Amplifiers known to be reliable for <4ohm speakers

[Mr Pig]

I don't know what 'small tweeks' Naim made, but to my ears 135's were far better sounding than the 250, although I wouldn't recommend them for difficult to drive speakers, they kept cutting out partnered with my Obelisk's if I decided to up the volume a little

You would expect them to sound better at twice the price! I've heard them many times but I've never heard a straight comparison between them and a 250. A friend uses 135s and a Klout in rotation but I think he marginally prefers the 135s. He had a 250 before and has an 82 pre.

Strange one. Years ago I went to a few Naim dems at the local dealers, Stereo Stereo in Glasgow. One of them was the original CDS I think and I happened to be there a bit early and was padding around while the Naim guys were still tweaking the system. They tried both 135 and 250 in an active system and decided to go with the 250s as they thought they sounded better. The same thing happened on the dem of something else, I can't remember what it was but they used 250s instead of 135s. I couldn't understand that at the time and still can't. I've not heard anyone who owns 135s say that they felt the 250 was better but my takaway is that they must be very close or there is no way you could prefer the 250.

If I remember correctly, the 135 has the fan, different earthing and some tightening up of the power supply. The big bits are all the same as the 250, minus a channel.

The 250 and 135s are not that powerful, the idea that they are is a myth. The 135 holds out slightly longer, it has a fan, but they both run out of puff pretty early. What I found was that the 250 got edgy and hard sounding long before the trip shut it down. I didn't have any complaints about the sound quality at lower volume but crank it up and it got nasty. With easier to drive speakers I guess it wouldn't be a problem.

The 105s are different. Yes, you've got about 40wpc more but you can still shut them down. I haven't done in for years as it takes night club volume but the main difference is that they still sound clean right up until the point where they shut down. At lower volume there is not much between them with the 105 sounding a little sweeter and more tuneful but they're very similar. I think later Meridian amps veered off a bit as you don't hear much about them but the 103 and 105s are good amps.

 

[Darth Vader]

Well, not quite so obvious to some may be the size of the transient peaks that can give sources like a well recorded haprsichord a fairly high peak/mean ratio.

This is what I have experienced and have been saying for a long time. Its those very fast but short transient peaks that give instruments their characteristic timbre and appeal.

I had at one time a NAP250 and a pair of NAP135s (which I had fettled by LesW to his stage 3) but neither could bring my old Shahinian Obelisks (now Obs 2)to life. However using a beefy Meridian amp The thing I noticed on the first opening track was the sound of a triangle - it was though it was just in front of me sparkling and tinkling away and just so very real. How much power did that need? However my Naim amps left it for dead.

Needless to say this amp and speaker combo has handled everything thrown at it. A church pipe organ is superb from its little tinkling pipes down to a full blown all stops out lower chords that you feel through the house foundations and yer bones.

Recorded instruments now have timbre and sound real. We have a piano in another room and it wasn't until I had this amp that I heard realistic piano through my HiFi. A piano can generate around 0.4 to 0.5W at full pelt. My speakers are 0.5% efficient (based on published figures) so I'd need an amp that can deliver undistorted 800 to 1000W for a piano at full pelt and thats why recorded piano now sounds very close to my joanna in the next room.

DV

 

[dalryc]

I've got a McIntosh MC275 driving Linn Saras (originals, passive).
They are connected to the 4 ohm speaker terminal (their are also 8 & 16 ohms terminals).

I've not had any issues with them - does the fact that tubes seem to have impedance specific terminals help in any way?

 

[Jim Audiomisc]

Wow 60C is way low! About 80-90C ish is typical of last resort thermal cut out on heatsinks.

My memory may well be in error. But IIRC we fitted 60C cutouts to the 732 because either the USA or Canada mandated this for any external metalwork. The cutouts reset when the temperature cools down, and the amp comes on again. But I've only managed to get it to trigger in a bench test.

FWIW IIRC the 732 I got second-hand had been used, bridged, by someone as a guitar amp. I can only imagine the power levels they got! 8-]

 

[Jim Audiomisc]

The ATC manual refers to the heatsink at 60 degrees. It says this in the manual. https://atc.audio/wp-content/uploads/2013/08/CDA-2-SIACD-Rev-C.pdf

Excessively high operating temperatures are potentially very damaging. The SIACD heatsink is fitted with a 60°C thermal switch and if the heatsink temperature exceeds this limit, the unit will shut down. Excessive operating temperature is only likely if the load is too great (speaker impedance too low) or, if the ventilation is not adequate. It will only be possible to restart the unit after enough time has passed that the heatsink temperature has fallen well below the 60°C threshold

Hence my question re amps able to operate with a tricky load (low ohms)

Just for info, There is no visible heatsink (ie fins) externally on my ATC unit - must be buried!

If you have good heatsinks reaching those temperatures shouldn't really happen. I had to run 1/3rd power contnuous sinewave for about 20 mins with the 732. Music isn't like that. (OK, I'll exclude Def Lep, etc, from that definition of music. 8-] )

 

[Jim Audiomisc]

This is what I have experienced and have been saying for a long time. Its those very fast but short transient peaks that give instruments their characteristic timbre and appeal.

I had at one time a NAP250 and a pair of NAP135s (which I had fettled by LesW to his stage 3) but neither could bring my old Shahinian Obelisks (now Obs 2)to life. However using a beefy Meridian amp The thing I noticed on the first opening track was the sound of a triangle - it was though it was just in front of me sparkling and tinkling away and just so very real. How much power did that need? However my Naim amps left it for dead.

Reading things like the above make me sigh with regret that so few people had a chance to hear the Armstrong. Alas, it was banjaxed by 'the reviewers' and their cluelessness. cf

https://www.audiomisc.co.uk/Armstrong/reviews/finale/1982.html

if you've not already read it. Heaven knows how many hours I spent staring at a scope screen as I listened to music driving 'difficult' speakers, etc. Oh well, decades later at least it gave me a chance to expose the unreliability of some reviews. Punters, take note.

 

[G.smith310]

I maybe wrong , but Denon market their higher end amps with 4ohm reading as their headline. That says to me they are confident with their ability to handle 4ohm…. Or maybe their are just being less careful. Either way, Denons sound excellent and handle Isobarik Neats well

 

[Arkless Electronics]

My memory may well be in error. But IIRC we fitted 60C cutouts to the 732 because either the USA or Canada mandated this for any external metalwork. The cutouts reset when the temperature cools down, and the amp comes on again. But I've only managed to get it to trigger in a bench test.

FWIW IIRC the 732 I got second-hand had been used, bridged, by someone as a guitar amp. I can only imagine the power levels they got! 8-]

60C is the figure I've always known as max legal temp for exposed metalwork yes. Musical Fidelity A1 top plate runs at approx 72C! As I said though 80 - 90C ish are about the most common safety cutout figures IME.

 

[Arkless Electronics]

I maybe wrong , but Denon market their higher end amps with 4ohm reading as their headline. That says to me they are confident with their ability to handle 4ohm…. Or maybe their are just being less careful. Either way, Denons sound excellent and handle Isobarik Neats well

A common "cheat"! It makes the amp seem much more powerful. Almost all amplifiers are spec'd to drive 4R loads.

 

[davidsrsb]

Most Yamaha amplifiers have a 4/8 Ohm selector that reduces supply rails to keep everything safe into the lower impedance loads.
The bigger Quad amplifiers can handle 4 Ohm with plenty of power

 

[Arkless Electronics]

I've got a McIntosh MC275 driving Linn Saras (originals, passive).
They are connected to the 4 ohm speaker terminal (their are also 8 & 16 ohms terminals).

I've not had any issues with them - does the fact that tubes seem to have impedance specific terminals help in any way?

A valve amp can have secondary taps to drive any load required...1 Ohm... 0.5 Ohm... no problem. This is NOT because they have better "load tolerance", "current reserves" etc etc by any means. The output transformer can be thought of as a gearbox and when driving say 1 Ohm (yes a very few amps had this for Apogee Scintillas IIRC) it is in a very low gear. A 30WPC valve amp can still only provide 30W no matter what load but it is "geared" for a specific load. It will give far less power if then used with the wrong impedance load ie a SS amp my be rated at 30W @ 8R but may give 56W @ 4R and 90W @2R but a valve amp of 30W set to 8R tap will much less power and higher distortion if used with 4R load.
A transformer can also be used with an SS amp to do the same job (a very few SS amps have output transformers like valves but it's not a good idea so is rarely used... Quad 50E, a few McIntosh models etc) and probably the most common example is the step up transformer in ESL speakers. Going in the other direction, I believe early Apogee Scintilla's had a matching transformer which could be used to drive their 1R load as a 4R load.

 

[Arkless Electronics]

Most Yamaha amplifiers have a 4/8 Ohm selector that reduces supply rails to keep everything safe into the lower impedance loads.
The bigger Quad amplifiers can handle 4 Ohm with plenty of power

Some of them yes and not only Yamaha.... it was "a thing" for a while on quite a few Japanese amps and a money saving cheat really. If you can switch the supply rails to a lower voltage so that it gives say 60W @ 8R on higher setting and 60W @ 4R on lower setting (rather than increasing to say 95W @4R if left on higher voltage setting) then money can be saved on mains transformer, output transistors and heatsinking.

 

[ToTo Man]

IIRC the 25wpc ML2 will do about 100wpc into 2R. The high voltages required to give 200WPC+ into 8R puts the output transistors under a lot of stress when giving the currents required to drive <4R loads, much more so with particularly awkward low impedance loads with big phase angles.

So, if I've understood correctly... a SS amp that's rated at 200wpc @ 8Ω stresses its output transistors more than an amp that's rated at 20wpc into 8Ω irrespective of the amount of power actually being called for by the programme material, because the supply rails are running at a higher voltage? Does this apply to both Class A and AB topologies?

 

[Cesare]

Unless i'm totally mistaken, isn't 4 ohms a totally normal load for amplifiers? I would be more interested to know if anyone can point at an amplifier which *doesn't* work driving 4 ohm speakers. I think the idea that speaker loads were 8 ohms stopped somewhere in the 70s, and amplifiers have been designed with the expectation that loads will drop, maybe not as low as 2 ohms, but certainly in the 4 to 6 ohm range.

 

[Arkless Electronics]

So, if I've understood correctly... a SS amp that's rated at 200wpc @ 8Ω stresses its output transistors more than an amp that's rated at 20wpc into 8Ω irrespective of the amount of power actually being called for by the programme material, because the supply rails are running at a higher voltage? Does this apply to both Class A and AB topologies?

Not necessarily no. Many factors come into it!! The most important is a phenomena called secondary breakdown.
Headline figures for power transistors of 200W, 20A and 250V are relatively common but what they don't tell you is that such a transistor would only be safe up to about 1A @ 100V!! 20A yeah but only at maybe 10V and 250V yeah but only at say 0.1A.... The Safe Operating Area (SOA or sometimes SOAR) graph must be studied to reveal the truth...

Highly reactive loads are the worst case as voltage and current can be out of step with each other and hence the transistor can be asked to deliver high voltage and current at the same time, which doesn't really happen with a resistive load.... unless it it gets too low in resistance of course!

The above is why protection is so important in many amps, why the Quad 405 MkI was limited by its protection from driving 4R and less loads well, why the Quad 303 gives 45WPC into 8R but only 25 into 4R, why it's a very bad idea to operate Avondale modules at as high a voltage as many people do and why muscle amps (Krell, Levinson etc etc) appear to have way more output transistors than seems necessary.

Obviously if you have a silver plated mains lead that was blessed by a shamen on a full moon non of this matters of course as that sort of thing completely outweighs mere electronics....

 

[Arkless Electronics]

Unless i'm totally mistaken, isn't 4 ohms a totally normal load for amplifiers? I would be more interested to know if anyone can point at an amplifier which *doesn't* work driving 4 ohm speakers. I think the idea that speaker loads were 8 ohms stopped somewhere in the 70s, and amplifiers have been designed with the expectation that loads will drop, maybe not as low as 2 ohms, but certainly in the 4 to 6 ohm range.

Yes it's a normal load. You will often see complete exaggerations from audiophiles in as much as an amp that sounds a tiny bit worse into 4R will often be claimed "not to work" with such a load...

Even something like a MkI Quad 405, which "infamously" was designed for best results into 8R would work fine into 4R at what most of us would regard as domestically acceptable volumes.

SS amps work best into higher impedance's hence THD etc is worse at 4R than at 8R and damping factor halves with load impedance.

 

[John Phillips]

... Highly reactive loads are the worst case as voltage and current can be out of step with each other and hence the transistor can be asked to deliver high voltage and current at the same time, which doesn't really happen with a resistive load.... unless it it gets too low in resistance of course! ...

A graphic example of which is in Fig.4b on @Jim Audiomisc's Society for Cruelty to AMPlifiers page.

There we see two trajectories over a single sine-wave cycle of voltage and current seen by an output transistor, compared to its SOA envelopes. One trajectory for a 4 Ohm resistive load (zero phase angle) but more stressfully to the amplifier one for a load of 5.65 Ohm at 45 degrees phase angle.

The OP asked about reliable amplifiers and one of the key things for playing loudly through insensitive real-world 4 Ohm 'speakers is enough heatsinking (and a thermal cut-out).

I have not seen the insides of the OP's ATC SIACD. However the outside photographs show a grille at the rear which will be above an internal heatsink. The internals of other ATC amplifiers as seen in online photographs (e.g. the SIA2-150 here) do have heatsinks of a good size under similar-looking grilles so my inclination would be to trust the SIACD. Note that ATC is known to bias its power amplifiers at a higher than normal standing current so you can expect them to be fairly warm even at no load.

 

[Jodet]

Exposure is good into 4 ohms.

 

[Arkless Electronics]

A graphic example of which is in Fig.4b on @Jim Audiomisc's Society for Cruelty to AMPlifiers page.

There we see two trajectories over a single sine-wave cycle of voltage and current seen by an output transistor, compared to its SOA envelopes. One trajectory for a 4 Ohm resistive load (zero phase angle) but more stressfully to the amplifier one for a load of 5.65 Ohm at 45 degrees phase angle.

The OP asked about reliable amplifiers and one of the key things for playing loudly through insensitive real-world 4 Ohm 'speakers is enough heatsinking (and a thermal cut-out).

I have not seen the insides of the OP's ATC SIACD. However the outside photographs show a grille at the rear which will be above an internal heatsink. The internals of other ATC amplifiers as seen in online photographs (e.g. the SIA2-150 here) do have heatsinks of a good size under similar-looking grilles so my inclination would be to trust the SIACD. Note that ATC is known to bias its power amplifiers at a higher than normal standing current so you can expect them to be fairly warm even at no load.

Good example John yes

IIRC (and I could be wrong) ATC use a Hafler like "swinging PSU, grounded output stage" topology and (more importantly) again IIRC they use old style lateral mosfets, which are the closest to an indestructible output device you can get. They like to run at least 100mA quiescent so do run warm yes.

I once had a prototype amp with lateral mosfets of the type I believe are used in the ATC go unstable on me and the mosfets got so hot so quick that the solder melted on the two pins of the To3 cases and the wires dropped off! Not the type of protection I had in mind (though Leak used it on some valve amps!) but don't try this with normal transistors as they would definitely fail. They still worked perfectly after this!

 

[Arkless Electronics]

Exposure is good into 4 ohms.

Not especially no. Varies a lot with model and age as they switched output transistor type at least 3 times plus "regulated" (they're not. They are stabilised) can get very hot driving 4R loads. A single pair of output transistors and driving 4R to powers much over 130W ish do not go together well.