Reddish 500 EQ

[dmp]

I don't mean this to sound glib, but you've just explained how the bass frequency is set

Ha, I guess I'm not understanding it then. I don't see the normal arrangement of components to make a HPF.
What's the 3dB freq formula for those orientations?

 

[dmp]

Including the Presence resistor divider

 

[dmp]

And the cut

 

[dmp]

To lower the bass frequency, raise the inductor value and raise all the cap values. Maybe @JMan knows the formula or just do it in a sim

 

[inf0]

My inexperienced self , asked google gemini for help. Is there anything correct with this? haha

 

[ruffrecords]

Looking at the schematic I don't see how the bass frequency is set? For boost there is a resistance in series with the 7H inductor going around the presence band, while for the cut there is a resistor and cap in parallel with the inductor out of circuit.

Here is my original REDDEQ schematic.



The big chain of resistors down the middle starting with the 1K5 at the top and ending with the 1K4 at the bottom is simply a potential divider. With everything else disconnected, the signal at the OUT terminal is about 14dB lower than the signal at the IN terminal. If a 14dB gain make up amplifier is connected to the OUT terminal then the overall gain is unity.

To achieve a boost, you can short the IN to the OUT in a frequency selective manner. If you short them together the output will increase by 14dB. This is how the bass boost works. At low frequencies the 7H inductor is a low impedance so the amount of boost is effectively set by the value of the series resistor selected by the boost switch. With the bass boost switch set to +10 you might expect the zero ohm resistor to create 14dB of boost but the internal dc resistance of the 7H inductor limits it to 10dB. At high frequencies the impedance of the inductor rises until there is no longer any boost.

The treble boost and mid boost work in exactly the same manner.

On way to achieve cut is to add resistance in series with the IN terminal so the potential divider has a greater loss. This is how the bass cut works. At low frequencies,the capacitors in parallel with the resistors in the bass cut switch are open circuit so at low frequencies the resistors are added in series with the IN thereby increasing the amount of cut. At high frequencies the capacitors are effectively short circuits so there is then no cut.

Cheers

Ian

 

[JMan]

So Ian, based on that explanation, how would one theoretically go about manipulating the frequency at which the bass boost/cut occurs?

Obviously it is a matter of changing component values. My instinct is that to lower the frequency for the boost, the first step would be to raise the value of the inductor used and then recalculate the resistor values to maintain similar gain steps; and for the cut, raise the values of the capacitors (and resistors?). How far off the mark am I in terms of a general starting point?

 

[dmp]

I don't think you need to change the resistors. In my simulation above increasing the inductor from 7 H to 20 H moved the frequency lower, but the boost is still about 6dB.

 

[JMan]

My only thought is that a 20H inductor is likely to be quite large, perhaps too large to hack into the circuit of a 500 series module. But maybe there are options out there that I'm not aware of that don't take up quite the same real estate...

 

[ruffrecords]

I don't think you need to change the resistors. In my simulation above increasing the inductor from 7 H to 20 H moved the frequency lower, but the boost is still about 6dB.

Correct, the resistors just set the amount of boost. The frequency depends on the value of the inductor AND the total resistance of the upper half of the potential divider. So if you wanted to achieve the same boost frequency with a smaller inductor or lower the frequency with the existing inductor you would you would need to scale down all the value of the pot divider. For example if you reduced the values of all the resistors in the pot divider by 10 you could use a 700mH inductor for the same frequency. However, the lower half of the pot divider would now total 510 ohms and this would be the worst case load your input would have to drive (this is roughly the way my mastering EQ works).

This EQ was designed to be driven directly via a 10K:10K transformer. The worst case load the EQ present to the secondary of this transformer is 5K1 (the sum of the resistors in the bottom half of the pot divider). If, for example, you decided to halve all the pot divider resistor values your worst case load would drop to 2K5 and you would probably need to add an active buffer stage to drive the EQ.

Cheers

Ian

 

[Forcemusicgroup]

Okay. It sounds like there might be an issue around R27. Both sides seem to be measuring far too low, and the real clue here is that the right leg is connected directly to the right leg of R28, so those should have pretty much identical readings, but look at how wildly different they are.

Can you check these measurements against your working unit?

On the working unit R27 = left leg is -2.9 ohms testing from audio ground to r27
right leg = -3.6 ohms

on the non working unit r27 left leg positive 4.2 ohms
right leg = 3.1 ohms

Im getting reverse polarity on the working unit. Going through the build to even check the resistor colors. Outside of that I'm stomped.

 

[Forcemusicgroup]

I did notice one trace under the opamp area. on the back on the pub there is a very small test point. upon tracing for continuity. nothing came... could be the issue. however the test point is under the op amp