Need an amp for low ohm speakers

[Ted G]

Hello,

Fulfilling my teenage dreams, I purchased a Pioneer SX 1280 receiver. Unfortunately, even with 200 watts per channel it can't run the recently acquired pair of Infinity RS-2.5 speakers at high volume for very long before cutting out on protection. After doing some research, I found that the speakers can run down under 3 ohms when the bass kicks in and the receiver can't handle it. I'm considering getting a purifi or hypex based class D amp. The alternative is a used Adcom 555 or similar. I'd love a beefy Mcintosh but I've already dropped a lot on what I have . Also, rather than using the receiver, what is an inexpensive alternative pre-amp? I am not a purist as you can tell with the Bose 901's. Please hifi guru's lend your expertise!

 

[staticV3]

The 3e Audio A7 has no trouble driving low frequencies into low impedance speakers:

'3e audio A7/A7 Mono Amplifier Review'

Apr 14, 2025

This is a review and detailed measurements of the A7 / "A7 Mono" stereo power amplifier with balanced inputs. It was sent to me by the company. I have no idea what it costs in US. Audiophonics lists it for 379,00 €tax incl.

While the A7 may look like its competitors, it has a heft to it that I don't recall others not having. It really feels like it is filled with lead or something! A nice volume control in the middle adjusts the gain but it can also be bypassed using a switch in the back. Pushing the volume control quickly switches between XLR balanced and RCA. Two...

• amirm
• Replies: 816
• Forum: Stereo and Multichannel Amplifier Reviews

• 

 

[ppataki]

I have 2-ohm DIY subwoofers and run them with two mono A7 units - they are marvelous!
Note: their auto signal sensing feature cannot be switched off and that is really really very annoying sometimes....

 

[staticV3]

Note: their auto signal sensing feature cannot be switched off and that is really really very annoying sometimes....

Can it be bypassed using the 12V trigger input?

 

[ppataki]

Can it be bypassed using the 12V trigger input?

It does not have one

 

[AcousticTheory]

This application is made for Class-D. I would echo the TPA3255 recommendations; that chip makes marvelous music, and it has a lower damping factor than the current state-of-the-art discrete Class-D amps like Hypex nCore, and in my listening its character is a slight bit warmer-sounding than the Hypex. That subjective comment will raise all kinds of outrage around here, but since you are needing a Class-D amp that is an alternative to vintage electronics which will have a naturally warm sound, a TPA3255-based amp is an excellent bet, and the prices are low enough that you won't lose much if you don't find it to be to your liking.

 

[JayGilb]

Hello,

Fulfilling my teenage dreams, I purchased a Pioneer SX 1280 receiver. Unfortunately, even with 200 watts per channel it can't run the recently acquired pair of Infinity RS-2.5 speakers at high volume for very long before cutting out on protection. After doing some research, I found that the speakers can run down under 3 ohms when the bass kicks in and the receiver can't handle it. I'm considering getting a purifi or hypex based class D amp. The alternative is a used Adcom 555 or similar. I'd love a beefy Mcintosh but I've already dropped a lot on what I have . Also, rather than using the receiver, what is an inexpensive alternative pre-amp? I am not a purist as you can tell with the Bose 901's. Please hifi guru's lend your expertise!

View attachment 463229

Fun setup !

 

[boXem]

lower damping factor than the current state-of-the-art discrete Class-D amps like Hypex nCore,

Would be curious to see the data backing this claim.

@Ted G if you are searching for class D delivering a lot of current, Hypex NC1200, Purifi 1ET7040SA, Purifi 1ET9040BA, with adequate power supply and proper cooling (especially Hypex).

 

[Ted G]

Fun setup !

Thanks- it's a problem

 

[Roland68]

Hello,

Fulfilling my teenage dreams, I purchased a Pioneer SX 1280 receiver. Unfortunately, even with 200 watts per channel it can't run the recently acquired pair of Infinity RS-2.5 speakers at high volume for very long before cutting out on protection. After doing some research, I found that the speakers can run down under 3 ohms when the bass kicks in and the receiver can't handle it. I'm considering getting a purifi or hypex based class D amp. The alternative is a used Adcom 555 or similar. I'd love a beefy Mcintosh but I've already dropped a lot on what I have . Also, rather than using the receiver, what is an inexpensive alternative pre-amp? I am not a purist as you can tell with the Bose 901's. Please hifi guru's lend your expertise!

View attachment 463229

The Pioneer SX 1280 receiver was only designed for 8-ohm speakers and couldn't even handle non-critical 4-ohm speakers, shutting down very quickly.

The 3E Audio A7 (not A7se) recommended by @staticV3 is ideal for such speakers, as it can continuously drive 2-ohm speakers, even with over 400 watts per channel (at least a 48V/10A power supply).
A friend of mine uses it to drive even larger and more impedance-critical old Infinity Kappa speakers.

 

[boXem]

2-ohm speakers, even with over 400 watts per channel (at least a 48V/10A power supply).

Sorry but the math don't math.
400 W in 2R is 14 A RMD / 20 A peak. Per channel.

3E audio products are excellent, but considering that they fulfill all needs seems a bit exaggerated.
There are people who listen speakers with tricky impedances, very loud, in well damped rooms. They need copious amounts of power, and 3E audio products do not target these users.

 

[Roland68]

Sorry but the math don't math.
400 W in 2R is 14 A RMD / 20 A peak. Per channel.

3E audio products are excellent, but considering that they fulfill all needs seems a bit exaggerated.
There are people who listen speakers with tricky impedances, very loud, in well damped rooms. They need copious amounts of power, and 3E audio products do not target these users.

Please don't take this personally, but I'm giving recommendations for real music listening, not for measurements with square-wave signals.
For normal music, a ratio of 1/4 to 1/8 for sine power/peak power to power supply is assumed. This is more than sufficient.

The A7 uses a TPA3255 as a PBTL for each channel. Many people underestimate this power, especially since it's only a small amplifier.
Many people use amplifiers like the PA7 or A7 with power-hungry and impedance-critical speakers very successfully.

Perhaps you overlooked the maximum power measured by Amir. These measurements were also taken with the standard power supply.
I would like to see the speaker that allows you to turn the A7 up to the maximum in a normal-sized room without damaging your hearing.

 

[Torin Krell]

Hello,

Fulfilling my teenage dreams, I purchased a Pioneer SX 1280 receiver. Unfortunately, even with 200 watts per channel it can't run the recently acquired pair of Infinity RS-2.5 speakers at high volume for very long before cutting out on protection. After doing some research, I found that the speakers can run down under 3 ohms when the bass kicks in and the receiver can't handle it. I'm considering getting a purifi or hypex based class D amp. The alternative is a used Adcom 555 or similar. I'd love a beefy Mcintosh but I've already dropped a lot on what I have . Also, rather than using the receiver, what is an inexpensive alternative pre-amp? I am not a purist as you can tell with the Bose 901's. Please hifi guru's lend your expertise!

View attachment 463229

Before you try any of the excellent but more costly solutions here, you may simply try placing a quiet but effective fan(s) directly above the 1280’s heatsinks.
Since as you indicate / imply the 1280 is able to drive your Infinity speakers at lower volume, and sound fine without clipping even at high volume settings before shutting down, the shutdowns are possibly just a heat dissipation issue (created by the low impedance load) which some well placed fans along with situating the 1280 in a way that maximizes ventilation will likely solve.
Ventilation for cooling can be improved by placing the 1280 on top of a shelf with nothing near it nor on top of it, and propping it up by extending its feet along with the use of one or two small quiet fans aimed at the heat sinks possibly with a shroud to reduce their noise and increase their directivity.

 

[Roland68]

Before you try any of the excellent but more costly solutions here, you may simply try placing a quiet but effective fan(s) directly above the 1280’s heatsinks.
Since as you indicate / imply the 1280 is able to drive your Infinity speakers at lower volume, and sound fine without clipping even at high volume settings before shutting down, the shutdowns are possibly just a heat dissipation issue (created by the low impedance load) which some well placed fans along with situating the 1280 in a way that maximizes ventilation will likely solve.
Ventilation for cooling can be improved by placing the 1280 on top of a shelf with nothing near it nor on top of it, and propping it up by extending its feet along with the use of one or two small quiet fans aimed at the heat sinks possibly with a shroud to reduce their noise and increase their directivity.

That's really not a good idea or recommendation.
It's well known that the Pioneer SX 1280 is only designed for 8 ohms and non-critical speakers and can't handle 4 ohms.
Tricking the protection circuit with a cooling system could lead to serious damage to the receiver and speakers.

 

[Mnyb]

Would be curious to see the data backing this claim.

@Ted G if you are searching for class D delivering a lot of current, Hypex NC1200, Purifi 1ET7040SA, Purifi 1ET9040BA, with adequate power supply and proper cooling (especially Hypex).

Damping factor >100 is meaningless with passive speakers anyway . You practically have to bolt the amp directly to a driver in an active design to utilize "better" damping factor , but in such design you use extensive eq anyway and can fine tune the mechanical damping together with amp ,so it would be meaningless even then .

Any kind of speaker wires and/or series inductance in the passive filters dominates over most modern amps output impedance ( DF is simply a ratio between 8Ohm/"output impedance" )

 

[Sokel]

Please don't take this personally, but I'm giving recommendations for real music listening, not for measurements with square-wave signals.
For normal music, a ratio of 1/4 to 1/8 for sine power/peak power to power supply is assumed. This is more than sufficient.

The A7 uses a TPA3255 as a PBTL for each channel. Many people underestimate this power, especially since it's only a small amplifier.
Many people use amplifiers like the PA7 or A7 with power-hungry and impedance-critical speakers very successfully.

Perhaps you overlooked the maximum power measured by Amir. These measurements were also taken with the standard power supply.
I would like to see the speaker that allows you to turn the A7 up to the maximum in a normal-sized room without damaging your hearing.

View attachment 463471

What's important is combining the below charts:

https://www.ti.com/lit/ds/symlink/tpa3255.pdf?ts=1752642631478&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTPA3255

2 Ohm is no joke, it needs some serious real estate for cooling and power losses are far greater.

 

[Hear Here]

Hello,

Fulfilling my teenage dreams, I purchased a Pioneer SX 1280 receiver. Unfortunately, even with 200 watts per channel it can't run the recently acquired pair of Infinity RS-2.5 speakers at high volume for very long before cutting out on protection. After doing some research, I found that the speakers can run down under 3 ohms when the bass kicks in and the receiver can't handle it. I'm considering getting a purifi or hypex based class D amp. The alternative is a used Adcom 555 or similar. I'd love a beefy Mcintosh but I've already dropped a lot on what I have . Also, rather than using the receiver, what is an inexpensive alternative pre-amp? I am not a purist as you can tell with the Bose 901's. Please hifi guru's lend your expertise!

View attachment 463229

That looks like a treasure trove of ancient kit best donated to an audio museum!

A quick count of boxes shows 10 of electronics and perhaps 6 speakers - and I can't imagine how many cables tie it all together! An impertinent question maybe, but do you want a museum collection, or do you want to close your eyes and just listen to music, at as good quality as your money can buy?

If the former, then continue with your quest for an ancient amp that is happy with low impedance speakers - there must be plenty to choose from and others can offer advice on models. If the latter, you'd really be better off with a simple but well-chosen all-in-one (streamer/amplifier and a pair of decent speakers - all bought "nearly new" of course. Ditch the record and CD collections too - you can hear it all from a subscription streaming service such as Quobuz or Tidal at as good or better quality than you enjoy now.

That's it - one box (no cables is another massive bonus and a big cost saving too) and a pair of good speakers chosen to work well in your particular room. or

 

[Torin Krell]

That's really not a good idea or recommendation.
It's well known that the Pioneer SX 1280 is only designed for 8 ohms and non-critical speakers and can't handle 4 ohms.
Tricking the protection circuit with a cooling system could lead to serious damage to the receiver and speakers.

I can understand while some may feel this way however after pushing amplifiers in clubs and PA systems beyond usual parameters I have found that additional cooling (usually through shrouded fans) can significantly and safely enable an amp to work into lower impedances without longterm reliability issues. In fact way back in college we added a fan to a similar Pioneer receiver whenever we pushed it hard for our parties with no ill effects. Fans do not "trick the protection circuit" but rather keep the amp from passing beyond its SOA for heat dissipation. To pass previous FTC RMS specifications, and especially to certify for 4 ohm loads, some amp and receiver manufacturers would simply add additional heatsink area or fans. Phase Linear and other amp manufacturers used to sell official shrouded fan kits for their amps so this useful strategy is well known in the engineering community. Of course things depend on the actual severity of the impedance curve over the full frequency range of these RS-2.5 speakers (which can be compounded by their aging condition) and how loud one wants to play them. It is true however that pushing vintage gear hard when repairs are often challenging may not be prudent. Just like babying a vintage car when one takes it out for a drive there is some wisdom in being gentle with old receivers which may have aging parts that are no longer 100%.

 

[boXem]

Please don't take this personally, but I'm giving recommendations for real music listening, not for measurements with square-wave signals.

"Please don't take it personally" + sarcasm = classy. The usual internet equation. Anyhow...

For normal music, a ratio of 1/4 to 1/8 for sine power/peak power to power supply is assumed. This is more than sufficient.

Care to explain a bit more? I guess you understood yourself when typing, but this means nothing to me.

The A7 uses a TPA3255 as a PBTL for each channel. Many people underestimate this power, especially since it's only a small amplifier.
Many people use amplifiers like the PA7 or A7 with power-hungry and impedance-critical speakers very successfully.

How loud do they listen? Which music style? In what kind of room?

Perhaps you overlooked the maximum power measured by Amir. These measurements were also taken with the standard power supply.
I would like to see the speaker that allows you to turn the A7 up to the maximum in a normal-sized room without damaging your hearing.

View attachment 463471

I didn't overlook this metric and noted that this is for a single channel. It looks like the power supply is the limiting component, so we can assume that in stereo mode each channel will output 200ish W in 2R. To compare with modules that I know very well, that's a bit better that an NC252MP.
For my room, my speakers, my listening tastes, an NC252MP is more than enough in terms of power. But I have witnessed NC252MP based amps getting out of steam at very reasonable levels with very normal speakers in well damped rooms. An NC252MP is not an universal solution, neither is a single 3E A7.

Still to compare with my own experience and the hundreds of feedbacks I received, the 2R output power in mono mode is close to the one from the 1ET400A with a proper power supply. I heard only one or two complaints of missing current delivery with speakers that had a messy impedance. So A7, why not (pending proper cooling), but one per Infinity RS-2.5, not one for two.

To finish on this, it's quite easy to calculate the listening levels reached knowing the listening distance, speaker minimal impedance, speaker efficiency and amplifier voltage/current capability. Would be nice if ASR members were giving an answer like "such amplifier will allow you to reach that many dB with your speakers" instead of promoting the flavor of the month as a generic solution.

Damping factor >100 is meaningless with passive speakers anyway . You practically have to bolt the amp directly to a driver in an active design to utilize "better" damping factor , but in such design you use extensive eq anyway and can fine tune the mechanical damping together with amp ,so it would be meaningless even then .

Any kind of speaker wires and/or series inductance in the passive filters dominates over most modern amps output impedance ( DF is simply a ratio between 8Ohm/"output impedance" )

Don't want to enter the debate around the importance or not of high damping factor, just asking for data to backup a claim that is technically very dubious.

 

[Torin Krell]

One of the pleasures of using a powerful vintage receiver is its nostalgia, elegance and simplicity. Perhaps instead of wiring in an extra amplifier it may be best to find a different and even better speaker system with a reasonably flat 8 ohm impedance. Using a new amp with a vintage receiver, to me, seems like missing the point of using vintage gear. I would suggest system matching over system patching.