[sdiy] Low voltage synthesis?

Mike Bryant mbryant at futurehorizons.com
Fri Jan 14 12:09:37 CET 2022


> 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.

Have a look at the Gainsil catalogue (through Google translate).   They don't quite match LTC's specs on MHz/uA (about 2:1), but they do work down to 1.8V.  I've used them on several products as they are easy to buy online at LCSC or Alibaba.

Cosine Semiconductor is another Chinese opamp supplier which have some better parts, but not all of them are available online to ship abroad.  Again they use about twice the power of LTC/MHz.


-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of rburnett at richieburnett.co.uk
Sent: 14 January 2022 10:39
To: Tom Wiltshire
Cc: synth-diy
Subject: Re: [sdiy] Low voltage synthesis?

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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