decoupling caps for dip8 opamp - are they just there for a regulated supply? - diyAudio
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Old 15th December 2006, 05:17 AM   #1
Archwn is offline Archwn  United Kingdom
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Question decoupling caps for dip8 opamp - are they just there for a regulated supply?

Hello.

I have an adverse effect putting 390uF Panasonic FC // 0.1 uF normal ceramic one on both +- supply to ground near the opamp supply pin. This is when I use 2x9V batteries.

The sound was getting darker and muddier as far as I know. Not transparent like without those caps. I can't say anything more coz I didn't clearly hear anything else different to be honest.

The schematic was just the same as normal headphone amplifier one (like cmoy, grado) the opamp used here was opa627 and opa637

Do you think 390uF is too much? Any math that is behind this would be really appreciated. I'm a newbie in e-world.

Now I build the regulated supply using simple 7815 and 7915 regulated ic. Do you think what will be appropriate value for the coupling caps at this position?

Thanks,

AK
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Old 15th December 2006, 06:18 AM   #2
Nordic is offline Nordic  South Africa
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Cmoys are quite revealing in "component sounds" even swapping with other brands of the same value cap, will change the sound.

The value is not too high, my favourite cmoy has 2x 1000uF on the rails that I ripped from a motherboard.

I preffer .01uf (10nf) close the the pins, with 10uf close to that, and then the large caps closer to my battery.

which chip did you use? I can't figure if the one you mentioned just apeared in the schematic you had or if that is what you are using.
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Old 16th December 2006, 01:16 AM   #3
Leolabs is offline Leolabs  Malaysia
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How's the condition of the batteries???
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Old 17th December 2006, 01:45 PM   #4
CarlosT is offline CarlosT  United States
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Are ceramics the right type for this application?
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Old 17th December 2006, 01:59 PM   #5
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Decopuling caps close the the op amp serves a purpose whatever power supply you use (ie. also when using batteries). However, 390 uF is much more than required for most op amps, since they have good PSRR for low frequencies. Start with the values recomended in the datasheet, which will usually land on something like a 10 uF electrolytic, possibly bypassed with a smaller cap.

Often the datasheets will say something like a 1uF tantalum or a 10 uF electrolytic. Most people would probably advise against tantalums for audio, and some would would even advise against them for any application. Similarly, the datasheets will often recommend a ceramic for the bypass cap, but that is also often considered bad for audio, since most ceramic caps are highly non linear. While it is less obvious that this matters for supply decoupling than for signal (de)coupling, it may be wise to go for a film cap instead.
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Old 17th December 2006, 02:32 PM   #6
singa is offline singa  Singapore
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Default Re: decoupling caps for dip8 opamp - are they just there for a regulated supply?

Quote:
Originally posted by Archwn
Hello.

I have an adverse effect putting 390uF Panasonic FC // 0.1 uF normal ceramic one on both +- supply to ground near the opamp supply pin. This is when I use 2x9V batteries.

The sound was getting darker and muddier as far as I know. Not transparent like without those caps. I can't say anything more coz I didn't clearly hear anything else different to be honest.

The schematic was just the same as normal headphone amplifier one (like cmoy, grado) the opamp used here was opa627 and opa637

Do you think 390uF is too much? Any math that is behind this would be really appreciated. I'm a newbie in e-world.

Now I build the regulated supply using simple 7815 and 7915 regulated ic. Do you think what will be appropriate value for the coupling caps at this position?

Thanks,

AK

This applies equally to OPA627/637.You need a low inductance stacked film cap of 1uF. across the supply.Better also with the normal 0.1uF and an E-cap too.
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Old 20th December 2006, 11:35 AM   #7
Archwn is offline Archwn  United Kingdom
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Thank you everyone helping me out.

Nordic - Thanks for your info. I will try that too. I used OPA627/637 with 1k/10k resistor feedback loop. sometimes i took the blocking cap off too.

Leolabs - The batteries was 2x8.4V GP200mAh or GP300mAh. Do you think this could be involved?

CarlosT - I'm quite sure it's not. From what I have read, it seems to be another 'specific' ceramic type you guys are using but I can not find more info on this. I use the normal ones.

Christer - Would be good if you could tell me the purpose of these decoupling cap. All I know now is it just reserve some energy and discharge more rapidly than batteries when needed. There must be something more right?

When you say 390uF is more than required, does that also mean if I put too much of it will ruin the sound?

Would you please tell me what will be pros and cons for using tantalum cap? Not only the data sheet say it but in this forum too. May be it is a special type as CarlosT said. I will try film cap anyway.

I've tried standard .22uF ceramic for signal coupling for fun. It's even darker and now the stage is closed.

singa - Thanks for your info. I will have a look carefully and will give them a try.

BTW Does anyone know where is the best price to buy black gate cap? I would like to try that too. And auricap doesn't be made from gold right? what are they made off?

Thank you all,

AK
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Old 20th December 2006, 04:52 PM   #8
Nordic is offline Nordic  South Africa
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Will try to tackle a few Q's somoeone else can do what I skip.

2 charged 9Vs should be enough power.

The only ceramics you should use are np0 types, you would only use ceramics where you don't have space for film caps.

The diffirence lies in linearity... I.e. a 100nf being 100nf at 100HZ as well as at 20KHz (in ideal world)

Tantalums are very good for some things none of them seem to have too much to do with audio, so as almost a rule you cold ignore their existance for the time being, you will know when you need them.

In the right place a 1uf tantalum can do the same work a 10uf electrolytic would....so it is not that they are bad per se.

You are going to kick yourself when you hear the dif between a cheapo ceramic and a nice film cap... forget about hype and branding, many decent film caps out there that don't cary massive prices just because of a cult name...

Getting my first $5 caps was quite an eye opener.... seemed like a lot of money at the time compared to a ceramic at a few cents....
But I wouldn't do it any other way now.
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