RCA Ba2A Pre amp qestions & answers

[Bren-hur]

Well I got the A25 dismantled, partly anyway... i very carefully and slowly drilled out the holes where the screws go in the bottom, on pressing the base plate with a screwdriver they were the only parts on the bottom that didn't move. Once i drilled about 1/8th of an inch in on each hole, the bottom popped right off! I desoldered the primary wires and checked them... unfortunately, it seems the transformer is well and truly dead....

dang it...

 

[CJ]

possibly a busted wire at the splice of the cloth leads/pri enamel wire.

i think the sec. is wound first,

so the pri will be on the outside of the coil,

this means you can get to the splices,

if the sec splice was bad, then you have to unwrap the whole pri to get to it and then rewind the pri, not fun..

so before you put it in the scrap heap or salvage it for lams for a diy transformer,
heat it up and gently unwrap it like an egyptian mummy,

pretend you are doing cosmetic surgery on madonna,

you will see a piece of cloth tape with a rivet in it for splicing small wires to big wires, sometimes too much tension snaps the small wire at this junction.

 

[Bren-hur]

wooaahhh!!! holy fckn shit... I feel like a super ninja!!  8)

Did as you said, cooked it for a bit, pulled out the coils (forgot to disconnect the ground so had a wee bit of a hearattack moment). I cleaned off some goo, carefully peeled back the tape till I got to the wire joins, and sure enough, one of them had come off. I thort I might've torn it opening it, but the other one was real secure... had another heart attack moment when i pulled a bit too hard and snapped off the wire that was still attached... peeled back some more tape to find it again, gently scraped both of the teeny wires to get rid of the coating and soldered on two new big wires... connected the multimeter... 1k! wot's upppppppp! it's back. A bit lower than spec DCR... i got 1K on primarys and 80ohm on secondaries, but shit damn! I can't believe it...

Only problem now is... i can't remember what wire went where on the bottom tabs, i didn't pay attention coz i was so sure the tranny was dead... help!!

There was a red wire on the topside and a black wire on the bottom, any clue which goes to P and which goes to B?

Thanks for the help...

 

[lassoharp]

Red is often a standard color code for the B+ wire but no idea if that translates 100% to UTC innards.

 

[Bren-hur]

Well, I put the a-25 back in my BA2 and it's cranking! no more crackles ( so far ! ) and it sounds a darn sight louder than it did before... I have a bit of a bad hum going on, from the power tranny, it's a temp one I had in the cupboard and is unshielded... the hum changes volume as i move the mic around like a hum sensor. Got a nice new transformer on the way and I'm going to rework my component positioning as was talked about earlier in this thread...

On a side note, what would be considered excess current for an A25? Like I said... the power tranny I used wasn't a pretty one, an old unsheilded RCA 270 0 270... I wanted to see if it was ME running excess current through it... :-[

 

[lassoharp]

On a side note, what would be considered excess current for an A25? Like I said... the power tranny I used wasn't a pretty one, an old unsheilded RCA 270 0 270... I wanted to see if it was ME running excess current through it...

It's rated for the quoted response at 8ma unbal DC, which is fairly high.  I don't know that the question of how is too much and for how long has been reliably answered other than to say I doubt, say 10ma of normal use time probably wouldn't hurt it a bit.  I've ran dead short - plate to ground with full B+, long enough (15 to 20 sec?) to heat the transformer to blistering hot and it's still kicking.  I = about 235V/whatever the DCR on the primary is = oh shit!

Your PT is higher HV than the stock so will draw slightly more.  Easiest to just measure current from cathode voltage.

 

[Bren-hur]

Well I've been waiting to ask some questions till I worked some stuff out myself... I got my BA2 Clone up and running and VERY quiet as per suggestions in this thread, rotating Power tranny and position of input to get zero hum... that truly was an awakening experience... I then got RMAA and started doing sweeps and noise tests... Now I kept getting really bad results, a bass rolloff starting at 300hz, I kept trying stuff to remedy, increase coupling cap, got the B+ voltage SPOT ON, got minor improvements but still not good enough... anyway, i read through this whole thread again and twigged onto what doug was saying about running tests with load resistors across the input pins when running these tests... so I tried that, 600 ohm wired into the ouput XLR and 150ohm wired into the input XLR.. and BAM... perfect results... ultra flat response beyond 10K and the noise floor dropped down to -100... with some niggly 60hz peaks at about -80, though I think these might be in my soundcard/cable setup, because I could still see the peaks, albeit smaller, when testing the transformers outside the case with teh unit unplugged... on listening tests in cubase, gain all the way up, headphones all the way up.. .it's really quiet, and at normal recording levels... silent.

ANYWAY... to get to the point... these load resistors got me thinking.... when recording, the input is loaded as in the tests, but if I'm going into my cheesy MAUDIO, the output is not loaded like in the test... now when I do proper recording, I use my friends RME UFX... which I imagine is better... but I start to wonder about the output loading and response...

I'm going to run some more tests now... take off the output loading resistor and see what I get... just thought I'd post my musings... and thank the fellas on this thread for the hum-killing advice... total gold.

 

[lassoharp]

when recording, the input is loaded as in the tests, 

Not exactly, as the mic is now the load or termination, so it's not purely resistive.  This assuming you are going directly in to the primary and not using a pad

the output is not loaded like in the test.

It should essentially be if you terminate the OT with a resistor.  I built switches for this in mine.  Sometimes you may like the differing response without the termination.  Sometimes it can sound horrible.  My BA2s don't seem to stray too far without termination going into Hi Z loads.

 

[Bren-hur]

What kind of value do you have on the switch? I get quite a bit of gain loss with 600ohm across the terminals...

Also, I'm trying to wrap my head around RMMA and the frequency graphs... I'm not sure if I'm reading things wrong, but looking at my graph in log view (no output termination)  I have a slope starting at 300hz, sloping from +0db to -10db @ 40hZ.... pretty extreme... but then it says on the numbers readout, the detail summary, that from 40Hz - 15Khz is +1db to -4db deviation... which is totally fine...

 

[lassoharp]

I think I used 620r on mine.  600-1K or so is a decent range.  As your tests seem to have shown, removing the termination has an effect the response, as it should in theory as loading on the output tube is changed.

It seems like a lot of people react unfavorably to the loss of gain from termination.  On most circuits I've listened to the response has been more balanced and less distorted with the resistor in place.  I've heard some say they like the sound of an unterminated output.  It's hard to say if they were responding to the difference in apparent gain/vol or other.

The difference may not show up until numerous tracks have been cut and the more subtle differences begin to stand out, whatever they may be. 

Your numbers sound about right for an A-25.  I think Doug may still have sweeps of mine on file to compare.  What inputs are on yours?

 

[Bren-hur]

I have a Cinemag 3440 as the input... I think that too has a bit to do with with the low end. I tested all the components out of the unit, the A25 was pretty much dead flat across the board, the cinemag on the other hand does have a drop in the bottom end, can't remember where, but I twigged that's where the loss may be occuring...

On the termination, I think I'll do as you say.. put it on a switch... terminated it did sound a lot smoother, unterminated had a much more open sound. On some instruments/mics I can see both working...

I actually have the tilt EQ setup in this unit too, on a hardbypass switch, I've been doing all my testing with it off... then I switched it in... and had a laugh... it has such an extreme affect on the sound... but sounds REALLY good... if you haven't tried it yet, I highly reccomend it, such a cool little EQ! 

 

[lassoharp]

I actually have the tilt EQ setup in this unit too

Do you mean the stock // resistor-cap network? or something else? 

 

[Bren-hur]

No... its from the MI-12241 preamp, pretty much identical to the BA2, except the tilt EQ section replaces the BA2 treble boost...

Doug kindly drew it out...

http://www.groupdiy.com/index.php?topic=16210.msg215423#msg215423

 

[emrr]

That Cinemag needs correct loading to sound right.  I recall they recommend something like 360K on the sec?  Whatever is in all the Quad Eight schematics.    It all sounds hosed if you run it wired 600 and feed it a 150 mic.  Really weird.  A real BA-2 doesn't care too much whether loaded or unloaded on output, and gets a treble boost with a typical mic pad on the front end. 

 

[lassoharp]

Oh, that one.  That is a fun one.

 

[Bren-hur]

It all sounds hosed if you run it wired 600 and feed it a 150 mic.  Really weird. 

That's exactly what I'm doing ... and why I keep on damn tinkering with it not quite satisified with the sound! I looked at the Cinemag schem's, and they say to put a 47K across the secondaries to match a phono preamp... I'll try that a bit later and see what I get....

 

[emrr]

set primaries for 150.  Load secondary at 360K.  If you load 47K you are making the primary reflect true 150, which you don't want. 

 

[Bren-hur]

Hey Doug, thanks for the info! I tried what you said... at least what I think you said (check my attached drawing). The frequency response remain unchanged, but I got a lot more gain, the larger step up from 150 instead of 600 i take it... problem is, I think it's too much, I'm getting a glassy, feedbacky sound in the top end, testing vocals... do I have it connected up right?

Interesting note.. it's a bit noiser, because of the gain i spose, but its far less susceptibale to power transformer hum pickup... anyway, pretty underwhelmed by the cinemag input at this point...  :-\

 

[emrr]

Gain should only be about 4 dB hotter wired for 150.  The Cinemag is a fine sounding transformer when set properly. 

 

[Bren-hur]

So on some more fiddling around, I got the noise down to a normal level, just input transformer positioning and making sure the case was touching the chassis and grounding. I was curious...  on the reflecting back of the 360K resistor... I'm curious on the theory behind it, what is the impedance seen by the mic? and how does the math work?

On another note... I was theorizing that back in the day, the BA2 was likely going out to another unit with an input transformer, and it was getting loaded up that way... I say this because my edcor's arrived today and just for the hell of it, I went out of the BA2 into the 600 side of the edcor, then out the 15k and ran it through RMAA... the response was exactly where it should be, pretty much dead flat from 40hz... like when I load up with a 600ohm resistor, but without as much gain loss... and on listening tests in cubase... FINALLY the Ba2 sounded solid, i A/B'd with and without the transformer load up... and without the transformer/load resistor going straight into the soundcard, it sounds real thin and honky... ie: not right....

Is there any value in having a 600 ohm : 600ohm transformer to load up the BA2? or is a resistor just as good... i like the *idea* of having a transformer at the back end... because i like transformers..