Posted: Sun Dec 09, 2018 2:37 am Post subject:
Just another Transistor Ladder Filter Subject description: part from the net, part from me
Yes, another transistor ladder filter.
A part from the net, a part from myself.
I have filters in my synth, built around a MAX26x programmable switched capacitor filter with a cv for frequency and q-factor (emphasis) as well midi input.
The downside is that when I set F0 to a lower frequency, the clock signal becomes audible, because F0=200 x Fclock.
So I decided to build a full analog filter, based on the moog transistor ladder filter.
When looking at the schematics of the well known transistor ladder filter I noticed that there always is a instrumentation amplifier made with either transistors or multiple opamps. So I thought why not use a real instrumentation amplifier?
So I added an instrumentation amplifier, an AD620, I know, concerning the price ....
The good news for me is that I can get samples from Analog Devices, so they cost me... nothing
I have not paired the transistors, but still it sounds good to me (untrained ears ) it is good ping-able, self oscillation is great too.
I have added a piece of perfboard to the pcb layout.
edit: updated schematic
FM.wav
Description:
FM modulation of the filter
Filesize:
7.92 MB
Duration:
0:47
Viewed:
2261 Time(s)
pinging.wav
Description:
just a short pulse (about 2 msec) to the Audio input
Filesize:
3.75 MB
Duration:
0:22
Viewed:
2284 Time(s)
filter_self_oscillating.wav
Description:
No audio input, just to the cv of the filter's F0, and playing with range and emplhasis potmeters
Like I wrote in the first post, a part from the net, a part from me, and the left part of this filter is ‘borrowed’ from the net what means that I am not sure how to answer your first few questions.
About the ‘fancy’ AD620 it cannot be exchanged by a single opamp like the TL072 because it is a true instrumentation amplifier, NOT an opamp.
It could be replaced by a number of opamp configured as an instrumentation amplifier.
like this:
but that is just what I wanted to inprove on. _________________ my synth
And I think that Grumble's circuit should work even with TL071 or almost any contemporary op amp instead of AD620. Instrumentation amplifier is simply differential amplifier with very low offset/drift and high input impedance. AD620 additionally has one-resistor-set gain, what is not important here. So simple op amp in "difference amplifier" configuration should work (although it will have worse some parameters like offset/drift and input impedance), at least for prototyping. Of course you should do some cosmetic changes to the schematic as standard diff amp schema is not the same as AD620 application.
BTW input op amp impedance in Grumble circuit doesn't matter as there are 220 Ohm resistors at the inputs of AD620. So only offset/drift is better.
BTW input op amp impedance in Grumble circuit doesn't matter as there are 220 Ohm resistors at the inputs of AD620. So only offset/drift is better.
Grumble wrote:
Oh just noticed that R31 and R32 are 100k NOT 220E ...
and:
Grumble wrote:
About the ‘fancy’ AD620 it cannot be exchanged by a single opamp like the TL072 because it is a true instrumentation amplifier, NOT an opamp.
You need a high input as seen from the ladder filter, even 100k is a little low, so I removed R31 and R32 and shorted C14 and C17 to get a better range.
These two resistors and C's where added because I thought that the difference between the transistors in the ladder would give a dc offset, but afterall it wasn't that bad, so I could remove the resistors and C's.
Also if you just replace the AD620 with a TL072 the gain is far to high, so my guess is that it wil oscillate. _________________ my synth
BTW input op amp impedance in Grumble circuit doesn't matter as there are 220 Ohm resistors at the inputs of AD620. So only offset/drift is better.
Grumble wrote:
Oh just noticed that R31 and R32 are 100k NOT 220E ...
and:
Grumble wrote:
About the ‘fancy’ AD620 it cannot be exchanged by a single opamp like the TL072 because it is a true instrumentation amplifier, NOT an opamp.
You need a high input as seen from the ladder filter, even 100k is a little low, so I removed R31 and R32 and shorted C14 and C17 to get a better range.
These two resistors and C's where added because I thought that the difference between the transistors in the ladder would give a dc offset, but afterall it wasn't that bad, so I could remove the resistors and C's.
Also if you just replace the AD620 with a TL072 the gain is far to high, so my guess is that it wil oscillate.
My bad with 220 Ohms but it doesn't change my conclusion. All JFET-input op amps have high enough input impedance so 100kOhm is not too much for them.
About gain you may be right. With 100kOhm at inputs you would need megaohms in loop what is not quite good. But TL072 and two-stage amp would do - first stage with low gain, second with desired gain as driven from low-impedance output. Oscillations can be prevented with small (10-50pF) capacitor in op amps feedback loop.
so I removed R31 and R32 and shorted C14 and C17 to get a better range.
Just saying
The whole thing was just to see if an inamp would fit there, and it did, by no means I would force someone to use an expensive inamp where one could use a cheap LM324. _________________ my synth
Joined: Jun 11, 2017 Posts: 57 Location: Grand Rapids MI
Posted: Sun Jan 27, 2019 2:56 pm Post subject:
Wow, I just came here to ask a question about transistor (diode) ladder filters and how to get the signal out of them without a noisy multiple-transistor amplifier!
Joined: Jun 11, 2017 Posts: 57 Location: Grand Rapids MI
Posted: Fri Feb 15, 2019 10:09 pm Post subject:
I was trolling the internet cursing my obsession with the 303 diode ladder VCF when I found an example of a transistor ladder that used a LM13700 OTA where you've got the instrumentation amplifier. I can't find the schematic right now, but I believe the inputs were connected straight to the uprights of the ladders through a pair of capacitors, no ground reference, the linearizing pin was ignored and the "amplification" pin (1 or 16) was just tied high through a 47K resistor.
And for some reason they ignored the on-chip darlington pair and buffered the output with... a transistor??? I wish I could find the schematic because why would anybody do that
I'm definitely going to build this filter soon. I *LOVE* the sound!
I can't find the schematic right now, but I believe the inputs were connected straight to the uprights of the ladders through a pair of capacitors, no ground reference,
You doesn’t need a ground reference resistor because the input resistance of a LM13700 is typical 26kohm, which, as I understand, is quite low as seen from the filter. _________________ my synth
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) it is good ping-able, self oscillation is great too.
