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 Forum index » DIY Hardware and Software » Thomas Henry designs
About VCA-1
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Arda Eden



Joined: Nov 14, 2020
Posts: 4
Location: Turkey

PostPosted: Sat Nov 14, 2020 8:44 am    Post subject: About VCA-1
Subject description: Strange Behaviour of my built VCA-1
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Hi all,

As many of you, I stepped into the fantastic world of modular synthesizers and the first module I built was the TH's VCO-1. It works perfectly and I already built the second one.



Next, I needed a VCA, and again TH's VCA-1 (https://www.birthofasynth.com/Thomas_Henry/Pages/VCA-1.html) worked, but in a strange way: The gate opens at negative voltages and closes on positive. When I press a key, gate is closed and no sound. On a depress, gate opens up. This is supposed to be the opposite way, isn't it?

I checked my circuit and it is exaclty the same with that TH designed.

What may have gone wrong?

Any ideas?

Thanks already.
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Arda Eden



Joined: Nov 14, 2020
Posts: 4
Location: Turkey

PostPosted: Sat Nov 14, 2020 1:21 pm    Post subject: Update Reply with quote  Mark this post and the followings unread

I figured out that:

1- TL071 is getting very hot and frying some time later. Even if I remove the TL071 from its socket, circuit keeps working in its abnormal behaviour. 071 has no function in my circuit.

2- Checked everything. No shorts or anything wrong.

Couldn't understand why TL071 is getting that hot and not working.
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drapdap



Joined: Oct 11, 2004
Posts: 204
Location: London
Audio files: 1
G2 patch files: 1

PostPosted: Sun Nov 15, 2020 3:18 am    Post subject: Re: Update Reply with quote  Mark this post and the followings unread

The ota you are using, 3080 or 13700?
I’ve built several vca-1’s with the original layout ages ago, and i had this behaviour with fake 3080’s!

I’d try swapping that after gettting the right psu voltages on ic sockets.
Also the 071, it could be fried now.
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Arda Eden



Joined: Nov 14, 2020
Posts: 4
Location: Turkey

PostPosted: Sun Nov 15, 2020 8:05 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
The ota you are using, 3080 or 13700?


It's a 3080, but note sure if it is fake or not. I will check. The same ones work on my VCO-1s without any problem.

Quote:
I’d try swapping that after gettting the right psu voltages on ic sockets.


I used 7815 and 7915 regulators on my 15v symmetrical PSU. I measure +15 and -15 on the transformer output, but -14,8 and +15,2 on the DC side. This is also interesting. I tried two circuits, and they did the same. This is the only problem with my PSU. I'll also change it.

Finally, I think I will put this VCA-1 aside for some while and find a different simple VCA circuit to build. And also a simple VCF and an ADSR.

Thank you.


IMG_20201115_174602.jpg
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My 3080
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IMG_20201115_174602.jpg


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marwatt



Joined: Jun 20, 2008
Posts: 3
Location: Italy

PostPosted: Mon Nov 16, 2020 10:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Arda Eden wrote:
Quote:
The ota you are using, 3080 or 13700?


It's a 3080, but note sure if it is fake or not. I will check...


Hi all...
regarding fake/not fake CA3080... how to test them on breadbord?
thank you!
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