Passive EQ suggestions

[RSRecords]

I've had this long term project of building a sidecar out of an electrodyne mixer and a rack of edyne line amps. Wiring up the line amps with a trim level in between is easy enough but the line amps have a minimum of 20dB gain. To avoid excess gain I need to reduce the level coming out of the preamp. I could just use a pad but the feature creep demon says what about a 2 band passive eq. Any suggestions for a starting circuit? Input impedance of the line amp is >15k and mic pre out is <50. Hoping to attenuate 14-20dB. Thinking 2 fixed shelves for very broad tone shaping.

Thanks

 

[ruffrecords]

You need a passive EQ with some insertion loss. You could just build a Pultec EQP1A without the gain make up stage.

Alternatively you could create a 600 ohm version of ny REDD EQ design ( Custom Tube Consoles - DIY and check out the REDDEQ folder). You would need to divide all the resistor and inductor values by 10 and multiply all the capacitor values by 10 to work at the edyne line amp impedances but ottehrwise it should be straightforward.

Cheers

Ian

 

[RSRecords]

You need a passive EQ with some insertion loss. You could just build a Pultec EQP1A without the gain make up stage.

Alternatively you could create a 600 ohm version of ny REDD EQ design ( Custom Tube Consoles - DIY and check out the REDDEQ folder). You would need to divide all the resistor and inductor values by 10 and multiply all the capacitor values by 10 to work at the edyne line amp impedances but ottehrwise it should be straightforward.

Cheers

Ian

Thanks Ian,
I’ll see what I can work out. I’m hoping to have just 2 fixed bands and use center detented pots instead of switches. Maybe that’s not accurate enough though.

 

[TwentyTrees]

How about something based on Ian's Passive Mastering EQ concept? In his scoping paper he sets out a two band shelving passive EQ topology from the early 70s that could perhaps be adapted to your requirements.

I love your instinct to attenuate the line amps with a passive EQ, by the way. I've built several passive EQs without make-up gain so as to give my mic preamps something to do during mixing!

 

[ruffrecords]

Thanks Ian,
I’ll see what I can work out. I’m hoping to have just 2 fixed bands and use center detented pots instead of switches. Maybe that’s not accurate enough though.

There is a very simple plus/minus 6dB bass and treble control used years ago by Audix that uses a pair of LIN pots. Would that do you?

Cheers

Ian

 

[RSRecords]

There is a very simple plus/minus 6dB bass and treble control used years ago by Audix that uses a pair of LIN pots. Would that do you?

Cheers

Ian

I was looking at that. Am I correct in that using 32k pots (if I can find them) would provide +/-10dB?

 

[ruffrecords]

I was looking at that. Am I correct in that using 32k pots (if I can find them) would provide +/-10dB?

Unfortunately not. With a LIN pot you always get 6dB each way with this circuit. If you want 10dB you have to change the law of the pot and you get different turnover frequencies for cut and boost. You can make it work with stepped switches instead of pots and that is exactly what my REDD EQ design does.

Cheers

Ian

 

[RSRecords]

Unfortunately not. With a LIN pot you always get 6dB each way with this circuit. If you want 10dB you have to change the law of the pot and you get different turnover frequencies for cut and boost. You can make it work with stepped switches instead of pots and that is exactly what my REDD EQ design does.

Cheers

Ian

oh ok...that clears things up a bit. I’ll whip a simple 6dB and see if that’s enough. In the Audix schematic is the inductor 2 henries? a Z? The 10k Fader pot is perfect too as this was already planned.

 

[ruffrecords]

It is 2 Henries. It is a David Rees design (who worked at Neve in the late 60s).

Cheers

Ian

 

[RSRecords]

Awesome. He designed a compressor/gate for Phoenix audio that I love. The N90-DRC.

 

[ruffrecords]

Awesome. He designed a compressor/gate for Phoenix audio that I love. The N90-DRC.

He also designed the Neve 2254.

Cheers

Ian

 

[RSRecords]

Ah yea but who's ever heard of that one.

 

[RSRecords]

Any recommendations for a 2H inductor? Having some trouble sourcing one.

 

[ruffrecords]

Any recommendations for a 2H inductor? Having some trouble sourcing one.

Carnhill VTB9050 or VTB9047 or VTB9055

Cheers

Ian

 

[TwentyTrees]

Any recommendations for a 2H inductor? Having some trouble sourcing one.

I was going to suggest Carnhill, though Ian's beaten me to it. Mouser also shows the 2H Hammond 154M for less than Carnhills usually go for, though it's currently out of stock there.

 

[volker]

I was going to suggest Carnhill, though Ian's beaten me to it. Mouser also shows the 2H Hammond 154M for less than Carnhills usually go for, though it's currently out of stock there.

You can't use a gapped inductor intended for a certain DC operating current for a filter. The inductance will be far off the nominal value.

 

[TwentyTrees]

You can't use a gapped inductor intended for a certain DC operating current for a filter. The inductance will be far off the nominal value.

Good to know, thanks. Makes sense!

 

[gyraf]

Wind one? Some 200 turns on a P30T38 core or so?

/Jakob E.

 

[RSRecords]

Wind one? Some 200 turns on a P30T38 core or so?

/Jakob E.

That easy? something like this? https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B65701W0000Y038/3913968

What about the wire?

 

[gyraf]

Yes, that easy.

Wire size is not really critical - it only determines the resulting resistance of the coil and thus the maximum Q for our purposes, and a handful of ohms either way does rarely matter. You could off course do the math about what maximum size wire you could fit the required number of turns into the bobbin - but again, you could also just use anything you have around anyway..

above P30T38 is specified at a A(L) of 28 µH, =28000nH/sqr-turns: Use Inductance Calculator – Power Magnetics to input this as AL value, the wanted 2000mH in the Inductance, press "calculate no. of turns", and it answers: 267.3 (ignore the .3, as winding turns is one of the VERY few places where nature actually insists on whole numbers)

Note that the core AL comes with quite a big tolerance of -30%, +40% - so if you need precision or matching, measure actual core AL (by measuring resulting inductance in clamped/grinded condition, see below, then using thed reverse AL calculation tool on Power Magnetics ) before calculating turn numbers.

Also note that to use this type core, you will need a bobbin like the https://www.digikey.dk/en/products/detail/epcos-tdk-electronics/B65702B0000T001/3913969 AND a mounting/clamp/holder kit like the https://www.digikey.dk/en/products/detail/epcos-tdk-electronics/B65705B0003X000/3913970

Wind 267 turns on the bobbin, put between core halves, mount clamp/holder (takes a bit of finger strength). Then note that the mounted halves can be moved slightly by pressing them sideways - this will do some grinding between halves, making surface-fit better, reducing the tiny airgap, raising inductance. If you have a inductance meter, massage it until you max out the resulting inductance.

..and before you ask, then yes - the T38 material is VERY suitable for our purposes, i.e. agreeable sonic imprint..

/Jakob E.