Cubie - small F5 variant with GR grade JFETs and LatFETs - diyAudio
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Old 4th July 2014, 04:06 PM   #1
juma is offline juma  Germany
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Default Cubie - small F5 variant with GR grade JFETs and LatFETs

In need for a nice 10-Watt amp I went through my inventory and found enough spare parts to make this little F5 variant.
Transformer is a 200VA, 4 x 12V AC (was saving it for a small circlotron but too lazy to obtain enough same-sex output parts).
Input JFETs are k170/j74 GR grade (Idss about 3mA) leftovers that come very handy here because with Rs=10R they run at Id of about 2.5mA which is high enough to drive properly Lateral MOSFETs and low enough to provide enough OLG (Rd is about 650R for LatFET's Vgs of 1.6V i.e. Id of 0.7A).
Laterals are k1058/j162, used without Source resistors so using the square law benefit to provide more class A headroom which is really noticeable when driving the low Z speakers.

The Cubie (kids are merciless) was extremely fast so I had to tame it a bit (that's why C19 was installed). The oscillograms show 24Vpeak-to-peak output (the clipping starts at about 26Vpeak-t0-peak) at 90kHz with 3R3 and 3R3||1uF loads and the amp's behavior is exemplary.
Thermal behavior is also very nice - heatsink size is 23 x 18 x 4 cm and after two hours they reach a temperature of 18 degrees Cels. above ambient (MOSFETs are 3 degrees hotter). DC offset drifts about 15mV from cold to hot.

It's worth mentioning: output MOSFETs' bias (Id) will depend on Idss of input JFETs and theirs Drain resistors (R16+R18 i.e. R17+R19) so wide range of values can be used but it's essential to measure JFETs Idss first.

The PCB doesn't show C19 because I added it later (prototype was tamer then the PCB version). I installed C20 on the RCA connectors - it's optional, I use it only because some of my signal sources leak DC. C20 is a combo of 1uF Philips MKC and 100nF WIMA MKP 10 which sounds very neutral to me.

Schottky rectifiers (MBR745) are used in PSU to minimize loses i.e. to avoid diode heatsink - too much work, there's 16 of them... Ripple voltage under the load is about 20mV_rms and the amp is dead silent on my 93dB/W/M speakers.

Finally, the sound surpassed my expectations - on my 4 Ohm speakers (Canton CT-1000 and Epos M5) it's even better than my "end of the road" amp (F5 variant with k2013/j313).
_____________________________________
Edit: there is a text error on the PCB drawing - the text states dimensions 60 x 25 mm but that's a typo - real dimensions are 65 x 25 mm
Attached Images
File Type: gif Cubie_PSU-AMP-PCB.gif (81.2 KB, 985 views)
File Type: jpg LookInside1.jpg (207.2 KB, 960 views)
File Type: jpg LookInside2.jpg (256.1 KB, 915 views)
File Type: jpg LookInside3.jpg (169.2 KB, 875 views)
File Type: jpg 90kHz@3R3.jpg (58.4 KB, 846 views)
File Type: jpg 90kHz@3R3+1uF.jpg (60.9 KB, 241 views)
File Type: jpg Front.jpg (95.3 KB, 254 views)
File Type: jpg Back.jpg (123.2 KB, 253 views)
File Type: jpg PCB.jpg (66.8 KB, 291 views)
File Type: jpg PSU.jpg (98.9 KB, 297 views)

Last edited by juma; 4th July 2014 at 10:59 PM.
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Old 4th July 2014, 04:40 PM   #2
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That's very nice. I have some similar pieces using the Alfet ALF08N16V and
ALF08P16V parts with good results.

I tamed the top end response with input resistor values larger than 1K.

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Old 4th July 2014, 04:40 PM   #3
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Nice as always
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Old 4th July 2014, 05:45 PM   #4
juma is offline juma  Germany
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Thank you guys

Quote:
Originally Posted by Nelson Pass View Post
...I tamed the top end response with input resistor values larger than 1K.

Yes, that would be more in the spirit of F5 - keep the caps out of the amp circuit.
But it was easier to add a cap than to change the resistor. After all, we are all adults here - who's afraid of a small cap ?
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Old 4th July 2014, 07:13 PM   #5
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My question is obvious; is it better than the Amp in post #66 of this thread LSK pre - BAF 2013

What when I just got some more parts for the latter amp? Thanks.
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Old 4th July 2014, 07:54 PM   #6
juma is offline juma  Germany
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If you mean the LSKpre with Laterals PP source follower added, it's different. I can't say which one sounds better - Cubie is a tiny bit more authoritative and the other one has a "velvety" quality a bit more pronounced...
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Old 4th July 2014, 08:45 PM   #7
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I like the description, '"velvety" quality. I hope the 'bit more pronounced' will be o.k. with my Fostex168Sigmas?!

Right now I am driving both the speakers connected in series and a single channel ACA. They sound more real that any amp I have had so far, with any speaker, for that matter.

The authoritative part for the Cubie is inviting, but I guess, more than 10 watts would allow for better dynamics.
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Old 4th July 2014, 10:03 PM   #8
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What`s the efficiency of the speakers you`re listening to and did you experienced clipping ?
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Old 4th July 2014, 10:12 PM   #9
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Hi why no power supply decoupling at the supply lines ? IMHO that would be good design practice.
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Old 4th July 2014, 10:33 PM   #10
juma is offline juma  Germany
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Quote:
Originally Posted by jean-paul View Post
Hi why no power supply decoupling at the supply lines ? IMHO that would be good design practice.
It usually is.
If you look at the schematic you'll see that every channel of the amp has independent PSU so inter-channel coupling through supply lines is practically nonexistant.
Also, schematic shows 1uF film caps (C4, C9...) which are stack-foil type (almost non-inductive) which are soldered beneath the PSU board. Putting them on the amp's PCB introduces the risk of creating a resonant circuit (together with inductance of wires that connect PSU to the amp) because this amp is extremely fast.
So, good design practices are implemented here...
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