[sdiy] Difficult 2164 behaviour between different sections.

[electronicpresskit]

Hi Tom,

What value did you start with for Rbias? Have you tried at 7k5R to see if it resolves the issue?
 
While a lower Rbias value results in more control feedthrough, it seems that should only affect the gain cell being controlled and not the other gain cells on the chip. Your troubleshooting shows the problem remaining with the chip, not with your external circuitry. Rbias/Mode is the only pin (aside from the rails) that affects all 4 gain cells so it does make sense to investigate this.

epk

> On May 30, 2018, at 11:15 AM, Tom Bugs <admin at bugbrand.co.uk> wrote:
> 
> Hi,
> 
> I've been farting around with a big design for 18 months already at least (!!) and turned up some behaviour in the 2164s that I've not encountered before, so wondered if anyone else on the list has such experiences or other comments - or posting as a potential warning..
> 
> The design uses two 2164s, so 8 VCAs used as follows:
> 1 for Tri-Core VCO plus one for temperature compensation
> 2 for SV-Filter plus one for temp comp
> 1 for VCA plus one for linearization
> 1 for VC-decay
> Most of the circuit approaches are pretty standard and ones which I have used many times before.
> 
> Having made what I thought was the final prototype and overall enjoying the results, I then noticed that the adjusting the VCA and VC-Decay settings were slightly altering the VCO frequency by about 10cents each - argh!
> In that version the TriCore + Temp Comp were on the same 2164 as the VC-Decay and the VCA (not the linearization section of it).
> 
> I did another prototype sharing the 2164 sections differently - so the VCO sections were on the chip with the SVF core (not the temp comp for it).
> It solved the original problem - but then shifted it elsewhere! Now adjusting the VCF cutoff would affect the VCO freq (by 40cents)
> 
> Voltage refs are stable, power lines are solid and bypassed, overall layout approaches are, I think, good - so I can only now put it down to interaction between 2164 sections.
> I have identified that the output of the VCO Temp-Compensation section is varying as VCF cutoff is adjusted (again, without the voltage sources or the temp-comp voltage varying).
> 
> One thing I have not massively investigated is the biasing of the 2164 - there is the graph on the datasheet suggesting that minimum CV bleedthrough should be achieved with something like a 560k resistor. Tried a bit, but without success.
> 
> I've already drawn out a new prototype with different temp-compensation approaches, but I wondered whether anyone has any smart ideas on something I may have missed.
> 
> Best! Tom
> 
> -- 
> BugBrand LTD
> UK company No. 07199808
> VAT No. GB 988 2629 57
> 1 Ninetree Hill
> BRISTOL BS1 3SB
> United Kingdom
> www.bugbrand.co.uk
> 
> 
> ---
> This email has been checked for viruses by AVG.
> https://www.avg.com
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy