[sdiy] Discrete OTA

[Tom Wiltshire]

On 3 Apr 2014, at 22:18, David G Dixon <dixon at mail.ubc.ca> wrote:

>> I'm sure the only reason for that is that Damian is still 
>> very curious about my OTA and I haven't told him how it works... 
> 
> I've been reading this thread (and responding) with interest all week.  I
> have one more comment on the original topic:
> 
> For 5 or 6 pounds ($8 to $10), that discrete OTA had better be absolutely
> the most amazing thing ever, because the current industry standard, LM13700,
> provides two OTAs in one package and can be had for about $0.3 in bulk, and
> is available in both SMD and through-hole formats.
> 
> I think that the market for such a thing is probably microscopic.  Module
> manufacturers are not going to buy it because it is too expensive, too
> large, the OTA function is well covered by cheap, widely available chips and
> the advantage of a discrete OTA circuit is debatable, potted circuits are a
> pain, and installing potted circuits would have to be done by hand and may
> thus add significantly to the manufacturering cost.
> 
> Now, a non-potted discrete circuit consisting of SMD parts on a tiny little
> board with pin headers (like Intellijel's picoVCA, three of which can be
> found plugged into the back of every Linix hex VCA module) might be popular,
> but you wouldn't be able to get much for it.

Why does the fact that something is potted or not make such a difference? I'm just curious. He's talking about a small SMD potted module, and you think that's not likely to succeed, but an unpotted one might? Or did I misunderstand you?

I can see good reasons for something potted being better - more robust, better protection from the environment, more thermally stable and consistent, etc. What are the advantages of "no potting" beyond "repairability" (which should be a once in twenty years thing if it's done right).

Either way, I basically agree that the market for a discrete OTA is tiny. If I wanted an OTA, I'd use a 13700, and I don't want an OTA because the S/N ratio they manage is poor by comparison with competing options like the THAT VCAs or the 2164. Any new OTA product has to be better than a linearised 2164 or there isn't much point.

2 cents, and only one person's view,
Tom