Diode VCA - built and measured. What output-amp is best?

jrp · 13th April 2007, 12:06 PM

Hi together!
I have put together this little diode-ring vca. It works very good, the couloring of the signal is quite pleasant. I want to design a compressor with this vca.
Diodes are BAT43.
In the schematic i have included some measurements at different points. AC and DC, as well as the current through the ring.
A 2Vpp Sine was applied to the input, the cv was adjusted for max output without clipping. When the coltage falls below the turn on voltage of the diodes this thing will sound very nasty...
But in this region it has a nice sound.
Output was 40mV for each end of the ring. those signals are 180° reversed, so we have a balanced signal.

What methode would you recomend to get the signal back to 2Vpp?
I have no idea what the source impedance of the ring is. Guess it will change with the current through the diodes.
Would a INA103 be a good choice? or maybe just a single opamp with the right circuitry?

If someone knows a little more about these things, help would be really appreciated!
cheers, jens

tINY · 13th April 2007, 05:35 PM

You need something with a really high input impedance.

An instrumentation amp or a pair of FET buffers feeding an op-amp might work

-tINY

Minion · 13th April 2007, 06:51 PM

You wouldn"t want to use the INA103 if you need really high input impedance as the Ina103 Likes the Input impedance to be under 50k....

You could put a Fet Buffer before the INA103 and that might work.....

You might even try the Discrete DIY Diode compression Opamp if it is for something like a Guitar...

http://www.diystompboxes.com/analoga...iodeopamp.html

Cheers

jrp · 13th April 2007, 07:45 PM

It´s not for guitar, and not intedet as an distortion effect.
I´m fascinated by dynamics so i´m checking out different vcas right now. This does give some distortion, but rather subtile. Different than everything else i´ve tried.
Ok, so i need high input impedance. Someone else told me that the output impedance of the ring should be very low, probably in the region of 10s of ohms.
Do you know any instrumention amps that are good for the job?
If i build one with opamps i should use 2 jfet opamp as inputbuffers.
OPA2132 is ok, right?
I´ll try the discrete fet buffers as well.
Don´t know if noise is a big concern. The noise of the diodes is propably much higher than the input noise of the buffers...

**Tim Farrant** · 14th April 2007, 04:10 AM

Work out how much actual gain you need in dB's. Given it's a high impedance input, a low noise FET or hybrid FET/BIPOLAR input opamp will work OK and the noise should be acceptable up to a gain of around 40dB or so.

jrp · 15th April 2007, 03:17 PM

thanks. You people have been great help!
I build an instrumention amp with opa2132 for the inputs. Works great.
I have made some samples, the compression settings are rather extreme.
This is just to start with, i´ll make a documentation with more sound and exact settings as well as original/processed once this is finished.
If someone likes to hear them:
http://www.electro-music.com/forum/v...=115948#115948

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14th April 2007, 04:10 AM	#5
Tim Farrant Moderator Join Date: Jun 2002 Location: New Zealand Posts: 1,833	Work out how much actual gain you need in dB's. Given it's a high impedance input, a low noise FET or hybrid FET/BIPOLAR input opamp will work OK and the noise should be acceptable up to a gain of around 40dB or so. __________________ innovative outboard processing

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13th April 2007, 12:06 PM	#1
jrp Gear Head Join Date: Oct 2006 Posts: 73	Diode VCA - built and measured. What output-amp is best? Hi together! I have put together this little diode-ring vca. It works very good, the couloring of the signal is quite pleasant. I want to design a compressor with this vca. Diodes are BAT43. In the schematic i have included some measurements at different points. AC and DC, as well as the current through the ring. A 2Vpp Sine was applied to the input, the cv was adjusted for max output without clipping. When the coltage falls below the turn on voltage of the diodes this thing will sound very nasty... But in this region it has a nice sound. Output was 40mV for each end of the ring. those signals are 180° reversed, so we have a balanced signal. What methode would you recomend to get the signal back to 2Vpp? I have no idea what the source impedance of the ring is. Guess it will change with the current through the diodes. Would a INA103 be a good choice? or maybe just a single opamp with the right circuitry? If someone knows a little more about these things, help would be really appreciated! cheers, jens Attached Thumbnails

13th April 2007, 05:35 PM	#2
tINY Lives for gear Join Date: Jul 2004 Location: Orygun Posts: 6,017	You need something with a really high input impedance. An instrumentation amp or a pair of FET buffers feeding an op-amp might work -tINY

13th April 2007, 06:51 PM	#3
Minion Gear maniac Join Date: May 2006 Location: Vancouver Island Posts: 185	You wouldn"t want to use the INA103 if you need really high input impedance as the Ina103 Likes the Input impedance to be under 50k.... You could put a Fet Buffer before the INA103 and that might work..... You might even try the Discrete DIY Diode compression Opamp if it is for something like a Guitar... http://www.diystompboxes.com/analoga...iodeopamp.html Cheers

13th April 2007, 07:45 PM	#4
jrp Gear Head Join Date: Oct 2006 Posts: 73	It´s not for guitar, and not intedet as an distortion effect. I´m fascinated by dynamics so i´m checking out different vcas right now. This does give some distortion, but rather subtile. Different than everything else i´ve tried. Ok, so i need high input impedance. Someone else told me that the output impedance of the ring should be very low, probably in the region of 10s of ohms. Do you know any instrumention amps that are good for the job? If i build one with opamps i should use 2 jfet opamp as inputbuffers. OPA2132 is ok, right? I´ll try the discrete fet buffers as well. Don´t know if noise is a big concern. The noise of the diodes is propably much higher than the input noise of the buffers...

15th April 2007, 03:17 PM	#6
jrp Gear Head Join Date: Oct 2006 Posts: 73	thanks. You people have been great help! I build an instrumention amp with opa2132 for the inputs. Works great. I have made some samples, the compression settings are rather extreme. This is just to start with, i´ll make a documentation with more sound and exact settings as well as original/processed once this is finished. If someone likes to hear them: http://www.electro-music.com/forum/v...=115948#115948