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 Forum index » DIY Hardware and Software » YuSynth
Few questions about VCA and VCF
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aka.kike1



Joined: Aug 06, 2014
Posts: 9
Location: croatia

PostPosted: Wed Aug 06, 2014 5:38 am    Post subject: Few questions about VCA and VCF Reply with quote  Mark this post and the followings unread

Hi Everyone

I have started building Yusinth.

I have build 2 VCOs, ADSR, VCA, SteinerVCF for now.

I didn't mached transistors on VCA and VCF.

So my question is this, can I use mached BC547b instead of BC547c?

I have a lot of BC547b laying around so I was wondering can I use them.
It is a long shipping time and I want to continue right now.

And SteinerVCF is producing a lot of noise when i connect the output to my mixer even if I don't connect anything to input.

I did't do any trimming yet because i need to match BC547

I have used all resistors 10k (R3*,R4*,R5*,R19*) where Yves told to adjust the resistors to obtain the desired input and output levels.

Can I reduce the noise with these resistors or I have a noisy OPamp or is it something else entirely?

If it is the resistor issue what resistor do I change and with what value?

Thanks in advance
Kike
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gdavis



Joined: Feb 27, 2013
Posts: 161
Location: San Diego

PostPosted: Fri Aug 08, 2014 10:56 am    Post subject: Reply with quote  Mark this post and the followings unread

The difference between B and C is the current gain. The VCF and VCA circuits may not perform correctly if you substitute one for the other.

Matching mainly affects temperature stability. These days, most transistors from the same batch will be fairly well matched. I've built the VCO, Moog VCF and VCA without matching and they all operated reasonably well, the VCO tuning is just sensitive to temp.

I would guess the noise is something else. Adjusting the 10k's is for dealing with different signal levels for different systems but shouldn't create significant noise, especially if you're using it with other Yusynth modules or any modules with levels close to 10Vpp.

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aka.kike1



Joined: Aug 06, 2014
Posts: 9
Location: croatia

PostPosted: Fri Aug 08, 2014 12:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thank you for the reply,

By the way, was looking other posts on the forum and have found the answer on my questions, so If anyone reading this, don't be lazy like I am and read couple of posts before asking questions Smile

I think i have found the problem with the noise.

Yves told some some guy on the forum that he must clean the flux of the PCB because VCF is sensitive. I did that and the noise disappeared.

Maybe it's not that but i don't have a noisy filter anymore.
Maybe it was cable or some stupid thing like that

But I think there maybe another problem with the filter, I am going to check everything before ask for help

Once again, thank you for your time
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aka.kike1



Joined: Aug 06, 2014
Posts: 9
Location: croatia

PostPosted: Sat Aug 16, 2014 11:02 am    Post subject: Reply with quote  Mark this post and the followings unread

I have noticed that the steiner-vcf has another input on the pcb

Can I use that input or Yves didn't connected it for some reason??

And, because the fine tune on the VCO is very sensitive, I was thinking to put a center-dented pot instead of the regular one.

Is that OK?

But i cant find 22k or 25k center-dented pot, is it possible to use 20k pot??
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gdavis



Joined: Feb 27, 2013
Posts: 161
Location: San Diego

PostPosted: Sat Aug 16, 2014 12:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

I don't know why the second input isn't hooked up, maybe just not enough room on the panel for another level control knob. You should be able to hook it up just like the first input.

I wouldn't use a center detent pot for fine tune. If anything I think it would exacerbate the problem of being too sensitive since it will snap to center when you want it just slightly off center.

There are several ways you can reduce the sensitivity:
1. increase the value of the 3.3M resistor - roughly double would probably be good if you can find something suitable. Using that large of a resistor might be less desirable due to noise, but wouldn't hurt to try and see if it works for you.

2. Put resistors in series with one or both of the end lugs of the pot. For example 12.5k on each end or 25k on one end.

3. Connect one end of the pot to ground instead of V+ or V-.

Any one of these changes should reduce the range of the fine tune by roughly half making it less sensitive. You can adjust the resistor values as you wish to decrease the amount of range/sensitivity however much you want.

Using a 20k pot for fine tune is fine.

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aka.kike1



Joined: Aug 06, 2014
Posts: 9
Location: croatia

PostPosted: Sat Aug 16, 2014 12:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thank you once again.
I am not going to use center dented pot but make the fine tune
less sensitive and see how that works.

Btw, i have seen your blog, nice.
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