Adding overdrive back to the Moog ladder filter

[facon]

I'm attempting to replicate the subtle overdrive that occurs in a Minimoog Model D's ladder filter. The Model D has a different sound that is smoother than most other filters in their synths. I've been told that this is in part due to the clipping that occurs throughout the regeneration and VCA. I'm hoping to find a way to add this to a later Moog circuit that used op amps instead of transistors in the VCA and regen path. I'd like to avoid a complete overhaul of the entire regen and VCA if I can get away with it, but am not completely opposed to it.

Since I can add overdrive after the synth and replicate some of the clipping that would occur within the VCA, I think it would be best to focus on the regeneration path that occurs from U12 back to the filter. A some ideas came to mind.
1) Replace the LM741 with transistors.
2) Replace the LF353 and LM741 op amps with transistors.
3) Add diode clipping after the op amp.

However, I'm treading into territory that I'm pretty unfamiliar with. Could I just replace an op amp with transistors and if so, would I be able to calculate a similar amount of gain? If I replaced them, would they even have an effect on the sound at all? If I went the diode clipping route, is the signal strong enough?  According to the schematic, it shows the waveform at 14mV, but that seems really low since germanium diodes clip at 250mV and silicon at 500mV. This is strange to me because the output of the synth is very high compared to a guitar pedal.

Last but not least, any other ideas?

Here is the circuit that I'm working with:


Here is the original Model D schematic:
http://www.experimentalistsanonymous.com/diy/Schematics/Filters%20Wahs%20and%20VCFs/Minimoog%20ladder%20VCF%202.gif

Thank you!

[alanp]

http://yusynth.net/Modular/EN/MOOGVCF/index.html

Here's another datapoint for you. I've found that this filter works if you have all the transistors in but forget to put the opamp in due to excitement.

Overdriving the input is also fun.

[peterc]

One of the popular techniques to overdrive this filter when playing the Mini was to route the headphone out back into the External Input, and use the Ext In level to get the right amount of overdrive.

So I tried doing this (kinda) by boosting the level into the filter, and found it to be very similar in nature to the Headphone overdrive.

Peter

[facon]

Thanks guys. I'll try over driving the input of the filter to see how it sounds as is.

[teemuk]

1) Replace the LM741 with transistors.

It's a differential amp that provides feedback to the ladder. What would you gain from replacing it with equivalent but discrete transistor circuit?

2) Replace the LF353 and LM741 op amps with transistors.

Again pretty much the same thing. They are unity gain buffers so you could sub them with a plain cathode follower. What would you actually gain from that?

3) Add diode clipping after the op amp.

There's a 3080 OTA following everything. It's running open loop so it will clip much softer than any diode. Oh, and with much, much lower threshold voltages than any diode.

Soft clipping of the ladder filter in general...? Well that's largely a characteristic of that differential amp. The very core of the design. It clips softly when overdriven, and even more so because the following stages sum two out of phase signals, which means asymmetric (even moderately hard) clipping in these two out of phase signals sums up to be symmetric soft clipping when the signals are combined. There's not much to modify because every Moog "ladder" filter is based on that differential amp.