[sdiy] Coolaudio VCAs: V2162 noisier than V2164?

[Mattias Rickardsson]

On 22 December 2015 at 11:56, Stewart Pye <stewpye at optusnet.com.au> wrote:
>
> I haven't used the 2162, but I'm sure (at least with the THAT part) it will
> live up to the data sheet.
> It appears that the 2162 (in single ended mode) is capable of 20dB more gain
> and 20dB less attenuation than the 2164, which could result in more noise.
> Also, read the THAT 2162 datasheet carefully, especially noise
> considerations on page 8.

Those considerations should be cool, but one thing that I've *not*
done like the datasheet is the input resistor of 20k. Since I used a
soundcard directly as the test tone generator, with a signal of just a
couple of volts, I put 2k5 as input resistance and recalculated the
voltage values to give me the same input currents as in the datasheet.

I found the clip level where I expected it, so the signal calculations
should be alright - but does it matter that I've changed the input
resistor? Does the 2162 input behave differently (noise-wise) if it
feels a lower impedance there?

(Btw also the output resistor has been lowered for the same reason,
but that is situated in the op-amp feedback path beyond a virtual
earth summing node, so I guess it can't affect things.)

> This may have the info you need:
> http://www.thatcorp.com/THAT_Alternatives_to_SSM2164.shtml

Interestingly, that page says
"For best noise performance with the 2162, we recommend 20 kΩ for the
input resistor (Rin) and for the output (Rout) resistor in the
current-to-voltage converter."
This could either mean just that you should avoid higher resistances
as they give a lower signal current and worse noise due to lower
dynamic range, *or* they could mean that the input resistor actually
affects the behaviour of the VCA. I'm not sure! Are you? :-)

/mr



> On 12/22/2015 8:04 PM, Mattias Rickardsson wrote:
>>
>> Hi Chris,
>>
>> I'm using a MOTU UltraLite Mk3 Hybrid sound card and Arta software on a
>> PC.
>> The 2164 and 2162 have I/V converters built from a NE5532 low-noise
>> op-amp.
>>
>> I've tried to eliminate all sources of stupidity (except myself) and
>> compared different ways of disconnecting & comparing the sources of
>> noise. With the 2164, the generator (MOTU) is noisier than the VCA,
>> which is noisier than the op-amp, which is noisier than the analyzer
>> (MOTU). With the 2162, the VCA is noisier than both the generator, the
>> op-amp and the analyzer. I've also tried to eliminate all noise on the
>> CV signal and tried to avoid HF oscillations.
>>
>> Is the 2162 used by anybody or in any product out there?
>>
>> /mr
>>
>>
>>
>> On 21 December 2015 at 17:40, Chris Juried <cjuried at yahoo.com> wrote:
>>>
>>> Hi Mattias,
>>>
>>> What instrument(s) are you using for you SNR measurements?
>>>
>>> Sincerely,
>>>
>>> Chris Juried
>>> Audio Engineering Society (AES) Member
>>> InfoComm-Recognized AV Technologist
>>> http://www.JuriedEngineering.com (Juried Engineering, LLC.)
>>> http://www.TubeEquipment.com (Tube Equipment Corporation)
>>> http://www.HistoryOfRecording.com (History of Recording)
>>>
>>>
>>>
>>> ________________________________
>>> From: Mattias Rickardsson <mr at analogue.org>
>>> To: synthdiy diy <synth-diy at dropmix.xs4all.nl>
>>> Sent: Monday, December 21, 2015 10:21 AM
>>> Subject: [sdiy] Coolaudio VCAs: V2162 noisier than V2164?
>>>
>>> Hi there VCA lovers,
>>>
>>> I've done some measurements of SNR & THD of the 2164 and 2162 VCA ICs
>>> from Coolaudio, and I'm very surprised with the noise results I'm
>>> getting. With the 2162 I cannot reach a SNR over 95 dB before
>>> clipping, at any gain. With the 2164 at gain=1 I reach 120 dB SNR.
>>> That's a huge difference!
>>>
>>> When trimmed properly the 2162 does have slightly better THD, but the
>>> noise seems to always be there in the background. I hope my
>>> measurements are correct... the "-97 dBV" noise spec in the V2164
>>> datasheet seems a bit odd to me compared to the "-94 dBu" in the V2164
>>> dataheet - or I've misunderstood them.
>>>
>>> Has anyone else tried the 2162 (either from THAT or from Coolaudio)
>>> and compared it to the 2164 or other VCAs?
>>>
>>> /mr