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<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>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.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><b><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma;font-weight:bold'>From:</span></font></b><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'> Synth-diy
[mailto:synth-diy-bounces@synth-diy.org] <b><span style='font-weight:bold'>On
Behalf Of </span></b>Neil Johnson<br>
<b><span style='font-weight:bold'>Sent:</span></b> Monday, July 15, 2019 12:14
PM<br>
<b><span style='font-weight:bold'>To:</span></b> electronicpresskit<br>
<b><span style='font-weight:bold'>Cc:</span></b> SDIY List<br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [sdiy] Difficult 2164
behaviour between different sections.</span></font><o:p></o:p></p>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>The 2164 is quite temperature sensitive - there is a reason the data
sheet mentions a warm up time as part of the device spec. <o:p></o:p></span></font></p>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>It is also why <a href="http://temp.co">temp.co</a> 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.<o:p></o:p></span></font></p>
</div>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>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.<o:p></o:p></span></font></p>
</div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Neil<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<div>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>On Mon, 15 Jul 2019, 19:39 Erik Dower, <<a
href="mailto:electronicpresskit@gmail.com">electronicpresskit@gmail.com</a>>
wrote:<o:p></o:p></span></font></p>
</div>
<blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;
margin-left:4.8pt;margin-right:0in'>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>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.<br>
<br>
epk<br>
<br>
> On Jul 15, 2019, at 4:57 PM, Tom Bugs <<a
href="mailto:admin@bugbrand.co.uk" target="_blank">admin@bugbrand.co.uk</a>>
wrote:<br>
> <br>
> I didn't exactly - I rejigged things so that different sections were used
for the critical parts.<br>
> <br>
> Best, Tom<br>
> <br>
> On 14/07/2019 21:05, Erik Dower wrote:<br>
>> Hi Tom,<br>
>> <br>
>> Did you ever resolve this issue? Unfortunately I find myself in a
similar situation.<br>
>> <br>
>> Thanks,<br>
>> <br>
>> epk<br>
>> <br>
>>> On May 30, 2018, at 10:29 PM, Tom Bugs <<a
href="mailto:admin@bugbrand.co.uk" target="_blank">admin@bugbrand.co.uk</a>>
wrote:<br>
>>> <br>
>>> Thanks all.<br>
>>> <br>
>>> 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!<br>
>>> <br>
>>> I will try Rbias some more for sure.<br>
>>> <br>
>>> 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.<br>
>>> <br>
>>> I'll report back..<br>
>>> <br>
>>> Cheers - Tom<br>
>>> <br>
>>> <br>
>>> On 30/05/2018 19:41, David G Dixon wrote:<br>
>>>> That's an excellent idea. I was thinking something
similar, because if<br>
>>>> there is any external impedance at that pin, then those
control inputs are<br>
>>>> going to interact because of those internal 5k
impedances. That's probably<br>
>>>> the issue -- your ground isn't really ground.<br>
>>>> <br>
>>>>> -----Original Message-----<br>
>>>>> From: Synth-diy [mailto:<a
href="mailto:synth-diy-bounces@synth-diy.org" target="_blank">synth-diy-bounces@synth-diy.org</a>]
On<br>
>>>>> Behalf Of Oakley Sound<br>
>>>>> Sent: Wednesday, May 30, 2018 9:11 AM<br>
>>>>> To: Synth DIY<br>
>>>>> Subject: Re: [sdiy] Difficult 2164 behaviour between<br>
>>>>> different sections.<br>
>>>>> <br>
>>>>> It could well be internal to the 2164 but have you tried<br>
>>>>> lowering the the resistance to the 0V pin? Temporarily
cludge<br>
>>>>> a ruddy big wire from the PSU's 0V directly to pin 8, in<br>
>>>>> parallel with what you've already got on the board, and
see<br>
>>>>> what happens.<br>
>>>>> <br>
>>>>> Tony<br>
>>>>> <br>
>>>>> <a href="http://www.oakleysound.com" target="_blank">www.oakleysound.com</a><br>
>>>>> _______________________________________________<br>
>>>>> Synth-diy mailing list<br>
>>>>> <a href="mailto:Synth-diy@synth-diy.org" target="_blank">Synth-diy@synth-diy.org</a><br>
>>>>> <a href="http://synth-diy.org/mailman/listinfo/synth-diy"
target="_blank">http://synth-diy.org/mailman/listinfo/synth-diy</a><br>
>>>> _______________________________________________<br>
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