VCA That2180

Monte McGuire

Member
Paid Member
2013-07-28 2:58 am
Not many. The 2180 is a very mature, well optimized VCA. If you want a VCA that has a constant dB per volt control law, there's no better alternative. There are other types of gain control devices such as digitally programmable attenuators and the like, but they're not actually VCAs.

THAT sells some other derivatives of the same basic VCA, and some provide their very useful RMS detector as well as the VCA, but in terms of raw VCA performance, the 2180 series is the best available. To better it, tricks like paralleling multiple VCAs have to be used.
 

mchambin

Member
2011-03-13 8:21 pm
Thanks, so, the 2180 is a good way to go.
Tell me more about paralleling 2180s. To decrease noise I guess.
I found PGA4311 a VCA with like an 8 bits DAC included, giving 255 gains in 0.5dB increments.
What do you think about PGA4311 ? Is there a way to make thinner increments ?
 
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I developed and tested a couple of revisions of a PCB for a V2164 quad VCA (has four channels). I was planning to use this as a 4-ch gain control for a project but now don't have that need anymore. You might look into that VCA IC, which I felt has about the same performance as the THAT2180. In my application I control the level of all four channels together (e.g. the attenuation or gain is the same for each channel). One can easily create a stereo, 3-, or 4-channel volume control in that configuration. Channel-to-channel level matching for the V2164 is about 1dB or less IIRC.

If you are interested in my PCB drop me a line. There is a thread on it in this forum where I describe it and provide a bunch of measurements - info here:
LINK TO INFO ABOUT MY VCA BUILD
 
Very interesting. Unfortunately, I see SSM2164 NLA at Mouser and Farnell ( C14 element ).
By any chance is there a new version or similar.
Quad is nice.
What do you think about THD: 2180 versus 2164 ?

Not the SSM2164 - that's no longer in production as you discovered. It's the V2164 by Cool Audio Semiconductors, which is in current production and is essentially the exact same part. Evidently Cool Audio is reproducing a few parts like this one and the Perkin Elmer VTL5C3 Vactrol "LDR" (LED+LDR combo) under their own name, I guess having bought the rights to do so. You can see their list and get info (e.g. datasheets) here:
:: Official website of Coolaudio Semiconductors (COOLAUDIO International Limited) - www.coolaudio.com
Just scroll down or do a page search and you will find the V2164. One thing that I really like about this part is that it's analog and it is from, and its outputs can be driven to, analog power supply rails of for instance +/-15Vdc. This means you can deliver as much output voltage as you would ever need (e.g. to drive a low input sensitivity amp) unlike many digital volume controls that operate from a single 5Vdc supply.

As far as the THD is concerned, look at my thread for measurements of a real world (and probably not even as good as it can get) application of the 2164. The THD depends on both the amount of gain or boost applied by the VCA as well as the input voltage supplied to the chip. I tried to investigate this parameter space in the design phase of my VCA board and optimize things a bit but it was not straightforward. In the latest rev I added multiturn trimpots to adjust the bias and other settings that affect THD and noise so that one could easily tweak these settings and measure the resulting performance in order to to change/optimize it. Under the right conditions I was able to get THD down to about 0.02% around 0dB gain, which I think is pretty good for this technology. If you need another zero or two between the decimal point and the first digit then look into the PGA2133/PGA4311 volume controls. They are better performing but I believe you need a microcontroller to run it.
 
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mchambin

Member
2011-03-13 8:21 pm
Thanks, great infos, a lot to read.
Coolaudio has V2164 ( quad VCA ) V2162 ( dual VCA ) and V2181 ( single VCA ). This V2181 is the same as THAT2181 I guess.
They don't have the 2180 which is a pre-trimmed version of the 2181. Pre-trimmed does not need Sym adjust at a worthwhile small over price.
I will run the VCAs from a Raspberry Pi, so I am looking at 12 bits DACs I2C or serial. A slow DAC will do, I might need more than 12 bits, +15v -15v would be convenient about power supplier.
I know PGA4311 ( It does 0.5dB steps, 8 bits DAC like, I need much smaller steps ), I do not find PGA2133.
 
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Thanks, great infos, a lot to read.
Coolaudio has V2164 ( quad VCA ) V2162 ( dual VCA ) and V2181 ( single VCA ). This V2181 is the same as THAT2181 I guess.
They don't have the 2180 which is a pre-trimmed version of the 2181. Pre-trimmed does not need Sym adjust at a worthwhile small over price.
I will run the VCAs from a Raspberry Pi, so I am looking at 12 bits DACs I2C or serial. A slow DAC will do, I might need more than 12 bits, +15v -15v would be convenient about power supplier.
I know PGA4311 ( It does 0.5dB steps, 8 bits DAC like, I need much smaller steps ), I do not find PGA2133.

Here is a DAC that will work well for your purposes:
https://learn.adafruit.com/mcp4725-12-bit-dac-with-raspberry-pi/overview

Yeah, I did this same thing a couple of years ago except using an Arduino (see below). At some point I abandoned the project because I switched over from doing DSP using the miniDSP to doing it in software on the Raspberry Pi itself and from sending SPDIF over analog RF wireless to streaming PCM over WiFi. But I did get the concept working with a lot of spaghetti wiring and breadbording, etc.

437164d1410045006-proposal-wireless-digital-loudspeaker-remote-volume-off-control-initial-working-preamp-etc.jpg
 
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maxw

Member
2004-06-10 7:23 am
Berlin
Given you need digital control, why not use an MDAC as the gain control element directly? Of course multiplying DACs don't give direct dB gain control.

If you are prepared to sacrifice some granularity in the dB steps at low levels this option can give you very small increments. I've made a few with a 16 bit R2R DAC ICs that work really well. 65536 linear steps. Controlled via SPI.
 

mchambin

Member
2011-03-13 8:21 pm
Thermal stability ?
The THAT2180 data sheet claims no thermal drift at 0dB gain setting.
From given data:
At +30 dB gain setting and 25 °C temperature increase: The gain shifts to 32.5dB.
At -30dB gain setting and 25 °C temperature increase; The gain shifts to -32.5dB

In a THAT design note they give a compensation circuit, that gives a perfect temp cpmpensation. I am very suspicious, this is exact, but rely on VCAs and temperature sensor all at the same temperature.
While one can see, in the data sheet, the gain control constant 6.1mV/ dB spécified at :gasp: Ta=25°C ( Tchip ~= 35°C ) :scratch1:.

What is to expect in the real world about VCAs temperature compensation ?

I presume MDACs give much better temperature stability. Maxw, which 16 bits R2R DAC ?
 
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Monte McGuire

Member
Paid Member
2013-07-28 2:58 am
THAT now only sells their RMS detector with a VCA, so for those parts, such as the 4301, they are on the same die, so they should track well.

When using this detector with a 2180 however, they will not be on the same die, so measures should be taken to thermally couple the detector to the VCA.
 

mchambin

Member
2011-03-13 8:21 pm
Charlie Laub,
I need more than 8 bits resolution, 14 bits will do ( there are plenty in the slow lane with I2C ), giving 0.007dB steps. A step is 44 microvolt at the gain command signal. This is easy, on paper. May be out of "the real world".
How warm your VCAs get ? does that depend of the audio level ?
 
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Charlie Laub,
I need more than 8 bits resolution, 14 bits will do ( there are plenty in the slow lane with I2C ), giving 0.007dB steps. A step is 44 microvolt at the gain command signal. This is easy, on paper. May be out of "the real world".
How warm your VCAs get ? does that depend of the audio level ?

8 bit???

In my post I linked to a tutorial for a 12 bit DAC that is very easy to implement. Here is a link to the product itself:
https://www.adafruit.com/products/935
Here is a 16-bit 4ch DAC:
https://www.adafruit.com/products/1085

The analog VCA should be capable of "infinite" resolution so it's limited by the DAC resolution. On the other hand it will have worse noise and THD figures compared to most of the current integrated volume control ICs.

Haven't fired up the VCA board in quite a while so I can't say about heat, but I don't recall any using +/-15Vdc rails.