Help modifying EQ

[spaceludwig]

I need some help and advice modifying the bare-bones EQ (high, mid, low at 100, 1K & 10K respectively) on the console I finished upgrading (recapped, upgraded opamps, changed input transformers)

Schematic (rotated for ease) of the EQ section below:

If I am not mistaken, I can change the value of the caps tied to the wiper of the 50K pots to change the frequency, correct? My problem is the values I get when I calculate the frequency the components already there are supposed to give me they do not jive with their supposed corner frequencies i.e.,

• high: 470R &0.022uF =15.4kHz
• Mid:  0.18H & 0.056uF = 1.6 KHz
• Low: 1.2H & 10uF =46Hz

These figure are off by a factor of 1.5, more than 2x for the low frequency. I am obviously making some very basic mistake.

So, before I figure out what values I need and how I will physically integrate this into the existing circuit I was hoping some of the knowledgeable posters could explain where I am going wrong in my calculations, what I am overlooking, forgetting, don't know, etc to lead me to the correct figures. I am happy to do this step by step rather than rushing through the whole procedure.

ultimately, I would just like to add a few values to each band that I can select with some kind of switch, probably rotary.

Thank you all in advance, looking forward to reading your comments/feedback.

Cheers

 

[gyraf]

..specifying eq frequencies is not an exact science - see a recent thread in the brewery..

simply try out various value capacitors, use the values you like best - and don't bother what theoretical frequencies you hit...

Jakob E.

 

[spaceludwig]

Jakob,

Really? I would have thought there was some method to the madness. It seems so strange to just go out and buy a whole range of values and then plugging them in one by one without some foundation for the choice.

I searched in vain for the thread you mention above in the Brewery. Would you be kind enough to at least provide the name of the thread, please?

Thanks

 

[gyraf]

sorry, not the brewery - the drawing board.

Mainly http://www.groupdiy.com/index.php?topic=43095.0

but also possibly
http://www.groupdiy.com/index.php?topic=44202.0
or
http://www.groupdiy.com/index.php?topic=44803.0

Jakob E.

 

[abbey road d enfer]

• high: 470R &0.022uF =15.4kHz
• Mid:  0.18H & 0.056uF = 1.6 KHz
• Low: 1.2H & 10uF =46Hz

These figure are off by a factor of 1.5, more than 2x for the low frequency. I am obviously making some very basic mistake.

You have to understand that the frequency where a shelving EQ is defined is NOT the corner frequency.
Typically, a high shelf will have its corner frequency at about 1kHz, that means the B/C is +/-3dB at 1k, but the specified frequency is probably about 10kHz, where the B/C is about +/-15dB.
And the RC you that defines the frequency is not the 470R, it's the 8k2 resistors that go to the direct or NFB paths. The characteristic frequency is then ca. 880Hz. the 470R is there to limit the amount of B/C at ultrasonic frequencies.
The issue is somewhat different with the low shelf, because it's an RLC circuit, but again, the 10uF cap is just there to limit excessive boost at infrasonics.
The actual corner frequency is defined by the inductor and the 8k2 res, which compute to about 1kHz, and seems about right with +/- 18dB at 100Hz.

 

[spaceludwig]

sorry, not the brewery - the drawing board...

Thanks

 

[spaceludwig]

You have to understand that the frequency where a shelving EQ is defined is NOT the corner frequency.
Typically, a high shelf will have its corner frequency at about 1kHz, that means the B/C is +/-3dB at 1k, but the specified frequency is probably about 10kHz, where the B/C is about +/-15dB.
And the RC you that defines the frequency is not the 470R, it's the 8k2 resistors that go to the direct or NFB paths. The characteristic frequency is then ca. 880Hz. the 470R is there to limit the amount of B/C at ultrasonic frequencies.
The issue is somewhat different with the low shelf, because it's an RLC circuit, but again, the 10uF cap is just there to limit excessive boost at infrasonics.
The actual corner frequency is defined by the inductor and the 8k2 res, which compute to about 1kHz, and seems about right with +/- 18dB at 100Hz.

Thank you for the explanation. If I understand you correctly from this and a post you made in another thread I would have to change the cap of the high shelf to achieve roughly 1/10th the target +/- 15db plateau? The resulting factor in this particular circuit is 11.33X for the 10K. So if I wanted a 7K shelf we have:

7000 / 11.33 = 617hz
617 from 8200 R = 31nF

Does this sound like a reasonable approach?

There is no mention about the mid frequency. The math is giving me 1.5K though the EQ is supposed to B/C at 1K. Does 22nF act as a parallel capacitor?

As for the low frequency, I take it I would have to add inductors to change the B/C target?

 

[spaceludwig]

Any suggestions on what kind of switch I could use to jump from one cap value to the other? Also, what would be the best way to wire the switch to the PCB where the original cap (ex, the 0.022 for the high frequency) is placed?

 

[jsteiger]

Just curious, what console is this from?

A 100uF cap will drop the low band frequency a little bit but not much. Anything larger than that is redundant.

 

[abbey road d enfer]

Thank you for the explanation. If I understand you correctly from this and a post you made in another thread I would have to change the cap of the high shelf to achieve roughly 1/10th the target +/- 15db plateau? The resulting factor in this particular circuit is 11.33X for the 10K. So if I wanted a 7K shelf we have:

7000 / 11.33 = 617hz
617 from 8200 R = 31nF

Does this sound like a reasonable approach? [/quote] Correct.

There is no mention about the mid frequency. The math is giving me 1.5K though the EQ is supposed to B/C at 1K. Does 22nF act as a parallel capacitor?

for the mids, it's the LC product that governs the center frequency F = 1/2pi.sqrtLC. The relation of the C (or L) value with the 8k2 res determines the skirts, in other words, the bandwidth.

As for the low frequency, I take it I would have to add inductors to change the B/C target?

Yes.

 

[spaceludwig]

Just curious, what console is this from?

A 100uF cap will drop the low band frequency a little bit but not much. Anything larger than that is redundant.

It's a Yamaha PM-700 which I recently recapped, substituted old IC's for modern opamps, and changed the stock transformers for the ones used in the PM2000 desks. I would like to add a little flexibility to the EQ so that I have the option of making gentle boost/cut at more than 2 frequencies.

I hope I'm not opening a can of worms here but I wonder what determines the quality of the EQ, for example the High frequency (in this instance by quality I mean boosts would be silky, not harsh and grating)? Is it the capacitor, the resistor, the opamp circuit it is feeding into? Would I get more pleasing results changing these for components made of a different material? Would transistors that have better noise specs make a difference? I am curious but don't have the infinite resources and time to sit there and swap parts in and out and listen for changes. So any advice on components known to have a quantifiable effect would be appreciated.

 

[spaceludwig]

for the mids, it's the LC product that governs the center frequency F = 1/2pi.sqrtLC. The relation of the C (or L) value with the 8k2 res determines the skirts, in other words, the bandwidth.

So, though Yamaha is claiming a 1K center it is really 1.5?  Or is it possible that there is some other component there shifting this figure over that I am not taking into account?

 

[jsteiger]

Looks the manual does say 1.5k for the mid. The sweep looks like 1.5k also.

http://www.manualowl.com/m/Yamaha/PM-700/Manual/153294

 

[spaceludwig]

Looks the manual does say 1.5k for the mid. The sweep looks like 1.5k also.

I sit corrected, my apologies.

 

[spaceludwig]

Ok, I've done all my calculations and found all the values I need from an online distributor.

My question is what kind of switch would I get so that I can select between values? Would a 2 pole rotary as such

(though not this particular model) be ok?  I'm thinking I could place the caps between two adjacent lugs and switch between values.

Any advice (especially that leading to a frugal mod) would be appreciated.

Cheers

 

[spaceludwig]

Just to be clear, I'd like a switch that will allow me to change the value of the 0.022 cap in the high shelf and the 0.056 cap in the mid band.

I'm not sure how to approach this as  I have no practical experience with rotary switches - I don't even know the proper terminology for the type of switch I would need, making it difficult to do research as the word "switch" is such a generic term. Perhaps someone has a more practical solition based on one of the myriad clones/projects that proliferate these pages.

 

[abbey road d enfer]

Although a rotary would be operationally correct, you may use a toggle switch, if only two positions are needed. You could even have 3 positions with an ON-Off-ON SP3T. Toggles are much smaller than rotaries.
In the middle position, only one C is connected, so that's the highest frequency; in the other positions, the frequency value is governed by the addition of either additional cap to the default one.

 

[spaceludwig]

Although a rotary would be operationally correct, you may use a toggle switch, if only two positions are needed. You could even have 3 positions with an ON-Off-ON SP3T. Toggles are much smaller than rotaries.
In the middle position, only one C is connected, so that's the highest frequency; in the other positions, the frequency value is governed by the addition of either additional cap to the default one.

Thanks, Abbey, thats great!  I need more than 3 positions but I realize I could put 2 SP3T's side by side.

Is there anything particular I need to watch out for when hooking up the switches, like stray capacitance? Should the wires be twisted?

 

[MikeClev]

Any stray capacitance will be in the order of a few pico farads at most, and will be completely swamped by the many nano farads of the capacitors in the circuit. The tolerance of the capacitors (+/- 10% ?) will be more than any stray capacitance...  so no problem!

 

[spaceludwig]

Any stray capacitance will be in the order of a few pico farads at most, and will be completely swamped by the many nano farads of the capacitors in the circuit. The tolerance of the capacitors (+/- 10% ?) will be more than any stray capacitance...  so no problem!

Ok, good to know.

Is there any way of making the low shelf a peak?