Bigger amps and <4 Ohm designs

[fjhuerta]

I never really gave much thought about the differences between amps, since I'm not really convinced that different amps can sound *that* different (you know, the Julian Hirsch school of thought)...

Anyway, someone offered me a pair of Channel Islands D-200 amplifiers for a great price, and since they are 2 Ohm stable, I thought it was a good opportunity to get a nice pair of monoblocks. I wasn't expecting any sonic difference between my current amp (an old Adcom GFA-5400) and these....

I've been using the amps for about two weeks now, and I'm a bit surprised. As much as I thought there would be no difference whatsoever, I can't help it but think the newer amps have quite a bit more bass and midbass response. I'm using them with my Monolith design (two Peerlesss 8" Exclusives, 1 Seas 27TBFC). Impedance drops to 3.5 Ohm at places.

Am I correct in thinking that when using speakers with less than 4 Ohm impedances, hefty amps can make a bigger difference?

[joecarrow]

I think you're very correct. The shortcoming of wimpier amps isn't that they don't reach the required voltage for a given signal- it's that they can't hold that voltage up while supplying a lot of current.

There's another side to this- the transient power requirement of an audio signal. For very brief moments, a signal can be 20 db or more above the "average", and so the amp has to be able to handle that. A lot of it has to do with the power supply, and there are a good number of people who have modified their amps to improve the power supplies.

It's not just low impedance speakers either- I was using the Modula MT (easy load!), and I could definitely hear an audible difference between a Hafler 500 (capable of 500+ watts into well below 4 ohms) and my present receiver, a Panasonic XR-55. The Hafler sounded like the bass was better, like the percussive sounds "hit harder". I could have been fooled by the increased dynamic range of the Hafler- although I was listening at levels that shouldn't have taken more than 10 watts on average, those peaks I was mentioning very well could have been going past my receiver's capabilities. It could have purely come down to watts, but I have a feeling that current supplying capability makes a difference as well.

[Curt C]

There is much more to a good amp design than how much power it will output into 4 ohms. To use Joe’s example an amp running 10 watts average, and then faithfully reproducing a 20 dB transient will require 1000 watts during that transient. Not only do the output devices have to be able to pass that current, but also the power supply will need to supply 65+ amps (per channel) assuming a 4 ohm load. Obviously you are not going to pull this instantaneous current out of the 15A wall outlet. That is the duty of the filter caps to supply this short term current. The power transformer also needs to be large enough to quickly resupply the caps. Big torroidal transformers and large banks of filter caps are a significant cost and take up valuable space, so many mfg’s will cut corners here.

Consequently, you could easily have two amplifiers ‘sound’ different solely due to their power supply design...

Likewise, a low THD in itself does not adequately describe an amplifiers dynamic distortion performance, so it is entirely plausible for significant differences in two amplifiers to be audible even though they have very similar or identical distortion specifications.

-And note I didn’t use the work subjective even once… :W

C

[servicetech]

I'm surprised that power amps haven't gone to the much more efficient switching power supplies like every other piece of electronics now uses. The transformers are 1/10th the size due for the same power to the higher frequencies used. As an added bonus they generate much less heat. Car audio amps have been using them since the 80's.

[whoaru99]

There is much more to a good amp design than how much power it will output into 4 ohms. To use Joe’s example an amp running 10 watts average, and then faithfully reproducing a 20 dB transient will require 1000 watts during that transient. Not only do the output devices have to be able to pass that current, but also the power supply will need to supply 65+ amps (per channel) assuming a 4 ohm load. Obviously you are not going to pull this instantaneous current out of the 15A wall outlet. That is the duty of the filter caps to supply this short term current. The power transformer also needs to be large enough to quickly resupply the caps. Big torroidal transformers and large banks of filter caps are a significant cost and take up valuable space, so many mfg’s will cut corners here.

Consequently, you could easily have two amplifiers ‘sound’ different solely due to their power supply design...



C

In general, I do agree that amps can sound different because of their current/voltage capacity - provided the load makes use of this difference or one drives the amps beyond the capabilities of the lesser.

Something else though...

A 15A circuit will supply many times that on an short-term basis. The typical Square-D 15A breaker will supply approximately 2.5x to 4x it's rated current for about 5 seconds according to the trip curves. If time is dropped back to 1 second, the current goes up to about 5x to 10x rated before trip.

[whoaru99]

I'm surprised that power amps haven't gone to the much more efficient switching power supplies like every other piece of electronics now uses. The transformers are 1/10th the size due for the same power to the higher frequencies used. As an added bonus they generate much less heat. Car audio amps have been using them since the 80's.

Many do, especially in the pro audio world.

My QSC PLX 3402 (1,100 wpc @ 4 ohms ~21lbs) has a switching power supply as do my two Crown CE4000s (1,200 wpc @ 4 ohms ~30lbs).

[ch83575]

Linn is one of only "audiophile" manufacturers that use a switching supply with a standard class AB output stage (at least that I know of). When they were making the switch (mid -> late 90s ?) there were a few products you could take to your Linn dealer and have the big toroid ps replaced with their switch mode supply... the Kairn preamp comes to mind. When you got it back the difference was night and day, and in a pre there aren't even the huge current demands like in power amps. Now, I am pretty sure, every product they make uses a switch mode supply, and they sound pretty good in my opinion.

-Chad

[pedroskova]

High efficiency loudspeakers largely do away with this need for megawatt power.

To use Joe’s example an amp running 10 watts average, and then faithfully reproducing a 20 dB transient will require 1000 watts during that transient.

8O 8O 8O 8O

10 watts is close to the full output of my PP 6V6 amp. Lucky for me, my 95bB/w/m Infinity's (with powered sub) only need ~0.5W for my listening habits. Orchestral crescendos don't seem to be a problem.

[Gir]

I'm no expert by any means, but I believe another large factor in how an amp sounds is where the distortion occurs, not just how much. For instance the difference between class A/B and class A amps is that class A amps distort in the even harmonics whilst the A/B amps will distort in both even and odd harmonics (if memory serves). As I've come to understand, even harmonic distortion is more pleasant to the ear, thus many class A amps sound better than the A/B equivalents.

Now if only I could smuggle that 190 kW class A liquid cooled amp out from NASA...

[whoaru99]

High efficiency loudspeakers largely do away with this need for megawatt power.



8O 8O 8O 8O

10 watts is close to the full output of my PP 6V6 amp. Lucky for me, my 95bB/w/m Infinity's (with powered sub) only need ~0.5W for my listening habits. Orchestral crescendos don't seem to be a problem.

Hmmm... those big amps I previously mentioned? They're used with 99dB and 100dB sensitivity speakers. 8O

[servicetech]

Many do, especially in the pro audio world.

My QSC PLX 3402 (1,100 wpc @ 4 ohms ~21lbs) has a switching power supply as do my two Crown CE4000s (1,200 wpc @ 4 ohms ~30lbs).

A switching power supply also does a MUCH better job of keep the rail voltages constant. It's not untypical for the caps to be recharged 50,000 times per second vs. 120 times per second on a standard power supply. Switching power supplies constantly monitor output voltage and make instant adjustments to the power needed. They are lighter and cost less.

I guess there are still those out there that judge quality by weight, hence the continued use of the old 60hz supplies...

[littlesaint]

A switching power supply also does a MUCH better job of keep the rail voltages constant. It's not untypical for the caps to be recharged 50,000 times per second vs. 120 times per second on a standard power supply. Switching power supplies constantly monitor output voltage and make instant adjustments to the power needed. They are lighter and cost less.

I guess there are still those out there that judge quality by weight, hence the continued use of the old 60hz supplies...

Many feel that switching supplies don't sound as good, particularly at higher frequencies. You typically find switching power supplies in pro audio equipment where fidelity isn't as important as volume. That said, I've heard some very good switching power supply amps, but I still always find myself preferring the sound of old-school amps.

[whoaru99]

You typically find switching power supplies in pro audio equipment where fidelity isn't as important as volume. That said, I've heard some very good switching power supply amps, but I still always find myself preferring the sound of old-school amps.

Fidelity is very important in pro audio. However, it's easy to make it sound crappy pushing a system to it's limits. Same goes for any system though.

[littlesaint]

I've heard complaints on both sides of the spectrum. At lower frequencies you have more dynamic power requirements and a poorly designed switching supply can have issues there. I use a switching supply amp fro my sub and haven't noticed any problems, bu that's not a "pro audio" application. At the higher frequencies, I've heard people say the switching frequency can be heard (as noise) at the output terminals. Again, in the "pro" world, probably not a big deal. At home, I have heard this and it can be quite annoying.

[whoaru99]

It's not untypical for the caps to be recharged 50,000 times per second vs. 120 times per second on a standard power supply. Switching power supplies constantly monitor output voltage and make instant adjustments to the power needed. They are lighter and cost less.

QSC says 230,000 times per second for their "PowerWave" switching supplies such as that in the PL and PLX series of amps. :E

[littlesaint]

Fidelity is very important in pro audio. However, it's easy to make it sound crappy pushing a system to it's limits. Same goes for any system though.

I generally think of pro audio as live performance applications where it's hard to have high fidelity no matter what equipment your using.

[whoaru99]

Guess that depends on whether you're looking for studio fidelity or live performance fidelity.

[servicetech]

You would think if switching supplies would make a difference you could hear that you could SEE the effect of any ripple/poor regulation in video displays. I've never seen a the effects of a switching supply in the video which is MUCH more sensitive to interference than audio. Of course if a cap is going bad in the supply all bets are off.

[littlesaint]

You would think if switching supplies would make a difference you could hear that you could SEE the effect of any ripple/poor regulation in video displays. I've never seen a the effects of a switching supply in the video which is MUCH more sensitive to interference than audio. Of course if a cap is going bad in the supply all bets are off.

Not so much ripples as just noise in the signal at the switching frequency and its harmonics. I'm really splitting hairs on this one as the frequencies are usually not in the audible spectrum, but I have heard designs where it is recognizable once someone points out what to listen for. Don't you hate when someone does that. :B

[servicetech]

Yeah, just like people who can tell the difference between 16ga zip cord and $100 speaker wires. Guess my ears just aren't that good.

[fjhuerta]

Thanks for all the opinions.

After even more listening with different speakers, I'm completely convinced I'm not imagining things. The CI amps really sound completely different in the bass than the Rotel RB-1070, the Adcom GFA-5400, and the Little Dot T_150 (the amps I used before).

It's not only quantity - it's "control", for the lack of a better word. The bass has a bit more accuracy, or definition. I suppose that the other amps were a bit more current limited than the ones I have now.

I have to admit, though - I heard no difference when using my Monitor Audio speakers. I suppose higher impedance speakers don't impose such heavy demands on amps.

[kevmurray]

I had a pair of KEF Uni-Q speakers which were 4 ohm so I wanted to see how they'd sound driven with a car audio amp. The amp was advertised as being good down to 2 ohms and was rated at 100wrms/ch. Compared to my 35wrms/ch Harmon Kardon it was crap. No bass. Now I know the HK was designed right, the car amp I guess was not. I got rid of the car amp long before I knew what I was doing around electronics so I guess I'll never know why. I just remember the sound was undistorted but had no bass.

Regarding the SMPS issue, many manufacturers would rather stick to the reliability of a linear supply. Switchers have so many parts they are notorious for failing early.

[Dennis H]

FWIW, JonMarsh designs SMPS as part of his day job but he prefers plain old linear supplies for audio. I think the biggest issue is HF/RF noise. SPMS generates a lot of it and it's hard to keep it from getting into the audio signal and/or back into the power cord. That said, QSC seems to do a good job with SMPS.