High output impedance emulator with CMOS

[dschwartz]

Hi all:
I have been simulating various types of filters to get the typical response of a high output impedance power amplifier (like tube amps). I tried the Marshall output impedance simulator (http://www.freepatentsonline.com/5467400.html), but instead of a opamp, i used a transistor as a gyrator, and some other values for components. The results were nice, but the peak wasn´t sharp enough. I liked the possibility to add a "mix" control replacing the opamp feedback resistor for a pot. But i don´t feel comfortable using a patent as a source for my projects..

i tried to do the same thing with CMOS inverters and twin-t filters on feedback, after a lot of value changing and experimenting, i got a very nice emulation, with a sharper Q. But implementing a mix control on it without other active components may be complicated.

This is the schem:


upper inverter is a resonant peak filter tuned to ~110Hz with a gain of 10dB with a reasonably nice Q.
Lower inverter is a hi pass filter.
Both are mixed through resistors, balancing the response.. Probably will need a buffer after due to high output impedance (ironic, isn´t it?) of cmos inverters.
I like inverters because they are cheap, available, they auto bias, have nice clipping characteristics and need little extra components to make them work.

this is the frequency response:


i like it more than the marshall ckt, since the 110Hz peak is sharper, and nearer an actual speaker response..but i miss the "mix" capability of the marshall ckt..any idea how to implement it easily here?
also, i´d like to just use 1 inverter for this.. i tried using different feedback configurations, but the resonant peak eats everything else..

[earthtonesaudio]

It would be easy to make the high-pass passive.  Just replace everything in the lower path with a cap (perhaps from the right side of C1 to the left side of R8).  You might have to adjust the gain of the bandpass stage, so to keep its response I'd suggest doing the "mixing" with the output resistors.

[dschwartz]

yes i though of that, but then the signals will be inverted from each other (band pass inverted, high pass not inverted), i don´t know if that may cause troubles, bout surely sounds strange to me

[davidallancole]

http://sound.westhost.com/project56.htm

Have you seen this before?  I don't know if its along the lines your after.

[dschwartz]

Yes David, i have seen that before, but mixed feedback configuration works only with speaker loads to get the response i´m seeking. My goal is to get that response without a speaker. Some marshall and Randall amps use this kind of filtering when the power amp topology makes difficult to implement mixed mode feedback (for example, BTL configs). Also, mixed mode feedback limits the output power of the amp to some extent.

The idea is to use this to simulate that response with linear-high power simple power amps, for example 2  LM3886 on BTL., or to use it as a pedal to make the effect more noticeable.

I reproduced the response with my graphic eq into my bandit amp, and the result is a nice "thump" effect with extended clarity, it really improves the sound.
I want to add a mix control, so tube amp users can use the effect just a little to emphatize the response and not get a huge THUMP! kick when playing loud.

[R.G.]

CMOS inverters are one version of low gain opamps. If it were me and I had a hex inverter package and unused inverters, and needed a mixer, I would use one of the spare inverter packages as an inverting mixer. In this case, I think it also re-inverts your signal so lows are still nominally in phase (ignoring what the filtering is actually doing to them).

[dschwartz]

thanks RG, good idea..