[sdiy] Difficult 2164 behaviour between different sections.

[David G Dixon]

To further add to Neil's comment, in an Irwin cell, the 2164s really heat up
when the VCA is off - i.e., when the control voltage is 0V.  I have two
tricks I always apply to these VCAs: 1) I put -5V to the CV summer through a
10M resistor to ensure that the VCA actually goes off completely at a CV of
0V (to compensate for opamp offset voltage - my beloved TL07Xs can have
enough offset voltage to keep the VCA slightly on at 0V), and 2) I put a
4.7k resistor between the output of the CV summer and the 2164 control pins
to limit the current into those pins.  In the Irwin circuit, when the CV is
0V, the CV summer actually hits the positive rail.  Given the 5k input
impedance of the V2164 (I know it's different for the SSI 2164, but I can't
remember what it is), this will put a current of about 2.7mA into both VC
pins, or 5.4mA total into the chip.  If the unit is a dual VCA, and they are
both off, then the 2164 is getting nearly 11mA into its VC pins - it heats
up in a hurry.  The 4.7k resistor limits the voltage at the VC pins to about
4.7V (coincidentally), and the total current into the chip is only about
0.9mA per pin - about three times less.  With that resistor in place, the
2164 doesn't heat up at all, it has no effect whatsoever on linearity, and
4.7V is still enough to shut the VCAs off pretty hard.

 

  _____  

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Neil
Johnson
Sent: Monday, July 15, 2019 12:14 PM
To: electronicpresskit
Cc: SDIY List
Subject: Re: [sdiy] Difficult 2164 behaviour between different sections.

 

The 2164 is quite temperature sensitive - there is a reason the data sheet
mentions a warm up time as part of the device spec. 

 

It is also why temp.co schemes work quite well, and also why it is the
cardinal rule that any compensation gain cell is on the same silicon as the
cell it is compensating. The same goes for the Irwin linearised VCA.

 

It is worth bearing in mind that any control voltage more than a couple of
volts is going to have a noticeable heating effect on the silicon and that
will affect the gain coefficient of the surrounding cells.

 

Neil

 

On Mon, 15 Jul 2019, 19:39 Erik Dower, <electronicpresskit at gmail.com> wrote:

I was thinking that might be the way to go. On my circuit, there is a very
small amount of fm happening when the gain cell is attenuating, but a very
distinct jump in frequency once it starts amplifying.

epk

> On Jul 15, 2019, at 4:57 PM, Tom Bugs <admin at bugbrand.co.uk> wrote:
> 
> I didn't exactly - I rejigged things so that different sections were used
for the critical parts.
> 
> Best, Tom
> 
> On 14/07/2019 21:05, Erik Dower wrote:
>> Hi Tom,
>> 
>> Did you ever resolve this issue? Unfortunately I find myself in a similar
situation.
>> 
>> Thanks,
>> 
>> epk
>> 
>>> On May 30, 2018, at 10:29 PM, Tom Bugs <admin at bugbrand.co.uk> wrote:
>>> 
>>> Thanks all.
>>> 
>>> Beefing up the 0V is something I tried on another point of the circuit,
but I don't think I tried it on the 0V pin itself - good point!
>>> 
>>> I will try Rbias some more for sure.
>>> 
>>> And agree with many of the points from David - it has been an eye opener
& I will have a few more deep sessions with it.
>>> 
>>> I'll report back..
>>> 
>>> Cheers - Tom
>>> 
>>> 
>>> On 30/05/2018 19:41, David G Dixon wrote:
>>>> That's an excellent idea.  I was thinking something similar, because if
>>>> there is any external impedance at that pin, then those control inputs
are
>>>> going to interact because of those internal 5k impedances.  That's
probably
>>>> the issue -- your ground isn't really ground.
>>>> 
>>>>> -----Original Message-----
>>>>> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On
>>>>> Behalf Of Oakley Sound
>>>>> Sent: Wednesday, May 30, 2018 9:11 AM
>>>>> To: Synth DIY
>>>>> Subject: Re: [sdiy] Difficult 2164 behaviour between
>>>>> different sections.
>>>>> 
>>>>> It could well be internal to the 2164 but have you tried
>>>>> lowering the the resistance to the 0V pin? Temporarily cludge
>>>>> a ruddy big wire from the PSU's 0V directly to pin 8, in
>>>>> parallel with what you've already got on the board, and see
>>>>> what happens.
>>>>> 
>>>>> Tony
>>>>> 
>>>>> www.oakleysound.com
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