Hi again,
Many thanks PRR for the simple clarification. I just used a TLO72 model and got it running in simulation nicely indeed. I did use appropriate load resistors etc to ensure I got close to the original. Its my understanding now that the gain pot controls the feedback around the amplifier along with the filter network. I've played quite a bit in the simulator to see how everything interacts.
Dave are you saying that the originals would sound subjectively 'better' (or be more useful) with a wider BW?
After simulation, apart from the lowest of my proposed new frequencies, they are pretty wide IMO.
Here is my HTML version of the spreadsheet for all MF freq values with each Wima cap. So if there are any freqs you would rather have, I can simulate them for you. Like I said earlier I tried to pick an even spacing much like a 1073. As a general rule, once you get to about 1kHz, go for the 1.1 or 1.2H values and at about >4kHz go for the 0.45 or 0.7H values. That will get you close to the original curves (0.7H will be wider).
My first pick was (in Hz) 360, 760, 1k5, 3k4 and 6k1, which looks like this:
I then did some for the other Sowter inductor (9805) which is 2, 1.2 and 0.7H - and should lead to wider curves (at the 1.2 and 0.7H taps).
This one was (in Hz) 360, 760, 1k4, 3k3 and 6k, which looks like this:
The 3k3 is a fair bit wider than the 3k4 in the first and the 6k is fookin' huge - too much I think. I think I prefer the first ones.
Here is another one to see a few different freqs. If you guys are worried that the 360Hz is too narrow then you could start at around 520Hz (like the Avedis E15) and get smoother transitions.
I think both inductors have the same pin-out, so hopefully you can have somewhat 'custom' curves by picking your own values.
I played with the 'Q' resistor (which is normally 18k in Lo-Q mode on the original) and have settled upon two resistor values that can be switched between instead of switching the resistor out to achieve Hi-Q as in the original.
By using 47k as the 'Q' resistor, the curves are very similar to Hi-Q in the original and it also limits the range to +/-15dB. This EQ is like some Neve EQs I think. Hi-Q mode does nothing but add more gain range (from +/-12db to +/-16dB in the original). If you back off the gain in Hi-Q mode you pretty much land on the same curve as Lo-Q.
I found that another 'Q' resistor value of 8k6 was pretty cool. It limits the range to +/-9dB but actually does make the curve broader and less 'peaky'. My suggestion is to toggle between 8k6 and 47k for either +/-9dB or +/-15dB of MF EQ. the 8k6 mode should be really nice for smooth vocal lifts etc. The following graph shows that the 8k6 shape cannot be matched by backing off the 47k curve. Its subtle but more of a substantial difference than the original.
The following two plots show the 'Q' resistors effect @ 360Hz and 3k2:
Next two show their effects on max cut & boost for the five mid positions:
Narrow BW
Wide BW
What do ya reckon?
Here are plots for each individual mid position - I think this kind of filter is called "Proportional Q"??
360Hz
760Hz
1k5
3k4
6k1
Any thoughts?
I also plotted the LF shelf curves:
Which seem fine. Here is the cut & boost range at 100Hz - looks like the filter adds a small resonant peak near the pass band.
I did the HF shelf too, but it seems a bit weird, there is a broadband loss or gain of about 0.5-1dB depending upon position - is this normal? The full original schematic can be found here. I also noticed what I think is a schematic error too, C9 and C14 in the HF should be the same value, not 12n3 and 3n3. I set them both to 3n3 for the sim.
And here for the cut & boost range @ 6k8 shelf...
OK so I guess I'll redraw the EQ schematic next for a 6 pos switch and remove the HPF and LPF filters. I'd like to talk a bit about caps with you guys. It looks like the original uses Polystyrene and Polyester, how is it with you if I lay out boards for Wima Polyesters and Props?
I think they will be available for a long time to come and they sound good apparently!
Let me know your thoughts on practical mid-frequencies. We all have different uses.
Cheers Tom
