BBC AMC/4 impedance converter for Neumann M7

Pretty interesting design
Happy sunday!

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12at7/Ecc81 circuit which could be easily adapted into most chinese ccda mikes

Have you tried it? 

I don't recall if there's a BBC paper for the Neumann amplifier.  Perhaps there is, and it contains arguments for improving the circuit.  Or perhaps this is purely a cost-based in-house design.  Has anyone found this combination?  Has anyone tried or compared it?  The output transformer poses a design problem, more than any other part. 

Bv.12 might work ...?

Thanks ln76d!  I combined the pngs to a pdf and added it to the Technical Docs section.

https://groupdiy.com/index.php?topic=58823.msg873468#msg873468

MagnetoSound said:

Have you tried it? 

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Nope, but in the next two months i will definately test that circuit

emrr said:

I don't recall if there's a BBC get paper for the Neumann amplifier.  Perhaps there is, and it contains arguments for improving the circuit.  Or perhaps this is purely a cost-based in-house design.  Has anyone found this combination?  Has anyone tried or compared it?  The output transformer poses a design problem, more than any other part. 

Click to expand...

There's a lot of papers from BBC

Rather not the "cost-based", but practical-based - Neumann never make that small tube impedance converter.
This is BBC in house design, like many other nice stuff.
If i read transformer ratio correctly, then it's ca. 4,5:1 - then i don't see the problem

MagnetoSound said:

Bv.12 might work ...?

Click to expand...

If i'm right for the ratio then BV12 rather isn't best candidate

mjrippe said:

Thanks ln76d!  I combined the pngs to a pdf and added it to the Technical Docs section.

https://groupdiy.com/index.php?topic=58823.msg873468#msg873468

Click to expand...

Thanks! I have 120 pages full document about microphones from bbc - this is just one microphone from it.
In a days i will upload full and put the link here.

*like*

gyraf said:

*like*

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+1

Tertiary feedback winding on many of the BBC's transformers , something off the shelf is never going to have that .Maybe Canford Audio can still get some of the BBC designs made to order ,worth checking out .

Good point, didn't found in canford webstore, i will ask them when i'll be ordering cables.
I will also ask Brian Sowter
There's one question how read 40:9 ratio? 4.5:1? Then how the windings are divided? Equal?

ln76d said:

There's one question how read 40:9 ratio? 4.5:1? Then how the windings are divided? Equal?

Click to expand...

I think that is the point that Doug was making - we don't know.

There's no ratio I see mentioned for the tertiary at all.  Ian or CJ or PRR or DaveP can probably run the math in their sleep and tell us what it probably is. 

I took a fast look at the article.
I don't get the "WHY" of the design, what did the designer(s) want? I get the ribbon output level because of the time this was built, but I don't get why it was designed this way.

Why build something like this today then reduce the output with the EQ pad to a ribbon level?

You will need the choke in the  fil supply for low noise for the 1st tube biasing or a very low noise fil circuit without a choke

Also the capsule voltage/feedback resistor around the odd first stage(was the first stage done this way to save not using more biasing resistors and a cap?)  and grounded cathode contact bias 2nd stage WHY?

leaving out information about the transformer windings?

Still didn't investigated, most of is probably matter of some bbc standards at the time. Didn't had timeo read more articles. Maybe there's more answers in the full document
I will back to this after easter.

Here's full document:

https://files.fm/u/xvhsnmtp