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 Forum index » DIY Hardware and Software » Developers' Corner
MS20 filter clone - help needed
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CHRISKELLY



Joined: Apr 08, 2018
Posts: 84
Location: England
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PostPosted: Sun Dec 23, 2018 2:32 am    Post subject: MS20 filter clone - help needed Reply with quote  Mark this post and the followings unread

Hi
I've built a MS20 LPF/HPF clone based on the Tim Escobedo 9V circuit (layout attached)

Works great in LP mode. But in HP mode the output sounds attenuated for some reason. Almost like it needs an op-amp follower to boost the signal.

Could anyone help suggest any tweaks to the original circuit that might help with this?

Cheers
Chris


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ixtern



Joined: Jun 25, 2018
Posts: 76
Location: Poland

PostPosted: Sun Dec 23, 2018 7:43 am    Post subject: Re: MS20 filter clone - help needed Reply with quote  Mark this post and the followings unread

CHRISKELLY wrote:
Hi
I've built a MS20 LPF/HPF clone based on the Tim Escobedo 9V circuit (layout attached)

Works great in LP mode. But in HP mode the output sounds attenuated for some reason. Almost like it needs an op-amp follower to boost the signal.

Could anyone help suggest any tweaks to the original circuit that might help with this?

Cheers
Chris

Original was LFP only:

I don't like how this LPF/HPF scheme looks. 680pF in HPF configuration is too small value for me - perhaps it attenuates sound too much.
Look at the better version in vcf8e (although different supply) - 2000pF is used there.


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JovianPyx



Joined: Nov 20, 2007
Posts: 1822
Location: West Red Spot, Jupiter
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PostPosted: Sun Dec 23, 2018 8:16 am    Post subject: Reply with quote  Mark this post and the followings unread

Considering distortion, I can't help but look at the Vr generating circuit. LED forward voltage can be anywhere between 1.8 and 3.3 volts. So Vr can be from 3.6 volts to 6.6 volts depending on at least LED color. There is no indication on the schematic for what Vr ought to be, but Vr definitely will influence the clipping point. It may be worthwhile to mess with the LEDs. A Vr voltage of 4.5 volts will give the most headroom.

Also, in the OP's schematic, there is a note "point A for LFO" which appears to be an input for modulation by an LFO. If the Freq pot is turned all the way down, the LFO's output is grounded and if turned all the way up, it connects to the 9V rail. This may or may not damage a connected LFO, but at the least it constrains the range of the LFO's usefulness.

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Last edited by JovianPyx on Sun Dec 23, 2018 8:41 am; edited 1 time in total
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AlanP



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PostPosted: Sun Dec 23, 2018 8:38 am    Post subject: Reply with quote  Mark this post and the followings unread

What power supply are you using? This is a single-supply circuit (knowing Tim, probably intended for a guitar pedal or something.)

Rene Schmitz' MS20 circuit sounds quite good, and the HPF input cap is 1nF in his. Strongly recommend that circuit.
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CHRISKELLY



Joined: Apr 08, 2018
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PostPosted: Sun Dec 23, 2018 9:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for all the replies

Ixtern - I tried swapping out the 680pf but it didn't affect the levels. I think that the caps are matched to help with resonance (I'm probably wrong about this though)

Jovian - I'll make sure any LFO has a minimum 10k input to limit the current to the 13700 pins 1 and 2.
You're right about the LED's being weird. I tried a different combo to get closer to 4.5V. Could I just use resistors instead? It'd be easier to get it spot on.

Alan - i'm running 9V single supply. I tried Rene's clone using CA3080's but struggled to get it to work. I've got a few 13700's so could I just swap out the 3080's for 13700's? Not sure if Rene's version would be any good at 9V though
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JovianPyx



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PostPosted: Sun Dec 23, 2018 10:05 am    Post subject: Reply with quote  Mark this post and the followings unread

CHRISKELLY wrote:
Thanks for all the replies

Ixtern - I tried swapping out the 680pf but it didn't affect the levels. I think that the caps are matched to help with resonance (I'm probably wrong about this though)

Jovian - I'll make sure any LFO has a minimum 10k input to limit the current to the 13700 pins 1 and 2.
You're right about the LED's being weird. I tried a different combo to get closer to 4.5V. Could I just use resistors instead? It'd be easier to get it spot on.

Alan - i'm running 9V single supply. I tried Rene's clone using CA3080's but struggled to get it to work. I've got a few 13700's so could I just swap out the 3080's for 13700's? Not sure if Rene's version would be any good at 9V though


I think it's worth a try to use resistors to make 4.5V. Make sure you put a cap from Vr to ground (like 10uF, perhaps more). The size of the resistors used is up to you. The larger they are the less current they draw from the 9v supply, however, larger resistors means a less stable Vr rail. If this is a walwart 9v supply you can get away with smaller ones (like 1K). If a battery, then you'll want 10K or more.

What I find a bit odd is there's no indication of the input and output reference. The reference (or what is called "signal ground" is usually the point between the power supply rails - in this case Vr. But it looks like the reference here is the ground symbol which is really the negative side of the supply. That could also be a source of clipping.

If this were my project, I'd toss the power supply stuff and do it with a real dual supply such as + and - 5V. That gives a highly stable ground with proper input and output referencing.

If batteries are necessary, this could also be done with two 9v batteries arranged in series with the connection between the batteries being Vr (which is actually ground). This will give a huge amount of headroom.

_________________
FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.
Fruit flies when you're having fun.
BTW, Do these genes make my ass look fat?
corruptio optimi pessima
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CHRISKELLY



Joined: Apr 08, 2018
Posts: 84
Location: England
Audio files: 3

PostPosted: Mon Dec 24, 2018 4:10 am    Post subject: Reply with quote  Mark this post and the followings unread

I've used resistors now to get a virtual ground of 4.5V.

Afraid I'm committed to use 9V now Crying or Very sad but for my next project I'll definitely build a decent split power supply.

Thanks for the help again[/quote]
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