The 2011 Richard C. Heyser Memorial Lecture: "Where Did the Negative Frequencies Go?" Case Study 1: Recording

Case Study 1: Recording

Back in 1987, the AES published the anthology pictured above of historic papers on "Stereo." It includes a document (celebrating its 80th anniversary this year) that pretty much defined the whole field of stereo reproduction, including the 45°/45° stereo groove and the moving-magnet stereo cartridge. That document, a 1931 British Patent Application written by the English engineer Alan Dower Blumlein, is worth quoting at length:

"The fundamental object of the invention is to provide a sound recording, reproducing and/or transmission system whereby there is conveyed to the listener a realistic impression that the intelligence is being communicated to him over two acoustic paths in the same manner as he experiences in listening to everyday acoustic intercourse and this object embraces also the idea of conveying to the listener a true directional impression. . . . An observer in the room is listening with two ears, so that echoes reach him with the directional significance which he associates with the music performed in such a room. . . . When the music is reproduced through a single channel the echoes arrive from the same direction as the direct sound so that confusion results. It is a subsidiary object of this invention so to give directional significance to the sounds that when reproduced the echoes are perceived as such."

In other words, if you can record not only a sound but the direction in space it comes from, and can do so for every sound wave making up the soundstage, including all the reflected sound waves (the reverberation or "echoes"), then you will be able to reproduce a facsimile of the original soundstage, accurate in every detail. In addition, because the spatial relationship between the direct and the reflected sounds will be preserved, that reproduced soundstage will give a realistic illusion of depth.

Incidentally, I mentioned earlier Hermann Bondi, one of Hoyle's collaborators on the Static Universe Hypothesis. Like Blumlein, Bondi had worked on the British development of radar in World War II. When I worked in the research lab developing LEDs, in a corner office was a charming elderly gentleman, Dr. Henry Boot. Only years later did I learn that Henry was one of the people who invented the cavity magnetron, which was fundamental to the British development of radar. I suppose you could therefore say that there are just two degrees of separation between me and Alan Dower Blumlein.

The Blumlein Patent Application mentions that, when recording for playback over headphones, the simplest way of carrying out the preservation of the soundstage is to use two microphones spaced as far apart as the average pair of human ears: the "binaural" technique. This, however, makes headphone listening mandatory; until recently, headphones have been about as popular as a head cold for relaxed, social listening. Blumlein was concerned with a system for playback over loudspeakers, and proposed a method of recording directional information as a ratio of amplitude differences between the two signal channels.

The ear/brain, of course, uses more than amplitude information to determine the direction of sound sources. It uses the amplitude difference between the signals reaching the two ears above about 2kHz, but below about 700Hz, it determines direction by looking at the phase difference between the signals; ie, it uses time-of-arrival information. (Both frequencies are proportional to the head size, so there will be a spread among individuals.)

Things get a bit ambiguous between those two frequencies, but there are two other mechanisms also at work: first, the frequency-response modifications due to the shape of the pinnae differ according to the direction of the perceived sound; and second, the head is in continual lateral motion, sharpening up all the mechanisms by introducing second-order (rate-of-change) information. The result is that human beings—and animals—are very good at determining where sounds come from (unless they happen to consist of pure tones in the "forbidden" region between 700 and 2000Hz, which is why birds, for example, use such tones as warning signals).

Blumlein's genius lay in the fact that he realized that the low-frequency phase information can be replaced by corresponding amplitude information. If you have two independent information channels, each feeding its own loudspeaker, then the ratio of the signal amplitudes between those two loudspeakers will define the position of a virtual sound source for a centrally placed listener equidistant from them. For any ratio of the sound levels of the two speakers, this virtual source occupies a dimensionless point somewhere on the line joining their acoustic centers. The continuum of these points, from that represented by maximum-left/zero-right to that represented by zero-left/maximum-right, makes up the conventional stereo image. If there is no reverberant information, then the brain will place the virtual image of the sound source in the plane of the speakers; if there is reverberation recorded with the correct spatial relationship to the corresponding direct sound—that is, if it is "coherent"—then the brain places the virtual image behind the speakers, the exact distance depending on the ratio of recorded direct sound to recorded reverberant sound.

Thus, by recording and playing back just the amplitude information in a two-channel system, we can create a virtual soundstage between and behind the loudspeakers (footnote 6). And if, instead of capturing an original event, we record many individual sounds in mono and assign each one a lateral position in the stereo image with a panpot (along with any added reverberation or echo), when we mix down to stereo, again we have a true amplitude-stereo recording. It is fair to say that 99.99% of all recordings are made in this way. It is so fundamental to how recordings are now made that I doubt if anyone thinks about the fact that it is based on psychoacoustic sleight of hand: the substitution of amplitude ratios for time-of-arrival differences in the midrange and bass.

For many years, I was a hard-line Blumlein purist when it came to classical recording. I was attracted by the theoretical elegance of the M-S technique—a sideways-facing microphone with a cosine or figure-8 pickup pattern is spatially coincident with a forward-facing mike; sum-and-differencing the mike outputs gives you true amplitude stereo—and of two figure-8 microphones horizontally coincident at 90°, each positioned at 45° to the forward direction. Of all the "simple" techniques used to capture live acoustic music, these two, in all their ramifications, are the only ones to produce real stereo imaging from loudspeakers.

I used to dismiss with a snort recordings made with spaced microphones. After all, if the microphones are separated in space by a distance larger than the wavelength of most of the musical sounds—10', say—unless an instrument or voice is exactly halfway between the two microphones, there will be, in addition to the amplitude information, a time delay introduced between the electrical signal that voice or instrument produces in one channel and the signal it produces in the other. Such time information pulls the image of the source farther toward the nearest speaker, resulting in an instability of central imaging and a tendency for sources to "clump" around the speakers. Add to that the fact that the interchannel amplitude differences produced by spaced microphones do not have a linear relationship with the angular directions of the sound sources, and it is hard to see how a pair of spaced microphones can produce any image at all.

Yet . . .

In 1992, we were recording two concerts for Stereophile featuring classical pianist Robert Silverman.

The main pickup was with a single stereo microphone, but I had put up a pair of omnis that I fed to a separate recorder. After the concert, it became apparent that the stereo mike had failed, so I was forced to use the spaced-omni recording for the CD release. Here is a short track from that album, Schubert's Moment Musicaux No.3:

[Play Schubert Moment Musicaux No.3, from Concert CD, Stereophile STPH005-2 (1994)]

There are two things intriguing about this recording. For this lecture, one minute in, I flipped the right channel's polarity. I doubt that anyone noticed—there is so much time disparity between the two channels that it cannot be considered a stereo recording at all; rather, it is two different recordings of the same performance that happen to be played back simultaneously. The second thing is that, despite that theoretical imperfection—for which I was duly castigated on Usenet—the CD sold quite well. People liked the sound.

I wasn't too surprised by that. Theoretically perfect amplitude stereo has served us well, but when I played people some of my classical recordings made in the appropriately purist manner, they often described the sound as "thin" or "cold" or "lacking bloom."

As I said earlier, when people say they like or dislike something, you should take notice. And in this instance, the late Michael Gerzon had discussed the matter in a paper he gave to the London AES Convention in 1987. Specifically, he had postulated that Blumlein's substitution of amplitude for phase differences at low frequencies is inadequate, that people prefer the sound when there is some time-difference information between the channels, presumably because the information their brains use to synthesize a model of the stereo image now has more in common with what they would have heard at the original event.

Gerzon had floated the idea of using two pairs of microphones to capture all the information the brain requires: spaced omnis below 1kHz; coincident figure-8s above, with a crossover between the two. I tried that, with disappointing results. However, after 1992, I used a similar miking technique with which I thought I could get the best of both worlds: the good amplitude stereo from coincident or quasi-coincident mikes, and the lower-frequency bloom from spaced omnis. Both mike pairs were used more or less full-range; the only EQ was a touch of top-octave rolloff on the omnis, and some first-order low-frequency boost on the cardioids to compensate for their premature bass rolloff when used distant from the source.

It was the acquisition of a Sonic Solutions Digital Audio Workstation in 1993 that allowed me to fine-tune this technique, because it became apparent that the two pairs of mikes needed to be time-aligned for the resultant stereo image to lock into place. This time alignment of mikes had been used by Denon and was described in an early 1990s AES convention paper, but I had no way of easily implementing this until I could slide individual tracks backward and forward in time to get the required synchronization.

Since then, I have made all my classical recordings in this manner. Here is a typical example: Minnesotan male-voice choir Cantus singing Eric Whitacre's Lux Aurumque in the glorious acoustic of Sauder Hall, at Goshen College, in Indiana. You can see the two pairs of mikes in this photograph.

Not shown in the photo is a third pair of mikes, omnis on a Jecklin disc, farther away from the singers, which I used in case it turned out that the main pickup was too dry. (When you are on location and the clock is ticking away your money, you cover your bases.)

[Play Cantus: Eric Whitacre, Lux Aurumque (excerpt), from While You Are Alive CD, Cantus CTS-1208 (2008)]

If you listen critically to this recording, you will hear that acoustic objects get a little larger, the farther away they are from the center of the stage. However, their spatial positions in the image are correct.

I tell this tale because it illustrates one of my points: that thinking you are right about something in audio doesn't mean you are right. No matter how much you think you know, there will always be new things that upset your world view. Einstein, for example, would be astonished to find that his "biggest blunder," the Cosmological Constant, turns out to be real—that we now are aware that something completely unknown to science is causing the expansion of the universe to accelerate. Physicists call it "Dark Energy," but that's just scientific shorthand for "We have no idea what it is."



Footnote 6: this creation of a virtual soundstage only works for soundsources to the front of the listener and a two-channel system. However, when the mixing engineer requires a virtual image to be placed to the listener's side in multichannel audio, it fails for the simple reason that we do not have a pair of ears on the front and back of our heads.
Share | |
Comments
JohnnyR's picture
Sure is a Lot of............

......blind testing mentioned by a person that doesn't believe in doing them. Aren't they "too difficult" according to yourself? Wait......nevermind just don't answer that because I know it will end up with you Mr Atkinson making some EXCUSE as to why you can't do blind tests for the magazine when it comes to reviewing products. ZzzzzzzzzzZzzzzz.

John Atkinson's picture
Paul Simon to the white courtesy telephone, please

JohnnyR wrote:
Sure is a lot of blind testing mentioned by a person that doesn't believe in doing them.

A man sees what he wants to see and disregards the rest, eh? In the 15,000 words of this preprint, there are just a few paragraphs devoted to a discussion of blind testing.

JohnnyR wrote:
Aren't they "too difficult" according to yourself?

As I say, I took part in my first blind test 35 years ago and since then have been involved in well over a 100 such tests, as listener, proctor, or organizer. My opinion on their efficacy and how difficult it is to get valid results and not false negatives - ie reporting that no difference could be heard when a small but real audible difference exists - was formed as the result of that experience.

John Atkinson

Editor, Stereophile

GeorgeHolland's picture
In other words JohnnyR, Mr

In other words JohnnyR, Mr Atkinson DID find excuses not to use blind testing but found them worthwhile enough to mention them in his Heyser speech funnily enough.

JohnnyR's picture
Atkinson's Career..........

A. Join a band as a second rate bass player

B. Fail to get a record contract

C.Pretend he's an electrical engineer

D.Get an offer to ruin.....ermm I meant run Stereophile

E. Proceed to ruin........ermmmmm excuse me......run Stereophile like Hitler would.

F. Spend his time talking online instead of actually doing any real work.

G. Make excuss about his busy schedule and why he can't do DBTs

H Profit!!!

Regadude's picture
Johnny's career...

Johnny makes plywood boxes in his mother's basement, and then claims he is a speaker expert.

Trolls audio websites. 

 

That is all.

ChrisS's picture
JRusskie's trajectory ->

JRusskie seems to have become a failed audio (and DBT) expert in one easy step...

 

A. Failed...

dalethorn's picture
Hitler? I don't know whether

Hitler? I don't know whether to laugh or cry.

Ariel Bitran's picture
Cry

also, brings both of these to mind:

http://en.wikipedia.org/wiki/Reductio_ad_Hitlerum

http://en.wikipedia.org/wiki/Godwin's_law

ChrisS's picture
Whiff...

There it is, again...

ChrisS's picture
Poof...

I think a synapse just exploded...

dalethorn's picture
Blind testing

I asked a question of Harman Testing Lab in the Computer Audiophile forum, and post #11 is the answer, which I thought was perfect. i.e., let the testee control the switch, so whatever adjustment time they need they can accomodate.

http://www.computeraudiophile.com/f12-headphones-and-speakers/behind-harman-testing-lab-14112/

Other observation: The comments in this Stereophile article about the noise floating free of the recording due apparently to having a higher playback bandwith - that made my day.

On R.C. Heyser: His description of the 1971 Sylmar quake, hearing a low-frequency "tone" prior to the shaking, got him started on comparing earthquakes and woofers and low-frequency playback in rooms, and he got beat up on a lot for those writings. But having been involved with both, those observations of his were like gold in the bank. Saved me lots of money.

Regadude's picture
SRH940 vs Senn 800HD

Aren't you the dude who "tested" the Shure SRH 940 headphones and claimed they were almost as good as the 800HDs from Sennheiser?

I own the SRH940s. I was somewhat willing to believe you, but when I saw that your source was an Ipod, I just flicked your review in the trash.

You chose a 200$ source to test 325$ and 1500$ headphones?  And you believe your results mean something?

 

no

dalethorn's picture
My review of the 940, agreed

My review of the 940, agreed upon by more than 95 percent of respondents, was vetted with specific music tracks listed in the review and subsequent comments, using ipods alone, ipods with analog amps, and desktop DACs and amps. Some of the naysayers are either impatient or just grumpy and disagreeable, which the comments in this Stereophile article perfectly illustrate.

Regadude's picture
Touchy are we?

How nice of you to call people who question your methodology grumpy, disagreable, etc. I question your methodology because it makes no sense to test 325$ headphones with an ipod. It makes even less sense to test 1500$ headphones with an ipod...

I actually listened to the Shure SRH940 and HD800 today, using my ipod Nano. You are correct, they both sound almost the same. They both sound like shit!

I know from experience that these headphones really shine with a good source, and especially, proper amplification. As good as the Shure is (I own them), it is no match for the HD800 when both are hooked up to decent equipment.

Your test is like testing the acceleration of a Ferrari and a Prius in your 12' driveway. You won't get much better results from the Ferrari in such a limited distance.

As for the 95% who agree with you, what gear did they use (ipod)? Were those 95% SRH940 owners and fanboys? 

dalethorn's picture
Saying "they both sound like

Saying "they both sound like shit" when one of those is a revered $1500 headphone of great pedigree and popularity among actual audiophiles is telling of your judgement. I would suggest taking a deep breath, pick a headphone and some music tracks you like, then tune out the matrix and just enjoy your music. You'll be less grumpy that way.

Regadude's picture
Your covering your arse

Don't try and hide behind my words! They both sound like shit through an ipod! What a weasel you are. I did not say the 800HD sounded like shit... BUT IT SOUNDED LIKE SHIT THROUGH AN IPOD. You rigged your test, Dale. 

Again, you use insults...

 

dalethorn's picture
A long time ago one person on

A long time ago one person on a big audio site challenged people like you to be more specific instead of throwing words like s**t around. There were just two persons who offered test tracks with which to compare those headphones, but everyone else (like you) just threw out expletives, and a couple members even made serious threats. Now I have over 30 years experience comparing headphones like the Stax and Sennheiser series, and quite a bit of time invested with the Audio Engineering Society as well as reading Stereophile magazine, so even if nobody in the world were to agree with my conclusions, I'm certainly qualified to test and evaluate these devices. I'd suggest you refrain from purely ranting with words like s**t and offer specific testable facts, or just go away.

This article may help you:

http://dalethorn.com/Headphone_Ipod_Versus_Amp.txt

dalethorn's picture
BTW, and speaking of Prius

BTW, and speaking of Prius and Ferrari, you may be one of the better off who don't mind paying $1500 for a headphone and then shelling out $2000 for a headphone amp. But let's pretend for a moment that you have to pinch pennies to get those luxurious goods, so when you finally have them you really appreciate what you have. In this latter case, were you to stumble across an ipod touch or iphone of recent manufacture, with the new Earpods ($29 purchased variety only) being driven by the Dirac DSP player, and had an opportunity to listen at length, it might make you ill to realize what you got for your $3500.

Regadude's picture
400$

My headphone amp is a whopping 400$ (CI Audio VHP 2).

I do pinch pennies. The VHP 2 and the Shure 940 are mine.The Sennsheiser 800HD I borrowed from a friend.

Stop trying to cloud the issue with non relavant issues.

Regadude's picture
Stereophile test it!

How about we let Stereophile test both headphones! First through an ipod, then with decent equipment!

Both headphones sound similar through an ipod.

Plug them into a receiver, they sound less like shit.

Plug them into a decent headphone amp, and they both sound MUCH BETTER. BUT the Sennsheiser pulls away and wipes the floor with the Shure.

Buy yourself a CI Audio headphone amp (the one I use), or any other decent headphone amp Dale... Before buying a 2500$ DAC to install on an ipod.

DUH...........................

GeorgeHolland's picture
The continuing saga of ChrisS

The continuing saga of ChrisS being a troll and all around noncontributing poster. Classy act there ChrisS.

Mostly what I got from Mr Atkinson's article was the amount of photos of himself rather than Mr Heyser. I suppose a memorial lecture should be all about the presenter and how he sees fit to turn it into reasons why Stereophile does what it does regarding testing and the lack there of.

"Yes, what you think you are hearing might by dismissed as being imagination, but as the ghost of Professor Dumbledore says in Harry Potter and the Deathly Hallows, "Of course it's all happening in your head, Harry Potter, but why on earth should that mean that it is not real?"

Oh I see now. Stereophile's philosophy is based upon a ficticious Harry Potter character who spouts that what we hear is real.I'll be sure to let those people being treated for such voices in their heads that all is well and medication is not necessary.

"Footnote 2: For a long time, I've felt that the difference between an "objectivist" and a "subjectivist" is that the latter has had, at one time in his or her life, a mentor who could show them what to listen for. Raymond was just one of the many from whom I learned what to listen for."

Well if only that was true. Your readers tend to all be subjectivsts who aren't trained what to listen to but blindly ( I made a pun there since blind testing is not wanted at Stereophile) believe whatever they think they hear as the truth when in fact measuremnts will show that either they heard nothing related to the measurements or perhaps it's Dumbledore once more in their heads, make believe or wishful hearing.

All in all I found your presentation 90% about yourself and Stereophile and reasons why you defend subjective listening and gloss over things like cables and doing serous testing and throwing in various quotes and ambiguous stories barely related to the subect you were supposed to be talking about, unless that subject was muddling the understanding of Stereophile and telling everyone your own personal history instead of Mr Heyser's.

"Third, one well-known skeptic sitting in the audience tonight criticized my abstract a few weeks back on the grounds that I am just offering "hypotheses about stuff that might be just to stir the pot, while offering no real explanations."

Class act there Mr Atkinson, speak out and criticise someone you don't like or agree with in the audience who then has no opportunity to speak up and address the audience themselves. Very childish and immature.I already see how you do the same on here. "My way or the highway"

JohnnyR is once more sadly correct, Blind testing is not and wil not be used by Stereophile. Just because you Mr Atkinson, know how to push a few buttons to measure speakers, does not make you even close to the same league as Mr Heyser was. He furthered the cause of objectivity and made Audo magazine the best audio magizine there ever was. The Audio Critic is a very close second. Mr Aczel used to be one of the worst subjectivists around untill he admitted to being such and saw the light and spoke out against foolish subjective reviews. Testing will always be the most important part of audio magazines. Simply letting reviewers spout off about "blacker backgrounds" "lifted veils" or other such nonsense belittles both the reader and the so called reviewer not to mention the product under "test". Truth in testing is what Mr Heyser strove for not some silly review about rainbow foil, magic bowls or pebbles.

John Atkinson's picture
Correct: the lecture was not about Richard Heyser

George Holland wrote:
Mostly what I got from Mr Atkinson's article was the amount of photos of himself rather than Mr Heyser. I suppose a memorial lecture should be all about the presenter and how he sees fit to turn it into reasons why Stereophile does what it does regarding testing and the lack there of. I found your presentation 90% about yourself and Stereophile...throwing in various quotes and ambiguous stories barely related to the subect you were supposed to be talking about, unless that subject was muddling the understanding of Stereophile and telling everyone your own personal history instead of Mr Heyser's.

You have misunderstood the nature of the lecture, GeorgeHolland. The Richard C. Heyser Memorial Lectures are not intended to be _about_ Dick Heyser but instead are offered to honor his memory. The invitation for me to be the October 2011 lecturer clearly stated that I was free to choose any subject I felt appropriate; my triple career as a musician/audio engineer/audio editor and the conclusions I have formed as a result of my 4 decades' experience in those careers were felt by the AES Technical Council members to be an eminently suitable subject. 

George Holland wrote:
Class act there Mr Atkinson, speak out and criticise someone you don't like or agree with in the audience who then has no opportunity to speak up and address the audience themselves. Very childish and immature.

The skeptic in question had emailed me before the lecture to let me know he would be in the audience and would take an active part in the anticipated Q&A session. I included this mention in the preprint to give him the necessary opening. As it turned out, he didn't attend, but I saw no reason to delete the point he made.

George Holland wrote:
Just because you Mr Atkinson, know how to push a few buttons to measure speakers, does not make you even close to the same league as Mr Heyser was.

I agree. As you can tell from my discussions of Dick Heyser's writing and thoughts in the preprint, I have an enormous amount of respect for what he achieved. I have never claimed to be his equal, nor would I. But again I must emphasize that the AES Heyser lectures are not intended to be _about_ Dick Heyser. That is your misunderstanding, I am afraid.

John Atkinson

Editor, Stereophile

Rolli666's picture
Trolls do not only live under bridges

I am not sure as to why a person would even want to respond to these posts. Some problems really are better ignored as to invoke the spirit of go away.  I for one appreciated this lecture for what it is thank you.

John Atkinson's picture
You're Welcome

Rolli666 wrote:
I for one appreciated this lecture for what it is thank you.

Appreciate your comment. It is rare that a magazine writer has the opportunity to gather all his thoughts in one place and I owe a debt of thanks to the Audio Engineering Society for inviting me to do so.

John Atkinson

Editor, Stereophile

 

ChrisS's picture
Whizz, sput, sput...

Whoa! There goes another synapse! Now they're both brain dead... I duelled extensively with JRusskie re: blind testing on the Forums and I daresay, he shot himself many times with his own arguments.

Saga? Can't wait for the movie version!

Sorry, Ariel... I'll try not to let it happen again.

Dunadan's picture
The whole is more than the sum of its parts...

...which is a valuable insight, I think, from this article that Mr. Atkinson has generously preprinted. His stories--which, though I am not a longtime reader, I presume also reflect the history of Stereophile--mirror the tensions found in the philosophy of science between "reality" and "perception", and the question of whether our observations are truly disinterested or if our minds shape how and what we see (or hear). And, really, what exactly does a blind test resolve? That the reviewer likes one piece of equipment over another, presumably without factoring in aesthetics or cost? But "liking" one thing better than another is still a subjective judgment, and who's to say YOU would share that judgment? Never mind the problem of translating sounds heard by another into words on the magazine's page and then back again into the sounds heard by the reader when he or she auditions the gear. Or the issue of whether sound quality can be adequately described by a series of measurements, any more than colour can be described strictly in terms of wavelength, or consciousness in terms of vibrating atoms in your brain. That's not to say these things aren't significant, but to say that they add up to the sum of our human experience is absurd.

Thanks for a reflective and enjoyable article (at least in my mind).

GeorgeHolland's picture
The purpose of blind testing

The purpose of blind testing is soley to see if you can hear a difference between two units. After all the subjective rants of "oh this lifted several veils of crud from the liquid sound" or "I heard a definite improvement" I find it sad that Sterophile keeps saying that DBT or even SBT are not a valid way to test those claims. Either you can hear a difference or you can't yet those that claim they do shy away from proving that fact with a blind test. Afaid they might be shown to be wrong is my opinion, so Stereophile allows them to go merrily along in their own self delusions while spending way too much money on sham products.

ChrisS's picture
Do you hear what I hear?

In a properly set up DBT, let's say for two amplifiers, what exactly will the listener be listening for?

 

(And what if the lamb heard a difference, but not the shepherd boy?)

GeorgeHolland's picture
Are you seriously so dense

Are you seriously so dense that you can't figure it out for yourself? Perhaps that's why you depend upon Stereophile to tell you what to buy?

http://home.provide.net/~djcarlst/abx.htm

http://www.matrixhifi.com/contenedor_ppec_eng.htm

Read and learn.

ChrisS's picture
May I Call You Georgie?

So Georgie,

If the lamb can hear that Amp A can make the test system sing in a range of 10hz-60khz, but the shepherd boy can't hear in that range so can't tell the difference between Amp A and Amp B, which amp should the shepherd boy buy?

Site Map / Direct Links