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PostPosted: 16 Oct 2010, 11:47 
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Hi,

Yup thats right. I did use the light bulb test when I first tested the amp last week so was pretty confident in readjusting it today since it is going to be in a chassis now. With a silly slip of the probe, bang!

All is ok now.

I have tested my small 20W 18V transformer and it can power up 2 CPU fans, the speaker protector board and preamp module without breaking a sweat. Going to be a long, late night of wiring today ! :mrgreen:

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PostPosted: 16 Oct 2010, 14:18 
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Here is the mains wiring (hardly comparable to Benny's :mrgreen: )...

Chassis grounded close to the mains ground. (white wire secured onto a washer and nut.)
All exposed internal metal parts (tranny body/heatsink) are in electrical contact with the chassis, thus they are grounded. The double pole, single throw switch turns on both the amplifier and the preamp + DC protection module at the same time. Mains fuse is 2.5A for the amplifier circuit, while 0.5A is used for the smaller transformer.


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PostPosted: 17 Oct 2010, 08:33 
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Here is the 75% completed amp. The only thing left to do is the input and output wiring to the binding posts and RCA connectors... 8-)


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PostPosted: 18 Oct 2010, 14:28 
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After wiring up, I had some problems. The speaker protector which is on the top right corner requires 18V AC power, so the AC wiring caused a bad hum no matter where I tried to position it. In the end I removed it. There is a slight buzz now. I am unsure of the grounding procedure.

Is a wire connection from PS ground to IEC earth/chassis ground necessary ?

Listening impressions :

The bass is a standout. Finally it can dig as deep as aussie amps offering slightly less damping but full of authority as before. Detail is all there, but presented in a non fatiguing way. It is a cross between a valve amp, ZCA and aussie amps. The midrange is hyper detailed but a little warm and relaxing, highs are softer, silken and tones down the utter agressiveness of the Scanspeak tweeters. Overall tone is rich like a SET valve amp. Extremely pleased with this amp ! The sound has become more coherent, soundstage depth has improved. All of this from a very late night listening session and I can't crank it up to really hear the differences; but I can say that this amp belongs in another league. It is not the case where you have 2 amps, and one offers a slightly different presentation to the other. This is just another level, another 5 rungs up the audio ladder enabling me to get closer and closer to lifelike sound.

The P3A + P37A DoZ preamp is sure a hell of a performer.

Edit: The bass performance is better now compared to when I used 60,000uF per channel. Looks like overkill capacitance doesn't make much of a diff in class AB amps.


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PostPosted: 18 Oct 2010, 18:17 
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Hi Dinesh,

Make sure all AC wire pairs are twisted together. Where a signal wire pair (twist together) has to cross an AC wire (which should also be twisted) do so at a right angle. Don't run them parallel to each other. Try run the AC wires in the corner of the chassis.

So how is your grounding layout? Did you use a star grounding system? Can you post some photos of the wiring and ground?

Cheers

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PostPosted: 19 Oct 2010, 08:33 
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Hi Gio,

I have solved the hum issue. I connected the PS ground to the chassis ground directly and the hum has been eliminated. There is a slight hiss which I find inaudible from listening position. There is something amiss with my speaker protector and it created a massive hum. I have since removed it.

I guess I will follow this grounding style next time. I have used it in the ZCA and it remains the quietest amp I have built.

I am not 100% sure what a star ground is even after reading about it..
Anyway each stage has it's own ground. The amp module has a star ground, as well as the preamp module. So I guess the star ground is already present in the PCB.

The first pic shows the white wire connecting from the middle of the brown MKT caps (Power ground)
to the chassis ground.

The main difference in this amp vs my older version is the bass power. Previously, I had an output cap on the preamp feeding into another input cap at the power amp's input. Thus, a 22uF cap is in series with a 1uF cap which might cause some bass roll off. Without the 22uF cap, the sound is noticeably more transparent and the stereo image is more defined.

To sum up, here are the changes compared to my older build :

1. All resistors in the preamp are carbon film vs metal film.
2. Output cap of preamp removed.
3. Input cap of preamp changed from 1uF Wima MKT to 3.3uF Mundorf M-Cap.
4. Ultra low leakage small signal diodes in the preamp PS.

Will be bringing this to Singapore to have a comparison with a friends Krell KSA 100. That is one hell of a heavy amp running in pure class A. Tremendous bass control but the midrange is slightly too relaxed.


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PostPosted: 19 Oct 2010, 09:18 
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Here are a couple more shots from the top...

One more pic will be taken with the top plate fitted tomorrow...


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PostPosted: 19 Oct 2010, 20:25 
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Awesome! Can't wait to hear about the comparison! ;)
Cheers

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PostPosted: 21 Oct 2010, 08:58 
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Ok here goes....the shootout of P3A fully integrated vs Krell KSA 100 + Adcom preamp

Both amps sounded quite similar, but P3A had a more forward image location and depth. The Krell was more 2 dimensional but broad. We played music from Queen - Another one bites the dust and the Krell had a more massive grip on the bass guitars. Dynamics of the tracks were strangely won by the P3A. All of this from a mere 10,000uF per channel compared to Krells "coke sized can" caps. Probably 1 Farad or so. The P3A exercised the cones of the 18W midwoofer to the limits. 2cm point to point excursion was seen on the cones ! What an incredible midwoofer. The distortion (if any) was hardly audible.

Overall tone of the Krell was leaner and a touch more detailed. Saxophones sounded thinner in comparison. P3A was overall more musical, with vocals having a warmer tone. On complex orchestral pieces, both amps were able to resolve the fine details, but the Krell had a slight advantage in layering of instruments (violin and violas were more easily identified).

The main difference is in the bass. Considering that the amps were run on his demanding 86dB 3 way speakers - (6.5'' Scanspeak 18W, 12M and R2904 tweeter), I expected the Krell to perform better, and it did. The P3A finds my 8'' woofers a breeze to drive.

The final comments were " Can compete in the $2k range". He was quite impressed that a DIY amp could sound very good without any hiss/buzz. Considering that the KRell is $4000 new, and that he purchased it used for $2000, it did compete very well with the Krelll. I told him that my total cost was $250 and again he was flabbergasted.

How about another project that competes with amps costing $10K or more !! :confused: :eek:

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PostPosted: 21 Oct 2010, 21:49 
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Great listening notes Dinesh. Sounds like a big thumbs up for the P3A! :up:
Cheers

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