Variable Impedance schematic?

Does anyone have or know where to find a schemo that shows a variable impedance circuit?Something like the Groove Tubes Vipre or the Art TPS II.I know transformers with split ratios could do this but can you achieve this with no tranny's.I would like to add this feature to a SSM2019 based mic pre I'm building but not sure if it will work in front of an i.c. based input.The common input of the SSM2019 has a 10k resistor tied to ground. Any help would help..... :guinness: :?

dumb question or no thaughts?

I think that a pot or a rotary switch across the + and - terminals would work as an variable impedance in this case, but should also be seen as an attenuator... not really sure, I´m just writing this because nobody else did.

lima loaaaauw TIMMY!

> dumb question or no thaughts?

Well, IMHO variable impedance is a bad idea, a gimmick.

There are two meanings I can imagine:

1) simple resistor across the inputs. Try 300Ω and 75Ω. Signal will be weaker and will be bent according to the mike's impedance/frequency curve.

2) custom input transformer with many primary taps. If the secondary is loaded (resistance or capacitance), this will screw with the frequency response of mike and transformer, without quite so much loss of gain.

While I know there is a well-regarded "variable impedance" mike-amp, and that the knob does something, I don't think we can imitate it without a very intense intimate dissection or lengthy trial-and-change. I don't think a schematic would tell all the secrets.

No, it's

"Living a lie! Timmy!"

[quote author="PRR"]
There are two meanings I can imagine:

1) simple resistor across the inputs. Try 300Ω and 75Ω. Signal will be weaker and will be bent according to the mike's impedance/frequency curve.

2) custom input transformer with many primary taps. If the secondary is loaded (resistance or capacitance), this will screw with the frequency response of mike and transformer, without quite so much loss of gain.
[/quote]
and third meaning:
Paralel negative feedback.
Feedback is useful to do impedance and noise matching together
(~ van der Ziel)
xvlk

I wanna know how Gordon Instruments does their variable impedance Output stage?

Variable impedance.......mmmmm........could this create a useful sound effect if continually varied?
Stephen

Look at Telefunken v672. Feedback done through input trafo,
that seems to be bifilar wound toroid. This thing supposed to work
as "black-hole" (active) summing or micpre, input impedance variable.
Trafo with taps on pri is best IMHO.

Look at Telefunken v672

Click to expand...

I had problems using my V672/2 until someone pointed out that they operate on what they call zero ohm input impedance ! This means that you have to put a pair of resitors before the input transformer, these are the input impedance & without them the impedance is zero (& they sound terrible).

Just curious, can a TX with a two split bifilar wound primaries be used for both series and parallel hookup of those primaries ?

I guess it'll work and the input-Z varies by a ratio of four,
but I'm not sure I'm overlooking something and that one of both hookups perhaps gives some signal-cancellation.
Since bifilar it not about winding direction but just the 'location' of one winding w.r.t. the other it'll both work, correct ?

(Whether it serves a function is indeed something else, as stated above)

Thanks,

Peter

Hey, how about variable impedance input and output that changes with the performer's body temperature, kind of like a mood-ring? That should be a real edge in the gimmickery department. :green:

[quote author="tk@halmi"]Hey, how about variable impedance input and output that changes with the performer's body temperature, kind of like a mood-ring? That should be a real edge in the gimmickery department. :green:[/quote]
What did you have in mind, morphing or hard-switching ? :wink:

But kidding aside, 'two split bifilar wound primaries' do exist, and I was just wondering if I understand that stuff correctly :thumb:

[quote author="clintrubber"]Just curious, can a TX with a two split bifilar wound primaries be used for both series and parallel hookup of those primaries ?
[/quote]

Hey Peter,

I think you are thinking about stuff which I'm thinking. :wink:

I've got my split bifilar transformers coming in the post.

I'm still not sure what it means - bifilar just means two separate coils. I asked in another thread about paralleling these to change the ratio, but as usual, I was completely wrong!

Thing is - the transformer ratio remains the same but the DC resistance is paralleled (I think). Is this useful?

A variable input impedance circuit for guitar was recently discussed here...
http://www.diystompboxes.com/smfforum/index.php?topic=39559.0

Someone other than I would have to comment on modding it for balanced (mic) I/O, tho.

~ Charlie

Hi Roddy,

[quote author="rodabod"][quote author="clintrubber"]Just curious, can a TX with a two split bifilar wound primaries be used for both series and parallel hookup of those primaries ?
[/quote]

Hey Peter,

I think you are thinking about stuff which I'm thinking. :wink:
[/quote]I was thinking that while at it, why not have two split primaries so that you can toy around with series or parallel of those primaries ?

I'm still not sure what it means - bifilar just means two separate coils. I asked in another thread about paralleling these to change the ratio, but as usual, I was completely wrong!

Thing is - the transformer ratio remains the same but the DC resistance is paralleled (I think). Is this useful?

Click to expand...

As I hope and am reasonably sure the situation is like this:

Connecting them in series gives you a centre tap to which you can or won't be connecting phantom by what was it, a 3k4 ? Ratio will then be 1:N.
By connecting them in parallel the ratio will be 1/2:N, so 1:2N (and the impedance-ratio will scale fourfold).

As I understand bifilar, it's just that two windings are wound at once, so they're kind of twins i.s.o. one after the other (with 9 months delay :wink: ). For the principal functioning of the TX it won't matter I figure.

I've got my split bifilar transformers coming in the post.

Click to expand...

Interesting ! Which types ? Please let us know how they turn(...) out !

Bye,

Peter

By the way, impedance switching on a Cinemag CM-75101APC with Fabio?s 312 here.
These links are interesting too:

• Looking for a variable impedance transformer.
• variable impedance theory
• who wants to see the inside of my Vipre?(gear porn)
• Variable Impedence Mic Input

'Bifilar' is a winding method where dual wires are wound instead of single; that is, you get 2 windings that are identical in every respect. This is particularly valuable in audio transformers where high CMMR is important; the windings can be configured as a single centre-tapped winding with identical inter-winding capacitances...

[quote author="clintrubber"]
As I hope and am reasonably sure the situation is like this:

Connecting them in series gives you a centre tap to which you can or won't be connecting phantom by what was it, a 3k4 ? Ratio will then be 1:N.
By connecting them in parallel the ratio will be 1/2:N, so 1:2N (and the impedance-ratio will scale fourfold).

As I understand bifilar, it's just that two windings are wound at once, so they're kind of twins i.s.o. one after the other (with 9 months delay :wink: ). For the principal functioning of the TX it won't matter I figure.
[/quote]

Hi Peter,

Is having a bifilar twin primary the same as having a split primary?

I asked what would happen when paralleling a split primary (because I thought it would halve the ratio) but this is what CJ said:

[quote author="CJ The Transformer Murderer"]Imagine if you used 100 strand litz wire to wind a primary. How many turns do you have if you wrap the litz wire around the core one time?

1 turn. Even though that turn consists of 100 individual strands of wire.

So if you parallel a primary, you are just doubling the strands in your wire, not doubling the turns.

So the ratio stays the same. [/quote]

So, will the ratio not just be the same if you parallel these bifilar primaries? The turns number on the primary would just be the same, no?

Of course, it is easy to double the ratio with the twin primary...

P.S, Peter, those puns are getting worse and worse....

[quote author="Peter The Comedian"]they're kind of twins i.s.o. one after the other (with 9 months delay :wink: )...

Please let us know how they turn(...) out ! [/quote]