Author Topic: Amplifier caps, what they do, and where they are...  (Read 7881 times)

Offline Folsom

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Re: Amplifier caps, what they do, and where they are...
« Reply #15 on: September 18, 2019, 10:28:19 PM »
Not necessarily. It's common for digital devices to put out DC though.

If you're 100% sure you have no DC coming from anything then you can remove input capacitors on an amplifier. But sometimes the sound may not be what you want as capacitors seem to have an effect that stops fatiguing sound, too.

C1 doesn't remove DC, it simply won't pass it.

Offline HAL

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Re: Amplifier caps, what they do, and where they are...
« Reply #16 on: September 19, 2019, 04:46:04 AM »
Scott,
The C1 coupling cap as stated removes any DC offset from the source device, but also stops the DC bias from the tube input from going back to the source.  The grid leakage current on the input resistance will cause a DC voltage as well.  Small, but that may cause some DC coupled source output stages problems. 

If the source is AC coupled (has an output capacitor), then you can bypass the input C1 by shorting it with a jumper.  Easily reversed if needed.


Offline steve

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Re: Amplifier caps, what they do, and where they are...
« Reply #17 on: September 19, 2019, 12:51:45 PM »
So is there usually incoming DC from the signal input that C1 needs to remove?

Scott,

I would suggest the easiest method of understanding is to obtain a copy of the Radio Amateurs Handook,
say 1955, or 64, if into ss, then 69 issue, or another source of basic electronics such as a book from Parts Express. They are relatively inexpensive and go into the audio basics. I will use Scott's schematic amplifier in my below example.



The second image shows, in red lines, for future reference.

C3 is the power supply filter, for both minimizing 120hz signal from rectification and, from the tube's side, see my second schematic and red lines, "seeing" the musical signal and "handling" the musical signal current from V1p to ground.

C2 is the coupling capacitor between the two tube stages. C2, along with R2, forms a high pass 1st order filter, whose slope is 6db/octave. C1/R2 should be sufficient to cover the audio band.

Hal did a wonderful job describing C1's functions. I would just like to add that C1, along with volume control total resistance, also forms a 6db/octave high pass filter.

One important point is that the -db of both 1st order filters add. As a simple example, suppose each filter has a
-0,1db point at 20hz. Together, the response is -0,2db at 20hz. When we add the -db points from the source, preamp, and speaker, the response is much more than -0,2db. We should consider the whole system's response and therefore capacitor's values.


cheers

steve
« Last Edit: September 19, 2019, 02:32:43 PM by steve »
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Offline S Clark

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Re: Amplifier caps, what they do, and where they are...
« Reply #18 on: September 19, 2019, 02:18:48 PM »
Ok, I've got a 1961 copy on the way. 
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Offline Folsom

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Re: Amplifier caps, what they do, and where they are...
« Reply #19 on: September 19, 2019, 07:01:56 PM »
Good point about biasing up the line. This is a mixed bag because if your preamp has output capacitors it's not a problem. But some things are DC coupled so it is a problem :rofl: . Hence why most put the capacitors in.

Electrolytics also can get biased and sound different than film in the position of input capacitor. It's really weird, but if they're being biased you can put a film inline with the lytic and it'll sound way better :shock:

Offline steve

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Re: Amplifier caps, what they do, and where they are...
« Reply #20 on: September 19, 2019, 09:39:01 PM »
Ok, I've got a 1961 copy on the way.

I hope you enjoy the book. One chapter addresses power supply introduction, different rectifier types, types of filters, capacitive input, inductor input, regulators, voltage regulation etc.

Another chapter addresses speech ampllifiers, transformer coupled stages, capacitor coupled, etc. High fidelity stages differ by using larger and higher quality coupling capacitors, wider bandwidth transformers for wider bandwidth. I think it is a good overall book and hope it helps you. Parts express might have a good book, but it costs $25.

Capacitor quality is addressed in previous posts, ESR, DA etc, the article "picking capacitors" by Walter Jung and Richard Marsh. I might add that materials used/quality are also important.

cheers
steve

 
« Last Edit: September 19, 2019, 09:46:38 PM by steve »
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Offline Folsom

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Re: Amplifier caps, what they do, and where they are...
« Reply #21 on: September 19, 2019, 10:01:20 PM »
The problem with all the types of articles on "picking capacitors" is that there isn't a useful conclusion, not really.

Experience with sound and why the sound is the way it is, is what really matters.

My favorite smaller capacitor doesn't sound right in everything. But with the right control over certain things it excels.

I wish it were as simple as always choosing the "best" but when you go down that road it get complicated because the closer you get, the more everything that isn't stands out.

Offline Guy 13

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Amplifier caps, what they do, and where they are...
« Reply #22 on: September 19, 2019, 11:10:53 PM »
The problem with all the types of articles on "picking capacitors" is that there isn't a useful conclusion, not really.

Experience with sound and why the sound is the way it is, is what really matters.

My favorite smaller capacitor doesn't sound right in everything. But with the right control over certain things it excels.

I wish it were as simple as always choosing the "best" but when you go down that road it get complicated because the closer you get, the more everything that isn't stands out.

 :thumb:
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Offline Guy 13

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Amplifier caps, what they do, and where they are...
« Reply #23 on: September 19, 2019, 11:14:33 PM »
Hi Folsom and all.
I am not an expert in audio capacitors,
however, I know a thing or two on photographic flash capacitors,
with capacities that varies from 100uF to 6400uf at 350 to 600 volts.
Just want to say.
I also have a basic course in electronics at Teccart Institute in Montreal,
I have to admit that was a very long time ago (1967)
when we study manly tubes and very little transistors.

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Offline tmazz

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Re: Amplifier caps, what they do, and where they are...
« Reply #24 on: September 20, 2019, 08:08:46 AM »
The problem with all the types of articles on "picking capacitors" is that there isn't a useful conclusion, not really.

Experience with sound and why the sound is the way it is, is what really matters.

My favorite smaller capacitor doesn't sound right in everything. But with the right control over certain things it excels.

I wish it were as simple as always choosing the "best" but when you go down that road it get complicated because the closer you get, the more everything that isn't stands out.

Which is why designing high end audio equipment is at times as much of an art as it is a science.

Heck if everything was done "by the book" nobody would be listening to tube amps at all because they cannot match the distortion specs of their solid state counterparts.

The stuff they taught us in engineering school and that is in the books is just a start. A computer can be programed to blindly apply rules and produce a workable product. But the real engineering starts when the engineer is cut loose to step outside of the box and creatively solve problems using his/her intuition and gut instinct. And that is when you separate the true engineers from those that are simply CAD operators. It is also where you get innovation and advancement in the state of the art.  Because if you program a computer to design things using yesterday's rules you will never do anything but replicate yesterday's design's.

« Last Edit: September 20, 2019, 09:00:37 AM by tmazz »
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Offline P.I.

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Re: Amplifier caps, what they do, and where they are...
« Reply #25 on: September 20, 2019, 11:14:00 AM »
The problem with all the types of articles on "picking capacitors" is that there isn't a useful conclusion, not really.

Experience with sound and why the sound is the way it is, is what really matters.

My favorite smaller capacitor doesn't sound right in everything. But with the right control over certain things it excels.

I wish it were as simple as always choosing the "best" but when you go down that road it get complicated because the closer you get, the more everything that isn't stands out.

Which is why designing high end audio equipment is at times as much of an art as it is a science.

Heck if everything was done "by the book" nobody would be listening to tube amps at all because they cannot match the distortion specs of their solid state counterparts.

The stuff they taught us in engineering school and that is in the books is just a start. A computer can be programed to blindly apply rules and produce a workable product. But the real engineering starts when the engineer is cut loose to step outside of the box and creatively solve problems using his/her intuition and gut instinct. And that is when you separate the true engineers from those that are simply CAD operators. It is also where you get innovation and advancement in the state of the art.  Because if you program a computer to design things using yesterday's rules you will never do anything but replicate yesterday's design's.
You guys killed it!  Good job.

There is no "one size fits all" device in audio.  I like a particular op amp for use in some line driver applications. It is a good piece in almost any application when properly implemented and a GREAT op amp when used in 'some' applications.  Again, it has to be treated properly and with respect to get "there".

Same goes for any other audio component. Pick the right one for the right application and you'll have a winner.  You can't just willy-nilly take a bunch of devices that may excel in certain applications, string them all together and be guaranteed a winner.
"A man with an experience is never at the mercy of a man with an argument." - Hilmar von Campe

Offline steve

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Re: Amplifier caps, what they do, and where they are...
« Reply #26 on: September 20, 2019, 12:47:24 PM »
I guess I take a different view point, to some extent on this string. I have seen so much confusion and mis information on the internet. How does confusion create order. I will address "synergy" further down.

My reply would be there are better capacitors than others, including accurate ones, but how is one suppose know what to believe? And a truly accurate component will reproduce all kinds of music accurately/naturally. If the recording is bad, well...

A very simple example of mis information. I have seen capacitor testing using a single piece of gear, one size capacitor (the wrong value ufd) and then the author rates the capacitors. Of course the most accurate capacitors are never rated well because the wrong value ufd is used, and the quality of the rest of the circuit is questionable. So the accurate/good capacitor becomes extinct while the inaccurate/bad capacitors flourish.
One cannot make a silk purse from a pig's ear.

Say one uses a 1.0uf capacitor with a 100k ohm load, often called the grid resistor. The reactance of 1.0uf at 20hz is ~8,000 ohms. The frequency response has just dropped off 7.5% at 20hz. At 80hz 1.87%, at 160hz,
~1%, at 320hz 0,5%, and at 640hz the frequency response has dropped off 0,25%. And that is just one high pass filter, one coupling stage. There are many more in an audio system.

Let's take a look at the opposite. If all the prolyprope film capacitors were designed correctly, for accuracy, all the designers would produce the same high quality products. However, different capacitors do affect the sound differently, due to DA, ESR, materials used, thickness of insulation, termination techniques, physical size (L vs D), shape etc. So obviously not all designers/manufacturers understand how to properly design. In fact, very few.
One cannot make a silk purse from a sow's ear.

Resistors the same story as capacitors, they vary the "sound" due to materials used, termination techniques, internal inductance due to physical size, shape etc, maybe even internal capacitance. Crappy parts result in poor sonic quality. Can't make a silk purse from a pig's ear.

What about circuit design. Are all circuits accurate? Nope. Are just the ones with low harmonic distortion accurate. Nope. There are many different kinds of distortion according to the RCA Radiotron Designers Handbook, written by 26 EEs, 1960.

1. Non linear distortion and harmonics. Harmonic Distortion, HD.
2. Intermodulation, IMD, related by not harmonic but sums and differences. It is very generally 3 times that of HD, but sometimes near zero if extremely high figures of negative feedback are used.
3. Frequency distortion, frequency response weaknesses
4. Phase distortion
5. Transient distortion.
6. Dynamic range and its limitation
7. Scale distortion
8. Frequency modulation distortion
9. Variation of frequency response with output level

How about smearing caused by a part as number 10.

So going by the lowest harmonic distortion figures is only one criteria.

Probably the worst flaw of an inaccurate part is that there is NOT just one flaw, but multiple. Synergy will never be optimized using the flawed concept of synergy. Optimum Synergy is only attained when all the parts, components, conditions are optimized. This includes room conditions. One cannot make a silk purse from a sow's ear.

The toughest component to design is an accurate speaker/tranducer. However, my contention is that another huge bottleneck is still the poorly designed recording equipment that is almost always used. We have all seen the long, long consoles with all those sliders, switches. Think of a table radio quality parts or a $100 stereo. The musical signal can "travel" through as many as 100 transistors, and nearly as many stages. It amazes me how the sound quality is as good as it is. Limited amount can be done with original recordings.

Lastly, budget is something we all contend with. We can only purchase what we can afford. But I think knowledge is power, to understand allows one to help his fellow audiophile/music lover to avoid the pitfalls.

Cheers, and great weekend to all.

steve
« Last Edit: September 20, 2019, 01:04:58 PM by steve »
Steve Sammet (Owner, Electron Eng, SAS Audio Labs, Ret)
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Offline Guy 13

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Amplifier caps, what they do, and where they are...
« Reply #27 on: September 20, 2019, 02:26:35 PM »
Hi Steve.
Thanks for this very informative write up.
By what you wrote, I can see that you know what your are talking about.

Guy 13
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Offline S Clark

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Re: Amplifier caps, what they do, and where they are...
« Reply #28 on: September 20, 2019, 05:39:08 PM »
Having taught for decades you emphasize that there are no "stupid" questions (usually true).  So bear with me please,  if C1 and C2 are "coupling" caps ---- I see where C1 could "couple" the input to the power tube.  What is the "coupling" that happens with C2 which appears to bypass the power tube?  I get that every cap acts as a high frequency filter, depending on value... but why add unamplified high frequency back to the circuit?? Or am I looking at it backwards?  What's coupled?
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Offline HAL

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Re: Amplifier caps, what they do, and where they are...
« Reply #29 on: September 20, 2019, 06:23:42 PM »
The tube is a triode and pentode in one.  The C2 couples the output of the triode plate to the grid of the pentode for output.