Digital Amplifier Questions

I've been wondering the following:

1. Do digital amps benefit from being left on 24/7in the same way the ss amps do?

2. Is plugging a digital amp into a different outlet as your preamp as crucial compared to a ss amp?

3. Do digital amps have small transformers? If so, is distancing equipment from one another less important? (My peachtre Nova Pre/220 look best stacked)

4. When upgrading power cords on a digital amp, do you look for cords that do well with digital gear like your preamp and dac, or do you still go for a beefy cord?

I apologize ahead of time for my ignorance on the matter! Thanks!
B limo,

By 'digital amps' do you mean class d amps? 'digital amps' is a bit of a misnomer probably persisting because people believe 'class d' refers to digital, but it was just the next letter in the alphabet used to designate various types of audio amplifiers using different methods of amplification. Class A defines amplifiers that use a more traditional circuit that has input devices that constantly remain 'on', resulting in excellent sound but produce excessive heat as a byproduct. Class D amps use 'switch mode power supplies' (smps) that are able to switch on and off extremely quickly, which produces much less wasted heat as a byproduct. I use a pair of class d mono-blocks that sound very good, output over 1k watts and barely get warm to the touch no matter how hard they're driven. I leave them on 24/7 since they only require a few watts of electricity at idle.

I'm no expert on amplifier design, so others may need to answer your other questions. So, if by 'digital' you meant class d, then yes I would advise to leave them on constantly. I thought it would be useful to clarify your question before your thread progresses.

Let us know,
I can only offer my experiences in my case. Different products may always behave and perform differently.

Do digital amps benefit from being left on 24/7in the same way the ss amps do?

I don't notice too much difference in teh sound quality either way but I leave mine on most of teh time unless I know I will not be listening for a day or more mostly out of convenience. Power draw idle is small.

2. Is plugging a digital amp into a different outlet as your preamp as crucial compared to a ss amp?

Digital (Class D) amps have their own unique noise characteristics due to their high speed switching nature. I find using shielded ICs to be beneficial in this regard but also use unshielded ones but these tend to be more sensitive to noise issues.

3. Do digital amps have small transformers? If so, is distancing equipment from one another less important? (My peachtre Nova Pre/220 look best stacked).

Mine uses a dedicated power supply circuit board internally. No large or heavy parts involved. It works well...the amp never sounds even slightly strained or runs our of steam or clips that I have heard. Some other designs I have read about do appear to still use overbuilt large and heavy power supplies that may work well also but seems to defeat the purpose of being small and lightweight.

4. When upgrading power cords on a digital amp, do you look for cords that do well with digital gear like your preamp and dac, or do you still go for a beefy cord?

I use upgraded cords on my line leve gear to control noise but still use stock cords on my amps in that I have no clear reason to change. They seem to do teh job needed well. I might try better ones someday just to see but stock cords are not a problem as best I can tell.

I apologize ahead of time for my ignorance on the matter! Thanks!
Whoops in response to question 2, I do plug my amps into a different wall outlet from the rest of my gear mostly as a convenient way to utilize the outlets I have available. My line level gear plugs into a conditioning strip and my amps plug into a different wall outlet directly. Seems to work well. Noise levels are non existent and everything sounds most clean smooth and dynamic.
In my system pic, you will notice that I do keep my amps on their own stand physically separated from other line level gear to help insure against noise. Can only help and not hurt. One can always experiment either way to see if it makes a difference. I also keep some space between amps as additional physical isolation between the two.

ALso not that it seems there have been strides made with newer Class D amps in regards to noise levels. Mine are a generation old and are quieter than what proceeded them. The latest and greatest are likely even better in taht regard, but of course every product is different. Its just worth noting that these things can produce a fair amount of RF noise if not addressed in the design.

Also not that Class D amps are switching amps, not digital, however both use very high frequency oscillators that produce RF noise and that must be controlled, either by design or otherwise. Most modern devices like Class D amps and computers that can suffer from noise issues designed for hifi audio use tend to have these things largely in control due to their potential negative effects on sound quality but just be aware that it can be a real issue that can limit performance if not under control somehow.
I prefer Class D left disconnected at all times.
Once is enough!!
I mean class d, which I thought was some type of digital amp...I just briefly read about that lastnight. I'm going to go read up for a minute and see if I can get this...
Ok, now I remember why I can't get this...

Can someone dumb it down for me as far as Class A, Class A/B, Class D?

I really only know that Class A amps are usually lower wattage, inefficient (whatever that means aside from drawing more electricity from the outlet...), run hot, but sound good (right?).

Class A/B is some type of dual mode where it runs in Class A for the first few watts then switches to Class B? Now this doesn't really make any sense to me. Wouldn't the transition be audible?

Then Class D... some type of switching amp, where it switches on and off? This makes no sense to me either. And why the heck did I think it was a Digital amp?

Anyhow, when I go to research what a Class D amp is, or any amp/preamp/dac/electronics, my research instantly turns into me looking up 15 words in the first paragraph which leads me to look up 100 more words, trying to wrap my head around it all, then steam comes out my ears and I start twitching.

I'd like to have a better understanding of all this so maybe someone can describe it in layman's terms?

PS, Thanks Mapman for your input. I've noticed you've been raving about your Belcantos for awhile now, not quite as much as ohms, but close! Good to see you enjoying the setup
I'll try the most basic explanation.
I really only know that Class A amps are usually lower wattage, inefficient (whatever that means aside from drawing more electricity from the outlet...), run hot, but sound good (right?).
The output is always on driving both the positive and negative side of the signal.
Class A/B is some type of dual mode where it runs in Class A for the first few watts then switches to Class B? Now this doesn't really make any sense to me. Wouldn't the transition be audible?
Not at all. Class A/B has 2 outputs where one side only passes the positive part of the signal and the other for the negative side. You also just doubled the output power. Can you hear the switch over? Um no. Some manufacturers will start in class A then switch to A/B for more power.
Then Class D... some type of switching amp, where it switches on and off? This makes no sense to me either. And why the heck did I think it was a Digital amp?
The outputs are turned on and off very fast (usually in the mega hertz or way out of the audible range) and then filtered. So the output is on as much as it is turned off. But the resulting output like the input is analog (sine wave). Digital is a bunch of square wave pulses that needs to be converted to analog. With Class D the signal is analog with the power being turned on and off.

Of course it gets much more complicated than that but it's my most basic understanding of the different classes of amplification.
Xt explained it pretty well I would say.

THe Wikipedia article is a good one and highlights key advantages and disadvantages of each.

The main advantage of Class D is its efficiency, and often a resulting smaller package. Class D amps are a practical alternative in cases where a large heavy and expensive monster class A or class A/B amp would be needed for best results otherwise due to speaker power and current demands. Its a common case these days that smaller inefficient speakers with extended bass are designed to fit tighter quarters. A Class D amp fits this need well. Some good quality 250 w/ch Class D amps with decent current delivery capabilities can be had for less than $1000 these days. Most smaller ineffficient speakers capable of extended bass in larger rooms will likely benefit sonically from teh extra power and current.

In my case I find the high power Class Ds to be uniquely capable of driving the larger OHM speakers that I like, hence my enthusiasm over that pairing in particular. They also benefit my smaller Dynaudio monitors compared to other higher power SS amps I have used in the past, but the benefits are greatest whenever more power and current out of a smaller more efficient package is what the doctor ordered.
B_Limo, in a lot of cases, the answer is... "It depends"... But here are some specifics as they have applied to class D amp I have used... ROwland M312, Merrill Veritas monos, Belcanto REF100M and REF500M, Rowland M925.

Q: Leave on 24/7?
A: Yes, in all cases... Takes at least 24 hours of playing time for all class D amps in my test set to reach peak performance after being off for a spell.

Q: Plug in its own AC circuit?
A: It depends.... Amps with high degree of power supply isolation like Rowland M925 do not inject much noise back into the AC line, others do. In all cases, you may get slightly better dynamics by plugging power amps in a dedicated outlet.

Q: Stacking or not.... RF leaks?
A: It depends again on degree of isolation... Newer amps like Rowland M925 and Merrill Veritas do not leak RF... Try moving an FM radio on top of them... You will not hear any additional hash on the radio... IMO stacking is OK with these... Both amps are based on Hypex NCore NC1200 modules.

Q: What power cords?
A. Use high current capable power cords... with class D amps, instant current swings/transients can be huge. Do not use analog cords for low power applications, nor old style "digital" cords like the old Shunyata VX.... They will "caramelize" the sound. Note that some cable manufacturers might have today new styles of digital-ready PCs that handle high current class D amps... E.g. Check with the Shunyata factory for their recommendations.

Let me know if you have further questions. G.
One other thing I would note about Class D amps is that they typically seem to have very high damping factors. That will play well with larger speakers taht can benefit with more control of the driver movements as a result of the high damping factor. My large OHm F5s use a 12" walsh style driver known to benefit from high damping in terms of bass control and articulation and that is in fact the result.

Due to high damping, Class D amps can come off as somewhat lean in other speakers. That is the case with my smaller Triangle monitors in particular. I have those set up accordingly to compensate and the results are still quite top notch. My Dynaudio monitors are more in between. I like the clean taut articulate bass that results with them as set up (see my system pics) but I could easily see where some might consider the sound a bit too lean despite the fact that the bas is all there and very detailed and accurate. it just won't be shaking the rafters, at least on most recordings.
OK Guido, now you've accelerated my desire to add a pair of pangea ac9 cords to my amps to go with the AC14ses on my line level gear.

I guess it never ends. :^}
B, I think excellent recommendations from Guidocorona. I know he has a big interest in Class D amps and probably has actually heard and spent time comparing the most of just about anyone I am aware of in these parts.
Hi Mapman, what is fascinating is that the NCore NC1200 modules do not seem to leak RF into the air regardless of power supply design.... Merrill Veritas use the basic non regulated Hypex SMPS that matches the power conversion module... While Rowland M925 uses a custom massive multi-regulated SMPS.... Neither amps generate any extra noise in an FM tuner operating near them.

Perhaps Tim can do the test on his D-Sonic powered by the AbleTech module and tell us results.

All, please note that if some RF leak occurs, that does not mean at all that the particular class D amp does not generate good music... the ICEpower ASP1000-based Bel Canto Ref1000 Mk.2 and Rowland M312 remain absolutely marvelous amps... Just do not stack an FM tuner just on top of them.

That's interesting to hear.

I know the technology continues to evolve and noise isolation is an important part of the overall equation.

I find with the BC ref1000m, with just a little physical isolation from other components, as seen in my system picture, the only noise issues I encounter is with unshielded ICs. The DNM Reson ICs are my preferred ones in my main system and I am able to use those to good effect from pre-amp to amps, but I do have to play with the physical location/path of teh wires to avoid RF noise. With other more common shielded ICS, it is not an issue. I With things set up properly, I can put my ear up to the isotar tweeters in the Dynaudios and hear nothing with no music playing. So its just a matter of proper setup to provide teh extra touch of isolation needed, which is never a bad thing to consider doing.

Any familiarity with the Class D audio amps? These seem like they must be a great bargain. If teh noise isolation is up to par with the latest and greatest as well, that might just be the icing on the cake.
Hi Mapman, no I have never had an amp manufactured by Class D Audio in my system.
Every component that I've ever had, preamp, amp, dac, etc, has benefited sonically by leaving it on. Even tubes, I eventually turned off my tube gear to save on life, but on long lasting pre amp tubes, I did experiment over a few weeks never turning them off and you could hear it.
Yep... Worth pointing out that my old Rowland M7 monos, which were almost class A amps, reached best performance after one week (yes, that is 7 days) of operation... Tried that one fine Christmas break... The creatures sucked up 720W per side for some three weeks... By the end of the first week they did sound really golden... When wifie received the AC bill she almost threw me out of the house.

May Gaia bless class D... Rowland M925 monos are on 24/7, and I never heard one peep of complaint from Mrs. Better Half.

Have you had any experience with the Peachtree 220? If not, maybe you could pull up the specs and let me know what you think?

Mapman, Guidocorona, Timlub and Xti, thank you all for your helpful and informative posts! I appreciate it and hopefully others will benefit from this also.

Ps, Mapman, I researched Bel Canto last night and a pair of the Ref500m monoblocks and a Dac 3.7 or Dac 2.7 looks like that be an awesome system. You've got me interested in Bel Canto now :)
There's also a pair of Ohm Walsh 4's for sale local to me; if I had the $ for them I'd go listen to them.
If you look at second hand OHMS, be sure to identify what version/generation drivers on them. They all tend to look similar on teh outside but each version will sound differnet with the most current ones being the most refined. Mine are series 3 which is one generation old now (current sereis are designated X000) and superceded original X series (including Walsh 4's) and mkII series. Also custom mods to all of those have been done over the years. So unfortunately it can be very difficult to know for sure sometimes unless the source is trusted or maybe John Strohbeen at OHM can help confirm.
Also with Bel Canto, newer ref1000m and ref500m include teh BC custom power board internally as well as custom input board with higher input impedance to work better with tube preamps. These make a difference and go for a premium accordingly. PRior ref1000 and ref500 models were basically stock Icepower modules in a nice box and can be had for much less accordingly. Also I believe 500m uses a newer version of Icepower than 1000m last time I checked about a year ago FWIW.
My experience has taught me that there is a vast difference in the approach that some manufacturers have taken with Class D technology versus others. An example would be Rotel vs Jeff Roland. Albeit, that we are talking about two companies targeting different market segments, but spending time with each has caused me to love one and despise the other. I'll let you guess which is which!

And yes, it loves to be kept on all the time.
RF leak thru power cord depends on the type of power supply. Rowland's supplies have zero current switching and power factor correction presenting load similar to resistive. No narrow current pulses. Ref1000m, a second generation uses also power factor correction. In contrast linear power supplies are, in reality, a very primitive switchers operating at 120Hz. They produce a lot of electrical noise since switching is done at max voltage (producing narrow current spikes of high amplitude). Higher switching frequencies of SMPS are easier to filter. Linear supplies in power amps require a lot of capacitors not only to filter out 120Hz but also to reduce voltage drops since they aren't regulated (line or load). For that reason Jeff Rowland uses zero current switching (+PFC) supplies operating at 1MHz, even in class AB amplifiers (Models 625, 725).

One comment on Damping Factor. Speakers are mostly resistive meaning that 4ohm rated speaker will measure likely 3ohm with DC ohmeter. This resistance is in the circuit limiting effective DF to less than 3. Adding 1/10 (0.3ohm) equivalent to DF=27 should not affect the sound. Above that it is only spectmanship. In class D amps very high DF, especially for low frequencies, is caused by the fact that one of the output switchers (Mosfets) is always connected to low impedance point (GND or Vs), making low output impedance to start with. Feedback reduces it even further.

Also, transition from A to B in class AB is audible - it is only matter of extend. It is because trans-conductance (Iout/Vin) is different for one transistor (class B) operating at given time vs two transistors (class A). It is called "gm doubling" and it adds to distortions in class AB amplifiers. It is also the reason why extending area of class A operation (increasing bias) increases distortions.

I had a Peachtree 220, 220se, and Grand Integrated. The 220se has the best overall sound. With the exception of the Grand, initially I enjoyed the sound of both the 220 and 220se... I never liked the Grand. After a couple of months, to my ears the Peachtree had a thin sound and something just never sounded right.
Recently I've listened to a friends Jeff Rowland 125 and with the exception of bass impact, it was one of the best amplifiers I've ever heard. I confess that's the only amplifier that I've had in my system that presented a three dimensional image.
Kijanki...what a great response. Hearing and tech analysis agreeing on something. What a concept!
Hi Richard, I suspect that the perceived lack of bass impact with Rowland M125 in your system might have been a meer lack of overall power rather than an architecture issue in the device... Assuming that the overall size of images and stage was not too far from desired when using a single unit, If you used a pair of M125s in bridged configuration you would likely to achieve correct base response, and overall excellent imaging and staging proportions. G.

I agree it was just a lack of overall power(my room is 20'x26'). The image size and stage was the best I've ever heard. We had this conversation privately. My plans are to get a 525. I really love the Jeff Rowland sound.
Hi B_limo,

I've got the Peachtree Grand Integrated X-1, which combines the pre, amp and dac, so questions 2 and 3 won't apply.

For question 1, I honestly don't hear sound difference between sitting down to listen after the unit has been powered for 10 minutes vs. 5 hours, so I shut mine down most of the time when not in use just to save electricity.

For question 4, before I bought the Peachtree I called them up with some questions. I don't remember whether I asked them, or they asked me what my plan was for the power cord, but the question came up and I said that I was just planning to use the stock cord it came with. Their answer, and I'm only paraphrasing minimally, was "good, don't waste your money on a power cord, the one it comes with is all you need". That's just what they told me, so use that information however you like.

Just for kicks, I have tried the power cord from my Audio Research integrated and I can't hear a difference so I just use the stock Peachtree cord. For what it's worth, the Grand X-1 sounds as good as the Audio Research - my wife and I can't tell them apart sonically.

I hope you're enjoying your Nova Pre/220 combo as much as I enjoy the Grand X-1. Peachtree makes some good gear.
Hi Richard, if a single M125 created a great stage and images in your large room, M525 is sure to be amazing for those parameters.... In addition, M525 is known for inducing a great bass experience.

Hi Bcgator! I have been enjoying the Nova pre/220 combo. It sounds pretty good but I still prefer my last setup (Classe CA-200, Paradound Z Dac, Lightspeed attenuator) more. The Volume control on the Lightspeed was way better, especially at low volumes.

I haven't heard a difference really with power cords on the Peachtree gear either, that's partly why I asked that question. I could definitely hear a difference when I upgraded my power cords on my last set up so I was wondering if it was because the Class D amps draw less current.

Anyhow, thanks again to everyone who's responded! I appreciate it.
back to the Op - W4S manual said to leave the ST1000 on all of the time.
Hi Guido,

In an earlier post, you asked if I would perform your FM radio test on my D-Sonic M3-600M class D mono-blocs. Yes, I will test them and report back results on this thread when done.

I'm currently use 3 class D amps in my combo 2-ch and 5.1 ht system:

D-Sonic M3-600M monos for R+L mains

ClassD Audio SDS-440CS stereo amp for rear surrounds

Emerald Physics EP100.2SE stereo amp, bridged to mono, for center channel.

So, I will test them all soon (including the Dayton Audio SA-1000 950 watt class A/B amp that now powers my 4 sub bass system.

You remain the class D Guru in my estimation. Like you, I have a strong interest in class D happenings and its evolution. Now that I've heard 3 class D amps in my system, although only 2 were compared on my mains with music, I can say they all have similar traits:

Extremely low noise floor- music and sound emerge from a dead quiet background. Without an input signal, I've turned the volume to near maximum and listened with my ear an inch from the speaker and it's still dead quiet.

Very detailed and neutral- the low noise floor is probably at least partially responsible for the high level of detail. All of these amps impart no particular sound of their own and present a clear and honest portrayal of recordings as well as the quality of upstream components. This results in great system performance when recordings and upstream components are of good quality. If either recordings or upstream components are subpar, however, these amps will make it obvious.

Very good bass response- muddy or loose bass is never an issue; as Mapman stated, the high damping factor typical on class D amps results in clean, taut bass without exaggeration that easily enables differentiation of the instruments producing it.

Relaxed and effortless- these amps never sound strained, overtaxed or restricted. This could be a function of high power, since they range from 440 to 1.200 watts into 4 ohms, but it may be a result of the topology, I'm not sure.

As others have mentioned, the other benefits of class D include small size, light weight, low heat and very low wattage at idle so they can be left on 24/7.


In a prior reply, you asked:

"Any familiarity with the Class D audio amps? These seem like they must be a great bargain. If the noise isolation is up to par with the latest and greatest as well, that might just be the icing on the cake."

Prior to buying the D-Sonics, I used a ClassD Audio SDS-440CS amp to power my mains for both 2-ch and ht for over 6 months. I would definitely classify this amp as a great bargain. For about $650 you get 220/440 watts into 8/4 ohms, respectively, along with all the virtues mentioned above. The 440 and the D-Sonics are similar in that they both are very neutral, detailed with solid bass and smooth midranges and trebles that are never harsh or fatiguing. The D-Sonics are more detailed with an even more effortless presentation and better dynamics. Since the price difference is about $1,200, I'd say that makes the ClassD Audio an excellent bargain. I'll report back on whether they have rf interference/leakage issues.


You replied:

"I prefer Class D left disconnected at all times."

I consider this both funny and sad; funny because it's a humorous and good line but sad because your fairly obvious bias against class D amplification will likely prevent you from giving them an honest audition in your system.

I assume you currently use another competing amp topology that your allegiance to has resulted in closing your mind to possible better performance. Your choice and your loss.

Thank you Tim, keep me posted... But do not worry too much if you hear a bit of increased hash on the tuner... IMO the impact of a little bit of RF leakage from a class D amp does not necessarily translate into a perceivable degradation of overall system performance. E.g. ICEpower modules seem to leak a bit... Yet, Bel Canto REF1000M and Rowland M312 remain marvelous sounding amps in their own rights. G.
I never thought changing from an Parasound A21 to the 525 would yield so many improvements...the most noticeable is spacing between instruments.In my system the highs are musical,real,great tonality, and the mids are even better. The bass compared to the A21 is very different...A21 sounds bigger, 525 tight and detailed. I don't know how much better the 525 will get, but I'm already satisfied. I've reached a point that I'm enjoying music and done buying for now.
Hi Richard... Congrats for M525!!!!

Is the unit new or used? If used, how many hours on it? Overtight bass is likely to relax with more playing time.

Could you also give us initial comments on other Rowland M525 parameters compared to Parasound A21....

Overall stage and image sizes;
Transients distortion and speed;
Macro and microdynamics;
Harmonic complexity;
treble sweetness in complex passages;
Sense of the recording venue surrounding instruments.

Saluti, G.
Not of real importance, but Jeff Rowland gear is absolutely beautiful. I saw his gear at RMAF a few years back and if I remember correctly, the chassis is machined out of solid billets of aircraft aluminum. Jeff also lives in the same city as I do, probably less than 20 minutes away from me... maybe someday I will be able to afford some of his gear :)
B_limo, if you live around the Springs, there is a Rowland dealer with vast experience in JRDG gear -- My friend Ron of Black Forest Audio keeps a super selection of new and pre-owned gear in his stash, ranging from late 1980s classics to Jeff's latest amps and preamps for you to listen to... 719.475.7501.

The M525 is used; however it only has approximately 60 hours on it. In a couple of days I will post my thoughts.
That's exciting Richard.... You basically got an essentially brand new M525 for the priced of a used one! Let us know how things develop. G.

There are a few things that were obvious from the beginning that I can address:

Overall stage and image sizes: compared to the A21, image size appears to be the correct height/size with the M525...the A21 is relatively flat and images appeared below eye level, as if I was looking down. One of the most significant difference is the overall stage...this is where the M525 really shines, outstanding separation of instruments, with the best front to back placement of instruments that I've ever heard in my system

Authority-very hard to explain because the A21 and M525 are very different, I need more time to provide honest feedback

Transients distortion and speed- M525 has a lower noise floor, where music emerges out of a completely dark background

treble sweetness in complex passages-the A21 was closed in and lacking detail compared to the M525, cymbals sound so real and natural with the M525

Well, I finally tested my amps for RF interference today; sorry it took so long.

The only FM radio I had for testing was my bedroom alarm clock. I tuned it to an FM station that had very good reception and then positioned it within 2 inches along each amp's sides, top, front and back. I then increased the distance, to a distance of about 6 inches and then multiple distances trying to find any location where the signal was affected. I even tested my class A/B bass system amp even though I knew there should be no RF interference/leakage on a non-class D amp.

The results indicated there was absolutely zero affect on FM radio reception on any of my amps at any position or distance I tested. I understand your comment that, even if there was some RF interference/leakage from an amp, it would not affect the sound quality produced by the amp.

I was still pleased with the lack of RF interference, however, since I consider this a validation of the amps being well designed and the designers' attention to details. As a reminder For anyone interested, the 3 amps I tested were the following:

D-Sonic M3-600-M mono-block amps
ClassD Audio SDS-440-CS stereo amp
Emerald Physics EP-100.2-SE stereo amp

I highly recommend all 3 of these amps for sound quality. Not having any apparent RF leakage issues is just a bonus for those concerned.
Guido & Noble100,

Why an FM radio? I would expect that an AM radio would provide a much more sensitive and potentially more meaningful test.

As you probably realize, FM is inherently vastly more immune to RF interference than AM, since the audio that ends up being reproduced corresponds to frequency variations in the airborne signal rather than to amplitude variations. Also, in contrast to the AM broadcast band the 88 to 108 MHz frequencies of the FM band (and even its 10.7 MHz IF frequency which follows down-conversion within the radio) are vastly higher than the bandwidths of any audio component, and are therefore much less likely to be relevant.

I suspect that if an FM radio audibly responds to airborne RFI generated by an audio component it is likely not what is picked up by its built-in antenna that results in the audible interference, but rather what is picked up by downstream circuitry within the radio that operates at much lower frequencies. That response probably being much less sensitive than an AM radio's response would be to interference picked up by its antenna.

FWIW, my guess is that you'll hear significant interference when an AM radio, preferably tuned near the low end of the band (e.g. 540 kHz), is placed close to the amp. But what will be of interest is how quickly the interference drops off as the distance increases.

Best regards,
-- Al
Thank you Al, I conducted the test with an ancient tuner a few years ago.... Device is a rather low end NAD from approx. 1979. The tuner has a button that causes it to reject interference in FM... If the switch is in the off position, the device picks up very significant statics whenever tuned to weaker stations, and in some days, FM is plainly unlistenable, regardless of station or switch setting... or amp connected to the system. Call it a hyperdistractable/ADD FM tuner... Or just a plainly crappy tuner.

But I might have conducted the test in AM... I just can't remember.

Al and Guido,

I'll test again using a station low on the AM dial and report back on the results. I Don't know why but I thought any RF leakage from class D amps would be closer to the FM band range.

I Don't know why but I thought any RF leakage from class D amps would be closer to the FM band range.

Noble100, don't know why you were thinking this???
FM operates in the 88MHz - 108MHz range i.e. 88 million cycles per second --> 108 million cycles per second.
the class-D switching amp is probably running at 1MHz switching freq (if that high at all. more likely in the 500KHz region). I.E. 500,000 cycles per second --> 1 million cycles per second.
As you can see, 500,000 --> 1 million cyc/sec is far away from 88 million --> 108 cyc/sec.
Let's say that the class-D amp was switching at 1MHz, then the 89th harmonic of this 1MHz switching freq would be 89MHz but it's power would very, very low to the point that even if it interfered with the FM radio station tuned to 89MHz it would produce a very low amplitude distortion that would most likely be un-hearable.
(Just wanted to point out that the switching waveform of the class-D amp is most likely a square wave & square waves have only odd harmonics).

Like Almarg pointed out the AM freq would be right in the vicinity of the class-D switching frequency & much more likely to create interference.
My class D amp switches at 500kHz. This square wave is filtered by the Zobel network at about 60kHz. Harmonics are most likely filtered out and main problem is about 1% residue of 500kHz switching frequency present on the speaker cable. This 500kHz frequency represents wavelength of 600m. Antenna below 1/10 of the wavelength becomes very ineffective - and that is 60m. Typical 3m cable is practically non-radiating. It is still a good idea to keep cables apart to avoid capacitive coupling.