Dsd without D/A stage....

[angel]

Originally posted by stolbovoy
Didn’t anybody think about making high current DSD "DAC" to feed loudspeaker directly, controlling volume by altering rail voltage. Pretty interesting and simple at first glance…

I've mentioned this idea on the forum before, I think.

It is similar to what Tact used in their Millenium amplifier, which used a class D output stage, where the volume control set the rail voltage.

My suggestion is that this approach should be used with electrostatic panels. Essentially, this would let the air perform most of the integration, which is about as ideal as it gets, IMHO.

Try it out with a set of "cheap" Stax 202 headphones. 100Vrms delivers 100dB, so 10Vrms should give you 80dB. Pretty okay for a prototype, and simpler than trying to deliver 1KVrms (120dB) peak at 2.8MHz into a 120pF load..

[Hooligan]

I'm very interested in matter of driving conventional loudspeakers by digital signal, currently i'm finishing my PhD work - digital loudpeaker - and 1-bit sigmadelta (DSD) stream seems to be an alternative to driving high count of small transducers with PCM decomposed signal. So, if anyone finish any suitable amplifier for DSD, let me know :-) thanks

[stolbovoy]

Originally posted by kevinkr
In effect you are talking about a power dac, and yes it can be done by varying the raw dc voltage to the power amplifier - note that you still need a post filter to reconstruct the signal after the digital amplifier. I think in this case an air core inductor of a couple of mH and a capacitor such that your cut off frequency is around 50kHz would work well..

What will happen if you let loudspeaker to do the "reconstruction" part of the job?

[stolbovoy]

Originally posted by kevinkr
The power supply would be the most complex part of the equation and would need very good regulation and stability. If you desire to avoid using a large output capacitor either a bridge configuration could be used or tightly tracking bipolar supplies.

It sounds as power supply for this solution is much more critical than for conventional design. Why?

[kevinkr]

The reactance of the speaker voice coil is not well defined at rf, and the interwinding capacitance, not to mention strays result in unexpected resonances which can result in significant self heating. In addition depending on the amount of inductance present the impedance of the driver can be quite high resulting in extreme voltage drive requirements or high currents if there is a lot of capacitance in the voice coil.

The driver's impedance is not defined at these frequencies so it would be hard to know what amount of power would be delivered to it, and hence its efficiency.

A woofer driven through a second order LC based cross-over could work fairly well depending on how lossy the inductor is at higher frequencies and where the self resonance is.

A tweeter might not survive depending on the impedance across its voice coil at the switching frequency..

Also levels of rf could be quite high and some rf would be radiated off of your speaker cables which could create local rfi problems for your hifi and telephone.

Designing a well regulated and stable supply at the currents required is not a huge problem, making sure that the impedance of the supply is low at the switching frequency so as to minimize losses and rfi is an issue. If you contemplate using bipolar supplies they will need to track closely in order to eliminate variable dc offsets in the output.

[stolbovoy]

kevinkr ,
Thanks a lot for detailed answer!