transformer question (sowter 4257/BBC LG 96A)

[jwk1]

Hi,

I've got a bunch of these transformers, 300:30k ratio 1.29:1

Datasheet attached to this post.

Originally used in BBC line amps.  I was just wondering what I could do with them... Would they make good input transformers for a Gyraf Pultec EQ, wired 30k:300r?

Thanks for any advice,

josh

 

[mrclunk]

I've some of the bbc line amps with these inputs too.
The amp doc's state that the line amp has an 18K input impedance.
I'm a bit noobie with transformers, but i think this must be the 'reflected'? load thu the transformer.

30K:300 would be good for a Pultec inter-stage i reckon.

I was actually planning on using the line amps as gain stages for a pultec type eq.

 

[abbey road d enfer]

The 300:30k indication brings confusion.
According to the inductance values, 57H and 96H, I would think the definition was 18k:30k, but designed to operate with low impedance sources.

 

[jwk1]

Thanks very much for the replies, that's very helpful.  Would this therefore be a useful substitute for a 10k:10k line input transformer, if loaded appropriately?

Josh

 

[ruffrecords]

Thanks very much for the replies, that's very helpful.  Would this therefore be a useful substitute for a 10k:10k line input transformer, if loaded appropriately?

Josh

Pretty much. Load the secondary with 30K and it will 'look like' about 18K on the primary - perfectly OK for a bridging input.  It will give you about 2.25dB of gain.

Cheers

Ian

 

[MagnetoSound]

Hey josh, where did you get that data sheet? I could use a similar one for the LG/94ASC that is used in the AM9/19B, if you might know where it could be found? Or anyone else?

 

[rodabod]

Did I not post most of the common transformer specs. somewhere here? I can't remember. I took a cell-phone photo from a book we have at work.

 

[jwk1]

HI Dan,

I got that datasheet from the Sowter website.

There's some bbc transformer info (but not for LG/94ASC unfortunately) in this thread:

http://www.groupdiy.com/index.php?topic=24105.msg349554#msg349554

Thanks,

Josh

 

[MagnetoSound]

Thanks Josh, I was just perusing that thread earlier today ...

There's nothing at Sowter's on the LG/94 either.

Here's what's baffling me - the AM9/19 is stated as having 2k input impedance and the trafo is 1:3.5 - yet the attenuator ladder loading the sec totals 20k, which works out at 1.6k on the primary. I just wonder if i am missing something wrt the trafo, or if there is some convention at the BBC regarding the way they put things across in their spec sheets that accounts for the confusion ...

I'm playing with using a 10k pot instead of the ladder, but I'm a little concerned that the impedance will fall below 1k, hence seeking clarification here. 

 

[abbey road d enfer]

The specs say 2k approx.. If you take into account the DCR of the windings, let's say 200r for the pri and 1k for the sec, that shouldn't be too far.

 

[MagnetoSound]

2k approx! - you're right, it does say that ...  :

 

[CJ]

start with the total turns from the datasheet

1696/2200 =1:1.297 turns ratio as seen on the datasheet

so Sowter did the math right. round it up to 1:1.3

so it is kind of a bridging transformer with a bit of a step up,

the step up could be there to compensate for signal loss

or it could be there to provide a bit better match into the transistors,

inductance squares as the turns are doubled, just like impedance,
so you do not have to add twice as many turns to get twice the inductance,
more like root 2, which is 1.414.

1.3 is close to 1.414, so you would expect the henries to be almost double, but not quite.

we are lucky to have inductance figures on the datasheet, as this is the first time i have seen it on a transformer spec sheet.

they list the henries as 57 and 96.
almost double, so it looks like their L meter matches up with the turns count.

now we can get the impedance from the henries

take the pri

57 henries

the impedance of the pri is just the reactance in ohms from the  formula

Xl = 2 pi f L

so Z = 2 pi f (57 henries)

Z= 6.28 f (57 henries)

Z = 357.96 f

(just punch the numbers into your search window, no need to pull up an online calculator)

ok we have the answer but there is a problem,

Z =  358 ohms f  , but at what frequency?


when you talk impedance, you need to know at what frequency.

1 hz? no,

20 k hz?  no

ahh, it says right on the sheet, L taken at 50 hz,

so plug in 50 for f and you get

50* 358 = 17,900 ohms

call that 18 k input impedance

do the same for the sec, or use the turns squared to get the ratio.

the sheet says 30 k load, lets check this,

what is our impedance ratio?

same as the turns ratio only squared.

1:1.3 turns will equal about  1:1.69  Z ratio, round that number to

1:1.7  impedance ratio.so multiply 18 K by 1.7 and we get 30,600 ohms.

so the datasheet is right,

pri Z = 18 K,  sec Z = 30 K,

if you do not want to load down your source, you make the load at least 10 times higher than the source.

so if the source is 300 ohms, as they say on the sheet,

it looks like they have a load factor of 18,000/300..

60 to 1.

this is a lot higher then 10:1, maybe they have a sensitive source, like a microphone?

or maybe a passive filter?

so you could use this as a solid state  mic pre input, or a solid state Pultec input transformer,

single ended transistor amp only, as there is no center tap for push pull operation.

18 db input level, that could handle a condenser mic output,

you could also figure this all out by taking the turns count, looking up the specs for the core, and figuring out how many turns equals how many henries for that core,

we were lucky not to do that as we have the specs already measured.

so to summarize,

Sowter 4257/BBC LG 96A

1:1.3 Turns Ratio
18 K Pri Imp.
30K Sec Imp.
Max Headroom (remember him? )  18 db

 

[mrclunk]

Thanks for explanation CJ! One of your more lucid moments?!