No disrespect intended, at all, just basic Electronics and Math.
Please calculate the impedance of a saturated transistor , which is what the bias transistor is: a few ohm at most.
Or consider it a string of 2/3/4 diodes, whatever the BC and BE resistors ratio ends up becoming.
OR what base biasing string impedance/Hfe (transistor current gain) gives you.
WHY? .... because the mystery capacitor is in parallel with it.
3 ways of calculating which give same result so we´ll use the third because it´s more intuitive and easier to grasp.
Tr411 base is fed from a resistor string: R408a // (in parallel with) VR402a + R409a
Not sure what value VR402a is set to but since biasing voltage (hence effective Vce) is about "4 diodes" (we have 4 junctions to forward bias at TR407-409-408-410) then thye lower branch must be set to about 1/3 of R408 so about 1300 ohm.
TR411 base will "see" about 1k so will show about 1000/Hfe impedance.
Assuming Hfe around 100 Tr411 will be equivalent to a 10 ohm resistor
Please do the Math as to what frequency .047uF C405 can start having any influence.
To save you time I´ll do it: 338627 Hz
Now you know why I used the word "irrelevant"
But there´s more: being a saturated diode, voltage variation cross it is way less than the "equivalent" resistance would indicate.
Even worse (is that possible?), there *could* be some Audio voltage drop across it IF there was varying current through it BUT it´s basically fed constant current, so ....
As you see it does not depend on "opinion", not even mine of course, but basic Electronics.
EDIT: back to your original question: it may be "nice to know" what kind of capacitor is it, why not?, we Humans are curious by nature, but please rest assured that whatever you use there, by a wide margin, sound does not change in any audible way.
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@JMFahey
I have a different question, NOT relating to the cap in my original post which is certainly not NP0.
When did NP0 ceramics begin to be used in amplifiers?
I have asked this question many times and have received different answers. I am curious because the amps that I restore are from the 1970's and early 1980's and contain many small value, also physically small ceramics at locations such as Miller cap on the VAS, in parallel with the larger NFB resistor, and several other positions.
Do you believe (or know) these small pf ceramics at critical positions....in amps made during the time frame mentioned above.... to be NP0 or something other than NP0?
I have a different question, NOT relating to the cap in my original post which is certainly not NP0.
When did NP0 ceramics begin to be used in amplifiers?
I have asked this question many times and have received different answers. I am curious because the amps that I restore are from the 1970's and early 1980's and contain many small value, also physically small ceramics at locations such as Miller cap on the VAS, in parallel with the larger NFB resistor, and several other positions.
Do you believe (or know) these small pf ceramics at critical positions....in amps made during the time frame mentioned above.... to be NP0 or something other than NP0?
If the amplifier was of any quality at all, critical places such as Miller comp, lead comp, lag comp, or input RF shunt would have been NPO, even back in those days. C0G, which is roughly equivalent, came along much later. Any other place would have been a Z5U. X5R/X7R came along much later - and even now have a hard time going above 50 volts.
As to whether the .047 on the vbe multiplier is even audible, for the most part it’s not. It’s job is to keep the output stage vbe bias stable and low impedance during high frequency transient artifacts such as slewing and clipping. It does become audible then - and certainly measurable in terms of reduction in output stage cross conduction and “sticking”. At this point the music is already experiencing some distortion - you just don’t want your output transistors to heat up and die.
As to whether the .047 on the vbe multiplier is even audible, for the most part it’s not. It’s job is to keep the output stage vbe bias stable and low impedance during high frequency transient artifacts such as slewing and clipping. It does become audible then - and certainly measurable in terms of reduction in output stage cross conduction and “sticking”. At this point the music is already experiencing some distortion - you just don’t want your output transistors to heat up and die.
As for whether that cap in my OP makes an audible difference...I am not replacing that cap for any reason other than I need to remove the original anyway...to get room to loosen the Vbe multiplier transistor from the heat sink to replace thermal compound, and I had already purchased a possible replacement for it. I wasn't even thinking of asking whether it affected sound, but as mentioned a few posts back...people started volunteering that information (here and elsewhere), and their opinions were contradictory. So then I did become curious.
But I am moving on from that and, based to a large degree on your posts (thank you)
,... I feel comfortable with putting a new C0G in there and forgetting about it.---------------------------------------------------------
And now I am curious about understanding whether NP0 caps were used in those critical locations in the amps that I am likely to work on, for obvious reasons
I don't know your definition of "any quality at all". The amps I work with are consumer grade Kenwood, Sherwood, Sansui and similar. Would those manufacturers have used NP0 at critical positions as early as the early 1970's?
And if NP0 was used in critical positions in those amps, might its quality back then have been less than modern NP0 (or C0G) caps?
As mentioned above in my question to JMFahey, I have asked these questions previously, multiple times, and received a range of responses...
Amps like that are quality enough where caps would be paid attention to. Soundesign, and the like, not so much. If I were to find a class 2 someplace it shouldn’t be in a “real” amp or receiver, it would be outta there. I might put in a silver mica, mainly because i keep them on hand *for* compensation cap duty. If I had the right value. If not some C0Gs would be on the next order, whenever that is.
Size, if the marking doesn’t give it away. Class 1 caps are much larger for a given pF/voltage than class 2. Small values (under 100 pF) are usually class 1 for voltages under 250 V. Could be class 2 for higher voltages - if it looks “too small” to be class 1 then suspect class 2. If they are class 1 with a nonzero tempco, they are almost always marked as such (ie, N750). Such caps are often used where temp compensation is required (not usual for audio circuits). It does take a little experience because there are no absolutes.
I would like to add my comment, 403 means 40nF (0.040uF), so I am confused why many people here wrote some stupid mismatch, like 403 is 0.004uF.
403 means 40,000pF (or 40nF, or 0.040uF).
On schematic it clearly state that this C is 47nF capacitor. 40nF or 47nF is not so important.
So, there is no need to replace this capacitor and it will not affect any freq response with better quality or make.
403 means 40,000pF (or 40nF, or 0.040uF).
On schematic it clearly state that this C is 47nF capacitor. 40nF or 47nF is not so important.
So, there is no need to replace this capacitor and it will not affect any freq response with better quality or make.
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