I'm fairly serious about it and to motivate myself to actually start it, I've bought a chassis:
I plan to build most of it using turret boards. It is kinda small but I have put a whole ODS into a chassis that is even smaller https://ampgarage.com/forum/viewtopic.php?p=413604. So, I think it can be done.
The one thing that concerns me is the graphic equalizer on this thing. It is pretty central to the whole vibe of this amp. So leaving it out it not an option. But it does use rather large inductors. These inductors take up alot of space, are hard to find and tend to inject tons of 60/120 cycle hum into things (from demos I've seen on the net). So, I'd rather not use them. Luckily it looks like they can be replaced with a circuit called a gyrator. The above schematic includes an alternate eq using these gyrators in place of the inductors.
To figure out what component values would be needed I created a small python script which calculates the center frequencies, Qs and bandwidth in octaves for both the original inductor based eq and my new gyrator one. The script has the equations for both the LRC and gyrator filters.
Below is the output of this script. The label frequency is on the left. Note that either my calculations are incorrect (a good possibility) or the original filters are not exactly a good match for the labels on the front panel. So, I decided to make my gyrator eq match what the math says and not the front panel. It is kinda a tricky process because you don't have much choice of cap values and changing resistors in the gyrator modifies both the Q and the central frequency. With a bunch of iteration I came up with these:
Code: Select all
Mesa filters:
80 l=1.000H, r=609ohm, c=3.300uf, f0=87Hz, Q=0.90, bwoct=1.8 [47-246]Hz
240 l=0.390H, r=609ohm, c=0.470uf, f0=371Hz, Q=1.50, bwoct=2.4 [145-1276]Hz
750 l=0.220H, r=609ohm, c=0.220uf, f0=723Hz, Q=1.64, bwoct=2.6 [246-2630]Hz
2200 l=0.068H, r=1139ohm, c=0.150uf, f0=1575Hz, Q=0.59, bwoct=2.1 [740-4915]Hz
6600 l=0.033H, r=1139ohm, c=0.033uf, f0=4822Hz, Q=0.88, bwoct=1.8 [2643-13541]Hz
Gyrator filters:
80 l=0.984H, r=560ohm, c=3.300uf, f0=88Hz, Q=0.98, bwoct=1.8 [47-250]Hz
240 l=0.393H, r=610ohm, c=0.470uf, f0=370Hz, Q=1.50, bwoct=2.4 [144-1273]Hz
750 l=0.212H, r=620ohm, c=0.220uf, f0=737Hz, Q=1.58, bwoct=2.6 [266-2620]Hz
2200 l=0.073H, r=1200ohm, c=0.150uf, f0=1521Hz, Q=0.58, bwoct=2.1 [705-4786]Hz
6600 l=0.033H, r=1200ohm, c=0.033uf, f0=4820Hz, Q=0.83, bwoct=1.8 [2638-13550]Hz
Gyrator values:
80: r1=560, r2=18000, c1=0.1, c2=3.3
240: r1=610, r2=20000, c1=0.033, c2=0.47
750: r1=620, r2=16000, c1=0.022, c2=0.22
2200: r1=1200, r2=10000, c1=0.0068, c2=0.15
6600: r1=1200, r2=9400, c1=0.0033, c2=0.033
Next I mocked up the circuit on a breadboard just to see if this had any shot of working at all. Here is the gyrator eq on a breadboard:
And below I'm driving it with a sweep of frequencies from around 100Hz to 8khz. Note that I've set the pots for that scooped mids that this is known for. You can see the effect of the eq in the scope which is set for a very slow 2s sweep.
This does seem to work ok. So, next up it is time to figure out how to make it fit into an amp. And that means I will need to make a circuit board. The last circuit board I made was decades ago using a pcb kit from radio shack which had resist pens, acid and bare copper boards. Things have changed a bit since then
.I've used kicad for a while to create schematics for my builds. But never the pcb editor. I broke out the eq portion of the above schematic into its own project and after a couple of days learning the pcb editor I came up with this:
The kicad project is here:
The 35mm sliders are mounted on the back and everything else is on the front. The board is 55mm tall which should fit on the front panel no problem. I think I can still squish things togeather a little tighter to reduce the width some more. But it looks close. The 3d viewer is kinda nice. Here is a shot of the front and back:
After I tweak it a bit more, I will have to investigate what is involved in getting it created. I'd love any advice as far as which company to use and other things I've no doubt overlooked or just plain messed up.
Mike
EDIT: update kicad project files (mbeq.zip) [1/30/25]
