STK4392 IC - Class A or A/B

[dmx2020]

Here's a trivial question for the audio gurus in here

I've got this amp based on the Sanyo STK4392 amplifier chip (which is 15W +15W RMS - see below) running a pair of Pioneer S-Z83D speakers (75W max*).

Looking at the specs, you wouldn't really expect the amp to be capable of driving the speakers with any grunt but here's the wierd part --> the amp is easily running the speakers with floor rattling bass and it seems to have more bass power (oomph) than you would expect out of any old 15x2W amp.

My guess is either the amp has is either a class A amp which would explain this. It has been running for quite a few years now so its quite stable so no issues with under powering etc.

The full specs are below:

Amp based on STK4392 (from datasheet my guess is these are all RMS values)
15W +15W Min @8 Ohms 1kHz THD 1.0
7.5W +7.5W Min @8 Ohms 30-20kHz THD 1.0
15W + 15W Max @8 Ohms 30-20kHz THD 1.0

Speakers Pioneer S-Z83D
6 Ohm / 75W Maximum (RMS rating is unknown - may be someone who's owns a pair of these can shed some light)

The internal circuit of the chip is attached so can someone pl shed some light on whether this amp is a class A or a A/B or could there be any other reason for this.

[abraxalito]

The quiescent current spec is the one that reveals whether its class A or not. Here its typically 60mA so definitely not class A.

[dmx2020]

Also i noticed that the STK4392 uses only a single power supply line (ie: +35VDC) whereas some of the higher rated ICs of the STK family uses a dual supply line (usually +35VDC & -35VDC)
would this make any effect on the sound output/quality of the amp? Also whats the need to have a dual line as oppose to a single line.

Thanks for your input.

[abraxalito]

Single line amps usually have an output capacitor in series with the speaker - this can have an effect on sound quality, but not sound quantity unless its poorly specified. Bridged amps can also operate with a single line and not need an output cap.

Operating single or dual lines is just a design choice, no need for one over the other.

[dmx2020]

abraxalito, thank you so much for your input.
So a dual line amp does not need an output capacitor (i've seen dual line systems use inductors on the output) Any reason for this?

And finally, do you think the speakers are matched adequately for the amp, given the values above. Are 75W max 6ohm speakers a bit 'too much' for this small amp. What would be fair guess of the RMS of this pioneer speaker? thx

[abraxalito]

You're welcome - I'm interested in helping newbies understand amps.

A dual line amp doesn't need a series output capacitor no, because in its no signal state its output is hopefully sitting at, or very near 0V. Unlike a single line amp where its output is sitting around half the supply - apply this DC to your speaker and something will get hot very quickly.

Both single and dual line systems might employ an output inductor - this is for stability reasons where the output load is not known (or can't be known in advance). The inductor is there to make any capacitive load not degrade the phase margin too much. Phase margin is just a measure of how stable a feedback amplifier is.

Speaker matching to load is mainly a question of providing the right impedance, not so much about power. If your ears are telling you its loud enough then the match is fine. The usual reason for matching the power of the speaker to the amp is to stop people trying to turn up the wick too much on an underpowered amp and thereby damaging the tweeters (tweeters don't take too kindly to quasi-squarewaves).

The RMS rating of the speaker is anyone's guess - my guess would be 10-20W.