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Chip Amps Amplifiers based on integrated circuits

Project Cute 16w
Project Cute 16w
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Old 15th October 2017, 09:48 PM   #1
flyingfishtw is offline flyingfishtw  Taiwan
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Default Project Cute 16w

I design a tiny amp which provides around 16Vp-p output swing on 8 ohm load, which is also 16 watt, cute little amp for both headphone and bookshelf speakers.

The main configuration is class A op amp gain x7 push darlington current stage. DC couple with offset adj pot.

For reducing johnston noise I use 10k volume pot, followed by opa1642, which is excellent in common mode distortion, it will handle the input impedance change well while adjusting voulme. Darlington will make the load extremely light for the opa1642, so I can push the potential of open loop gain to limit, and might be able to keep the chip cool.

On gain stage with unbalanced current source, which will force opa1642 operate -3mA in class A mode. Power supply will be simple regulated 18V for op amp, unregulated power for darlington stage.

Output stage I wanted to find something around 3A, 2SA19xx series, but A1940 was the only one I can get from local distributor, Now I'm a bit concerned to hook headphone in this system.

I'm concerning about stability issue, since opa1642 is rail to rail output, the output impedance varies, I don't know how will it interact with the capacitance between Vbe multiplier and in Bjt transistor. Also output transistor's BW is only 30Mhz, with 30mA quiescent current, it will be lower, I'm afraid it will create a second pole before close loop gain intersect with loop gain.

I've made PCB, some faults here and there. How bad will it be? that I didn't add 0.1uf decouple capacitor for current source.

What do you guys think about this cute 16 project?
Attached Images
File Type: jpg cute16 Zout.jpg (58.6 KB, 516 views)
File Type: jpg opa1642aol.jpg (23.1 KB, 497 views)
File Type: jpg cute16.jpg (248.4 KB, 540 views)
File Type: jpg cute16top.jpg (511.9 KB, 513 views)
File Type: jpg cute16btm.jpg (363.1 KB, 479 views)

Last edited by flyingfishtw; 15th October 2017 at 09:58 PM.
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Old 15th October 2017, 10:12 PM   #2
flyingfishtw is offline flyingfishtw  Taiwan
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cute 16watt, on the PCB I haven't decide how to connect different sections of GND yet. So everything is still scattering around.

For installing output transistor, I'll screw Q1 2SD669A on top of Q2 2SC5197, ,Vbe multiplier in between 2 output transistor, in simulation when Q1, Q2 has same temperature, the quiescent current(working current?) will be more stable when temperature changes. Trouble is Q1 has Pc around 1W, but in simulation it will consume maximum 1.1W, so I hope Q2 can help and dissipate some heat.
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Old 15th October 2017, 11:03 PM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by flyingfishtw View Post
I design a tiny amp which provides around 16Vp-p output swing on 8 ohm load, which is also 16 watt
For 16V p-p into 8R, the power is ( 16 / 2x1.414 )^2 / 8 = 4W.
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Old 16th October 2017, 03:50 AM   #4
PRR is offline PRR  United States
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Quote:
Originally Posted by flyingfishtw View Post
{snip}...opa1642 is rail to rail output, the output impedance varies, I don't know how will it interact with the capacitance between Vbe multiplier and in Bjt transistor. Also output transistor's BW is only 30Mhz ..... I didn't add 0.1uf decouple capacitor
You mean C3? That does not load the opamp at all.

The specified opamp is 11MHz gain-bandwidth, used at gain of 7. So closed-loop gain is 1.57MHz. The 30MHz output transistors, used as emitter followers, can be approximated as unity-gain to 30MHz, maybe 10MHz at low current. The output section does not appear to have a major 2nd pole inside the opamp's GBW.

The output transistors' hFE will fall with frequency, reducing the impedance seen by the opamp. At low frequency, 8r*100*100 or 80K, "no load at all". At 30MHz, 8r*1*1, heavy load. The opamp impedance is never as high as 1K. We need hFE*hFE> 120 to make 8r reflect as 1K. So hFE of 11 in each device. This is 2.7MHz, still well above where the opamp will go.

I have no doubt it "may" oscillate. Multi-stage amplifiers love to do that. Often not the circuit you designed, but the physical layout, including bypass caps at key points.
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Old 16th October 2017, 03:28 PM   #5
flyingfishtw is offline flyingfishtw  Taiwan
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Quote:
Originally Posted by rayma View Post
For 16V p-p into 8R, the power is ( 16 / 2x1.414 )^2 / 8 = 4W.
4W won't be cute, will be mini, you're sharp. I meant to say 32Vp-p. It's a good output power for bookshelf speakers. but for headphone, all I can say is , it's going to be powerful
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Old 16th October 2017, 04:04 PM   #6
flyingfishtw is offline flyingfishtw  Taiwan
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Originally Posted by PRR View Post
to make 8r reflect as 1K. So hFE of 11 in each device. This is 2.7MHz, still well above where the opamp will go.

I have no doubt it "may" oscillate. Multi-stage amplifiers love to do that. Often not the circuit you designed, but the physical layout, including bypass caps at key points.
Thank you for the detailed reply, I've tried to find the answer for this kind of question in Chinese forum with no luck.You're a savior I wrote in the schematic in mandarin, make sure ft>10*ACL BW. Guess it will be difficult to achieve.

what do you mean make 8r reflect as 1K, this part I don't quite understand. so op amp will see load as equal impedance and get half of the voltage?

Since I worried about the phase margin stuff, So I don't have the balls to put base stopper. I guess it will be a good idea to put it before q1. how would you recommend to decouple the darlington transistors, one 0.1uf for both, or one each?
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Old 21st October 2017, 05:45 PM   #7
flyingfishtw is offline flyingfishtw  Taiwan
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Done, it sounds pretty awesome. The photos I took was when I build it half way.

One small question, I adjust the output Iq 12mV, about 3x mA. After full loading 10mins, the measurement will be increased to 26mV, 8xmA, once stopped blasting, it will recover to 12mV in 1 min.

Is this okay heat stability?
Attached Images
File Type: jpg Cute 16s-3568.jpg (218.3 KB, 177 views)
File Type: jpg Cute 16s-3583.jpg (230.7 KB, 153 views)

Last edited by flyingfishtw; 21st October 2017 at 05:51 PM.
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Old 22nd October 2017, 11:17 AM   #8
AndrewT is offline AndrewT  Scotland
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Why did you set your RF filtering to F-3dB=1.5MHz?
I would drop that down two, or three octaves.
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Old 22nd October 2017, 11:47 AM   #9
flyingfishtw is offline flyingfishtw  Taiwan
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Originally Posted by AndrewT View Post
Why did you set your RF filtering to F-3dB=1.5MHz?
I would drop that down two, or three octaves.
thankx, for both stage I will change it to around 400k ~ 250k hz, is that too aggressive? When the Pot is maximum I can hear some buzz on sensitive headphone, does that have anything to do with the low pass?
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Old 22nd October 2017, 11:52 AM   #10
AndrewT is offline AndrewT  Scotland
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Many here use between 40kHz and 1MHz
But the consensus seems to be around 150kHz to 300kHz.
I use 160kHz (RC=1us) to 234kHz (RC=680us), or ~ one decade above 20kHz

400r + 270pF gives RC=108us
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