Tuesday, November 1, 2011

Aragon 8008 BB...A Groovy Class A Amplifier



On Halloween 2011, after all the kids had come and gone, I finally finished measuring and then hooking up my newly acquired Aragon 8008 BB amp.  At first, it seemed a trifle less detailed than the Parasound HCA-1500A I had been using.  And I had considerable difficulty setting the proper level, as during the set up process I turned the volume controls on the Parasound so had no proper measurement of the previous as-adjusted gain level.  I guessed I had the Parasound attenuated by 6 or 12dB, or somewhere in between.

It didn't take long, however, to realize how wonderful this amp sounded.  Like the Krell FPB 300 (which is sitting in the corner waiting to be repaired) it has a wonderful 3-dimensional deeply layered soundstage.  The soundstage is also wider to the left and right than the Parasound, and phase tricks in the recording (as done frequently on the first album I listened to, We Want To Be Loved by Grouse) became mind-blowing.  The mids and lows are palpably real, percussion impact is incredible.  I can feel whacked back into my chair.  Deep bass was surreal.  And this despite the fact I cross over the Acoustat 1+1's at 85 Hz to self-amplifying SVS PB13 subs, so you wouldn't think the amplifier for the panels would make that much difference on deep bass.  But the improved bass seemed to be there at any levels I set the Behringer digital gain adjustment to.  Finally, while listening to Hotel California on DVD-Audio I made the last gain adjustment, to get the highs in proper balance to the subwoofer bass, but I only raised (?) the mid level by 1dB.  In between Grouse and Eagles I listened to Imaginary Day by Pat Metheny on DVD-Audio.  This amp sounded so good I just couldn't quit listening until 4am.



I left the amp turned on overnight, and found the heatsinks to be 130 degrees F in the morning, measured with my IR probe set to "max".  That surprised me at first, but would be about right for an amp biased up to 26 watts class A, as Klipsch technical support said.  Nelson Pass designs his amplifiers to have 55 C temperature at the heatsink fins.  He says that is 60 C at the transistor itself, and you can run hotter, but he prefers not to.  55 C is 131 degrees F.  It looks like whoever last set the bias on this amp, either at the factory or elsewhere, knew what they were doing.

I didn't mean to suggest that this is an engineer's idea of a true Class A amplifier, but 26 watts of Class A power is a lot compared with most Class AB amplifiers which have just a few watts of Class A power.  Even Nelson's Class A amplifiers don't stay in Class A up to maximum output.  Only my Krell comes anywhere near this.   The Krell plateau biasing maintains class A operation up to full rated output, 600W per channel, into 4 ohm loads.

Stereophile also uses the Class A designation to describe the best audio products.  Actually, I think they only rated the 8008 ST as Class B, but IMO the 8008 BB deserves a Class A.

*****

Before the amp was delivered last week, I had reason to worry I wasn't actually being sent an 8008 BB.  UPS indicated the shipping weight was only 68 pounds.  The 8008 ST is said to weigh 67 pounds and the 8008 BB is said to weigh 76 pounds.  The ebay seller told me he would take the amplifier back if it proved to be the wrong model.  The first thing I did when the amp arrived was try to weigh it by the "net" method, standing on scale either holding the packaged amplfier or not.  This was not easy, as you might imagine.  But I tried this in both front entry hall and the kitchen and both times I came to a net packaged weight of 81 pounds.  That would be perfectly consistent with a 77 pound amplifier, in double (but light) boxes with bubble wrap and styrofoam pellet cushioning.  (Those pellets are a pain to deal with, and I don't blame Krell for charging an "environmental disposal fee" if people incorrectly ship an amp to them that way.)  Despite somewhat flimsy boxing, neither the boxes nor the amp was damaged during shipping.  UPS must be getting better about that.  And don't tell them about the scale error they apparently made.  I think the box must have been resting slightly on the sides of the scale.




Then after unboxing I counted the transistors while observing through ventilation holes in the bottom.  I counted 12 large (very chunky looking) Toshiba transistors per channel, just as the BB version is supposed to have.  Then I took the cover off, and verified that there are two separate power transformers, and 4 x 35,000 microfarad capacitors.  Very nice big Cornell Dublier capacitors, mounted to PCB with 4 diode bridges.  I had already seen from the eBay photos that the amp has XLR and RCA inputs.  So there is no doubt it is either the BB version or equivalent in every way.  The circuit boards look to be a nicer material than typically seen in electronic equipment.  All in all, the amplifier looks first class on the inside.  It looks fine on the outside too, though fairly basic except for nice 1/3" thick faceplate.




The top cover (which covers the transformer and capacitors) has some damping material applied.  Fortunately that part doesn't get very hot, but the damping material should be examined once and awhile; it was probably added by previous owner.  Also, the factory feet had been replaced with single felt pads.  I felt that didn't allow enough ventilation underneath, so I added 3 more felt pads to each foot.



Before moving the amp into place, I measured power output into an 8 ohm load using Tektronix scope and B&K sine generator.  I connected scope probe to hot side but didn't connect ground, expecting amp to be grounded through common grounds.  However I was wrong about that, though I was able to do clipping measurements as intended the lack of ground connection meant that high frequencies were very rolled off and therefore at lower levels in the scope display.  I found the amplifier put out 45 volts from 20 - 20khz.  But I was still concerned about high frequency roll off in the amp itself.

A couple days later when I set the amp into place, it's only a little wider than the Parasound, I did a dedicated frequency response measurement measuring both the input and output of the amplifier at 20, 200, 2k, and 20khz.  I also did the same tests on the Parasound.  I still detected more HF loss than expected, at -0.13dB.  The curve posted at Stereophile shows only about that much loss at 50kHz.  However, the Parasound measured even more loss, at -0.15dB.  Then I realized that my B&K oscillator has about 3% distortion, probably mostly third harmonic, and what is happening is that it is the harmonics that are getting rolled off more than the fundamental.  To do this measurement properly, one needs to use a very low distortion oscillator or a good filter to prevent the harmonics from contributing differentially to the response measurements.  I do have such an oscillator, my ST1410A, but it's hard to move because of all the junk in my computer room at the moment.  I suspect the rolloff at 20kHz is far less than 0.13dB, and even if it were 0.13dB, that would be less than any 44.1kHz sampled digital sources.

At 2kHz, the Parasound with volume controls turned fully up had 1.5dB more gain (8 ohms,  about 5v output).  Usefully, the Aragon takes larger input voltages to achieve the same output level, though not by much.  Unfortunately I didn't measure the Parasound before turning the volume controls up, I could only guess they had been turned down 6-12dB.  So I had to set the crossover level controls by guess and ear.  I also played pink noise and did a 1/3 octave measurement, which was looking flatter than I remembered.



During testing, I made the amp mute relay shut off output when I inadvertently inserted DC levels of about 2V.  The B&K generator has a DC offset adjustment, and when the upper output level is chosen, the offset increases by 10x.  I previously had misadjusted the DC level to 200mV in the lower level.  With no input, the Aragon servo loop maintains very low DC offset, about 1mV.  But if significant DC is applied to the input, the amp quickly mutes.  When the DC is removed, the amp takes a few seconds to unmute.  This is actually a very useful protection feature.





2 comments:

  1. Greetings, While searching the web I stumbled upon this blog. I am in need of advice. If you can help or tell me where to find info I would be sincerely grateful. I obtained an Aragon 8008st, Aragon 18K Mark II preamp, Aragon D2A2 digital to analog converter, Aragon Isolated Power Supply, Pioneer F-93 Digital Synthesizer Tuner, Pioneer PD-65 Compact Disc Player, and a pair of Infinity Sigma speakers.
    My question is what is the external amp for and in what sequence should all this be connected. The manuals for the pre-amp and converter are missing. I am a novice and when I went to Custom Audio & TV for info I was treated with arrogance and indifference as I am not a twenty thousand dollar customer for them!!
    I donated the system to Music Cares a non profit for kids and they don't have time to research this and as I said I am just a guy trying to do the right thing.

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  2. Hi, I have the 8008ST and looking at yours with the two power transformers and balanced inputs it is definitely a 8008BB. You seem very knowledgable sO I have a question maybe you can help. My amp just stopped working. No output either channel. Main power is OK but F2 (8A +70V left channel) keeps blowing. Despite only left channel fuse blowing, right channel also has no output. I do not hear the speaker delay switch(s) activate so that may be the cause of no output on either channel. Any ideas you may have or help you can provide will be highly appreciated.
    thanks
    Derek

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