Total Passive EQ

[Nickos]

Maybe is a stupid idea...
but what about a passive eq without the amplifier stage?
It is possible to compensate the loss of gain going to the mic pre instead of the line pre of the consolle...

Something like:
input iron-->passive eq-->output iron

Maybe you can use it also between mic and pre... but i'm not sure of impedance...

Tell me if i need to sleep more :shock:
:grin:

Nickos

 

[NewYorkDave]

It's not a crazy idea, I've done exactly what you're describing.

This box is totally passive and can be used with the amplifier of your choice (including mic preamps):

It doesn't look too pretty--it was just a prototype I whipped up inside of an old Shure mixer chassis.

 

[Nickos]

Wow!
And what about impedance?
Can i just put 2 irons (600ohm in&out)?

Thank you very much.

Nickos

 

[NewYorkDave]

The concept of a totally passive EQ (with no amplifier built-in) is actually very old. Western Electric was making "line equalizers" for telephone work long before most of us were born. Variable high-low EQs for sound recording became fairly commonplace by the '50s; a famous example is the Cinema Engineering 4031-B and its descendant, the Langevin EQ-251A. These were designed for an input and output impedance of 600 ohms and used constant-impedance bridged-T circuits.

Passive variable cut filters were (and sometimes still are) used extensively in public address/sound reinforcement systems, typical examples being the Altec "Acousta-Voice" system and several units made by White Instruments. The concept still has a lot of merit.

To answer your other question: since passive equalizers possess a certain amount of insertion loss, it's generally not a good idea to insert them between a microphone and preamplifier.

And yes, you can use 600:600 transformers if balanced I/O is required. (Oftentimes it is not!).

 

[drpat]

I've always liked this idea, because, you don't really need your mic pres when you mix, so it's easy to free one up for amp duties on a passive EQ. I just finished up a prototype (passive EQ) that has a switch to bypass the internal amplifier, and route it through a different transformer to send it to a mic pre. It's amazing how many tonal possibilities it opened up.

Hey Dave, now that I've finally seen your passive EQ for the first time, I've never seen the schematics. Do I need a secret password or something?

 

[craigmorris74]

drpat,
What kind of output transformer did you use to send the signal to a mic pre?
Craig

 

[NewYorkDave]

[quote author="drpat"]I've always liked this idea, because, you don't really need your mic pres when you mix, so it's easy to free one up for amp duties on a passive EQ.[/quote]

Right, I think there could be a market for such a device, which is why...

I've never seen the schematics.

...I've never published them. I did have an early version up the web for a while but it doesn't bear much resemblance to what I ended up using.

 

[JohnRoberts]

Passive EQ + make up gain was just the way it was done way back when. I believe Baxandall was credited with being one of the first to incorporate the EQ into a feedback network which also provided the feature of symmetrical boost/cut.

My sense is that S/N is generally better with a feedback style topology. Many early EQs were limited to modest amounts of boost, probably to keep noise manageable. A passive EQ with only 6dB of available boost could be similar in S/N to the feedback design which has a noise gain of 2x when set flat, but feedback EQ cuts noise too when cutting.

If all you need is 10 dB of make up gain, a mic preamp may not be the best choice.

One of my pet theories explaining the high regard for old style EQ is the modest available boost, making it harder to make bad EQ choices.

JR

 

[gevermil]

insertion loss on a 600 ohm input output passive eq is something like 20 dbs isnt it ?

 

[craigmorris74]

Sorry for the uniformed question, but when would it be necessary to use transformers for interaction with other gear?

 

[burdij]

Wouldn't you have insertion loss of at least as much as the amount of "gain boost" in each stage offering it, since the "0" position is actually set at 0 - gain boost?

Why do you need transformers at all, unless you are planning to use the input t to provide some noise free voltage gain (not a 1:1 ratio, rather 1:2 or 1:4)? No part of the circuit would need to be referenced to ground so it should hang nicely on a 600 Ohm balanced line. Or am I missing something here?

 

[CJ]

have you ever used a forty foot guitar chord?
what did id sound like?
transformers in - transformers out.
thats the Law.

 

[NewYorkDave]

Red herring. HF loss from a guitar pickup driving a long cable has everything to do with the magnitude of the source impedance, not the fact that it's unbalanced. Putting a 1:1 transformer on the output of something won't reduce its source impedance, in fact it'll increase it a little bit. Now, if you're talking about the merits of using xfmrs for impedance transformation, that's a different topic.

 

[Kit]

Red herring.

:?: :?:

 

[CJ]

i was thinkin more in terms of hummmmmmmmm.
which we do no tolerate in studio electronics, right?
fine for guitar slim.
we need - 140 db humm.

yes i know about the les paul recording being ok for 40 feet.

hey speaking of hum, can anybody figure this out?
Click on number one:

http://www.livescience.com/strangenews/top10_unexplained_phenomena-1.html

 

[burdij]

I think the equalizing network only sees the balance of its inputs and outputs. The circuit is still only two wires. Because of that, WRT ground (the rest of the world) it still appears balanced. All the equalizing activity occurs between the two wires.

have you ever used a forty foot guitar chord?

I can't even use regular guitar chords . . .

 

[Michael Tibes]

The old german W86 Eqs were a completely passive design, intended for a V72 to be used as makeup amp. This one is shelving (bass / treble)
Kubi has it on it's site:

http://audio.kubarth.com/rundfunk/getfile.cgi?f=@,3$Y,34R-S<X,U]W+V5C:VUI;&QE<E]W.#9A+FIP96<`

Eingang = Input, Ausgang = Output

There was also a midrange unit (some sort of presence dip?) , but I haven't found that one on Kubis site. If there was a serious interest I could scan the manual.

I haven't really heard these though, don't know if it's anything desirable.

Michael

 

[CJ]

Kubi has timed out, but i guess if there is a transformer from the source and one at the destination, like a V72, and cables are short, then yes, maybe no xfmr is required at either end.
try and see.
600 ohm enviro, so no sweat maybe.

 

[Michael Tibes]

Sorry, didn't know this link was gonna time out.

http://audio.kubarth.com/rundfunk/index.cgi#w should get you to the schemo of the W86a (on top of the page),
http://audio.kubarth.com/rundfunk/index.cgi has a more detailed documentation in german for the W86 and W86a, 2nd and 3rd item on the page.

The presencefilter is called PF5/H, and it's in fact a boost, not a cut. I don't know if it's possible to combine it with the W86 / W86a, I guess so, but I'm not sure.
Also passive from the same family: HS10 / TS10 hicut and locut. Those are without insertion loss, if I got things right.

Michael

 

[[silent:arts]]

[quote author="Michael Tibes"]Also passive from the same family: HS10 / TS10 hicut and locut. Those are without insertion loss, if I got things right.[/quote]
that's right. I have them - and I like them :!: - for FX.
they don't like todays studio levels, and that makes their own sound :grin: