am9 preamp (b*b*c*)

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clintrubber said:
W.r.t. broadcast-specs, I can understand, but more from the BBC-label than neccesarily from this circuit-topology, which is actually not that unlike as used by many other brands (I figure those octal Altec-micpre-plugs won't be considered broadcast-quality). But the devil is in the detail of course, I'm not saying that 'quite alike' topologies sound the same.

I totally agree, Peter. The main thing which I think makes these modules (and all the others which are used) stand out is the quality transformers.

Davo said:
So if I were to use the Lundahl ll1530... configured @ 1:7 (200:10K)..... is that too much of a jump in turns ratio?....
I'm thinking in terms of "gain" w/ the turns ratios.... whereas I should probably be thinking just in terms of matching impedances
between source and load, as you stated.

Well, what's going to happen here is, the secondary is going to be connected to the same impedance in this circuit as if you had used a 1:3.5 model. So what will happen is, with a 1:7 transformer, the input impedance is going to be four times smaller (difference in turns ratio squared). A quick glimpse at the schematic and I'd guesstimate the unit has ~2K input impedance. Four times less at 500R doesn't sound like a great idea to me. You might get away ok with say 1:5 though if you are really struggling to find suitable models.

so is 10k too far off from 8k (on the input tx,s secondary) to be usable in this instance?

Remember you quoted the 1:3.5 as 600:8K...... Well, that would actually be 200:2.5K if you are talking with respect to what it does to a 200 Ohm source.

Roddy
 
Thanks Roddy!  I'm gonna have to read through your post a few times to really digest.... that definitely helps!!

dave
 
I was thinking of using an Altran 3402-2 for the input.. It's 1+1:8.15... This can be wired 1:4 by running the primaries in series, right? But on their site, Altran list the impedances as 600/150 primary, 10K secondary... What does this mean for this circuit? I'm always confused by impedances in trafos.. I can do the math part, but I'm wondering if this is maybe made for a different load? I'm using them in my 312s, loaded with a 100K resistor, and they seem fine to me.
 
Now I'm slightly confused ::) What I thought was "8K" at the input traffo is really "BK"

So I think that Roddy was right when he said that should be closer to 200:2.5K for the 200ohm source.
At the bottom of the BBC schemo, they list 300ohms as a source (noise figure)... so that would roughly a 300:3750 traffo.

I'm being picky at this point, but trying to figure out what my secondary impedance for the input traffo would be.... any opinions?
 
These are the traffos that David @ cinemag is recommending for input and output:

Input:  CMMI-3.5C ($66.08)
Output: CMOQ-2x quadfilar transformer "In steel ("S" suffix) they are $26.83 apiece.  In 50% high-nickel + 50% steel ("L") they are $41.25.  In all high-nickel ("H") they are $55.74

Here's a link to their notes on the different lams:  http://cinemag.biz/application_notes/AN-104.pdf
 
Hi,

Can't help wondering, I guess these could be nicely suited as input-transformers ?
At least as a decent My First Input TransformerTM for these micpre's ?
(If so, then please don't buy them all  ;) )

http://www.groupdiy.com/index.php?topic=31411.0

PIKATRON Übertrager

RÜP 817M/1

Mikrofon - Eingangsübertrager

Ü = 1 : 3.21 + 3.21

1-2 , 3-4 , 5-6

Grundfläche 43mm

Höhe 26,5 mm

Bye,

  Peter



 
ghihi i was thinking of buying these, but then i went and lost my wallet a few days ago so now i have to wait for my replacement creditcard, bankcard etc.... arrggghhhh  :mad: ::)

don't buy em all before i get my hands on a few!!!  :p
 
Some misc notes:

* Toyed around a bit with these (calculator & sim), looks like the BC109 is fine for the TX-driver since current is even a bit less than the non-CCS first section. each section of the two seems to run at some 10..11mA in total, so no direct need for other types of transistors (but it wouldn't hurt if T3 & T9 at 50% of max. dissip are not your thing.

* I can't match these currents with the specs though, which state that each 12V rail 'does' 40mA IIRC.

* The 'simple schematic' without the notch still has R11/2k7, which will have been the source-resistance for the filter. It could be skipped I'd say, but keeping PCB-artwork & use a link might be best.

* 1:3.5 input-TX, hmm iso going cheap with OEP maybe some Beyer TR/BV would be nice here..., need to dig up specs.

Bye,

  Peter 
 
hobiesound said:
hello,

does anybody know if L2 and L4 can just be left of. i assume that they are used to filter RF content?!?

Without much further thinking for now I think they perform an alike (but now single sided) function as the two 20uH coils in the 990 DOA do (a trick for which a patent was granted but in fact preceeded by the ADI 121 opamp from 1966, but that's another story).

Without going into much further detail for now, note the alike ratios: Jensen 990: R = 30.1 Ohm, L = 20 uH and BBC AM9: R = 2k2, L = 1mH.

Long story short: don't skip L2 & L4.

Bye,

  Peter
 

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