Passive EQ suggestions

[thomasdf]

How did the experimenting go ? I am curious about this design

 

[Michael Tibes]

This is a passive 'HIFI'-EQ I got from a Chinese eBay seller. I haven't tried it yet, but I think 10 k is a typical input impedance for HIFI stuff, so it might fit your setup. Afaik this might do a 6 dB 'boost', so it might have less insertion loss than what you are looking for. It's called 'James tone control' or something alike. It's certainly being discussed here and elsewhere, here's more theory: James Tone Stack - Analysis
The tone-stack calculator might also help, it has a model for this circuit:
TSC
tsc online

This might be close to Ian's mastering eq design though.

Michael

 

[Michael Tibes]

It's cheap to try:
eBay 1
eBay 2

 

[ruffrecords]

It's cheap to try:
eBay 1
eBay

The circuit you posted is intended for use in tube circuits where the circuit ir feeds is a very high impedance tube input (a FET op amp would be a good substitute). It bears no relation whatsoever to my mastering EQ design

Cheers

Ian

 

[rock soderstrom]

The circuit you posted is intended for use in tube circuits where the circuit ir feeds is a very high impedance tube input (a FET op amp would be a good substitute). It bears no relation whatsoever to my mastering EQ design

Cheers

Ian

Would this EQ circuit make sense in a tube microphone amplifier in the constallation:

Input Transformer - SRPP - EQ (mono) - SRPP - Output Transformer

What is the insertion loss of such a passive EQ circuit?

 

[ruffrecords]

Would this EQ circuit make sense in a tube microphone amplifier in the constallation:

Input Transformer - SRPP - EQ (mono) - SRPP - Output Transformer

What is the insertion loss of such a passive EQ circuit?

Yes, it would work in that context. Insertion loss depends on the maximum boost required but would be typically 20dB.

Cheers

Ian

 

[rock soderstrom]

Thanks Ian!

 

[Dainiusb]

Hey @ruffrecords , I love your eq schematics! I’m interested in adding an eq stage to a tube preamp, I would love for it to be inductor based but so many of the schematic references I can find for tube eq’s are just rc or nfb circuits, which is cool, but I would love to add some inductor color in there as well. How could I adapt one of your eq’s to accept the high impedances present in tube circuits?

In general im finding it difficult to find out how a high impedance would affect a passive eq circuit. I’m pretty sure I’ve sourced inductors that can handle the job, 500H total, hypermalloy core toroidal with a TON of taps so my options are open in that respect. Also 6x 9 stepped attenuators that can handle some pretty serious voltage.

Would love any input you have!

 

[gyraf]

Your main problem when going the high-Z way with passive LC eq's is that even though you can source high-inductance inductors these days, they usually come with such high stray capacitance that they're not really usable for LC eq'ing after all - and this before even considering the low saturation headroom resulting from the many-many turns required. I ran into this limitation also while making the G14..

I'd recommend that you try with a simple mock-up first, to confirm real-life viability of your idea before committing too much energy here

/Jakob E.

(ps: there's always the GPultec project, that one is rather accessible and already proven..)

 

[ruffrecords]

I agree with Gyraf. The solution is to use a cathode follower to drive the EQ with a low impedance and then feed it into a regular tube gain make up stage.

Cheers

Ian

 

[RuudNL]

Hi Ian, just a question out of curiosity: do you know who designed the 33609?

 

[Dainiusb]

Your main problem when going the high-Z way with passive LC eq's is that even though you can source high-inductance inductors these days, they usually come with such high stray capacitance that they're not really usable for LC eq'ing after all - and this before even considering the low saturation headroom resulting from the many-many turns required. I ran into this limitation also while making the G14..

I'd recommend that you try with a simple mock-up first, to confirm real-life viability of your idea before committing too much energy here

/Jakob E.

(ps: there's always the GPultec project, that one is rather accessible and already proven..)

Absolutely star struck, I swear I just stare at your G pultec schematic most days! I mean there's a reason it just isn't done, and that makes sense, with the tiny capacitance needed to balance it, that the stray capacitance would mess it up. Do you guys know any equations or have recommended literature on how to calculate how impedance would affect an inductor eq circuit?

My main motivation for doing this is being thrifty with transformers and tubes so I can be very intentional about their quality and vintage, and also being able to hit some incoming signal with a sweet sounding EQ at a relatively early stage on the chain. @ruffrecords Ian that could be an option, i'd just have to make sure my power transformer has enough juice to add an additional tube stage, or enough gain to change the 2nd half of the input tube, and more than likely I'd have to modify the output tube stage which is currently a 12at7, into a 12ax7 for more gain.

It seems like the option of running mic signal directly into a passive eq is out because of how susceptible it is to noise.

I ordered a Gertsch Ratio Tran (that's in the mail now) for cheap, it has a huge "ultra linear" toroidal transformer a hypermalloy core and a ton of taps and, I won't be able to say for sure until I can measure it, but intentionally designed to have a high inductance that total at about 500H that I can use tiny sections of and 6 9 way switches that set the transformer ratio in digits, so I was hoping that could be a fun thing to get involved here, If I got a million capacitors to go along with it even maybe a crazy LCR band that could span much of the frequency range. Here's more info on these in general: GERTSCH RATIO TRANSFORMERS

@gyraf definitely planning on winding my own some day soon per your recommendations, I guess I'm just still caught up in this obsession with vintage metallurgy and getting it involved somehow. (I've been collecting pieces of old iron to maybe someday experiment with winding some weird inductors haha)

But yeah may be worth setting up a baxendall or something similar in the tube preamp and then just making a glorious G pultec. I'm attaching a schematic of the unit I'm modifying.

Thank you both for your brilliance!!

 

[ruffrecords]

Hi Ian, just a question out of curiosity: do you know who designed the 33609?

I think the 33609 is essentially a repackaged 2254 which was the Neve compressor when I was there in the mid 70s. It was designed by David Rees who had moved to Audix by the time I got to Neve. Not sure where he went after that.

Edit: More info here: Early RUPERT NEVE Consoles and their stories | PART FIVE: 1969-1970 | ‘Free-grouping comes to the Thames’ – POSTFADE

Cheers

Ian

 

[thor.zmt]

This is a passive 'HIFI'-EQ I got from a Chinese eBay seller.

View attachment 108147

It is "wrong" for a passive Baxandall. You need the correct type of Log pot's, this unit likely uses linear pots. Normally the "upper" components would be 1/10th value of the lower ones.

Have a look here:

ESP - Simple High Quality Hi-Fi Preamp

for details on such a tone control. The response looks like this:

You could of course replace all the components and turn this into a standard James/Baxandall passive tone control.

Here a tone control (and makeup gain Amp) I did for a commercial product, notice the Pot specification in the note. This tone control allows appx. +/-12dB boost/cut.



As good log pots are increasingly rare, you might want to try 12 position switches with 1 position blocked off and +/- 5 2dB Steps.

It would be easy to add one or two mid band's as well.

Thor

 

[Michael Tibes]

The circuit you posted ... bears no relation whatsoever to my mastering EQ design

Cheers

Ian

Hi Ian, I didn't mean to imply there was any relation to your designs. I just wanted to point to a very cheap alternative which might be interesting for experimentation ;-)

Michael