Improving the Coupling Capacitors for Better Tone Sounding

[ruffrecords]

It is a fact that capacitors with a large varying voltage across them will produce distortion. Steve Bench used 70V rms in his test which is about +60dBu!!!

Steve Bench's tests were conducted with 0.1uF capacitors. When a properly sized capacitor is used as a coupling between circuits, the voltage across it is miniscule. A typical 47uF coupling capacitor in a 10K circuit will have a voltage across it easily more than 120dB lower than this.

On the other hand, in an EQ, there are significant voltages across capacitors so non-linearities are more likely to be detectable.

Cheers

Ian

 

[GrannyGremlin]

What I enjoyed the most in this video, was this claim:

"Anybody who really know what they're doing in electronics will tell you that that that the coupling capacitors definitely are one of the more audible parts of any circuit"


I would love to know the opinion of people "that really know what they're doing in electronics" about this statement,
people that are very experienced and we can trust, even if just a few.

This is a complicated answer because it does not allow for a statement of magnitude, but just relativity, and in that sense the statement is correct simply because coupling caps are in the signal path, while almost all other caps are not (a few exceptions). Not only that, they are the biggest caps in the signal path (capacatively coupled outputs excepted, but those are not used in guitar amps that I have ever seen). But how much one actually hears caps, even when in the signal path is another matter.

Where he is wrong is that paper in oil caps are a good idea. We have already explained why they are not above. Like I've passed on guitars because the seller advertised that the tone caps were changed to vintage russian PiO (a right mess waiting to happen inside an expensive instrument that could damage other components or the finish, and since already old vs new prod, probably altready started that slow process; no thank you.

I do not believe (except with some limited exceptions re noise and other factors) that different caps types make any tonal difference. At the very best, this difference would be a point of diminishing returns. The only reason to change a cap is due to failure, needing/wanting a different value, or because the current cap type is suboptimal (old paper in oil, old electrolytic either of which may fail, and have likely already drifted significantly in value, or renmoving electrolytics from the signal path, and cheap ceramic discs, etc - tantalums can have a lifespam, just generaslly much longer than electros; noise issues w silver mica, but eg Fender nerds love them in the tone stack).

 

[opacheco]

It is a fact that capacitors with a large varying voltage across them will produce distortion. Steve Bench used 70V rms in his test which is about +60dBu!!!

Steve Bench's tests were conducted with 0.1uF capacitors. When a properly sized capacitor is used as a coupling between circuits, the voltage across it is miniscule. A typical 47uF coupling capacitor in a 10K circuit will have a voltage across it easily more than 120dB lower than this.

On the other hand, in an EQ, there are significant voltages across capacitors so non-linearities are more likely to be detectable.

Cheers

Ian

Dear Ian,
Steve is talking about the DC biasing can change the performance curve D-E in caps too. Here we can linearize this curve of not! and this is a posible in ours units.

Best regards
opacheco

 

[CalavoBob]

CalavoBob,
I would like to do some measurements and see what I actually get between different capacitors and their sonic changes (if any!), but I have some doubts with your schematic:
1- What do you connect in the Rload/Rload terminals?
2- Do you have any instructions in order to use this DA tester?

Thanks in advence
opacheco

Hi Opacheco and others interested in comparing science with what we hear.
1. The Rload/Rload terminals are used to connect a resistor that the Radj2 pot uses to adjust for the cap's ESR (equivalent series resistance). I have been using a 1 kohm 1% metal film resistor there, and it seems to allow me to set the pot at a useful position in it's rotation. This pot doesn't really change things much when I adjust it though.

2. Keep in mind that this test does not produce a numeric value for DA. It does however allow you to compare one cap to another so in generic tests, you need to have a really good reference cap. I went onto Mouser and collected a range of capacitor values from the more trusted manufacturers like Wima, F&T, Solen, etc. choosing what are recognized as the better dielectric types (teflon and metalized polypropylene) and looked for voltage ratings that will work in tube amplifiers (200V to 630V). You'll find very few manufacurers actually list the DA.
The oscillator source needs to be a square wave that allows you to alter the duty cycle. The article suggests a level of 1Vpp to 10Vpp. I've been using about 2Vpp to 4Vpp. The article also recommends setting the wave's pulse width (tw) between 1ms to 20ms, and the wave's period (T) to about 5x to 10x the pulse width. I've been using a pulse width of 10 ms and a period of 35 ms. This is a little shorter of a period than it suggests, but I got pretty good results. A longer period gives more time for the residue to settle.
I Y-branch the signal generator to feed the tester, and also channel 1 of the o'scope. The output of the tester is sent to channel 2 of the o'scope. Notice that the bandwidth limiting capacitor C1 causes a slight rolloff of the leading edge of the source pulse. This keeps the artifacts below 100 kHz.
In the tests that I have done, I have set the gain switch to 100. I use a +/- 15V bench power supply.
I put the high quality reference cap on the Cref terminals, and then the cap to compare to this reference is attached to the Ctest terminals. As seen in my posted pictures, I use banana posts for all the connections except the 2 signal jacks which are bnc connectors,

My oscilloscope is set with channel 1 at 500mV/div, and the time base at 5ms/div. Trigger to channel 1. Channel 2's vertical sensitivity will vary as you adjust the trim pots. Try starting with about 2V or 5V/div on channel 2, and adjust the coarse pot to get channel 2's signal to drop. Then use the mid and finally the fine adjust pot. Now you can try adjusting the Radj2 pot to see if it makes any diference. You'll need to increase the sensitiity of channel 2 as you get it trimmed to it's minimum ampitude.

So far, the best I've seen are the Solen polypropylene caps and the worst I've seen are very old large Sprague caps pulled out of old guitar amps.

Despite this lengthy description of how to use this box, the tests go very quickly, and you can go through a stack of caps in very little time. I've got quite a collection of old Fender tweeds that I want to do some listening tests with, to compare the very good DA caps with the very crappy ones to see if I can hear a difference. One thing I should add though, is that I'm an active blues guitar player and I have become fond of using preamp tubes that are just a little microphonic because it give the amp a little resonance that I find pretty musical. To me, tubes that have no microphonics at all make a guitar amp sound a little dull. Of course tubes that have rampant microphonics I find unuseable. There is a nice in between spot though that really sounds great.

 

[Newmarket]

Where he is wrong is that paper in oil caps are a good idea. We have already explained why they are not above. Like I've passed on guitars because the seller advertised that the tone caps were changed to vintage russian PiO (a right mess waiting to happen inside an expensive instrument that could damage other components or the finish, and since already old vs new prod, probably altready started that slow process; no thank you.

Yes. Just from a 'mechanical' point of view. It's OIL - sooner or later it's going to go somewhere it shouldn't.

.

 

[Ó Draighnáin]

When building valve circuits, like guitar amps and compressors, I generally used the Cornell Dubilier WMF Series axial polyester film/foil capacitors. Traditional I suppose, very well made, reliable, useful valve circuit values, and the tolerance is good.

In respect of PIO caps, I bought some years ago (along with teflon caps, polystyrene and others) for timing circuits and eqs, when they were dirt cheap. In 20yrs I have never seen one of those sealed PIOs leak. Values for those I purchased were generally pretty tight to spec. Size is a big issue with these caps.

I have heard of problems with wax melting in wax/paper caps.

Lately I have just been trying to use up whatever I have at hand because I want to save on costs.

A few years ago I was working on restoring an old B67 and had it open and witnessed one of the old tantalum caps go south. Quite spectacular (and PCB scorching).