am9 preamp (b*b*c*)

[RedNoise]

cool , got ' em , many thanks!

 

[clintrubber]

Thanks for (re-)posting 

As drawn, the schematic is a bit weird around TR10: there is no DC-path for the top of R30.
Initally I thought it required a gapped output-TX, but that won't be the case I guess (it'd be pretty 'waisteful' as well).

Is the top of R30 (= 'G') simply connected to '0V' perhaps ?
Someone with real units care to check ?

Thanks/bye,

  Peter

 

[clintrubber]

FWIW:

http://www.bbceng.info/EDI%20Sheets/10301.pdf

So let's have a look at that mentioned Designs Department Handbook No.3.194(77)

Roddy, do you have access to that by chance ?

Bye,

  Peter

 

[clintrubber]

For all you lurkers
You are missing a great circuit.

Can I say a 7 transistor mic pre (discrete that is).

...

And this thing will smoke all the commercial junk made today.

And no it will NOT sound like a plug-in.

Yeah yeah, discrete, class A, from the BBC, vintage vintage, rare, signed by the queen etc, but what we want to know is HOW it is better than all that other crap then?


FWIW, apart from the pre-gain attenuation, the basic topology of each gain-stage is not unlike various other designs
(to keep quoting that one, the Altec 1588B for instance comes to mind ( http://electronicdave.myhosting.net/miscimages/1588B-3.gif )). Come to think of it, the second 'BBC-stage' might be a nice one to complement a few of those Altec-octal-plugins* that won't do all gain on their own.

*:

And no it will NOT sound like a plug-in.

ehhh  : 8)

Bye,

  Peter

 

[peter purpose]

Is the top of R30 (= 'G') simply connected to '0V' perhaps ?

It most certainly is Mr. Rubber.
A simplified schematic, with sundry junk dump.

 

[rodabod]

So let's have a look at that mentioned Designs Department Handbook No.3.194(77)

Roddy, do you have access to that by chance ?

Let me have a look tomorrow. Several mic preamp designs were made. Some fancier than others.

What makes all of the BBC gear very good is, very high build-quality, great documentation with labelled test points on the PCBs, and gear that was designed by ear as well as pencil. The vast majority of transformers used are Sowter by the way.

 

[clintrubber]

So let's have a look at that mentioned Designs Department Handbook No.3.194(77)

Roddy, do you have access to that by chance ?

Let me have a look tomorrow. Several mic preamp designs were made. Some fancier than others.

Perfect, thanks (where's the 'thumb-up' emoticon when you need it ?)
While eventual additional info won't reveal nuclear secrets I assume, it's always nice to see more info, docs & thoughts behind a certain design.

FWIW, somewhere else I saw a few more references (of which the last one (10301) was already linked to above):

AM090019 AM9/19A&B

Microphone Amplifier. Level raising amplifier for use
zero level input studio desks. "A" version without filter
and "B" version with 15.625kHz notch filter.
CH1/65A.1976.
DDMI 3.635(77).
DDHB 3.194(77).
EDI 10301(1)JAN78.

What makes all of the BBC gear very good is, very high build-quality, great documentation with labelled test points on the PCBs, and gear that was designed by ear as well as pencil. The vast majority of transformers used are Sowter by the way.

Looks good stuff indeed. A few TXs, some perfboard & BC550x & 560x will yield a nice preamp I expect.
We don't want anybody to make PCBs, right ? (I bet somebody is already working on a dual-pre in lunchbox format  )

Bye/thanks,

  Peter

 

[peter purpose]

(I bet somebody is already working on a dual-pre in lunchbox format  )

 

[rodabod]

Just looking through the docs. I have at home, there is also a twp-stage mic amplifier named the AM9/10.

I'll try and source that tech. doc you were looking for when I'm back in work. Maybe I should ask about taking home the never used audio eq. lunchbox they have there too... 

 

[clintrubber]

Just looking through the docs. I have at home, there is also a twp-stage mic amplifier named the AM9/10.
I'll try and source that tech. doc you were looking for when I'm back in work.

Thanks already. Nice. That /10 a bit like the /19A,B ?

Maybe I should ask about taking home the never used audio eq. lunchbox they have there too... 

These are busy & uncertain times. Big institutions should reduce clutter & hassle by getting rid of irrelevant inventory ! 

Bye,

  Peter

 

[hobiesound]

hello,

does anybody know if L2 and L4 can just be left of. i assume that they are used to filter RF content?!? I've only got a few 1Mh inductors of the "resistor" type but i'm worried that the current rating on them is too low. because on the photo pictured earlier in this thread shows a few inductors of the larger variety.

if all goes well i'll be posting a pcb for this pre.

bye bye!

Thomas

 

[peter purpose]

They're resistor sized numbers. Leave em in.

 

[mitsos]

if all goes well i'll be posting a pcb for this pre.

Are you working on an original format PCB (like in Peter P's picture on page 1 or lunchbox or other?

Also, is there any sense in doing a Sowter group buy for the trafos?

 

[hobiesound]

if all goes well i'll be posting a pcb for this pre.

Are you working on an original format PCB (like in Peter P's picture on page 1 or lunchbox or other?

i'm not really interested in making it a lunchbox style pcb, since i don't own a lunchbox rack. All i want is maybe fit 4 of these or 2 of these and 2 others in a 1U/HE 19" rack. Since the layout is also in the schematic i'll probably base it on that in the beginning. 

 

Also, is there any sense in doing a Sowter group buy for the trafos?

I honestly don't know. I don't have the time or kudos to organise something like that so probably not  :-\

thanx peter for your reply btw.

Greetings,

Thomas

 

[mitsos]

Hobie, I'd be interested in seeing your PCB. If you make a self-etch available, that'd be cool.

And about the sowter thing, I was kind of throwing it out there to gauge interest, not intending that you do it. 

 

[hobiesound]

self etch will be available, but don't pin me down on a release date  ..

also can someone recommend me a input and output transformer from sowter. For input i found the "n*e*v*e" replacement the "A*P*I" replacement and a few other. But i can't seem to decide on which transformers i should go for especially the output xformer is still a blank over here so to speak. I was looking at the api ouput replacement and i'll be ordering a few any way for a different project,  but i'd rather use a can type as on the original am9/19. 

greetings,

Thomas

 

[rodabod]

Look here and here.

The transformers are fairly standard, so if you choose a standard Sowter model with similar ratio then you should be good. I'm not sure if you can use the 1:1+1 in reverse to allow 1:2. You could ask Brian as they are very helpful.

Otherwise, the API equivalents should probably do a good job too.

 

[peter purpose]

I'm going to try some Stevens & Billington jobbies with this circuit.
As is, I'm not entirely sure how it would drive 600 OHMS. I think a little beefing up of the output might be in order.

 

[peterc]

Hi all

What happens to the connection marked PLA13 (bottom right)?

 

[peter purpose]

Hi all

What happens to the connection marked PLA13 (bottom right)?

Take to 0V for extra gain.....