Mark IV EQ

[Gregg]

Hi,

A friend of mine recently wound some inductors for the Mark IV EQ but after I built it I noticed that it doesn't sound right. I don't mean Q-wise, but dB-wise. It doesn't cut and boost enough compared to the original.
The inductors are wound on EPCOS 18x11mm N48 core /ungapped/ and have very low resistance that's why I don't think their Q is the problem here. Maybe the EQ circuit /transistors' bias?/ needs to be tweaked or something?
Any help and advice are welcome.

[Ray Ivers]

Gregg,

These toroidal/potted inductors are available from Boogie, or at least they used to be; about $3 each IIRC.

My guess would be the inductance value of the replacements differ from the originals, lowering the Q of the entire RLC resonant circuit (of which the inductor is only one part) - the presence of the resistor also reduces the effect of the individual cap/inductor Q's. It's possible - although IMO unlikely - that the inductors may be saturating, but you'd probably need an in-circuit LCR meter to determine this for sure.

If this is what is happening, changing the cap values should bring the circuit either towards or further away from resonance, giving you more or less boost respectively - you could try a 1/2-value and 2x-value cap to test. Obviously the new resonant frequency will differ from the original, which may be a good thing; I always wished for 100Hz/240Hz/400Hz/1kHz/4kHz myself, with a fixed 12dB/octave high-treble rolloff on the hi-gain channel.

Ray

[Gregg]

Thanks for the quick reply.
The inductances have been measured with an LC meter before installation and the values were matching.
My inductors are wound on a ferrite core like this one /I couldn't find replacement inductors on Mesa's site/:

http://www.smallbearelec.com/Detail.bok?no=368

[Steve Conner]

I can think of two things:

1: Maybe the home-made inductors have a higher DC resistance than the Boogie originals. Toroidal cores need fewer turns to achieve a given inductance than E or pot cores.

2: If you're working from a Mesa service schematic, it wouldn't surprise me if Mesa deliberately printed wrong values for the voicing components, including the inductors. They strike me as that kind of snake oil outfit.

Personally I prefer gyrators to inductors. No worries about DCR or hum pickup :P

[Gregg]

1/ The DC resistance is of all inductors is within 22-45 Ohm limits.
2/ Yes, I'm using the service schematic. Calculated the center frequencies with the help of L-C filter calculator found here:

http://circuitcalculator.com/lcfilter.htm

Instead of the listed frequencies I'm getting 87, 372, 723, 1576 and 4823Hz. This is not big deal as it can be easily corrected by changing cap values. The problem is the amount of cut and boost.
Except for the inductors could the transistor circuit add to the problem?
Thanks for the suggestions so far.