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PostPosted: 31 May 2010, 17:59 
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Oatley of Australia have had a 6418 tube / LM1875 5W-10W power amp out for a while.

Image

I have been asked to review the kit. Usually I upgrade my kit parts but this time I decided to eval. the kit "as is". I even picked a flimsy metal case to place it in. The kit comes with an International voltage aware 24V switch mode power supply. Though the chip is capable of 30W or an easy 20Ws, in the kit's current form the amp only delivers 5W. 90db efficient speakers (minimum) must be used to get good volume levels. My KEF iQ30s work well with the amp.
Attachment:
OatleyK281SS.jpg

Apart from a thump (see fix later in the post) on power-up the amp plays exceptionally well.Sound stage is holographic and extremely well defined. Bass is very controlled, mids and highs clean and extended. The kit is dead simple to assemble. I had both boards finished in 30mins. Very easy. My construction was dual mono. Both amp modules are in the one case fed by two separate supplies. I had to put my own touch on this amp some where so I installed two UV leds on the front panel to indicate when the amp modules are powered-up. Yes I could have used blue leds like everyone else but low glowing UV leds looks very schmick!!
Attachment:
K281FrontSS.jpg

Attachment:
K281RearSS.jpg

To fix the power-on thump in the easiest of fashions. Install a standby switch, DPDT. Place one 5W 15ohm resistor in series with each speaker. With the switch off the thump is shared between the speaker (8ohms) and the 15ohm resistor. After a few seconds throw the switch to short the 15ohm resistors to provide full power to the speakers. Timing ccts. with relays would be an automatic way of doing this but for me the standby switch works.
Attachment:
K281-Instructions-6418-Tube-LM1875.pdf


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PostPosted: 31 May 2010, 18:39 
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This looks really good, I want one, too.


Keep it up, Mark.

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My DIY Audio Projects:
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PostPosted: 02 Jun 2010, 12:36 
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Looks like a nice fun kit Mark. How does it compare to your LM3875 builds? This LM1875 chipamp kit is different in that it uses a single supply and output capacitor.

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PostPosted: 02 Jun 2010, 17:52 
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Gio wrote:
Looks like a nice fun kit Mark. How does it compare to your LM3875 builds? This LM1875 chipamp kit is different in that it uses a single supply and output capacitor.

If it was the only amp you have you would be very happy. It sounds better on the KEF iQ30s than the big 3 way Osborn speakers. I did a comparison with my entire valve system and just the amp and CD player alone on the Osborns. I'm writing up the result now. I think I can attach a file to the forum.

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PostPosted: 03 Jun 2010, 16:53 
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After some comparisons with my main all valve system and the new Oatley Electronics K281 connected direct to my Oppo CD player I am happy to report the kit amp performed extremely well. I have written a lengthy article on testing procedure, what I found etc. which follows in the next message.

The article will appear in the Melbourne Audio Club magazine in July.

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PostPosted: 04 Jun 2010, 14:33 
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Tubes and Chips

David meets Goliath
I don't usually start with a shoot out between some newly constructed gear and a reference. Usually I talk of the build quite often starting with the philosophy of the build or at least what inspired it.

BUT this time I'm going to start with a direct comparison of a new tube and chip hybrid kit power amp I have recently constructed and my very commendable bread board (Be Bamp) all valve power amp.

The test goes a bit deeper than swapping-out one power amp for another. You may scream "unfair" when I tell you I am going to compare an all valve system: preamp and power amp to a hybrid integrated power amp directly connected to the same CD player. But let's see how David the tiny K281 amp weighing only a few grams can compete with the multi-kilo Goliath Be Bamp and valve preamp. Unfair? NO! The Osborn speakers and interconnects are common.

The selection of stones
The method of comparison was to play five or so varied CDs on one system then to play the same CDs and tracks on the second system and to compare the two. I was looking for more than the best bass extension and airiest highs. I wanted to know which system could convey the emotion and passion of the music best.

Tough ask of a new $29 kit from Oatley, in a flimsy metal can compared with an expensive system developed over many years. But could integrated, singular substrate technology and a Russian ex-military tiny tubes complete? And against technology from the forties??!!??

Selections were played and noted evaluations made. I played all selections through before swap in the K281. This was so I didn't hear the same track twice in a row on two systems which can fool the brain that the second play is better, regardless.

I started with the Be Bamp after giving it a long warm-up period. At the same time I had the K281 warming so no amp was dead cold at test time. I played the selections through taking only mental notes and aware how the music moved me. I then swapped out the total valve section and fed the Oppo CD player directly into the K281using the same interconnects used in the system before.

The CD selection was then played again at similar volume levels. At this point I had been brought to tears by a few selections. The K281 needed to do the same thing or at least come close before I would be convinced this David of the amp world could topple my resident Goliath.
Attachment:
Oatley-K281-6418-Tube-LM1875-DIY-Amp-kit-1.jpg

The results. No hanky required.
Most critics when comparing gear look at bass extension and control, depth and clarity of mids and extension detail of highs. So let's start with that and move onto the more emotionally subjective. My resident system has brought other listens to emotional highs in the past so it has moved others than myself.

Bass: The bass from the K281 was definitely more extended and with greater control than the Be Bamp. Though the bass sounded trimmed down leaving just the notes and bearing the notes with no support. The bass from the Be Bamp was fuller, thicker more complex though woolly and fluffy-not as defined. But far richer and with greater weight. I liked the Be Bamp bass better-a personal thing.

Mids: The mids were absolutely lovely from the K281. Piano notes were clean and bright, voices clear, tonal and fast. Osborn Eclipse speakers are possibly the perfect speakers for the audiophile purist who listens to predominantly classical, jazz, acoustic, opera, male and female voice and stays clear of heavy rock, blues and pop (all types) music. Mids are what Eclipse are good at.

Once more I felt there was too little weight behinds those perfect notes or am just a sucker for valve bloom?? Mids from the Be Bamp carry a big stick and are well articulated. More convincing more attention seeking, not necessarily better. For me I liked the mids from the Be Bamp.

Treble: The extension to treble was very obvious with the K281. With a frequency response extending to 500kHz there is NO upper frequency roll off in the audio domain. You could ask for little more from an amp. Bright, very extended highs without any shrillness. Excellent, clean, precise and un-colour with well controlled sibilance.

Treble in the hands of the Be Bamp is far more a controlled matter than a sheer show of upper-end brilliance. The Be Bamp top end extends to 35khz. More than enough you may say. But research has shown that even frequencies up to 200kHz influence the music we hear. So in this case the K281 holds all the cards.

The Be Bamp also has an excellent control of sibilance. Remember the Be Bamp has no negative feedback, local or global. Negative feedback can influence frequency extension but also alter the treble in a bad way. I would have to say I may prefer the treble from the K281 but that does not say there is anything wrong with the Be Bamp treble.

Timing and pace: This is where all the boys stand on one side of the room and the men with golden ears stand on the other. The Be Bamp has an iron fisted grip on rhythm, pace and timing. NOTHING slips, slides or smears timing on the Be Bamp's watch.

On a few tracks I felt the K281 had lost the timing altogether. Notes jumbled on top of notes, rhythm was lost and timing appeared to move forward and back but rarely on the down beat.

Emotion: If you think all the other subjective banter is hard to express, emotional essays re-writes subjectivity. I'll make it short though this is what I was looking for, more than anything else in the amp. The K281 just could not convey emotion. Bucket loads of all the other good things, a wonderful amp in many ways but could not convey emotion. To another listening on the same support gear, it may be a very different story but for me, no hanky required.

Needless to say the Be Bamp can make you consider taking shares in Kleenex. It oozes great globs of passion, power, dynamics, depth and intimacy. The K281 is a clean, "bare-it-all" amp and with the right support gear will surprise you even move you. But cause you to sob uncontrollably, I think not. But then not everyone wants this. Clinical or emotional-pick your poison.
Attachment:
Oatley-K281-6418-Tube-LM1875-DIY-Amp-kit-2.jpg

The build
Now we are at the end of the article let's start at the beginning. The kit comes from Oatley Electronics of Sydney Australia as a mono amp module with a Jan6418 (sub-mini tube) frontend and LM1875 backend or power end. Brilliantly crafted by Branko (owner of Oatley) himself.

The chip is capable of 30W but in this build, with 24VDC international mains voltage power tranni, only 5W into 8ohms is forth coming. It's not hard to up the output. Into 4ohms 10W will be delivered.

I decided to build "as is" out of the kit without my usual mods and upgrades. I can see a dozen ways to improve the kit but this would not show the kit for what it is. The only "Houston touch" is the installation of dual low-glow UV LEDs, not blue LEDs.

An inexpensive flimsy case was used to house both amp modules and dual power packs (which came with the kit) were used. One 100K pot was used for the attenuator. The amp is light and compact.
Attachment:
Oatley-K281-6418-Tube-LM1875-DIY-Amp-kit-3.jpg

Conclusion
So where do I see this great amp being best used. As a computer amp for a couple of inexpensive speakers? No this would be a great waist of a very clean sounding amplifier. As the main amp for a big audiophile system? No not quite the high-end audiophile amp in a number of ways. How about a surround amp in a massive theatre system driving the front speakers? Definitely not. So where does it fit in?

I have a second, smaller listening area. Here I have my KEF iQ30s, upgraded Oppo 980H player, Hitachi digital tuner and some good leads. Perfect. This is exactly where the K281 fits in. It is a den amp where critical listening/analysis is not involved. The amp must perform cleanly and have a very wide frequency range and be able to good sounding and engaging. The K281 can do that.

The den amp must be pleasant sounding and be totally honest in what it can do and not lie about what it can't. And it is. This is where it lives in my house and this is where it will stay for a long while. Where would it live in your home?

Opera Convener – Mark Houston.


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PostPosted: 05 Jun 2010, 04:06 
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The various tube/hybrid projects from oatley are much appreciated and bears witness that someone hasn't just had a lots of imagination but also a drive to satisfy hoards of tubeheads out there.
However, I am no big fan of the tube used. I have built the headphone amp and it doesn't impress me.
Couldn't the good Otaley folks used other tubes like ECC88-family or if size was an important factor other pensize tubes.
I sometimes see notes from people who wants to build a hybrid Grain Clown, but I guess an ECC81 or 76N1P is a better choice.

Besides, where did all the power go? The ICs are rated at 25-30W ...

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PostPosted: 05 Jun 2010, 08:45 
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Soundbrigade wrote:
The various tube/hybrid projects from oatley are much appreciated and bears witness that someone hasn't just had a lots of imagination but also a drive to satisfy hoards of tubeheads out there.
However, I am no big fan of the tube used. I have built the headphone amp and it doesn't impress me.
Couldn't the good Otaley folks used other tubes like ECC88-family or if size was an important factor other pensize tubes.
I sometimes see notes from people who wants to build a hybrid Grain Clown, but I guess an ECC81 or 76N1P is a better choice.

Besides, where did all the power go? The ICs are rated at 25-30W ...

It is easy to understand why Oatley do what they do the way they do it when I say they buy stuff. No I'm not being smart. They buy 1000 24VDC switch mode PSs. They buy 5000 JAN6418s, They buy 1000 LM1875s. Then they says what can we do with these???

OK a tube and chip amp. Single 24VDC supply=5W from a chip. So they buy stuff then think of a kit they can put together for the DIYer that could be fun. But the fun is taking an Oatley kit and making it your own.

I happen to like the portable headphone amp kit. I like the sound but then I changed quite a few parts in the kit and made it my own. With my Cowon i9 and Phiaton MS400s phones I think I have the best sound I have ever had. After saying that a customer at work listened to my Splinter HP driver (single 2N3055), the Oatley headphone amp and my original CMoy in a Cedar box with cotton coated silver wire. He gave me $220 for the CMoy and came back the next day and raved about to the point I had to throw him out of the shop.

Different strokes for different blokes. Hybird Clown - I like it.

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PostPosted: 05 Jun 2010, 08:55 
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Seeing it from that point, the kits are genious.

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“If you don't know where you are going, any road will get you there.”
―Lewis Carroll


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PostPosted: 05 Jun 2010, 09:05 
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Soundbrigade wrote:
Seeing it from that point, the kits are genious.

The owner, Branko, of Oatley is continually pestering me to come up with another kit idea around the parts he stocks. I helped him fine tune the phono preamp which has impressed about a dozens serious listeners so far.

I have a lot of respect for you. Make some suggestions what you would like to see come out of Oatley as a kit. I'll pass on the suggestions.

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