Tempco

[Magnus Danielson]

>>>>> "TC" == Tim Cockram <tim at redragon.demon.co.uk> writes:

 TC> In message <B18ED9F91130D111804D00805F25A00310EE7F at powerweb1.ipt.isd.sco
 ttishpower.plc.uk> , Fraser, Colin J <Colin.Fraser at scottishpower.plc.uk>
 TC> writes
 >>> 
 >> I used 12 bits for hz/v on my homemade convertors, but didn't include
 >> pitch bend so I didn't need the extra resolution.
 >> What I'm thinking about now is a digital exponential convertor for vcos.
 >> Has anyone tried something along the lines of a linear input to an a-d
 >> convertor driving a rom with a lookup table for exponentiation, the
 >> output of which drives a d-a convertor, all clocked at audio rates to
 >> allow audio fm ?
 >> Would this give me a temperature stable linear to exponential convertor
 >> without having to buy 1000 tempcos ?
 >> 
 >> Colin f

 TC> Hi 
 TC> This would work but you would need to use a high resoluton A/D , D/A
 TC> process to get the pitch resolution required. e.g. an 18bit convertor
 TC> would give a 0.038Hz resolution for a 10Hz to 10kHz oscillator (16 bits
 TC> would only give 0.15Hz resolution).  Could be expensive.  If you could
 TC> run the convertors fast enough you could multiplex the process though.

 TC> On the other hand you could make the whole thing linear like Yamaha or
 TC> Korg, but.........

 TC> So I guess the Tempco still rules (especially if Paul S. stocks them).

 TC> Heated or self compensating (Rene's or the minisonic) circuits still
 TC> have their attractions.  The Thats corporation who manufacture what used
 TC> to be the dbx vca's do transistor arrays with up to eight pnp or npn
 TC> transistors. www.thatcorp.com

 TC> I haven't read the data sheets yet so I'm not sure about the log
 TC> conformity or the thermal gradient accross the chip.

I have worked with one of their VCA cursuits and it was running HOT
under normal operating conditions. It was used for catastroph limiter
in large PA systems and I can only recall one of them failing on me.
With some attention it could preform pretty well, a large PA can when
well tuned have about 130 dB of dynamic range, so we pulled a few
stunts back then.

Cheers,
Magnus