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Thoughts on the little caps in this phono input wiring?


aussievintage

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This is the input wiring on the Luxman 700x I am playing around with.  I just noticed these 0.1 caps on each of the phono input sockets. They are right there on the official shematic as well.  The caps appear bypass the ground connection to the same ground connection !!

 

My theory - it quietens down some high frequency noise that either the inductance of the amps internal ground wiring lets through to the phono preamp, or bypasses noise being picked up internally on that same internal ground wiring.

 

 

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It is better to look at where 

 the components are attached. Each capacitor ( ideally ceramic ) should connect directly to the individual rca socket shroud and the other end directly to the chassis by the shortest possible route. This bypasses any RF signal that may be present, to ground. The  links between all the rca sockets indicate a  common ground buss that connects to common ground point elsewhere.

 

 

 

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27 minutes ago, VanArn said:

It is better to look at where 

 the components are attached. Each capacitor ( ideally ceramic ) should connect directly to the individual rca socket shroud and the other end directly to the chassis by the shortest possible route. This bypasses any RF signal that may be present, to ground. The  links between all the rca sockets indicate a  common ground buss that connects to common ground point elsewhere.

 

 

 

Yes, that's where they are and they are ceramic.    So why does the common ground bus need to be bypassed? Is it just the longer wiring has some inductance and hence does not adequately ground the RF?   If so, this is an idea I think would help in another preamp I have been having a little trouble with noise pickup on.

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RF signals can be picked up on the outer shielding of connecting cables as they act as aerials. If you allow the signal earth returns to be contaminated  circuits within the amplifier can be effected. The bypass capacitors stop this problem. Any RF pickup on the 'active' inputs are dealt with by a simple  resistor/capacitor network and sometimes  the addition of ferrite beads.

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6 hours ago, aussievintage said:

Yes, that's where they are and they are ceramic.    So why does the common ground bus need to be bypassed? 

When you look at RF.  Parasitic resistance comes into play,  a straight piece of wire can be 0 ohms at DC...  but at AC the impedance of the same length of wire is very different.

Edited by Addicted to music
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25 minutes ago, Addicted to music said:

When you look at RF.  Parasitic resistance comes into play,  a straight piece of wire can be 0 ohms at DC...  but at AC the impedance of the same length of wire is very different.

It's really the impedance due to inductance that varies with frequency and does not exist at DC.  Resistance of the wirewill be  constant.  But yeah, this seems to be what's going on.    As I said, the idea gives me a clue as to some noise I was picking up in a totally different preamp, and I bet this solution will work well there.  

 

6 hours ago, VanArn said:

RF signals can be picked up on the outer shielding of connecting cables as they act as aerials. If you allow the signal earth returns to be contaminated  circuits within the amplifier can be effected. The bypass capacitors stop this problem. 

 

and this is exactly it.  The noise, possibly even lower frequencies than RF,  on this other preamp is coming in on the shielding, I am sure.   I will be adding some ceramic "bypass" caps to it very soon.

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7 hours ago, aussievintage said:

 

and this is exactly it.  The noise, possibly even lower frequencies than RF,  on this other preamp is coming in on the shielding, I am sure.   I will be adding some ceramic "bypass" caps to it very soon.

Ceramic caps are not the best for audio, they are microphonic and will effect the circuit depending  where they are placed And deployed, I would removed them and use film equivalents  however all phono stages are extremely sensitive,  if you take the cover off on some the circuit has the potential to pick up any noise even stray radio frequency transmission, that’s because the cover will be grounded and acts like faraday cage. 

 

 

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Bypassing RF signals  at the input ground points with  ceramic  capacitors  is the correct method. Film capacitors have  more  inductance  and as a result are not as effective in this role. Microphonic issues   do not come into  this scenario as the caps.  are not in  series with an audio gain stage, where, certainly quality capacitors should be used.

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Bypassing the RF signal at the ground is only one method is only effective to that part of the circuit; the RCA input, . this doesn’t mean it will be effective as we do not know where the noise is being induced on his other preamp.  Assuming that what the OP thinks by using the same method as above in that circuit may not be the solution, all he can do is try it, suck it and see.    I did say that the  microphonic characteristics will effect the circuit will depend on where it’s deployed, if the op uses the same method in the circuit outlined then  no.    Just because film caps have more inductance is irrelevant, ceramics  cause more issues.  

Read up on the Silicon Chip Power amp serious and as this version progresses to the latest version,  there is a section as to why they removed all ceramic caps in the circuit for RF bypass and self oscillation,  interesting read And interesting results with and without using ceramics and why they went film.  And this is for a power amp, so you can imagine what effect it will have on a preamp.

 

Edited by Addicted to music
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6 hours ago, Addicted to music said:

this doesn’t mean it will be effective as we do not know where the noise is being induced on his other preamp.  

by elimination, I think it must be coming in on the shielding of interconnects - I suspect, from some digital sources I also have connected.

 

6 hours ago, Addicted to music said:

all he can do is try it, suck it and see.

I agree, and it's very easy to do

 

6 hours ago, Addicted to music said:

Read up on the Silicon Chip Power amp serious

 

??  Which one?  Is there a link, or do you have to subscribe to read it?

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4 hours ago, aussievintage said:

 

 

??  Which one?  Is there a link, or do you have to subscribe to read it?

You have to be a member to access the file, in which I’m not.  I use to borrow them from the local library and took pics of the article when it was reference for the month.  I don’t have any of them because I’ve upgraded the phone.   It’s a 3 part series, the latest was an up to date version on the convert to SMD for the VAS stage which is there issue of keeping up with the Jones.   I believe it’s the 2nd series and the ceramics originally used In the 1st published article was to prevent oscillation if memory serves me right but don’t quote me on it it’s almost a decade since I last saw it...   but moving on what I do clearly remember is that removing them from the circuit and using film caps improved the distortion.  

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@aussievintage

 

heres a direct copy of an reply from the Editor; Nicholas from Silicon Chip in 2011 when I was reading the article and I was making a few suggestions and enquires, I did a search on my personal email, have a read on the final paragraph on ceramic caps.  If you don’t believe me, PM your email address and I’ll forward you the email that’s time stamp.   

Edited: now it’s coming back to me, the article was there integrated version where they decided to fit the preamp/poweramp into the one box, so you need to get that edition...  

 

Regarding the number of inputs, we think most people these days will have digital audio signal sources anyway and few analog sources. So adding more analog inputs doesn't seem worthwhile for most people. If you plug the DAC project into one of the three stereo inputs on the Ultra-LD Mk.3 preamplifier then you get three digital inputs and two analog inputs, allowing connection of (say) a CD/DVD/Blu-ray player, computer, set top box, phono preamplifier and AM/FM radio. In the few cases where people may have more sources than that, external digital or analog switchers can be used. It doesn't seem worthwhile to add cost and size for extra inputs that few people would use.

The Studio Series preamplifier is designed to be a standalone project anyway whereas this one is designed to fit into a chassis with a stereo power amplifier. So the new preamplifier doesn't really replace the Studio Series (although a standalone version could be built) and the limited space in the chassis after the other components are installed dictates that this one have a relatively modest size.

I agree that volume control pots are a problem. I'm not sure what you mean by a "resistor ladder", whether you are referring to a mechanical switch loaded with resistors or a digital IC with an internal resistor ladder network. The former are awkward and can not be remote controlled while we already published a project using the latter a few years ago.

I'm not sure what you mean by "a better op amp". The op amp we used in the Ultra-LD Mk.3 preamplifier is the right one for the job and the result is very low distortion; significantly lower than the Class-A preamplifier that the design is based on, at frequencies above 1kHz. It also has the benefit of being commonly available and cheap. We tested several other more expensive and well-regarded models like the LM4562 and OPA2134 which gave worse results.

You say "Chasing absulute distortion figures where instuments cant detect is pointless if it is not detectable by the human ear and poinless to achieve" but if our goal should be something other than low distortion then what should it be? Clearly, presentation and features matter but in terms of performance, if the distortion is no longer detectable, what improvements can we make?

Yes, you can replace the ceramic capacitors in the audio path of previous preamplifier designs (and other audio circuits). In many cases it will give a measurable improvement in sound quality (whether it will be audible is debatable). Ceramics are probably OK for op amp compensation (ie, where there are separate compensation pins on the package) and RF filters (if the value is low enough, eg, 100#/100pF) but in most other cases should be avoided.


Nicholas

 

 

Edited by Addicted to music
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11 hours ago, Addicted to music said:

@aussievintage

 

heres a direct copy of an reply from the Editor; Nicholas from Silicon Chip in 2011 when I was reading the article and I was making a few suggestions and enquires, I did a search on my personal email, have a read on the final paragraph on ceramic caps.  If you don’t believe me, PM your email address and I’ll forward you the email that’s time stamp.   

Edited: now it’s coming back to me, the article was there integrated version where they decided to fit the preamp/poweramp into the one box, so you need to get that edition...  

 

Regarding the number of inputs, we think most people these days will have digital audio signal sources anyway and few analog sources. So adding more analog inputs doesn't seem worthwhile for most people. If you plug the DAC project into one of the three stereo inputs on the Ultra-LD Mk.3 preamplifier then you get three digital inputs and two analog inputs, allowing connection of (say) a CD/DVD/Blu-ray player, computer, set top box, phono preamplifier and AM/FM radio. In the few cases where people may have more sources than that, external digital or analog switchers can be used. It doesn't seem worthwhile to add cost and size for extra inputs that few people would use.

The Studio Series preamplifier is designed to be a standalone project anyway whereas this one is designed to fit into a chassis with a stereo power amplifier. So the new preamplifier doesn't really replace the Studio Series (although a standalone version could be built) and the limited space in the chassis after the other components are installed dictates that this one have a relatively modest size.

I agree that volume control pots are a problem. I'm not sure what you mean by a "resistor ladder", whether you are referring to a mechanical switch loaded with resistors or a digital IC with an internal resistor ladder network. The former are awkward and can not be remote controlled while we already published a project using the latter a few years ago.

I'm not sure what you mean by "a better op amp". The op amp we used in the Ultra-LD Mk.3 preamplifier is the right one for the job and the result is very low distortion; significantly lower than the Class-A preamplifier that the design is based on, at frequencies above 1kHz. It also has the benefit of being commonly available and cheap. We tested several other more expensive and well-regarded models like the LM4562 and OPA2134 which gave worse results.

You say "Chasing absulute distortion figures where instuments cant detect is pointless if it is not detectable by the human ear and poinless to achieve" but if our goal should be something other than low distortion then what should it be? Clearly, presentation and features matter but in terms of performance, if the distortion is no longer detectable, what improvements can we make?

Yes, you can replace the ceramic capacitors in the audio path of previous preamplifier designs (and other audio circuits). In many cases it will give a measurable improvement in sound quality (whether it will be audible is debatable). Ceramics are probably OK for op amp compensation (ie, where there are separate compensation pins on the package) and RF filters (if the value is low enough, eg, 100#/100pF) but in most other cases should be avoided.


Nicholas

 

 

 

 

Thanks for posting that.  Interesting stuff.  He makes the point about them being in the signal path.  In the use we are talking about above (bypassing the ground wiring) if the value is chosen correctly, it won't really be in the signal path,  because it is short circuited by the ground wiring at audio frequencies.

 

The opamp comment was interesting as well.

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1 hour ago, audiofeline said:

So (as a non-technician), I'm curious.  If adding cap's in this way removes unwanted RF from the system, why isn't this standard practice in preamp design?

 

I would think that if the ground from the connector only had short wiring on it, or soldered straight onto a big earth bus, or circuit board ground plane, then it wouldn't be as necessary.  

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