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Old 15th June 2006, 10:36 PM   #1
ash_dac is offline ash_dac  United Kingdom
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Question Gainclone - why use large value decoupling capacitors ?

Hi,

Why do most Gainclone designs use large capacitance on the positive and negative rails ? (100-1000uf)

The datasheets mentions 0.1uf decoupling caps, and I was thinking of using 0.1uf ceramic with 1.0uf tant's for decoupling.
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Old 16th June 2006, 12:40 AM   #2
preiter is offline preiter  United States
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The capacitors on the rails are there both to filter noise from the power supply and also to provide an extra repository of power to draw upon for transitory peaks in the output.
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Old 16th June 2006, 12:48 AM   #3
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Larger caps are needed because you are trying to deliver power to a low impedance load. 100-1000uF is not sufficient, either, unless you want to be one of the mindless horde that strictly adheres to the "original" gainclown "design" and then complains about anemic bass.

Try 10,00-20,000 uF on each rail for each amplifier. Many will disagree, but many do not understand the function of the caps and the meaning of a PSRR spec. Use 100-1000uF to bypass the larger caps, and use 1uF-10uF film caps to bypass those.

I know, that's not how Sakura Labs (or whoever originated this nonsense) makes them, but judging by their assembly skills from what I have seen in photos of their stuff, they aren't exactly experts.

If you want to make an amp that works reliably with whatever load you connect to it, I suggest read and follow the info in the data sheet for the chip. I know it's hard to believe, but the guys who designed the parts actually know what they are doing.

I_F
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Old 16th June 2006, 12:49 AM   #4
dfdye is offline dfdye  United States
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Moreover, with gainclones, there is not a lot of smoothing on the rails before it hits the large "decoupling" caps. In all actuality, these are the primary smoothing caps for the gainclones. If you were to insert another small 0.1uf cap to the chip's power pins, these would truly be decoupling caps.

As it stands, the incoming power to the chipamp.com and audiosector.com kits is rectified, but with no cap bank to get rid of the "humps" in the rectified power. This is why they use large caps to get rid of the ripple right before it hits the chip.

[disclaimer] At least this is my understanding of it. I haven't ever built one of the chips, but looking at the schematics, this is how it should work. [/disclaimer]
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Old 16th June 2006, 12:53 AM   #5
Eva is offline Eva  Spain
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0.1uF decoupling is only suitable when the chip and the big supply capacitors are in the same PCB and quite close together, which is the usual situation in consumer products.

However, never ever use 0.1uF decoupling capacitors alone in DIY projects. That's because in DIY the supply rails usually come to the amplifier boards through long and poorly laid out wires, whose inudctance will resonate like crazy together with the 0.1uF capacitors. In these circumstances, 10uF should be considered as a minium, being higher values like 1000uF advantageous because they reduce the amount of "chopped" class B "trebble" and "midrange" currents flowing through the supply wires.
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Old 16th June 2006, 12:54 AM   #6
dfdye is offline dfdye  United States
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Quote:
Originally posted by I_Forgot
100-1000uF is not sufficient, either, unless you want to be one of the mindless horde that strictly adheres to the "original" gainclown "design" and then complains about anemic bass.
YOu posted while I was typing!

I'll agree about liking more capatance on the rails, but I have found 4400uf per rail per chip was my sweet spot (just because I had a bunch of 2200 uf caps around!). This puts me close to the 10,000uf (why don't we ever change units on capatance????) IF recommends. Of course more won't hurt at all, at least in my experience. It definitely fills out the bass response better!
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Old 16th June 2006, 03:28 AM   #7
taj is offline taj
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Quote:
Originally posted by Eva
0.1uF decoupling is only suitable when the chip and the big supply capacitors are in the same PCB and quite close together, which is the usual situation in consumer products.

However, never ever use 0.1uF decoupling capacitors alone in DIY projects. That's because in DIY the supply rails usually come to the amplifier boards through long and poorly laid out wires, whose inudctance will resonate like crazy together with the 0.1uF capacitors. In these circumstances, 10uF should be considered as a minium, being higher values like 1000uF advantageous because they reduce the amount of "chopped" class B "trebble" and "midrange" currents flowing through the supply wires.
Interesting. Leach mentioned this too.

..Todd
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Old 16th June 2006, 05:28 AM   #8
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Keep in mind ALWAYS, that when you see a schematic, an example circuit in a datasheet, that example is NOT the entire "thing" you are building.

If you see (for example), "+35V" somewhere and only a 0.1uF cap in the subcircuit, that does NOT mean you only need a 0.1uF cap in the entire "thing" you are building, it means you need ALL of the parts necessary to have a relatively clean power supply of 35V getting TO that subcircuit the datasheet pictures.

Some parts are more tolerant of noisey power than others, have good PSRR spec for example, but until you know for certain this is the case, you have to assume you are supplying the rest of the design around the subcircuit provided.

This extends to most circuits, certainly not just Gainclones.
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Old 16th June 2006, 07:08 AM   #9
ash_dac is offline ash_dac  United Kingdom
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Quote:
Originally posted by Eva
0.1uF decoupling is only suitable when the chip and the big supply capacitors are in the same PCB and quite close together, which is the usual situation in consumer products.

However, never ever use 0.1uF decoupling capacitors alone in DIY projects. That's because in DIY the supply rails usually come to the amplifier boards through long and poorly laid out wires, whose inudctance will resonate like crazy together with the 0.1uF capacitors. In these circumstances, 10uF should be considered as a minium, being higher values like 1000uF advantageous because they reduce the amount of "chopped" class B "trebble" and "midrange" currents flowing through the supply wires.
Even on commercial designs such as by Naim, and Cyrus audio with external PSU's it was a head scratcher!

So It's better to think of these designs as a capacitor with 100cm leads...

Is there any difference between decoupling caps, and caps to provide current to the load ?

Click the image to open in full size.

http://www.6moons.com/audioreviews/4.../gaincard.html
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Old 16th June 2006, 07:47 AM   #10
real is offline real  Australia
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Hi all,

The whole point of decoupling caps is to negate (as much as possible) the effects of wire/track inductance. Think of it this way: the wires carrying the current to the load have some inductance (and resistance, but you can't do much about that). Due to the AC components of the current drawn by the load, there will be reactance in the wires from the inductance, which will cause the voltage rail to sag. However, by using decoupling capacitors, the higher frequency AC components in the signal will be shunted through this capacitor (instead of travelling through the wires), just leaving low frequency AC and DC that is travelling through the inductive wires. Hence there will be less loss on the wires.

Note that at audio frequencies decoupling doesn't have a massive effect (although side effects like reduced chip oscillation etc mandate decoupling), high speed digital switching is where it really matters. The value and type of capacitor is also important - you should check datasheets for the resonant frequency of the capacitor - this is where it decouples most effectively. However, when people are saying that you should use 10,000uF caps - this is not decoupling. This provides smoothing of the rectified AC, quite distinct from decoupling which aims to reduce AC components of the load current such that wire inductance doesn't have a significant effect.

Chris.
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