What Class D amps will drive a 2 ohm load

WHY!! they keep making low input impedance amps these days is beyond me, go back to 48k (the once recognized industry standard) or even 100k like it used to be with tubes, makes for a whole better range of preamps ,tubes SS or passive able to drive them properly.

The reason you see this has to do with the fact that a filter often has to be installed between the audio input and the encoder (comparitor or the like). The filter is there to remove high frequency noise that might otherwise get amplified and add to the noise floor of the amp. But good high speed comparitora have issues of their own and can be unstable with a high impedance inputs. Hence low input impedances if there is no input buffering.

There’s no need for it in many cases, as sources and preamp have enough gain these days. It’s just "another opamp in the signal path" that’s not needed in many cases if the input was high enough instead of 2.2k, who wants that in hi-end if it can be taken out and one less opamp yay!!! gotta be good for the sound, and the business.

This statement is incorrect. Of course there is a need for an input buffer in many cases- a lot depends on the encoding scheme of course (for example with PWM the gain is based on the difference between the amplitude of the triangle wave in the circuit and the input signal). Things might not work out in the design that you have enough gain- particularly if feedback is employed. So an opamp or instrumentation amplifier might be used to make up the needed gain. Quite often this isn't a lot of gain, maybe a gain of 2 to 10. You could do it with an input transformer which has certain advantages, but it should be obvious that an input transformer adds a whole set of problems of its own.


Our preamps can drive such low input impedances- any preamp that supports the balanced standard can, but in high end audio such preamps are quite rare.

Our preamps can drive such low input impedances-

Of course it can, how did I know that was coming, stick with tubes, your good at it, and built your rep on them.


The fact is this amp without buffer it has around 15db of gain around the same as the Nelson Pass First Watt F5 and with today’s high output sources and preamps, in most cases you don’t need another soul destroying opamp buffer in the signal path with or without gain.

Here is the comments from a manufacturer of these Purifi’s Class-D amps when I queried him on the unnecessary low 2.2k input impedance. I think it could be because of the use of bjt input stage instead of fet, I’ve yet to suss that one out.

Yes you are absolutely correct that you can use a high output DAC directly into this module. "That would be the ultimate solution"
And I am considering a buffer bypass mode. However most power amps will have gain in the region of 26dB to account for sources down to 2 Vrms. A typical DAC straight in is a good solution with digital volume

Most digital sources today have far more than 2v and owners preamps can certainly have enough for the Purifi without another buffer.
As Nelson Pass with his F5 said " If you need to turn the gain up on your preamp, then do so."

Cheers George

Here is the comments from a manufacturer of these Purifi’s Class-D amps when I queried him on the unnecessary low 2.2k input impedance. I think it could be because of the use of bjt input stage instead of fet, I’ve yet to suss that one out.

Yes you are absolutely correct that you can use a high output DAC directly into this module. "That would be the ultimate solution"
And I am considering a buffer bypass mode. However most power amps will have gain in the region of 26dB to account for sources down to 2 Vrms. A typical DAC straight in is a good solution with digital volume

I explained why already. This is a common issue and why you see many Class D amps with some sort of buffer input. Many opamps can drive lower impedances like 600 ohms no worries and since so much feedback (+30dB) is being used, the assumption is that the distortion added by the feedback on the opamp will be quite minimal. But there are some class D amps where the designer opted to simply have a low impedance input for the very simple reason of having less circuitry- a purist approach. But you have to have a source that can drive it. Its nothing to do with FETs or BJTs.

Whatever Ralph.
Seems like this could be the module your using, as you've turned up the force shields a notch higher on this one, if so just go with the buffer by-pass switch. 

I've mentioned before, we're not using anyone's modules.

It seems that what you aren't getting is that comparitors (a kind of chip or circuit used in many class D amps) can be unstable if used with high input impedances. To prevent oscillation, lower impedances are used at their inputs.