Alpha Nirvana 39w 8ohm Class A Amp

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#2,421
Hello,
I am at the gates of the construction of some new 3-way speakers.
The bass(15') will be powered by a Hypex 250w module up to 240Hz, for midrange(8') and treble(compression motor and horn). The filter is passive and
The minimum impedance is 6.3 ohms at 1350Hz (attached the impedance curve).

I was thinking of feeding the midrange and high end (92db 1w/m) with the Alpha Nirvana 20w/8ohms. I find the idea of getting rid of the heat of the original 39w version attractive ;-)

I think I'll have enough power at all frequencies, for listening in domestic settings, it won't be a disco, that's clear 🙂
I'm right?

Best regards

A73BF956-9BD1-4693-9C11-2867729B7E12.jpeg
 
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#2,423
The Alpha Nirvana in its stock form has a damping factor of 160, or an output impedance (referenced to 8 ohms) of 0.05 ohms. I am assuming this is measured close to 100Hz. I don’t know what the output impedance is at the 1khz or greater region but I would probably consult with Hugh (@AKSA ) on this. Generally speaking, most amplifiers will have a rising output impedance after a certain frequency (usually 1khz and above). Most of the time we are concerned with lower impedances, i.e. 4 ohms and lower.

Best,
Anand.
 
#2,424
Anand thank you for your reply.

The truth is that with the information I have about the amplifier and how it is designed, I am not entirely clear about its performance under high loads either.

As I have read, you have to be clear about the value of the load to connect to the output of Alpha Nirvana. In fact I have seen throughout the thread some modifications in the circuit for this reason.

Greetings
 
#2,426
Hi X, if it seems like the impedance curve of this speaker is benign for the Alpha Nirvana and not having to deal with low frequencies, it helps to have that reserve of power. The peak up to 70Hz, in another type of amplification, I think there would be no problem.

Perhaps Hugh (@AKSA ) can shed some light on the subject.

Regards
 
#2,428
For sure you will have enough power for home listening with such high sensitivity!
The amp has a relatively low output impedance, so the variations in impedance should hardly have any measurable effect on frequency response.
If you referred to my comments/experiments about output impedance, that is mainly focused on the bass performance. IMHO mid/treble can actually sound 'sweeter' with higher output impedance.
 
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#2,429
I think there may be some confusion with regards to some other amplifier designs that have a single ended output stage, such as single ended triodes that are transformer coupled which have output impedances of a few ohms not milliohms. In those amplifiers with high output impedances, the frequency response (and overall SPL at 2.83V/1m) will most definitely change with high and/or low impedance extremes. In your example, a single ended triode amp will have too much midrange especially because it will output MORE power into the higher impedance peak in comparison to the other frequencies where it will be more constrained.

Here is an example of a tube amp without feedback that faired quite horribly in a recent stereophile review. It isn’t a single ended output stage (it’s actually push pull!) however, just a poorly designed one imho. It also weighs 29kg, has a power consumption of 500W AND costs USD$16,000!

JA wrote:
With the high bias setting, the source impedance from the 16 ohm output tap was a very high 11.5 ohms at 20Hz, 13.6 ohms at 1kHz, and 11.3 ohms at 20kHz. The impedance from the 8 ohm output tap was 6 ohms at 20Hz and 1kHz, increasing slightly to 6.85 ohms at 20kHz. From the 4 ohm tap, the impedance was 3.2 ohms at 20Hz and 3.7 ohms at 1kHz and 20kHz. The medium-bias setting increased these impedances by 2 ohms from the 16 ohm and 8 ohm taps and by 0.5 ohms from the 4 ohm tap. The low bias setting added another 2 ohms, 1 ohm, and 0.5 ohm, respectively.

These are all very high output impedance values.
Click to expand...

IMHO, the Alpha Nirvana is the most responsible well designed solid state amplifier employing a single ended output stage. I actually tested it with a Legacy Focus speaker which has an impedance nadir of ~ 2 ohms. It faired quite well in my comparisons. With my 96dB sensitive speakers that have a 4.5 ohm impedance nadir, it‘s excellent.

If you are concerned (and if you want your impedance load to be more compatible with say, a tube amp), why not design an impedance twister circuit (i.e. Zobel) to drop that impedance peak? You can always do that and play with tube or solid state amps!

Best,
Anand.
 
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#2,430
Rallyfinnen, it was your comments and others in general that made me interested in this subject of load impedance, but that's okay, it always helps
;-)

Anand, thank you for your insights and experiences. In the end I think that with the Alpha nirvana 20w//8ohms version, you shouldn't be afraid of that impedance spike 💪

Greetings
 
#2,431
Karucho, I've come to this party too late; my apologies.

Generally loudspeakers are complex, impedance loads and increasingly a number of very expensive, large speakers are in the market with very low impedance troughs down to 1.8R.

These dips are generally below 100Hz and reflect low inductance reactance in crossover and voice coils. These dips measure very like DC. Since highest amplitudes in music occur below 100Hz, these low points can cause real embarrassment for power amps, particularly low power Class A. You will current starve on the AN at these points if you overdrive at these dips; and overdriving comes back to the efficiency and listening levels in the situation. I would expect issues with <2.5R on the AN on <90dB/watt/metre speakers at high levels in a middle size room. Rock, percussion and orchestral music can tax any amp, but to avoid this situation (which will not damage the amp, I might add) use speakers of higher sensitivity, cf. >92dB/watt/metre, and keep the levels down a little.

The AN39 has plenty of power with 92dB speakers of nominal 8R. With 4R speakers, there is a version of this amp for tougher speakers, and you need to increase the quiescent current. The 8R amp has 23.5dB of global feedback, and a Zout of about 50milliohms over a wide frequency range, and even lower below 500Hz. This is by design to allocate more drive at low frequencies.

When speaker impedances increase to the mentioned 70R, driving them is a walk in the park. This is a resonant point too, and the issue is to prevent the speaker over-emphasizing that frequency. Normally these peaks are absorbed within good crossover design, but you cannot count on this with all speaker designs.....

Thanks to XRK, Rally and Anand for their excellent answers.

Cheers,

Hugh Dean
 
#2,432
Hugh, thank you very much for providing more data and knowledge about AN amplifier.
I don't think you're late to the party, the party started in post #1# of this great thread about the AN and its versions ;-)

Thinking that this 70R peak is not a problem, I only have one question... what power can the AN 20w//8R version deliver in a 6R load impedance?

Greetings
 
#2,440
Hugh,
27V rails - 8R load - 39W makes perfectly sense (50Vpp swing is 17.66VRMS is 39W @8R).
I cannot follow your reasoning when I ask if 31Vpp is correct for 24V rails and you reply "No it is for 24V rails..."
What am I missing?
For 28Vpp we would need no more than some 16V rails; apply enough bias current for 16W class A, dependent on load impedance.
 
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