[Arm-netbook] goodbye LM13700, hello CM108AH

[luke.leighton]

On Tue, Oct 29, 2013 at 1:20 PM, Derek <dlahouss at mtu.edu> wrote:
> luke.leighton <luke.leighton <at> gmail.com> writes:
>
>>
>> it is with much sadness and regret that for parts in the flying
>> squirrel i say goodbye to the LM13700M, a part with a huge and rich
>> history, having seen its first emergence into the world around 1967
>> even predating the moon landing.  the reason is simple: the bias for
>> the diodes is 3x 0.7 volts and on rails of +/-2.5V it's simply not
>> enough.  i'm seeing silence at low volumes... and then huge clipping.
>> i simply don't have time to resolve this.
>
> For those of us with electronics experience but maybe not name recognition
> of ICs: the LM13700M is a dual op-amp,

 dual _transconductance_ op-amp.  normally used with +/- 15V rails but
can go down to +/- 5V, and there are reports of people managing to go
lower than that but losing some of the characteristics... which i
didn't know about before picking it.

>  which Luke may be trying to configure
> as a Class B amplifier (push-pull).

 ah no - it's going straight into the ADC.. ok it was ... the output's
only 2mA which is (was) more than enough for the STM32F or the ATSAM4.
 all i had to do was get it up to a 0-3.0V swing.

 basically i needed a voltage-controlled pre-amp for the microphone.

> Since his input voltage is so slight,
> the circuit is spending too much time in cutoff, except where it reaches
> saturation.

 basically yes.

> Luke, using a class B amp is going to result in crossover distortion.  I
> think that's what you're seeing.  So, you're replacing your homebrew power
> amp

 just an OTA.  the speaker phase was using a proper class B (TDA2822)
- i learned the hard way that the input for that is line-out levels
(and i was trying to put the DACs *directly* into it.  a 240k/20k
resistor divider solved that one...) but that's another story.


> with a C-Media USB Audio device?

 yes.  ok if conexant get back to me fast enough i'll use their part instead.

> Is this directly connected to the USB
> lines, or after a hub?

 through a hub - there's only one USB connection from EOMA68 so
everything (including the ATSAM4S) goes through a hub.

l.