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Old 16th March 2015, 10:33 PM   #1
juma is offline juma  Germany
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Default Cubie2

Taking of from here (post #485), I adopted this lovely gain stage as the part of a power amp - Cubie sounds great but Cubie2 manages to sound a bit better.

So Cubie2 is an extension of BAF 2013 preamp idea, with current mirror (Q3/Q5 and Q4/Q8) instead of resistor/CCS and introduction of negative feedback loop (R7/R16) which sets the gain at about 19dB. The LATFETs in the output stage (Q9/Q10) work in source-follower mode, without source resistors, providing some extended class A regime due to the square law transfer characteristic of MOSFETs.

The input JFET pair can be of any flavor (GR, BL or V), it's just important to dimension R11/R13 so to get about 10-12 mA flowing through the folded cascode branch (Q6/Q7/Rbias). Values on the sch. are based on JFETs' Id of about 6.5 mA. With such JFETs I got about 10mA through Rbias=330R which sets the LATFETs Vgs_Q10 + Vgs_Q9 = 3.3V and the output pair's Id at about 0.7A, but you can use the 1k pot instead of Rbias to set the output stage current to different value or to accommodate for LATFETs' differentiating Vgs/Id characteristic (my current batch of k1058s need 1.70-1.75V for Id of 0.7A and J162s need 1.55-1.60V for the same Id). P1 sets the 0V DC offset at the output, and it's very stable: +/-10mV from cold to hot and +/-2mV when hot. It's also very stable into reactive loads - testing it, I didn't succeed to make it misbehave.
R25 is there to load the folded cascode and determine the OLG (about 45dB) while the C3 shorts the gates of LATFETs for AC signal.
Full power before clipping is 34W_peak at 4R load with 1.3V_peak input signal and about 19W_peak at 8R load with 1.4V_peak at input.

The PCB dimensions are 70 x 25 mm, copper side view. The GND connection for decoupling caps (C5/C6) should be made with separate wires to main GND point, independent from signal GND on the PCB.

The PSU is same as here (post #62).

I tested the amp without the feedback loop (R7 and R25 go out, Rbias is split into 2 x 160R and from their middle point 1k connects to GND) and it sounds great too but with feedback loop it has a bit more definition and bass firmness. Those who enjoy "velvety" quality of BAF2013 preamp might like it more without the feedback loop.
Attached Images
File Type: gif Cubie2JFET-SCH.GIF (8.7 KB, 1924 views)
File Type: gif Cubie2JFET-PCB_Layout.GIF (17.7 KB, 1900 views)
File Type: jpg Cubie2-PCB.jpg (264.5 KB, 1780 views)
File Type: jpg Cubie2-modules.jpg (285.0 KB, 1704 views)
File Type: jpg Cubie2-test1.jpg (500.0 KB, 1652 views)
File Type: jpg Cubie2-boxed.jpg (388.1 KB, 836 views)
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Old 17th March 2015, 01:38 AM   #2
The Dastardly Dad of Three
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Juma - nice and cute thank you for another very nice gift to the diy community
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Old 17th March 2015, 08:04 AM   #3
Itsmee is offline Itsmee  England
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Nice work juma, was thinking along similar lines to mod for an F4 for some gain.
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Old 17th March 2015, 01:54 PM   #4
juma is offline juma  Germany
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Yes, with +/-23V rails you can have 20V of output swing with this circuit - enough to drive F4 to full power.
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Old 17th March 2015, 11:46 PM   #5
fab is online now fab  Canada
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Nice work Juma

I really like your approach with the simple designs. You have already inspired me a few times, thanks!
Fab
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Old 18th March 2015, 03:43 AM   #6
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Hi Juma,

I wonder if it's possible to build two channels with 'random' output devices
(ie, without matching of P to P and N to N mosfets between channels)?

Thanks,
Dennis
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Old 18th March 2015, 08:11 AM   #7
juma is offline juma  Germany
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Hi Dennis,
sure it's possible, just take care to set the bias in accordance with Vgs/Id characteristic of your LATFETs.
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Old 18th March 2015, 12:24 PM   #8
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Quote:
Originally Posted by juma View Post
Hi Dennis,
sure it's possible, just take care to set the bias in accordance with Vgs/Id characteristic of your LATFETs.
Thanks. Now to order parts.

Cheers,
Dennis
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Old 18th March 2015, 10:01 PM   #9
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Nice as usual Juma, I use similar gain stage with a few twists in my amp for over 3 years and it sounds very good.
I think Ayre uses also similar gain stage for their amps and preamps but without feedback loop, just shunt volume control on the output for I/V conversion.
Anyway from my experiments this kind of gain stage sound better with very low local current feedback 5-6dB in my case, then without feedback loop, the difference I heard is in the Hi tones, they are more transparent and soft with low feedback loop.
C7 for compensation in my case was problematic, I`d use just low value shunt cap over R25, but I use different OPS.
Cheers, Borko .
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Old 19th March 2015, 08:02 AM   #10
juma is offline juma  Germany
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Yes Borko, the possibilities are many with this symmetrical folded cascode circuit and none of them sounds bad so everyone can find out what he likes the most and tune it to personal preferences and the system/speakers he has.
Cheers !
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