My latest project - fantastic power amp "Sigma" - diyAudio
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Old 1st September 2008, 08:15 PM   #1
Renx is offline Renx  Croatia
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Default My latest project - fantastic power amp "Sigma"

Hi everybody,
I just wanted to show you all a truly amazing power amp I have been building for the last couple of months, with very good specs...

power.........................225w/4 ohm, 160w/8 ohm
slew rate...................115V/us!!!
freq. response.............10Hz-105kHz (-3dB)
THD............................0.004% 100W RMS/8 ohm
..................................0.008% 180W RMS/8 ohm
..................................0.009% 240W RMS/4 ohm
TIM.............................0.005% 140W RMS/8 ohm (50Hz/7KHz 4:1)
s/n..............................>100dB (100W RMS)
Damping factor..............>450 (150W RMS/8 ohm)

It is called "Sigma" and it has been designed by Dr. Borivoje Jagodic from Serbia. Unfortunately, his web site is mainly in serbian http://bas.elitesecurity.org/.
He is a very helpful and understanding person, also an excellent english speaker, you can contact him by mail.

Anyway here is the schematic http://bas.elitesecurity.org/sigmasema.pdf

Its amazing slew rate is almost twice as big as Brystons 4B SST (60V/us) which is twice as powerful as Sigma.
Minimum figures to meet Jung and Pass criterion for audio quality is 65V/us for an amp of 250Watt/4Ohm, 200kHz bandwidth.
As you can see Sigma is fantastic...


Click the image to open in full size.
The stabilized +-75V PSU. 12x4700uF, 35A rectifier...


Click the image to open in full size.
Populated PCB. The output transistors used are matched Hitachi 2sk1058 and 2sj162 power mosfets.


Click the image to open in full size.


Click the image to open in full size.
Both pcbs together, as a mono block with custom 220V:57V 500VA transformer.


Click the image to open in full size.
The two mono blocks finished driving my Dynaudio BM15 speakers.
The back panel is custom laser cut and engraved from plexi.
The cases are aluminium, also custom made...

I haven't done any measurements yet, since I have no admittance to the expensive equipment needed. I will post the results as soon as my university starts and the labs open.

Many thanks to Dr. Jagodic for this exceptional design, it truly sounds BEAUTIFUL...
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Old 2nd September 2008, 09:45 AM   #2
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Downloaded the schematic and like the design (differential VAS)
This amp brings back memories of a kit I assembled in 1989.
Yours uses the more modern 1058/162 outputs while
mine had the old TO-3 laterals.

The sound of them hitachi's were SWEET ,the highs were
the best to hear.

VERY nice board, did you draw/ layout them out by yourself??
If you could, post your softstart,DC protect and regulated PS
schematics for us.

It looks like you are TRUE DIY ,start to finish ,Good work..OS
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Old 2nd September 2008, 10:20 AM   #3
Renx is offline Renx  Croatia
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The PCB layout of the power amp was given to me by the designer, Dr. Jagodic.
The softstart was from Elliot Sound Projects http://sound.westhost.com/project39.htm
so was the DC protection circuit
http://sound.westhost.com/project33.htm

I couldn't get the dc protection circuit to work for a week, so I gave up.
Instead I bought the Velleman K4700 kit, which is fantastic. It protects from dc an oscilations, it also saved my speakers during testing.
The PCB layout for these two circuits, I made my self, I could mail them to anyone interested.

here is the +-75V PSU.
Click the image to open in full size.


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Old 2nd September 2008, 11:03 AM   #4
h_a is offline h_a  Europe
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Quote:
225w/4 ohm, 160w/8 ohm
Seems there's some fun going on.

The 75V rails are good for more than 300WRMS into 8 ohms, however the 6 A fuse cuts that down to about the half of that.

Of course the maximum power into low loads is then also severely cut down and 225W are not possible into 4 ohms, but only less than 100 WRMS?

What am I missing here? Or is that really a strange power concept?

Have fun, Hannes
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Old 2nd September 2008, 11:12 AM   #5
Renx is offline Renx  Croatia
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well, the specs are from the designers web site, I haven't made any measurements yet.
There is actually no fuse on the pcb layout so I haven't included it either, maybe I should...

anyhow you are right!
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Old 2nd September 2008, 11:26 AM   #6
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I already have have all of elliot sound, thought you designed
your own, but Im glad to see these circuits actually used
for I have not built them and might have use for them in
the future.

Thanks for reg. PS schema, OS
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Old 2nd September 2008, 05:16 PM   #7
syn08 is offline syn08  Canada
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I hate being the party pooper, but there's something very fishy with this project.

At 8ohm and 160W output you need about 100Vpp in the load. Allowing 5V reserve means all you need is +/-55V. Then why using +/-75V?

At 100Vpp in the load and 4ohm you need 25A peak. The 2SK1058/2SJ162 are rated at Idmax=7A, for two pairs that's 14A. Chances are high this thing is going to blow in your face at max power into 4ohm load and only the forgiving nature of the laterals will save you from a fire.

At 4ohm load the max power dissipation will occur at 50Vpp in the load and that's about, on top of my head, some 160W, that is, an average of 40W/tranny. That's a lot to cool down.

Under the same conditions, Vds is about 30V and Id about 6A or 3A per tranny. You are driving the trannies very close to the SOA limit, and that's before any power derating (it assumes that trannies are at 25 degrees which is certainly not going to happen). So chances are high the amp will blow well before reaching the max power into 4ohm.

At +/-75V don't even bother to calculate what's going to happen. The thing is going to blow in your face ASAP. The only survival chance is if the power supply has a significant sag, but this will drastically affect the amp performance. The PSRR is not very good in this design.

If you want to meet those numbers I would suggest adding at least two more pairs of output devices and reduce the supply voltage to max. +/-60V.
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Old 2nd September 2008, 05:41 PM   #8
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an amplifier that takes into account the crest factor of music!
It is intellectually pleasing to see this!
Yet 75V on this poor BC558?
regards
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Old 2nd September 2008, 05:45 PM   #9
h_a is offline h_a  Europe
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Syn08,

don't worry you're no party pooper, you just forgot the 6A rail fuse

Have fun, Hannes
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Old 2nd September 2008, 05:50 PM   #10
Mooly is offline Mooly  United Kingdom
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Hello Renx,
My first thought on seeing this was "Hitachi application note" and "Maplin kit" circa 1980 ? and examining a copy of that project does show a large similarity.
I am sure it sounds very good indeed, those lateral FET's are my device of choice.
As to the performance figures, I must say I find the slew rate figure hard to believe, and agree with syn08, a quarter of a killowatt from a pair of 7 amp laterals--maybe but for how long
It's perhaps a bit unfair to critise without actually building and measuring it. Nice job building it by the way-- makes a change to see a real project, not just a simulation
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