[sdiy] Low voltage synthesis?

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Fri Jan 14 11:39:18 CET 2022


Linear Technology (now part of Analog Devices) make some pretty neat 
low-voltage low-power rail-to-rail input/output op-amps...

LTC6261 (30MHz GBP for 245uA supply current)
LTC6255 (6.5MHz GBP for 65uA supply current)
LTC6258 (1.3MHz GBP for 20uA supply current)

We use these at work for various underwater acoustics stuff with just 
2.5V supply, but they are fully spec'ed down to 1.8V supply!

I don't know what tricks LT used in their design, but they are quite 
unique in terms of RRIO and the gain-bandwidth product that you get per 
microamp spent on quiescent supply current.  Just looking down LT/AD's 
combined op-amp product range, nothing else comes close to these in 
terms of GBP per mA.

But of course you can't actually get any of these during the current 
chip shortage!  Something that's causing us a bit of head scratching at 
the moment.

-Richie,




On 2022-01-13 19:13, Tom Wiltshire wrote:
> MCP6002 might be a modern op-amp, but the specs read like a 741. It's
> designed for rail-to-rail use at 5V, but I don't think they had audio
> in mind!
> 
> Not that I'm criticising your choice. I've used it too in similar
> situations for the same reasons.
> 
>> On 13 Jan 2022, at 18:42, Chris McDowell via Synth-diy 
>> <synth-diy at synth-diy.org> wrote:
>> 
>> I have dug into this some.
>> 
>> I built a VCO, VCF, and VCA from LM13700s running at 3.3V. While it 
>> was what I'd call "a little bit noisy", they sound "old" which to me 
>> often equates to "good"
>> 
>> I'd say build something and determine if it is too noisy. I used 
>> MCP6002 op amps and LM13700.
>> 
>>> On Jan 13, 2022, at 10:20 AM, cheater cheater via Synth-diy 
>>> <synth-diy at synth-diy.org> wrote:
>>> 
>>> Synths generally run off 15V or 12V power rails, which among others
>>> helps with SNR. However, it creates a bunch of power dissipation, 
>>> plus
>>> you can't use some of the tiniest components. Does it make sense to
>>> build synthesizer circuits that run on much lower voltages than that?
>>> Say, 1.5V rails? Are there techniques of suppressing noise that are
>>> better than just ramping up the voltage rails? Have the parts
>>> progressed enough nowadays that we can do this?
>>> 
>>> Never mind stuff like interfacing with existing modular synthesizer
>>> parts etc. Let's say you're building a modern 8-voice polysynth with 
>>> 4
>>> VCO, VCF, VCA, ENV, LFO per voice. Obviously that would generate 
>>> loads
>>> of heat and possibly tuning issues given the constraints of a
>>> keyboard, plus it would be huge, especially given the size of
>>> capacitors vs their rated voltage.
>>> 
>>> Where would noise creep in first if you started lowering rail voltage
>>> in your synthesizer designs? What other disadvantages in terms of
>>> signal integrity come from using low voltage rails for synths?
>>> 
>>> Thanks
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>> 
>> 
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