Power Supply Concerns

I've been testing some 12V 3 Amp switching power supplies. You can see results here.

PSU FAQ

This is a simple PSU with a 12V 1A transformer (replaced, see below), diode bridge, LM 340-12 regulator and caps. C1 and C2 are large can type electrolytic with screw terminals. Caps marked "SMD" are an assortment of non polarized surface mount caps, standard ceramics, metal film and one SMD tantalum per side.

This PSU is quiet. With no signal and the amp input turned all the way to "11" and my ear right next to the speaker, there is no noise coming out. No hiss, no rizz, no hum, no buzz. Just as clean as the battery sound. Good job, Dave! It does help that it's a low gain system. You'd never find that in kilowatt P.A.s. =)

The 1A transformer was the weak point. It had serious voltage drop when the amp is pulling a normal load . A 14 or 16VAC transformer is needed. 2A or more.

In my commercial version of the T-amp I use a 2.4 amp 15V transformer and a 3 amp regulated supply. This works wonders for the sound of the amp.

The onboard "tank" cap.

This little cap must be replaced. Use something like a 680uF 16V (min) Panasonic FM series, or Rubycon. This cap is important for controlling switching noise on the PCB. Adding a second cap at the point were you bring power onto the PCB will also help.

The waveform on the right shows the onset of soft clipping at kHz into an 8 ohm load. At this point the amp is drawing 1A from the PSU. Perhaps 1A is enough, as you wouldn't run the Sonic this hot, unless you like the sound of clipping.

With a "normal" musical signal (Jimmy Buffet) run hard enough to start peak clipping -the Sonic is drawing no more than 300 ma average. This is well within the specs of the LM 304-12 regulator. However this or another 1A regulator will cause the amp to clip a little early. A 3A regulator prevents this. Caps on the PSU output side help supply the transient currents, too.

The Sonic will consume more power if pushed into serious clipping (see WAVEFORM page). But this is far beyond what would be needed or wanted for music, so the extra current isn't need, IMO.

summary:

1) This is a bright, forward sounding amp. That may be why it is popular. It does very well on vocals and percussive sounds, especially bells, cymbals, vibes etc.

3) Max current use on music is 250~350mA (clipping)
Average is 150mA @8 ohms.

2) Idle current is 70mA.

4) The amp needs at least 12 volts to work well, or at least loudly. 13.5 volts might be worth trying. Changing the V+ by 4 volts does not seem to change the SPL. Maybe there is some sort of feedback that controls this?

5) Still seems that the little amp can't play very loud. With a good healthy level going, the peaks distort - and it's not pretty. More efficient speakers would help, less power would be needed giving more headroom. This is not a condemnation of the Tripath technology, just that such a small amp can't drive average speakers to a good loud level. No surprise. I have read that these amps round of the peaks in much the way tube amps do. So far I have heard quite the opposite.

The amp works well on 12-13.5 VDC. Don't go above this, you'll fry the chip.

Average musical draw into 8 ohms will be 150mA or less. Above that you are clipping.

Peak current draw is about 2.2A into 4 ohms. This is unlikely to happen on a musical signal.

A 3 amp transformer is more than enough for this amp. 2A should also work. A 2 or 3A regulated PSU works very well. Be sure you use a good regulated supply and large filter caps. I suggest 10000uF on each side of the regulator.

Panasonic FM series caps for onboard use are fantastic. They really cut the switching noise and add capacity for musical transients. 680uF 16V works well on board. You can decouple your large PSU caps with smaller Panasonics too. Use at least a 25V cap on the input.