A Zobel Question on STK4241II - diyAudio
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Old 17th August 2014, 12:14 PM   #1
Ronj is offline Ronj  Singapore
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Default A Zobel Question on STK4241II

HI,

I am replacing the fried powerstage of a Sansui A-60 with a STK4241 Hybrid Chip Amp. Rail volatges are +/-52V. So I understand I can only use 80hm Load. The PCB is exactly like the typical application circuit below

Click the image to open in full size.

I already wired up a board except the zobel part. I have used 0.33Ohm 5Watts resistor.
Is it okfor me to use a 10H inductor there at the output instead of the recommended 3H? I know that's quite a high value. But I have it ready in hand. I have the following from Panasonic.

Specification from element14 as below
CHOKE, 10UH, 3.9A, 0R026
Product Range: PANASONIC - ELC-10D Series
Inductance: 10H
Inductance Tolerance: 20%
DC Resistance Max: 0.026ohm
RMS Current (Irms): 3.9A
Saturation Current (Isat): -
Packaging: Each
Case Dimensions: 10mm x 15mm
Core Material: Ferrite
DC Current Rating: 3.9A
Inductor Case Style: Radial Leaded
Inductor Type: Choke Coil
Lead Spacing: 5mm
No. of Pins: 2
SVHC: No SVHC (16-Dec-2013)
Test Frequency: 10kHz


Is this good enough as the inductor in series to the speaker load? Can I still use a 4.7Ohm resistor to by pass it?

I assume I can also use another 4.7Ohm in series with.1uF to the ground across the speaker load just as in the circuit diagram? I read somewhere than when used with 4Ohm impedence loads(with out even considering the inductance/resistance of the speaker cables), 10uH will result in a 0.4dB attenuation at 20Khz? Will my high end be really affected? Or shall avoid the inductor whole together? But I assume it's necessary to avoid high frequency oscillations.

Sorry even if I know a bit about the LC and RC filter equations I am a bit lost here on which all values to consider when calculating. My network theory fundamentals are rather weak

Another question is what wattage should those 4.7Ohm resistors be?
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Old 18th August 2014, 01:53 AM   #2
Ronj is offline Ronj  Singapore
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No one wants to help me?
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Old 18th August 2014, 03:32 AM   #3
djoffe is offline djoffe  United States
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I assume p/n is ELC10D100E.

Here's a link to the data sheet:

http://www.mouser.com/catalog/specsheets/ELC_Series.pdf

From a power dissipation point of view, it's rated at 3.9 Amps DC. Once again, strictly from I-squared-R dissipation, that says that you could dissipate 3.9 Amps RMS. That comes to be 3.9^2*8=121.7 Watts with an 8 Ohm load.

With a plus/minus 50 volt supply and 8 ohm load, you can deliver max of about 150 Watts...That says, for power dissipation, in the worst case, it's not quite up to the task.

Perhaps of greater concern is that the inductance will vary with signal swing...that will certainly increase distortion. Unfortunately, they give no curves for inductance versus current (or temperature, for that matter).

so...you could use all the typical components and values around it, and it would be ok...just not totally blameless. That there's 10 uH...at 20 kHz, that looks like 1.25 Ohms at 20 kHz...there will be a little rolloff...about 0.1 dB (assumes 8 Ohm load).

So, unless you're trying for ultimate blameless performance, they should work ok.
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Old 18th August 2014, 06:22 AM   #4
Ronj is offline Ronj  Singapore
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Djoffe, thanks a lot for reaffirming my decision. I do not see myself running this amp beyond 50Watts per channel. With the current rail-voltage of +/- 43V I surely won't hit 100Watts even at max as there definitely will be a power supply sag also. And yeah I don't think I will use with a load with 8Ohm impedance at 20KHz either. It will be much more higher. So I guess I will be safe in this case.
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Old 18th August 2014, 06:34 AM   #5
Mooly is offline Mooly  United Kingdom
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Inductors for stability such as this one are usually (always ?) air spaced. Just turns of wire with no ferrite core.
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Old 18th August 2014, 06:39 AM   #6
Ronj is offline Ronj  Singapore
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So what happens if there is a ferrite core? Will it hurt stability?
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Old 18th August 2014, 06:48 AM   #7
Mooly is offline Mooly  United Kingdom
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It can/will saturate on transient peaks... and nonlinearity... its just not the done thing

Your amp circuit is a little unusual. The 0.22 ohm is the one of the main components to aid stability by isolating the amplifier output from capacitive loading. The amp also runs at very high gain (56k/560 feedback factor) which also helps stability. The 0.1uf and 4.7 ohm "Zobel" network isn't technically a Zobel. Its on the wrong side of the inductor for that. It terminates the speaker cable at hf but not the amplifier (as a Zobel would) because of the inductor. The design seems to not need a Zobel (which is unusual) but the other factors I outline obviously render it not required.
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Old 18th August 2014, 06:57 AM   #8
Ronj is offline Ronj  Singapore
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The circuit there is from Sanyo's Data sheet. And I am using it as is. The PCB I have, has provision for an inductor. But not for the bypassing resistor. So I am just soldering the resistor on to the inductor itself. Again the PCB I have doesn't have the RC filter(100nF, 4R7) provision so I am also doing a P2P for that from the output. I am doing all this after reading horror stories of STK chips oscillating. Anyway I am on my way to pick-up an old Oscilloscope and function generator. Few hours later I will power up and see what do I see when fed with a 20KHz sine wave. Including the attenuation. I am happy to have 50W per channel. Because anything above STK produces massive THD like 10%.
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Old 18th August 2014, 07:00 AM   #9
Ronj is offline Ronj  Singapore
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Oh yeah I wired it up now using 5W/4R7 carbon film resistor and the same 10uH coil.
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Old 18th August 2014, 10:53 AM   #10
Mooly is offline Mooly  United Kingdom
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That is something that shows well in simulation.

Here is your set up and the response with a 10uh and a 3uh coil. And squarewave of 50kHz into 8 ohms.

Also, here is the response at 5kHz with a squarewave using the 3uh value and when the amp is loaded capacitively. Look how the parallel resistor damps the ringing (last picture).
Attached Images
File Type: png Circuit.PNG (32.2 KB, 53 views)
File Type: png Response.PNG (29.4 KB, 53 views)
File Type: png Square.PNG (40.4 KB, 52 views)
File Type: png 5kHz square.PNG (60.2 KB, 50 views)
File Type: png Square plus R.PNG (40.7 KB, 8 views)
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