12 Ohm Speaker

I have the Amp Camp amp (the original) and I am looking to start on an F6. I am also looking at a pair of Zu Dirty Weekend Speakers that are 12 ohms with 97 db efficiency. This seems like a good pairing but I am concerned about the 12 ohms.

I'm a newby. Is there a mod I should make to either the F6 or Amp camp that would make it a better companion for the 12 ohm load?
I want to understand this as well from a numbers standpoint. Anecdotally, I have an F6 running off a Focusrite Solo into some 6R speakers with 88dB sensitivity and it plays loud enough to shake my two-car garage with the garage door open, volume knob at 2 o'clock. So I don't think you're going to have any problems.

I would love for someone to swoop in and correct me on the following:

From a numbers standpoint, the way I understand it the F6 is voltage-limited into 8R speakers, but can supply more current for 4R speakers up to that voltage limit. I don't know if the F6 has a max voltage-out that it is capable of with any voltage-in, but my understanding is that sqrt(25W * 8R) = V which puts the output voltage for achieving 25W into 8R speakers at ~14V. If that also represents a max v-out that the F6 can do, that means the max wattage it can put into 12R speakers would be 14V*14V/12R = 16.3W, and I = ~1.15A (which is nowhere near the F6's amperage out capability, so that's leaving electrons on the table). If that's really the case and you find that isn't enough, you could double that voltage by switching to XLR, converting it to a monoblock and building a second one, making it 28V*28V/12R = 65W. At that point it'd still only be using 2.3A so it still wouldn't be current-limited, since the F6 can provide 3.5A into 4R loads at the 50W/4R rating it has.

source: I'm just going by the formulas I found when googling for "electrical formula circle" and I'm assuming that's how you apply them. But I could also be sniffing glue.
Both of those amps should be very good with the 12 Ohms speakers. I've been interested in the Zu speaker line as well. Zu offers some ballast resistors which may be placed in parallel with your speakers to lower the impedance a bit, if that's something you want to try.

I've built a hotrod version of the F6. Reports are somewhat scattered among various F6 threads, but the most information is in F6 Amplifier and The diyAudio Firstwatt F6.


2011-04-29 8:37 pm
The F6, or any ss amp, is limited in peak output voltage by the power supply rail voltage.
The 24VDC rail in the F6 limits the output voltage to less than 24V peak (17Vrms sine).
On a 12R load that would be 2A peak output current (1.4Vrms sine), or 23.5W.
Some output voltage is lost due to drop in the output devices, so the actual maximum
peak output voltage would be less.

With a bridged F6, the peak output voltage would double, so the power into the 12R should
quadruple to 94W.
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Typically the Zu speakers have XOs that cause the speaker to have a quite unflat impedance. It may be a nominal 12Ω but that is a nominal value that tells you little.

Higher output impedance amplifiers like the ACA (and more so a balanced/bridged ACA) care should be taken to use speakers that have a flattish impedance or what a high output impedance amp (ie Fostex FExx6).


Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
I'm a newby. Is there a mod I should make to either the F6 or Amp camp that would make it a better companion for the 12 ohm load?

Both amplifiers are happy with higher impedance loads. If you want to
play around with adjusting the load you can acquire some power resistors
of different values and place them in parallel with the speaker.

Try 22 ohms 5 watts to start, which makes the load 8 ohms.


Paid Member
2010-01-08 10:49 pm
Portland, OR
I currently use monoblock ACA’s with balanced connections and Zu Druid 4 speakers. Before the Druids, I had a pair of Zu DW speakers. I tried both speakers with Zu’s parallel resistors to lower the impedance, but did not notice any difference. YMMV.

I do like the ACA/ Zu pairing and my next build is an F6. I’ve been following TungstenAudio’s mods/ thoughts on the F6, but I don’t understand it all, yet.

My monoblocks were built following TungstenAudio’s updates.


2011-04-29 8:37 pm
A resistor in parallel with the speaker won't give you any more volume, but the tonal balance may improve,
due to the total load impedance seen by the amplifier being more constant. This is especially important for
amplifiers with somewhat high output impedance. If your speaker already has fairly constant impedance,
it won't make much difference.
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