SMT ceramic cap found in critical audio path in high end AV amplifier, please advise? - diyAudio
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Old 22nd January 2016, 12:25 PM   #1
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Question SMT ceramic cap found in critical audio path in high end AV amplifier, please advise?

Hi, it's a while since I've posted on this forum but I would really appreciate some opinions from other experienced engineers on this one please if possible...

Attached are high quality photos and schematics of the circuit in question.

The outputs from the PGA2311 DCAVC are AC coupled to the inputs of the next stage via 2 x 1uF SMT ceramic caps. The legend on the schematic is incorrect, they are non-polarised caps.

One has been removed and measured. I believe they are not NP0/C0G types, or indeed any other different type such as film, due to their physical characteristics. As far as I can tell they are X7R or similar. The amplifier was designed approx. 10 years ago and the capacitors have not been changed since it was manufactured.

My immediate thoughts are I believe in line with conventional wisdom, that this type of capacitor should not be used in these locations. Does anyone have any different thoughts on this?

Many thanks, Jim

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Old 22nd January 2016, 01:28 PM   #2
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Looks like the capacitor is an excellent choice and the ESR is very good. One cannot expect to fit wire ended components on an SMD board and after all, the conductivity of SMD components usually exceed wire ended, as long as Lead Free solder is used to solder them or replace them, (it stops leaching of the tin coated conductors at each end of the component).
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Old 22nd January 2016, 01:46 PM   #3
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Hi Jon,
Thanks for the reply. I was not intending to replace with a through-hole component, rather upgrade to a SMT film type.

I can find very many references from reputable and classic sources, articles and books arguing against using ceramic caps in a signal path as shown, I realise they have improved over the years but I don't think this is a type which has been improved - the equipment is fairly old and it looks like a bog standard ceramic as would normally be used for supply bypassing, etc. and I have not seen such caps used in the signal path before, I don't think?

Thanks
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Old 22nd January 2016, 02:05 PM   #4
sangram is offline sangram  India
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You'd be surprised at how often caps like this appear in AV equipment.

Panasonic ECPU1C105MA5 is a good replacement, but beware the low voltage ratings of film capacitors in relatively small sizes. This limits the maximum input voltage (I think these are limited to 10VAC, check datasheet). These ones are 1210, which should just about fit into the existing 1210 pad. The presence of bias complicates matters as it reduces the overall voltage handling ability.

If you are aware of the parameters determining the possibility of using them, you can substitute the caps for film types. In this specific case you may have some issues because of the circuit itself.
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Old 22nd January 2016, 02:10 PM   #5
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These are nice caps and mostly 50V working.
Polyphenylene Sulphide Film Capacitors | RS Components
You may have to dig about a bit as the values are a bit limited.
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Old 22nd January 2016, 02:14 PM   #6
DF96 is offline DF96  England
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1uF feeding a load of 47k will have an LF cutoff around 3.3Hz. This may be low enough that there is little signal across the cap, so little scope for generating distortion due to dielectric nonlinearity.
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Old 22nd January 2016, 02:14 PM   #7
anatech is offline anatech  Canada
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Hi Jim,
Yes, use a film type surface mount. You're going to be further ahead if you pre-tin the capacitor and one pad. Once the new capacitor is positioned properly, solder the other side.

I use eutectic 63/37 solder - leaded. You need the lower temperature and superior wetting too. A touch of liquid flux can do wonders. In this way you can avoid damaged new parts.

I have a pair of Cyrus Mono-X amplifiers I plan to go through. I do expect a big improvement in performance after the changes. I have already purchased the replacement parts (long time ago). I will be following my own advice to you.

Think about replacing those TL072 chips with something like the 2134 or similar. Keep the J-Fet input type op amps in your search for better devices. You can do a lot better than the TL07x series chips.

-Chris
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Old 22nd January 2016, 03:09 PM   #8
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Thanks for the replies and suggestions

Yes this thread was mainly posted because in 20 years of working with audio equipment I don't think I have once seen ceramics used for AC coupling between high quality audio stages, and in fact in all that time so much advice has soaked in to the contrary so this came as a bit of a surprise. But I understand there may be minimal distortion for the above reason... it just seemed an unusual choice by the designer.

It is a leaded board and I'm experienced with working with SMT, but thanks for that additional advice all the same. I think I will replace with film as I originally thought, I'll do some careful selection considering the circuit topology and physical constraints.

The amp has already had a lot of work done to it over the past week - it is nearing completion and there is an upper limit to the amount of additional time I can spend doing additional upgrades such as OPAMPS, etc. because it belongs to a customer. The majority of OPAMPS elsewhere are OPA2134.

The majority of the repairs were in the digital stages, I've also renewed electrolytics throughout due to age. Some of the electrolytics for supply filtering were rated at 16V on a 15V supply, so the voltage rating of those has been increased during the repair.
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Old 22nd January 2016, 05:55 PM   #9
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I found huge improvement in replacing tantalum 5 uf input caps on my amp with 50 V cop ceramic input caps, aerovox gold to be precise. The original 1970 caps passed less bass, the tantalum replacements had frying pan popping hissing noise. I suspect my local parts store was selling rejects, as was common before e-bay consumed the stock of rejects.
Look at the capacitance versus voltage curve of ceramic, and imagine a 1 v signal impressed on a 50 v cap. The curve is a lot straighter if you use only one fiftieth of the curve.
In your circuit with no diode clamps to the power supply, I'd be more worried about static blowing the TL072 through a low voltage rated film cap. Small size is a limiter.
My amp had an input impedance of about 500kohms to be compatible with 12AX7 plate drive preamps. You might straighten out the curve by curring current flow. Change the input and feedback resistors to achieve an input impedance of 200 kohms or so. If you have to change to metal film resistors to minimize hiss, so be it. Above 100k the difference hiss between metal film and carbon comp resistors is very audible. I've never bought any carbon film resistors to see how they fare in the hiss dimension. A diy poster said metal film was the standard in hiss reduction. I find no reason to buy anything else.
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Old 22nd January 2016, 06:17 PM   #10
anatech is offline anatech  Canada
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Hi indianajo,
Quote:
I'd be more worried about static blowing the TL072 through a low voltage rated film cap.
Not an issue with this op amp.

Hi Jim,
Quote:
The majority of OPAMPS elsewhere are OPA2134.
For a good reason. I do believe that a change would be audible, but the customer is always the boss ... well, usually. SMT stuff is a quick swap though. I've been in that biz for >35 years now. I'll agree that ceramic capacitors are unusual to find as coupling capacitors. I have seen this in low end equipment, large 50V disc capacitors. I never stopped to worry about the exact flavor of ceramic as it wasn't NP0 / C0G, that much was certain.

If you don't mind me asking, what brand is that in? You can PM me if it is a sensitive issue.

-Chris
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