🚀 go-pugleaf

in article <3800@memqa.uucp>, r91400@memqa.uucp (Michael C. Grant) says:
> In article <3364.26832a9d@ccvax.ucd.ie>, b_haughey@ccvax.ucd.ie (Brian J Haughey) writes:
>> I'm looking for information on D/A converters that could directly supply 
>> an output to a speaker from low to high volume. (Is such a thing possible ?)
>> What are the problems associated with this, e.g. would the power supply and
>> scaling factors of the D/A have to change as the volume changes ?
> 
> Sounds to me like you want a D/A converter, a preamp with volume control,
> and a power amplifier.  It wouldn't make any sense to do it any other
> way, for practical reasons.

Actually he's got a great idea, although possibly not for the application in
mind. Voltage amplification is a major problem from the audiophile viewpoint.
Some designers even combine vacuum tube voltage amps with transistor current 
amps.

2 possibilities:

1. Use a resistor ladder voltage output DAC that can handle the maximum output
voltage. Except for FET's used as switches, not amps, the signal path is so far
passive. Resampling ("oversampling") filters must be used to allow 
simplistic output filtering. Passive filtering is used. A voltage follower
buffer drives a passive volume control. A voltage follower drives the speaker.

2. Apply to switching type power amplifier. In this case a "one-bit" type
of DAC principle is used The difference is the conversion is at full power.
Filtering is passive between amp and speaker. Resampling is required. 
Switching amps are sometimes called a "class-D" amplifiers. "Class G"
refers to the same thing with a peak power extension circuit; this is not
required because the dynamic range is known. The circuit is much different
than the analog version of switching amplification however because there is
no comparison with the input. RFI could be a problem.


(2) would be possible to build at home, (1) is not. While I find this
interesting, my interest is damped by the scary thought of digitizing 
my phono signal :-)

bill S.