Optimizing amplifier for specific frequency bands?

[Javachip]

If one was to design a multi-channel amplifier for an active loudspeaker, what would be the desirable attributes of the amplifier devoted to the treble, the midrange, and the bass? Could the circuitry be more simple than that for a full-range amplifier? Are design compromises made in a full-range amp, that could be eliminated in a narrow range amp?

[suzyj]

Very good question.

Much of the power in music is in the bass. You can make a powerful amplifier to drive the bass driver that doesn't need to slew nearly as fast as it would if it were driving the tweeter as well. That simplifies the drive section significantly.

For the tweeter, you often don't need much power (depending on the crossover frequency) so you can use smaller transistors with less capacitance, and hence faster slew rate for a given drive current.

An example of the philosophy is what I've got in mind for the two-way speakers in my study. These use a crossover frequency of 3KHz or so. I've got a design for the mid-bass (that I'm currently using to drive both bass and tweeter via a passive crossover) using large MOSFETs, that's capable of some 50W or so.

The driver for the tweeter is based on the TPA6120 headphone amp. This chip is designed for driving 32-64 Ohm headphones, to a watt or two. I worked out that if I put eight in parallel, I can get something like ten watts, at vanishingly low distortion levels, and with eye-popping slew rates.

A similar setup would be useful to replace the 100W amp that I use to drive my Infinity RS-5b's three way speakers, with crossover frequencies of 600Hz and 4KHz. I could use a 50W (single pair of FETs) amplifier for the bass driver, another for the mid, and the little 10W amp for the tweeter.

[lumanauw]

I've been wondering about this "myth". In full active system (3 amps), the best sound will come if all the amps are the same type from the same manufacturer.
If we use different amps, the sound will not blend nicely. (say for tweeter we use small classA amp, for mid we use chip amp, for bass we use classD amp)
Is this myth true?

[AndrewT]

Originally posted by lumanauw
I've been wondering about this "myth". In full active system (3 amps), the best sound will come if all the amps are the same type from the same manufacturer.
If we use different amps, the sound will not blend nicely. (say for tweeter we use small classA amp, for mid we use chip amp, for bass we use classD amp)
Is this myth true?

Hi,
I don't think the myths can be true , neither of them.
I'll go further, I am convinced the myth is blatantly untrue.

I accept that some amps are good in the midrange,
some are exceptional in the treble,
some are very good in the bass.
It is very rare that a single amplifier can be very good throughout the whole frequency range.

I think if amps were selected to do the job that they do best then the resulting amplifier/driver will sound better overall.
If the amplifier were designed to perform well in a narrower frequency range deliberately tailored to a particular driver (or frequency range) then the synergy may be even better.

By all means experiment with different amplifiers, there must be some combinations that will work better than others.

I think the area of biggest benefit will come from a no electrolytic treble amplifier. That target would be totally uneconomic for a bass amp and probably almost as unaffordable in a mid frequency range amp.

[Javachip]

For the tweeter, you often don't need much power (depending on the crossover frequency) so you can use smaller transistors with less capacitance, and hence faster slew rate for a given drive current.

So you are saying that a high slew rate is more important for treble than for bass. If that is true, then why do many people recommend low power tube amps for treble and higher power solid state amps for bass? Don't tube amps generally have much slower slew rates than SS?

[AndrewT]

Slew rate requirement is related to frequency and to maximum drive voltage.

I believe that treble requires the same drive voltage as mid and bass when all the drivers are the same impedance and the same sensitivity.

Following from that the treble requires a higher slew rate, or more accurately the bass requires less.

However, many treble drivers are much more sensitive than the other drivers in the speaker, particularly the the low Fs bass drivers. This then brings about the need for different levels of drive voltage and different slew requirements.
It may be when you do the numbers, that the slew requirements become very similar for typical combinations of chosen drivers.

[djk]

Amplifiers with no loop feedback (SET comes to mind) sound very different than amplifiers with feedback. They do not sound well together in a bi-amp system.

[WithTarragon]

Originally posted by djk
Amplifiers with no loop feedback (SET comes to mind) sound very different than amplifiers with feedback. They do not sound well together in a bi-amp system.

Hmmm, Maybe I should be re-thinking my project.

I would be bi-amping some two-way horns (after an electronic crossover) using an Adcom 555 for the lows. For the Highs, I had been thinking about some Class A, either: 1) Forte 1A, 2) Monarchy SM70Pro, 3)Threshold s150, 4) Sumo Nine. These have less than "typical" amounts of feedback.

Before this I have used the Adcom 555 on some Klipschorns and loved the bass (this is why I am set on the Adcom for the lows).

Food for thought,
-Tom

[WithTarragon]

Originally posted by WithTarragon


Hmmm, Maybe I should be re-thinking my project.

I would be bi-amping some two-way horns (after an electronic crossover) using an Adcom 555 for the lows. For the Highs, I had been thinking about some Class A, either: 1) Forte 1A, 2) Monarchy SM70Pro, 3)Threshold s150, 4) Sumo Nine. These have less than "typical" amounts of feedback.

Before this I have used the Adcom 555 on some Klipschorns and loved the bass (this is why I am set on the Adcom for the lows).

Food for thought,
-Tom

Gee...., Any thoughts or advice?

The Cabinets will be a clone of a Klipsch Jubilee: hornloaded bass bin with dual 12 in drivers (impedance can be as low as 3 Ohm). The Top is crossed at about 800 Hz (electronic xover with steep slopes and time-alignment). This will be a tractrix horn (store bought from Klipsch) and will not present a very low impedance to the amp. The efficiency on both section is good (so huge amounts of power are not required), although I never want to even come close to clipping.

I like the sound (bass) of the Adcom 555ii on a Klipschorn, but I would like something very detailed and accurate sounding for the top section (NOT "warm, lush & musical"). That is why I was considering a Class A design for the top (Forte 1A which is affordable, Monarchy SM70, Sumo Nine, Threshold S150 , which may not be affordable) .

The only other constraint on the amp for the top section is that I would like the input impedance to relatively high since I am using a passive "pre-amp" (10k pot conventionally wired and coming after the electornic crossover).

This is a chance to test the notion of mixing/matching amps to their relative strengths when bi-amping.


Thanks, & Any thoughts?
-Tom

[AndrewT]

Hi,
what is the sensitivity of the two drivers?
The passive pre should be BEFORE the active crossover.
An inverting input to the active crossover will allow the pot to be integrated into the (negative) gain of the input.