Building the Audio Note Kit 1 SET amp...


Hi, Folks,
If anyone's interested, I've started a blog with lots of photos, documenting my ongoing build of the Audio Note Kit 1 300B SET amp. If you've ever thought of building any kit before and want to get a feel for what it's like, you're welcome to have a look!
rebbi

Showing 20 responses by almarg

Regarding the possibility of adding a sub, in addition to the concerns about integration that have been raised I would have some concern that the high pass filter and other circuitry in the sub, that would be in the path between the preamp and the SET amp, might noticeably compromise the SET magic.

And then of course there is the cost of the sub, which would be purely an additional cost rather than a cost that could be offset, at least partially, by selling the existing speakers or amp.

FWIW, although under the great majority of circumstances I am in the "build the system around the speakers" camp, in this particular set of circumstances, all things considered, as I indicated in Charles' system description thread where this has been under discussion my vote would be for a speaker replacement.

Best regards,
-- Al
Rebbi, note that the price of the Triumph Extreme II is indicated as being in Canadian dollars, while the price of the stands is indicated as being in US dollars. If those prices are accurate, at current exchange rates the total cost of both be $3400 in US dollars.

Best regards,
-- Al
Rebbi's small complaint was that the DeCapo did not capture the lower octave on stuff with Synth bass with his 300B.
Rob, more precisely the complaint was that the amp clips when reproducing that and other material having high volume content at deep bass frequencies. Clipping, in turn, results in distortion of ALL frequencies that may be simultaneously present, including harmonics and other frequency components produced by the synth at much higher frequencies, as well as notes that may be simultaneously produced by other instruments or singers.

Resolving that problem while continuing to use the Kit 1 amp calls for a speaker having considerably greater efficiency than the De Capo provides. How well and to what degree the speaker can reproduce deep bass frequencies is a separate question.

Although the Klipsch's 99 db/1W/1m efficiency rating is arguably somewhat optimistic, as is pointed out in the review, it is undoubtedly MUCH more efficient than the De Capo.

My comment should not be interpreted, however, as advocacy for the Klipsch.

Best regards,
-- Al
To be precise, 8 watts is 10log(8/3) = 4.26 db greater than 3 watts, everything else being equal. To put that in perspective, the classic rule of thumb is that a difference of 10 db (10 times as much power) will be subjectively perceived as "twice as loud."

Regarding "everything else being equal," though: Assuming that those power ratings are based on an 8 ohm load, to the extent that the speaker impedance varies from 8 ohms at various frequencies, and to the extent that the output impedances of the two amplifiers are not the same, that 4.26 db difference may change significantly in either direction. Other design-related differences between the amps, such as in their power supplies, of course can also affect that value significantly, depending on the impedance characteristics of the speaker, the dynamics of the music, and other variables.

Sebrof, great job helping out! Rebbi, continued good luck.

Best regards,
-- Al
Hi Rob,

Although I haven't personally used any SET's, low powered or otherwise, in my own system, I'm sure that would be a wonderful pairing for most listeners with the great majority of recordings, and for many listeners with all of their recordings. However, I also feel certain that at least a few recordings that would be listened to by some listeners would cause the amp to clip, or at best to experience a substantial rise in distortion.

My collection includes many classical symphonic recordings on labels such as Telarc, Sheffield, Reference Recordings, Chesky, etc., which have brief dynamic peaks that reach SPL's of 100 to 105 db at my 11 foot listening distance. I can't conceive of an 8 watt amplifier, no matter how robustly designed, being able to comfortably handle those peaks, even with the 94-98 db efficiencies of the Daedalus speakers.

That is why I chose a VAC amp which uses four 300B's per channel, in a push-pull parallel configuration capable of 65 watts. I wanted to draw a compromise between the magic that a 300B can offer in a SET configuration, and having power capability that would not be marginal (or worse) under any circumstances that could conceivably occur with my particular listening habits. And I made that choice while realizing that the cost of upgrading eight 300B's from the VAC-supplied Chinese tubes would be prohibitive.

Another point to keep in mind is that although the impedance curves of the Daedalus speakers are exceptionally flat, and they are extremely benign loads in terms of phase angles, their impedances are in the 6 to 8 ohm area (6 ohms in the case of my Ulysses), as contrasted for example with the 14 ohm nominal impedance of Charles' Coincidents. So I suspect that his Franks are capable of providing a bit more power when used with his speakers than they would be capable of with mine.

But as I say, I suspect that for many and perhaps most people the Franks would do just fine with the Daedalus speakers.

Best regards,
-- Al
Rebbi, I spent a little while looking at the photo of the underside of the chassis of your build, which you provided on the "Finishing Touches" page of your blog, looking for anything amiss that might account for the problem.

Although it's hard to tell from the photo, the one thing I question is the value of the 3.3K resistors which connect to the grids of each of the 300B's. I can't reconcile the color code bands appearing on the resistors in the photo with that value. The band that appears to be gold in color, at the end of each resistor closest to the top of the photo, would correspond to the +/- tolerance, which is unimportant. Reading the other bands upwards from the bottom, though, I would expect to most likely see orange-orange-black-brown, or (less likely) just three bands colored orange-orange-red. It appears, though, that the band just below the tolerance band is black, which would correspond to a much lower resistance value.

If the resistor value is much lower than the 3.3K the design calls for, it could very well account for the symptoms you've described, due to a lack of what is called "grid stopping". (See especially the paragraph near the bottom which begins with "The main reason for limiting grid-current is to reduce blocking distortion").

Also, regarding the possibility of a defective tube, fwiw I'll mention that based on a look at the schematic it appears that the only tubes which can affect both channels are the 6SN7 input tube, and the 5U4/274B rectifier.

Best regards,
-- Al
Yes, the two green colored resistors going from the TAG strip to each of the 300B sockets.

-- Al
Oh, well. I just found the more detailed photos here, in which the resistors can clearly be seen to be correct (orange-orange-black-brown).

Strange that they look so different in the less detailed photo.

Never mind :-)

Best regards,
-- Al
Following up on my previous posts about the resistor value non-issue, I've found myself sufficiently captivated by all of this unfolding drama to complete a careful look at all of the photos at Rebbi's blog, looking for anything that might account for the problem.

I found nothing. As might be expected, everything looks to have been done meticulously and accurately. I even checked all resistor values against the schematics which are available on the Internet, with the exception of the four resistors on the filament supply board (those don't seem to appear on any of the schematics I found; apparently they were introduced during a relatively recent revision of the design).

I did note what MIGHT be a bit of heat damage on R21 of the driver board, but looking at the schematic I don't see how that could account for the specific symptoms in this case. And in any event that resistor would only affect one channel.

One possibility that seems very remote but I wouldn't totally rule out at this point is that the output transformers were manufactured with the wrong color coding on the wires. I say that in part because most (but not all) versions of the Kit 1 for which photos appear on the net have transformers which used a different set of colors for their secondaries than the green-blue-yellow colors of the ones that were supplied to Rebbi. And of course if the colors were mixed up such that the 4 ohm tap is being used, or the speaker connections are being made BETWEEN the 4 ohm and 8 ohm taps, that would easily account for the problem. The tech could probably verify the color coding by measuring resistances between the different possible combinations of the three wires.

Best regards,
-- Al
Thanks very much, Jeff, Rob, & Rebbi.

I should add to my previous post that one thing that couldn't be verified from the photos, in many cases, is that polarized electrolytic capacitors were installed with the proper polarity. But given the accuracy of everything that could be verified, including the polarity of some of those capacitors, I suspect that too was accomplished with perfect accuracy.

Best regards,
-- Al
We also discovered that I have four ceramic capacitors soldered in the wrong position on the driver board, although these are not in the signal path and would not be causing the symptoms we've been hearing. (Brian is going to send me a set of replacement capacitors.)
Rebbi, I don't quite understand that. The ceramic capacitors I see on the driver board in this photo from your blog are C1, C2, C3, and C4, and they all appear to be in the correct locations per the markings on the board.

Where is Brian saying they should be located?

Best regards,
-- Al
OK, I just noticed in this photo that there are printed circuit connections visible within the board forming a direct connection between what I (and you) had believed were the two pads/holes for C1, and likewise for C4. Given that, it becomes apparent that the uppermost ends of C1 through C4 should go to the pads at the top of the board that are adjacent to (and connected to) W20 through W23, respectively. And the reason you need replacements for the capacitors, of course, is that the leads on the original ones were trimmed for mounting in the more closely spaced holes, and will no longer reach the correct locations.

Wow! An understandable and easy to make error, if ever there was one.

Best regards,
-- Al
Ronnjay, here is a Kit 1 schematic which I believe corresponds pretty closely if not exactly to the configuration supplied to Rebbi. (See page 3 of the pdf for a clear presentation of the schematic, although you will have to refer to the first two pages for the reference designations of the components). As you will see, C1 through C4 provide noise filtering and decoupling for the B+ that is supplied to the small signal stages.

Consistent with Rebbi's explanation of the reason for the two holes and pads at one end of the capacitors, it appears that some versions of the kit were provided with physically large 0.015 uf capacitors (see Section 7 of the manual), and others (including Rebbi's) with smaller capacitors which are 4700 pf according to the schematic. Both values seem to me to be within reason, with the choice depending on the range of frequencies that are considered to be most likely in need of being filtered.

Rebbi & Mapman, thanks for the nice comments.

Best regards,
-- Al
Wow! Congratulations, Rebbi, and kudos to Brian and Pete.

I've never heard or read of anything like that happening, but you might find the second from the last page of this paper to be worth reading, and possibly suggestive of something that might have led to the problem. It was written by Bill Whitlock of Jensen Transformers, who is a renowned expert on audio transformers.

Also, as you are most probably aware, a tube amplifier having output transformers should not be operated without speakers or equivalent resistive loads being connected (assuming the tubes are installed, that is). If perchance you operated the amp at any point without loads being connected (following the initial voltage checks that were performed sans tubes), I'm not sure that it could result in magnetization of the output transformers, but it seems to me to be a conceivable possibility so I thought I'd mention it to be sure.

Best regards,
-- Al
Dan (Drubin), the main reason for the cautions about not operating tube amps without a load is unrelated to magnetization. The concern is mainly the possibility of damage to the insulation of the windings in the transformer, and possibly also to the output tubes, that may result from what is called "inductive kickback," aka "inductive kick." That is the same effect by which the ignition coil used in cars produces the thousands of volts needed to fire the spark plugs. The voltage across an inductor (a coil) at any instant of time equals its inductance times the rate of change of the current flowing through it (i.e., the change of current per unit time). In the absence of a load on the secondary side of a transformer that would absorb the energy applied on the primary side, the primary acts like an inductor rather than as the primary of a transformer, and an abrupt change of current resulting from fluctuating signals can produce very large and potentially damaging voltage transients.

Obviously that will be a concern mainly if a signal is being processed through the unloaded amp. But IMO (which which some others will differ), although damage is presumably unlikely if no signal is present it would still not be good practice to operate an unloaded tube amp having output transformers even if no signal is present, because conceivably turn-on or turn-off transients could in effect put a brief "signal" into them.

As far as magnetization is concerned, as I indicated I haven't previously seen any reports of that being a problem with any audio amplifiers. But it does seem more likely that it would occur with a SET amp than with a push-pull amp, because with a SET amp the DC bias current, and in fact all of the current that is put through the primary of the transformer, is always flowing in one direction. It's magnitude fluctuates with the signal, but not it's direction. So the magnetic field generated by that current is always exerting effects on the magnetizable material in the core of the transformer in one direction, in contrast to a push-pull design where it alternates. That is a significant consideration that has to be taken into account in the design of transformers that are to be used in SETs.

As I said in my previous post, though, I'm not at all certain that operating a SET amp unloaded could cause or contribute to magnetization of the transformer. And in any event I would feel safe in assuming that Rebbi was knowledgeable enough to not do that. But I mentioned it as a possibility that from a technical standpoint seems conceivable to me, analogously to the mention that was made in the Bill Whitlock paper I referenced to how an input transformer used with a microphone that is "phantom powered" with DC could be magnetized if the mic were connected or disconnected while power is present.

Best regards,
-- Al
Very glad to hear the good news, Rebbi.

Speaking of Chesky, and also of classical music, I recommend that you consider purchasing a used or new copy of Chesky CD31, Dvorak's "New World Symphony," Jascha Horenstein conducting the Royal Philharmonic Orchestra. A magnificent performance of great and immediately likeable music, which is also one of the best sounding orchestral recordings I have ever heard. And one which I suspect would be particularly revealing of whatever the differences may be between the sonics of your De Capos and the Katz Meow or any other speakers you may compare them to. You won't believe, btw, that this CD was created from a recording made in 1962!

Although it is out of print, the CD can be obtained from a number of sellers offering it at Amazon.

Best regards,
-- Al
Rebbi, regarding power cords you'll most likely find this post by Atmasphere (Ralph K.) to be of interest, and perhaps the rest of the thread as well.

I would add that the factors he cites as being most important in a power cord, namely minimal voltage drop and adequate bandwidth, while probably compromised to an audible degree by the cord you are presently using, are readily obtainable for far less than the megabucks some audiophiles invest in power cords. In that regard see Ralph's post here, and also my response in the second post after that one.

I would also add that the consequences of voltage drops and bandwidth constraints that may be imposed by a particular cord figure to be highly dependent on the design of the particular amplifier, including its class of operation (i.e., class A, AB, or D) among many other factors. And the effects of voltage drops figure to be highly dependent on the happenstance of the line voltage at the particular location.

One thing I would definitely avoid doing is extrapolating any expectations about the performance that is likely to result with a specific power cord from experiences that may be reported with class AB or class D amps, which differ greatly from SETs and other class A amps in terms of how their AC current draw fluctuates with the dynamics of the music.

I'll also say that there is often what I perceive to be an incorrect tendency among many audiophiles (and I am NOT referring to any of those who have posted above, whose opinions I greatly respect) to conflate the musical resolution of a component or system with the ability of the component or system to resolve differences in hardware, such as power cords. Obviously there is a relation between the two, to some degree, but from a technical standpoint I see no reason to exclude the possibility that depending on its particular design a component providing a higher degree of musical resolution might be less sensitive to differences between power cords and other cables than one that is not as musically resolving.

The bottom line to my thoughts on the subject, FWIW: Consider Signal Cable.

On another note, I'll add my congratulations to the many that have been expressed regarding the great results you have already achieved with this project. And kudos once again for the beautifully done blog.

Best regards,
-- Al
You're welcome, Rebbi. BTW, Signal Cable sometimes sells at Audiogon, as member Frank9740. As of today he has 1,237 positive feedbacks here, with no neutrals and no negatives.

Best regards,
-- Al
Thanks, Rebbi. Glad my suggestion worked out, and thank you kindly for the acknowledgement. Enjoy!

Best regards,
-- Al