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 Forum index » DIY Hardware and Software » Developers' Corner
Just another Transistor Ladder Filter
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Grumble



Joined: Nov 23, 2015
Posts: 876
Location: Netherlands
Audio files: 27

PostPosted: Sun Dec 09, 2018 2:37 am    Post subject: Just another Transistor Ladder Filter
Subject description: part from the net, part from me
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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 Laughing
I have not paired the transistors, but still it sounds good to me (untrained ears Rolling Eyes ) it is good ping-able, self oscillation is great too.
I have added a piece of perfboard to the pcb layout.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Oh just noticed that R31 and R32 are 100k NOT 220E ...
Posted Image, might have been reduced in size. Click Image to view fullscreen.


FM.wav
 Description:
FM modulation of the filter

Download
 Filename:  FM.wav
 Filesize:  7.92 MB
 Downloaded:  37 Time(s)


pinging.wav
 Description:
just a short pulse (about 2 msec) to the Audio input

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 Filename:  pinging.wav
 Filesize:  3.75 MB
 Downloaded:  34 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

Download
 Filename:  filter_self_oscillating.wav
 Filesize:  4.31 MB
 Downloaded:  34 Time(s)


filter_norrmal.wav
 Description:
normal use of the filter

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 Filename:  filter_norrmal.wav
 Filesize:  7.03 MB
 Downloaded:  34 Time(s)


SCHladderfilter.pdf
 Description:
the pdf of the diagram

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 Filename:  SCHladderfilter.pdf
 Filesize:  155.27 KB
 Downloaded:  15 Time(s)


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cslammy



Joined: Apr 27, 2018
Posts: 95
Location: USA

PostPosted: Tue Dec 11, 2018 1:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Looks good grumble!

Preguntas:

What would be involved you think in having this work with +/- 15V?

And have many CV ins (to set frequency)...for CV I guess I could slap a CV mixer (a TL072 and a few resistors) in front what is here?

Does the AD620 need to be something that "fancy"? Can it be say a TL071?

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Grumble



Joined: Nov 23, 2015
Posts: 876
Location: Netherlands
Audio files: 27

PostPosted: Tue Dec 11, 2018 2:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

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:
Posted Image, might have been reduced in size. Click Image to view fullscreen.
but that is just what I wanted to inprove on.

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ixtern



Joined: Jun 25, 2018
Posts: 53
Location: Poland

PostPosted: Wed Dec 12, 2018 12:30 am    Post subject: Reply with quote  Mark this post and the followings unread

cslammy wrote:
Looks good grumble!
Preguntas:
What would be involved you think in having this work with +/- 15V?

And have many CV ins (to set frequency)...for CV I guess I could slap a CV mixer (a TL072 and a few resistors) in front what is here?

Does the AD620 need to be something that "fancy"? Can it be say a TL071?

Just look at the other moog transistor ladder VCF clones. For example, yusynth version:
http://www.yusynth.net/Modular/Commun/MOOGVCF/Moogfilter-sch.jpg
You have +-15V and common TL072 plus four transistors instead of AD620.

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.
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Grumble



Joined: Nov 23, 2015
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Audio files: 27

PostPosted: Wed Dec 12, 2018 3:06 am    Post subject: Reply with quote  Mark this post and the followings unread

ixtern wrote:

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.

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ixtern



Joined: Jun 25, 2018
Posts: 53
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PostPosted: Wed Dec 12, 2018 3:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Grumble wrote:
ixtern wrote:

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.
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Grumble



Joined: Nov 23, 2015
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Audio files: 27

PostPosted: Wed Dec 12, 2018 5:19 am    Post subject: Reply with quote  Mark this post and the followings unread

Grumble wrote:
so I removed R31 and R32 and shorted C14 and C17 to get a better range.

Just saying Rolling Eyes
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.

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