to chomp that damn AIFF files from ProTuzzz...

:guinness:
Fabio
 
I've scoped the output of the opamp, and i see something like a strange saw wave @ 1Mhz... i've tried also different feedback caps, 120pf, 470pf etc... but no way to solve this...
Click to expand...

Neeno,

I've done some additional simulations on the 1731. You might try to increase the value of C3 (now 10pf) to at least 470pf to 680pf. This pulls down the bump a 1.5-2MHz on the simulations. Just to make sure we're talking about the correct cap here, on my schematic it's the cap tied between the collectors of Q3 and Q4.

It doesn't effect the bandwidth at high gains and this should allow you drop the feedback cap back to no more than 200pf. That should be plenty with a 20k feedback resistor.

Also make sure your gain potentiometer and series resistor add up to the value of your feedback resistor. If not, then at the lowest setting of your gain pot (actually the highest resistance) you could be getting down below a gain of 2 on the op-amp. I'm not sure this 1731 is stable at this gain.

Let me know if this helps.

Regards,
Jeff
 
You might try to increase the value of C3 (now 10pf) to at least 470pf to 680pf.
Click to expand...
If these needs to be increased so much, couldn't it be that there's something else wrong in that particulair opamp ?
Obvious one: are the DC-voltages OK ? (I guess there'll be some values out there already, haven't seen them myself though) As I understood you only run into trouble for more feedback, so likely all is fine DC-wise, but you never know. Keep us posted.

Regards,

Peter
 
Yes I agree there might still be an issue with layout or components and this should be investigated first! As I understand it, others have built the 1731 and have beeen able to get good results. But, the simulations still show a bode plot that indicates the possibility of problems on the top end. Results from simulations should be considered and not taken as fact without experiment and common sense but also should not be ignored. So, I am in complete agreement that all other options should be looked into to solve this problem first.

Once the problem is solved it would be interesting to test the op-amp with the original value for C3 and the higher value, frequency response, THD, etc.

Regards,
Jeff
 
Yesterday I tested the first 1731 I made (Jays PCBs) and indeed, it wasn't hard to get ~1MHz oscillation when using 22k & 22k gainsetting resistors (so a gain of 2). I did use the 120pF // R-feedback and loaded the output with cabling & and an additional 120pF. No resistive load.

So yes, based on the info in this thread etc the 1731 with its current dimensioning seems not up to the task for lower gains.
Does this observation agree with the original 1731's ?

I thought I measured around 8mA DC (quiescent) in the V+ supply lead. What do you all see flowing there ?

Regards,

Peter

PS
Jay, I like your PCBs - the 'through-hole metal' gives it a solid feel :thumb:
 
from Duka:
Used OEP ans Profile 4804
Click to expand...

What wiring did you use for the OEP A262A3E, both the prim-windings in series and the same for the sec windings ?
And what Zobel-dimensioning did you use for the OEP ?

It'll likely been discussed already, but I'm also wondering if it'd be worthwhile to add the possibility for putting the prim-windings in parallel or in series.

Thanks,

Peter
 

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