Mystery solved ...!!!! thanks PRR and Wilebee

[ToobieSnack]

hey people,
I think I have solved this mystery. (for me anyway)
As most know... I have been building a power supply (schematic below) for an RCA ba71 mic pre. BTW thanks for all the help.
here's some info:
I bough a transformer (120v pri, 12.6 / 0 / 12.6 ct sec) hooked it up to the PCB
started pulling voltage data from different points.
12.5 vdc from the output WTF ????????
hmmmm funny thing I'd only getting appox 12.5 volts everywhere.
that was ....12.5 vdcv...

HOWEVER:
i switched to ac to check the ac voltages
well first thing that caught my attention was that the output was exactly 30vac...hmmm
well ... could it be that these RCA ba71 units are being powered with 30vac?
I think this would be the obvious conclusion...mysyery solved?
not exactly sure
opinions?

schematic

complete schematic

http://www.waltzingbear.com/Schematics/RCA/BA-31A.htm


thanks
ts[/img]

 

[VacuumVoodoo]

This looks like a floating supply. Take your measurements relative to "common +30V". Notice that transformer's center tap is not on ground as would be expected.
Or am I missing something too :?:

 

[ToobieSnack]

Alex,
thanks for your reply

Notice that transformer's center tap is not on ground as would be expected.

the ba71 units do have a chassis ground on the actual mic pres themself.
I assume this is some attempt at a star ground. (or at least close enough for gr loop cancellation) It also is grounded on the amphenol connector ..no doubt back to a star ground in addition to the chassis gr)
Pin 3 (core) of the output transformer is also chassis grounded.

Or am I missing something too

Dont feel bad ...this is a bit of a mystery... to others too
I have received a lot of different theories as it relates to this schematic.
For one, the secondary voltage of the power transformer is not noted anywhere on the schematic, which adds to the mystery
Also... I went into this expecting to see DC current at the output ...too...
HOWEVER:
2 things come to mind for me.
1- don't we need 4 diodes to rectify AC to DC?
note: there are only 2 in this design.
2-I am actually getting exactly 30v AC at my output NOW!!
coincidence??? I think not....



I also think that this is why the scematic ps output is labeled +30v / -30v.
the 30- is not ground but the other half of the AC .....waveform???????
<my concept of this is not 100% ... so please feel free to correct me or elaborate.

Take your measurements relative to "common +30V

I took a measurement across the main output.....that's the end of the trail there...and point of main concern
I also took measurements from the point just past the diodes (neg side of the cap rail) to all points on the circuit.

I can post these results later if nec.


ALSO:
IF IN FACT THIS IS AN AC SUPPLY...
and my transformer is rated at 2 amps...
could i run 2 mic pre's from this single supply...only two...
some kind person has calculated the appx. load of each pre to be 1 amp...
(see my main ba71 post)
I should be ok right?
all help sincerely appreciated
thanks
TS

 

[Guest]

First, AC has no polarity so your assumption that the power supply is for ACV is incorrect.

The schematic indicates 60V at it's output, +30V and -30V at point B and P. If you don't measure that, in DV, then something is wrong. If you measure lots of AC, as you seem to be doing now, something is wrong.

The diodes simply form a rectifier for the negative cycle, taking the positive from center. The supply is further smoother and regulated with the rest of the circuit.

Hope this helps some.

 

[ToobieSnack]

Thank you very much

Do you think I don't have enough power at my secondary?
Now it's 24 ct 12 / 0 / 12
do you think I need a 48v ct? 24/ 0 /24
thanks
TS

PS should i measure from -30v out to ct ...and +30v out to center tap..reversing my leads as needed to determine correct polaity?
ts

 

[Guest]

You should measure from centertap. I think PRR had a detailed explanation of this circuit for you in another post. A 30-0-30 would probably be fine but anything less will get you less than the required 60V out. Take a look at the other post again and I think you will see.

 

[alk509]

[quote author="ToobieSnack"]IF IN FACT THIS IS AN AC SUPPLY...[/quote]

It's not. Get that out of your head.

What do you get when you try to measure DC voltage between points B and P?

Peace,
Al.

 

[PRR]

> The schematic indicates 60V at it's output, +30V and -30V at point B and P

No, no, it is 30V. (And DC, of course.)

Just like a car battery might be marked +12V and -12V.

That's the way we always did it, before these silly bipolar +/- supplies and DC-coupled op-amps came along.

" the output was exactly 30vac...hmmm

Hmmmmmm is right. If you really have 30VAC feeding that amp, all you will get is big 50/60Hz "hmmmmmm".

> I am actually getting exactly 30v AC at my output NOW!!

And does the amplifier work properly?

> don't we need 4 diodes to rectify AC to DC?

No. You can use 1, 2, 4, but in different ways. Your car has 6 or 12 rectifiers in the AC alternator feeding the DC battery.

> I bough a transformer (120v pri, 12.6 / 0 / 12.6 ct sec)

Where did you get those numbers? With that rectifier layout, you will get ~18VDC. If you have mis-connected leads or meters, who knows what you will get.

> the secondary voltage of the power transformer is not noted anywhere on the schematic

56VAC CT, or 28-0-28VAC. A large 48VCT transformer may work too. With this rectifier that will give around 36V, R1 R3 R4 R5 will divide this by 10/13 or about 30V.

This is all basic electricity. Not even "electronics", just Ohms Law and Sine-Peak concepts. You better master that before you smoke anything more.

 

[Guest]

No, no, it is 30V. (And DC, of course.)

Just like a car battery might be marked +12V and -12V.

Of course you are correct. I misread the +30 (Com).

 

[wilebee]

toobiesnack if I'm understanding you correctly you are trying to build a dc powersupply that is capible of -30vdc to power your ba71 preamps. in responce to you pm I used a 28vdc linear powesupply and adjusted the voltage up to 30vdc. I then inverted the postive and ground to give myself -30vdc. If you are trying to get a transformer to give you something close to 30vac before rectification then you shold be right in the ballpark just get yourself an lm337 and a couple 2200uf @35v caps the lm337 is a negative voltage regulator pull up the white sheet on it and there should be a schematic on how to build the circuit . my 2 cents
Wil

Wilebee

 

[ToobieSnack]

hey people,
had comp hd problems was unable to post for a few days.
PRR thanks for the additional info.
I think I just got a few bits of misinformation as it pertains to the trafo.(or maybe i didn't understand)
I quadruple checked my circuit and everything is right.

The only subs I made were with the 250uf cap...
I couldn't find one so I subbed a 230uf and a 35uf for a total of 255uf ..5uf shouldn't make a diff...should it

the other sub was the 7500 ohm res
I subbed 2 3k's and a 750. I measured it and it was real close to the req 7500ohm. (the 3k's were 1% when tested out of circuit and the 750 was 5%)
with the trafo I have now I did get something like you explained at the different r1 r2 r3 r4 junctions.
the output was just slightly above the 12.6 volts I was putting in so this sounds about right...if i just get the right trafo i think i will be ok.
i'm sure I just have the wrong trafo.

just get yourself an lm337 and a couple 2200uf @35v caps the lm337 is a negative voltage regulator pull up the white sheet on it and there should be a schematic on how to build the circuit

Wil:
can you post a link for the white papers?
is that a lundahl product?
I's like to get attenuation info too.
much thanks ebody
ts

 

[wilebee]

do a google search it should direct you to National semiconductor's web site where you'll find the data sheet on the lm137/337
http://www.national.com/pf/LM/LM137.html
as for the attenuators depending on the size of your bc7 consolette you should have 6 to 10 600-600ohm unbalanced daven attenuators. the center terminal is common the front terminal (the one closest to where the knob would go) is the in and the one is back is the outjus run it inline with you output for gain control Wil

Wilebee

 

[ToobieSnack]

thanks wil

I thought there was some complicated scheme (remote) that controlled output.
didn't know I could just insert an attenuator across the output...that's easy enough

ALSO:wil/Anyone:
I wanted to keep this to a single 1u rack space
could I sub the daven for something more modern/smaller?
what would be a good sub for this?

1more:
I can get a deal on some sifam al19's
the guy selling them says there "true" vu meters but sifam www says no.
they look so much like the console meters but smaller i would really like to use them....pro's / cons?

thanks
ts

 

[wilebee]

the only ones I can think of would fit in a 1u would be some Langvins you might be able to find on e-bay or you cous spend some good money on some goldpoint attenuators either way would be much more expensive. as for the meters you would have to modify the circuit because the isnt taps on the output transformer for a meter you would have to design a buffer circuit off the unballanced output. If you ran it of the output you would load down the output transformer and throw off your inpeadence. I would worry about a meter but thats just me. Wil

Wilebee

 

[ToobieSnack]

wil, thanks for your quick response.
I think I have one of those langvins you are talkin about ...thanks for reminding me.

I would worry about a meter but thats just me.

I am in dire need of cash for other projects so..
I wanted to put as many frill's on this unit to drive up the evilbay bids
you know how people like pretty lights n meters. lol
I'll probably add phantom and phase reversal too.
I can look throught the posts but if someone can post links for:
the phantom power
and meter buffer circuit
that would be great!!!
thanks all
ts

 

[PRR]

As long as you got a 12-0-12 transformer in your hand, and want to use four rectifiers (a slightly better plan, now that rectifiers are cheap), go get a Bridge Rectifier (four diodes in a hard-to-miswire package), over 100V and 1A, and wire like this:

 

[ToobieSnack]

::laughing joyously:: xlnt xlnt excellnt!!......(and sorry but 1 more question plz???

thank you sooooo much ! PRR ....u the man!
that will at leasty get the units fired up
the trafo is huge so I want to find another one smaller..mebbee a 5$ torroid.

....and damn.... sorry about this ....but...

just to clarify:
I am changin all the caps per your modifications (schematic)
BUT???:
am i just dropping the ct entirely and using the both 12 volt taps on the trafo to P and B
OR:
the CT and 1 of the 12 volt taps to P and B


thanks again
ts

 

[PRR]

> am i just dropping the ct entirely

Yes.

> I am changin all the caps per your modifications

No need to use my values, but no need to go to extremes to "match" RCA's values.

Power filter caps are not an exact science. Too small lets hum through, too big costs too much and the company goes out of business. In DIY, "too big" is not usually a problem, not for preamps. Also, electrolytic caps are low-precision things: often specified -20%/+40%. A "100uFd" cap may really be 80uFd or 140uFd.

So when replacing a "250uFd" cap, it is NOT necessary to cobble-up 250uFd out of odd values.

Why "250", which is no longer a standard value? The first electros were 4uFd, soon followed by 8 and 16uFd. Note that they didn't bother with 9 or 13uFd: power filtering is inexact and next-size-up is always better, as long as it does not blow the budget. (And Sprague's early electros were SO much cheaper than oil-caps that nobody complained about buying 16uFd when they only "needed" 12uFd.)

That went on to 32uFd, and then the industry switched to "rounder" numbers: 10, 20, 40uFd. Then 100, 250, 500, and 1,000uFd. That must be when RCA designed this beast.

Capacitors improved in tolerance, designers sharpened their pencils, boxes got smaller, and in-between sizes appeared to fit small gear or tight budgets. Somewhere along the line, electro caps switched to the number-system long used by resistors: 10, 15, 22, 27, 33, 47, 68... So today, if you want "250uFd or better", you buy a 270uFd. Or since the 1964 caps were such sloppy tolerance and the new caps are much tighter, a modern 220uFd would probably meet the original specs. However these are now two-bit caps, and price does not rise much with size, and today's dynamic-range goals are higher than in 1964, so why not go to 470uFd? The cost won't kill you, the size is probably smaller than a 250uFd from 1964, and the hum will be a little lower. You don't want to go too far this way if you are trying to get "that vintage sound", but from 220uFd to 470uFd will all be fine.

Likewise the "35uFd" caps could be modern 33uFd for "faithful", 100uFd for "overkill" at negligible increase of price (in DIY quantity).

> I subbed 2 3k's and a 750. I measured it and it was real close to the req 7500ohm. (the 3k's were 1% when tested out of circuit and the 750 was 5%)

I'm still trying to figure how 3,000+3,000+750= 7,500. I think you made a 6,750 ohm resistor. I think that is fine for initial testing: I think you will get a little less DC voltage, which is sure better than too much.

 

[ToobieSnack]

PRR thanks ...a..a..a..again!
first I most correct my error

I subbed 2 3k's and a 750. I measured it and it was real close to the req 7500ohm. (the 3k's were 1% when tested out of circuit and the 750 was 5%)

I'm still trying to figure how 3,000+3,000+750= 7,500. I think you made a 6,750 ohm resistor.

what i did was 2 x 3k's and 2 x 750's=7500...sorry for the error ...
in the pic of my circuit you will see that, at least, i did it right there ... lol

I absolutely love the history stuff you posted about capacitor values
that always makes it easier for me to remember if I have the history behind it. interesting stuff.

an I'm almost embarrased to ask any more questions but I guess I'm part cat and the curiousity will kill me. lol

totally and hopefully the last question on this subject:

If I go up in value at C1 do I have to go up in value with the other caps??

I think it would be a good idea too but I'm tryin to see if I can get away with being a bit lazy here and just change 1 cap...:wink:

thanks a million PRR and wilebee
ts