Passive EQ/Mixer

[terrapin]

I have already built a 10 channel passive mixing device from NYDave schematic.  The other day I was thinking about how great it would be to have EQ on each channel.  Since a passive EQ will knock the signal down several dB's like the resistor nextwork of my mixer, could I have 10 channels of passive EQ filters whose signals are summed together and sent to a stereo preamp/makeup amp?  Would love any thought/opinions you guys have on the subject.

Ron

 

[JohnRoberts]

Passive EQ, can impose reactive impedances onto the bus that could interact with other channels and perhaps alter the EQ of other sources in unintended ways.

IMO there are sound reasons for using active electronics in consoles.

JR

 

[lassoharp]

I have already built a 10 channel passive mixing device from NYDave schematic.  The other day I was thinking about how great it would be to have EQ on each channel.  Since a passive EQ will knock the signal down several dB's like the resistor nextwork of my mixer, could I have 10 channels of passive EQ filters whose signals are summed together and sent to a stereo preamp/makeup amp?  Would love any thought/opinions you guys have on the subject.

Ron

Hi,

Just finished building the very same device - 10 channels worth!  I was considering the possibilities of a simple 2 band shelving network for mine.  There are a number of simple circuits but the biggest issue I see for hi level EQ work would be constant impedance - so either look at a constant impedance design - somewhat involved and requiring T attenuators or,  pultec style which would seem to require matching transformers  - if only for interfacing balanced to unbalanced networks however that may work out.

If you plan to use your mixer with hefty line amps and buss amps for the master outs then makeup gain may not be an issue.  The non constant impedance of the classic tone control circuits may not be the worst thing either depending on your expectations/needs.  I'm using mine with 8-track analog so something for cutting mud/rumble and tape hiss would be very useful.  I still may look at adding that in at some point.

There's also the baxandall type tube version to consider.  That kind of covers what a Pultec does but the points are fixed and you also need a buffer to feed back into the mix lines at 600r.

For comprehensive EQ I'm using stand alone outboard units in a chain with the line amps and limiters   

 

[CJ]

just build the nyd passive eq,

you want to pm for the schematic, he does not want it on the net,

 

[lassoharp]

Hey CJ - good to see you back!

tried to PM - inbox full

 

[shabtek]

CJ?

 

[gyraf]

Welcome back!

 

[terrapin]

I was afraid impedance issues would be an issue.  Didn't older consoles (UA's) have passive EQ?  How did they deal with these issues?

How would the NYD EQ get around these issues?

 

[abbey road d enfer]

There's also the baxandall type tube version to consider.  That kind of covers what a Pultec does but the points are fixed

This is not cast in stone. The Bax can be made as flexible as a Pultec by switching the components (caps) that determine the turnover points. In fact it can be made even more flexible by using pots to control them.

 

[lassoharp]

I was afraid impedance issues would be an issue.  Didn't older consoles (UA's) have passive EQ?  How did they deal with these issues?

I looked for the full schematic but only the mic preamp is readily available AFAIK.

 

[sahib]

I was afraid impedance issues would be an issue.  Didn't older consoles (UA's) have passive EQ?  How did they deal with these issues?

How would the NYD EQ get around these issues?

By terminating the eq stage with correct impedance using a resistive network. When designing a passive filter network the output impedance of the previous stage and the load impedance are taken into account.

 

[JohnRoberts]

I think I'll design a passive mic preamp...  ;D

JR

 

[lassoharp]

How would the NYD EQ get around these issues?

Constant impedance design using T attenuators.  Look at something like the Langevin 251-A. 

so either look at a constant impedance design - somewhat involved and requiring T attenuators or, 

 

[sahib]

I think I'll design a passive mic preamp...  ;D

JR

Well. That would be a very quick design.

A step-up transformer. ;D

 

[CJ]

here ya go, leave it to the germans for all that audio stuff,

 

[CJ]

this is all i could find, can't vouch, never wired it,

simulates real nice,

for the low bands, if there were any?

i simulated 820 mH for 40 Hz, cut only
330 mH for 100 Hz, cut onl,
330 mH and 1 uF in series for 300 hz,

plus this, 15 K is cut only,



3

 

[CJ]

mid band:

 

[haima]

I was afraid impedance issues would be an issue.

as long as they don't impede CJ from his hacksaw 

sorry... but HEY CJ!! good to see you back around these parts.

 

[Moses]

I know NYD is (understandably!) cagey about his designs being flung around the forums, but I was wondering if someone would mind PMing or emailing me the full schematics for his passive EQ?

Thanks,

Mo

 

[JohnRoberts]

I think I'll design a passive mic preamp...  ;D

JR

Well. That would be a very quick design.

A step-up transformer. ;D

Great, so I can just put a step up transformer at output of my passive mixer and I'm good to go?  8)

No need worrying about all that messy active circuitry.

JR

Sorry.. I should just avoid this thread...  The customer is always right...