Improving the Coupling Capacitors for Better Tone Sounding

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Hi!

If the caps you want to replace are old and if they are without pedigree then an improvement will be had with either Sprague "Orange Drop" capacitors or WMF (I think they are CDE brand, aka Cornell Dublier). You will find these on ebay. I think both can be found new. WMF are expensive. Orange Drop can be polyprolpylene or polyester...I have used both....don't hear much difference. Guitar players like them. WMF has a great tone overall, big and warm...probably the best sounding cap ever. They tend to be expensive. Just say no to paper in oil (or anything from former USSR) or any paper , that's old tech that is often decaying like salmon that has spawned.

Don't buy electronic components on ebay (unless you know and trust the seller) - tonnes of fakes and QC rejects. That goes double for ICs.

CDE (yes Cornell Dubliner) orange drops are available new from reputable sources (in North America Digikey and Mouser as examples, who will also have Sprague electrolytics; both probably ship to SA as well). Sprague orange drops are available from most tube amp (hifi or guitar) parts suppliers.

I never heard about the NPO/CGO ceramics capacitors at least with those names!; do you have more info about the Scott Gurcer study or investigation? Where I can find this investigation paper or notes?

Thanks a lot,
opacheco

NP0/CG0 (those are zeros) is the dialectric used in some MLCCs (MultilLayer Ceramic Capacitors) - only use MLCCs of this type (if it doesn't say don't buy them). Old school ceramic discs (and other types of MLCCs) are bad - wide tollerance and drift significantly in value with changes in temperature (so like in a hot tube amp) so performance (trem rate as mentionned above, filters etc) will be inconsistant. They also get worse over time. NP0/CG0 MLCCs are not cheap (they won't break the bank but compared to other dielectric types and basic ceramic discs they cost significantly more).

Def replace ceramic discs with NP0/CG0 MLCCs for a performance improvement (or at least more consistent performance). Only buy them from reputable stores. (again Digikey/Mouser and the like). I use them in my guitar pedal builds. One good brand is Murata.

Polystyrene are also good for the pF range, but they tend to cost even more (use them if you need higher V handling - then they may be comparatively cheaper than NP0/CG0). They may be better in tube amps because they are axial vs radial like MLCCs.
 
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I could have sworn that the old cap factory I used to work at 40 years ago made NP0 ceramic discs. Those were quite stable as I recall. Or this could be early onset dementia?
 
I could have sworn that the old cap factory I used to work at 40 years ago made NP0 ceramic discs. Those were quite stable as I recall. Or this could be early onset dementia?
Npo or cog (?) have been around for long time but mainly only tiny values for use stabilizing clock timing crystals and applications like that.

Their recent popularity for use in audio has encouraged manufacturers to offer larger values (not sure which came first, larger values or wider use?).

This makes me happy (at least a little). ;)

JR
 
Leaded NP0/C0G caps for tube amps (250V +) are sorta rare, 0.15µF seem an upper limit unless you have a DoD budget.
This one looks like not too expensive:
https://www.digikey.com/en/products/detail/kemet/C331C104KAG5TA/6658912
It is uphill from there.
Hard to think of reason to use them for low frequency.
If you have space constraints and have to use ceramics, pick a voltage rating very much higher than the actual working voltage.
This is a 10V cap taking a 70% dump:
https://weblib.samsungsem.com/mlcc/mlcc-ec-data-sheet.do?partNumber=CL05Y105KP6VPN
Perhaps OK for an LF VCO ;-)

As decoupling caps, ceramics can provide a much better response at higher frequencies.
 
I haven't seen any replies about this, so I'd like to bring some attention to two characteristics of capacitors that actually can affect their audible and electrical performance.
1. The first is actual impedance versus theoretical impedance. Any passive or active component only operates properly within its expected frequency range, and if it is used at frequencies beyond that range they won't perform in the expected way. RF engineers are painfully aware of the fact that standard diodes and transistors just don't work when you get into the microwave range. Both resistors and capacitors however, can exhibit undesired performance at an eventual high frequency too, where their parasitic inductance becomes significant. Since a lot of capacitors are actually rolled up like cinnamon rolls to cram the two plates into that small space, there is indeed a frequency where the parasitic inductance takes over. If you plot the measured impedance of say, a large electrolytic capacitor, you will eventually find there is a frequency where this inductance kicks in, and it will look more like an inductor instead of a cap. Hopefully that happens well above the audio range so it doesn't screw with your beautifully designed crossover networks, and amplifier feedback loops. This is the real reason that some folks put small value parallel capacitors across those big electrolytic caps, and the rule of thumb often used is to choose a value that is 1/20th the value of the big cap. If you want to get it just right, go ahead and measure the actual impedance performance of those big electrolytic caps in the power supply and those used as coupling caps in the signal path. and choose the correct cap to put in parallel to allow the impedance to continue to drop like it's supposed to.
2. The other capacitor defects are dialectric absorption (DA), and dissipation factor. In simple terms, DA is when a cap that had a charge on it is expected to allow you to suddenly remove that charge. This happens thousands of time a second with audio signals, but caps that aren't so good can have a tendency to hang onto that charge instead of letting it go. This can be audible if it is significant enough, and you will find that very high quality caps will have low DA. Now its quite possible that some guitar amp users may like this sloppy cap performance and that's fine if it make the sound you're looking for. Measuring DA with actual signal sources is not often discussed, but there was an article in Audio Amateur issue 4/1985 page 22, that describes a circuit that you can build to see the effect right on your oscilloscope. I believe there is still a copyright holder on the articles from Audio Amateur, so I can't reprint it, but I will show you the schematic and photos of the one I made, and oscilloscope pictures of some of my measurements using it. Interestingly, John Roberts, one of our regular forum contributors is mentioned at the end of the article as someone who wrote a letter to Audio Amateur about this test.
This circuit uses a reference cap, and has a bridge network that allows you to compare a second cap to it, and display the residue difference on the oscilloscope. I used a Kemet metalized polypropylene 1 uF/630V cap for the reference, and compared it to an old tubular Wima, a new stacked Wima MKS, and finally an old Sprague tubular, probably mylar. The two Wimas are almost identical, but the Sprague has MUCH more residue, especially when you notice that the 'scope's vertical sensitivity is set at 8V/div instead of 500 mV/div. The signal source is a pulse generator with a pulse width of 10 ms, and a period of 30 ms. The source amplitude is about 12Vpp.
When I first built it, I used tiny trimmers for the pots, but that made it really annoying to use, so this version used regular pots and knobs, which made it much easier to adjust. The Radj2 is to compensate for the ESR, while the other 3 pots adjust for the time constant. C1 band limits the input signal. The differential instrumentation amp with precision gain shifting is pretty expensive at $59.29 from Mouser, but the rest of the parts are pretty cheap, and it makes a hell of a cap tester. It will tell you which caps are really really good, but it will also tell you what to look for in a weird cap that you have decided you really like in your guitar amp. Have fun.
 

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20 years ago when I was into building guitar amps, someone on one of the boards did a capacitor material shootout. Same spot in the circuit, same value, only the material was different. It may have been on 18watt or the now gone ax84 site.

What I do remember is that clay disks sound like trash. The highs are very shrill and piercing when compared to the smoother sound from other materials. Since then, I have used nothing but polyester for anything in the signal path (or a poly-something). The only time I don't is if I can't find the needed value in poly, or the price is more than I want to pay (I don't want to spend $6 for a 100pF cap).

Unfortunately, I don't recall any of the specifics for the other materials, just avoid clay disk in the signal path if possible.
 
What I do remember is that clay disks sound like trash. The highs are very shrill and piercing when compared to the smoother sound from other materials.
That's exactly what I hear.

I want to blabber further.

This thread was begun by someone who wanted good advice regarding capacitor replacement in some little amp....I suspect it was old. A few replies such as mine gave concrete suggestions( such as mine as welll as daschnoz)...direct answers...while many diverged off into Aspergerish tangents, essentially wasting the time of inquiring individual, the end result being that he may been put off or ended up making a bad decision. I suggest to all that an effort be made to avoid tangents.
 
Interestingly, John Roberts, one of our regular forum contributors is mentioned at the end of the article as someone who wrote a letter to Audio Amateur about this test.
Yes back in the day I participated in some of the capacitor discussions over at TAA. I also wrote about capacitor non-ideal characteristics in my "Audio Mythology" column in RE/P. This was all several decades ago (1970-80s) so IMO old news.

JR
 
That's exactly what I hear.

I want to blabber further.

This thread was begun by someone who wanted good advice regarding capacitor replacement in some little amp....I suspect it was old. A few replies such as mine gave concrete suggestions( such as mine as welll as daschnoz)...direct answers...while many diverged off into Aspergerish tangents, essentially wasting the time of inquiring individual, the end result being that he may been put off or ended up making a bad decision. I suggest to all that an effort be made to avoid tangents.
Sometimes there isn’t a simple one-size-fits-all answer

This forum is one with a longstanding reputation for keeping the focus on the empirical and measurable.

Subjective statements like “disc caps have shrill high end” are totally fine, they just don’t find much of an audience here, as a rule.

Which shouldn’t feel like any great hardship—the majority of online fora (e.g. gearspace, audiokarma) are plenty receptive to them
 
So these shrill capacitors. That would imply that these capacitors do not follow the standard reactance equation. In order for the difference to be apparent, they would have to divert wildly from this equation. So some figures please. Shrill implies extra hf. So how many extra dB at 10 kHz?
 
So these shrill capacitors. That would imply that these capacitors do not follow the standard reactance equation. In order for the difference to be apparent, they would have to divert wildly from this equation. So some figures please. Shrill implies extra hf. So how many extra dB at 10 kHz?
Lousy voltage coefficient could introduce some added distortion. I'm not sure how audible it would be. Most competent design engineers avoid such dielectrics for use in audio filters.

JR

PS: I have another Peavey story about this but nah.... even I'm tired of them. ;)

[edit- sorry I can't resist... Back last century while working at Peavey I discovered that one of the other engineering groups was using some cheap ceramic caps (with poor linearity) in audio filters. They would monitor the distortion at the final output and replace only the caps causing measurable distortion with more linear film caps (like mylar/polyester). I didn't dig any deeper, it wasn't my area of responsibility or authority. Depending upon where used in a circuit, non-linearities like voltage coefficient could be more or less likely to express. I proudly declare that I am cheap, but I am not that cheap. :rolleyes: /edit]
 
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So these shrill capacitors. That would imply that these capacitors do not follow the standard reactance equation. In order for the difference to be apparent, they would have to divert wildly from this equation. So some figures please. Shrill implies extra hf. So how many extra dB at 10 kHz?
Not necessarily extra HF; can also mean distorted HF, or added HF harmonics.
 
Hi to everyone!

I am trying to improve the tone quality of this tube amplifier at maximum! and I would like to know what kind of coupling and dis-coupling capacitors can I use in order to achieve this superior sound quality, instance to the actuals Mica cheap capacitors used actually?

I have attached the schematic for your considerations and advices.

Thanks in advance!
opacheco
Avoid polyester capacitors. They made som distortion in the middle band. Douglas Self wrote about this problem.
Best regards!
 
Look for the article series “Capacitor Sound” by Cyril Bateman

Polyester has relatively less distortion than most types, but relatively (slightly) more than polypropylene, C0G/NP0, and a select few other technologies
 

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