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Old 7th May 2014, 12:03 PM   #121
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Quote:
Originally Posted by sbrook View Post
Hazard ... I think you have a preamp ... it's just spread across different devices.
I'm not following. I've got 3 sources, and 2 power amps. Each source is optimised for 2 volts output. The power amp is optimised for 2 volts input.Where is this preamp you speak of?

Sure you can say the function is spread across several devices, but if I had an integrated amp would you say I had a pre-amp? No, you would say the functions of the pre-amp and power amp were integrated. Thats why its called an integrated amp. Its not called a pre-amp. And nor is my power amp callled a pre-amp.

Last edited by hazard500; 7th May 2014 at 12:15 PM.
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Old 7th May 2014, 12:12 PM   #122
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Originally Posted by fdegrove View Post
Hi,



You've just provided the answer to your own question.



Now imagine you do not have this phono pre with its built in line preamp, but a CDP instead. You'd still want a preamp to set the volume, no?

Ciao,
Not sure what the answer is. I DO have a CDP (actually it plays SACDs and DVD-As. I'm pretty sure I mentioned this in my post. And no I don't need a pre-amp to set the volume.

Anyways, some of you are probably thinking that adding an attenuator to my power amp is cheating. But this is what I'm gonna do to prove you don't need a pre-amp.
1. My phono stage has a full range gain control. Amplifiers used to label their attenuators as gain controls. I am talking of a true gain control. The cartridge goes straight to the phono stage, then through the source selector, then the power amp. No attenuator involved.
2. My SACDP and sound card both have fixed output and for these I use the attenuator in my power amp for volume control. But when I assemble my by buffalo dac (still in bubble wrap after 12 months ) I'm gonna build a I-V convertor using similar topology to my phono stage. The design features variable gain and 2 volts output. I will then sell my attenuators on ebay.
3. Look ma. No pre-amp.
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Old 7th May 2014, 12:58 PM   #123
popilin is offline popilin  Argentina
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Quote:
Originally Posted by hazard500 View Post
Why on earth does any self respecting DIY person need a pre-amp?
Sorry, but I am a self respecting DIY person and I need a preamp, because among other things, I have a turntable and I want to play some vinyl.

Quote:
Originally Posted by hazard500 View Post
I have designed and built my phono stage.
Seems to me that you don't need a preamp because you already has a preamp.

Maybe it is just semantic, but when I was young, a "phono stage" was called a "phono preamp"
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Last edited by popilin; 7th May 2014 at 01:01 PM.
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Old 7th May 2014, 01:49 PM   #124
popilin is offline popilin  Argentina
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Quote:
Originally Posted by gingertube View Post
Since you are talking about capacitors - my experience (FWIW)
Hey, sorry, but I call this "wisdom" more than "experience"

The best synthesis about capacitors...on a single post !

Chapeau !!!

Quote:
Originally Posted by gingertube View Post
Once you restrict your discussion to film caps (no ceramics or electrolytics which both have crap dielectric absorbtion) then the most significant problems with capacitors can be traced to mechanical stuff, compressability of the dielectric etc. The problems manifest themselves particularly due to hystersis effects as the electric field reverses with signal.
Totally agree !

Quote:
Originally Posted by gingertube View Post
If you design such that there is always significant DC bias across the cap, such that the electric field never reverses (dielectric is always stressed in the same direction), then you will not be able to tell any audible difference between a good but garden variety $1 polypropylene and a $100 "audiophool jewelry" capacitor.
Totally agree !

In this case the dielectric is already polarized by DC bias.

Quote:
Originally Posted by gingertube View Post
If you have no DC bias across the cap and the electric field is constantly reversing with signal then any capacitor "warts" will stand out out like "canine testicles".
Redesign the circuit to make sure you have a DC bias on the cap sufficient to accommodate the full peak to peak signal swing.

Cheers,
Ian
Totally agree !

In this case the dielectric needs to be polarized cycle by cycle, this means zero crossing.
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Last edited by popilin; 7th May 2014 at 01:54 PM.
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Old 7th May 2014, 06:06 PM   #125
wicked1 is offline wicked1  United States
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Quote:
Originally Posted by hazard500 View Post
Why on earth does any self respecting DIY person need a pre-amp?
Some people design their systems with a preamp in mind. Their phono amps don't have enough stages to get the signal to two volts, and their power amps might want 6v to reach full output. Think about designing a relatively low gain DHT system from source to speaker, and a preamp makes sense. Plus, I've got modern commercial gear (not high end, laptops, mp3 players, etc) which don't put out more than 1v.
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Old 29th June 2014, 01:36 AM   #126
popilin is offline popilin  Argentina
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Quote:
Originally Posted by Merlinb View Post
It is well known that capacitor distortion is minimised by minmising the signal voltage across the capacitor. Therefore, enormous capacitors are actually better. Even the worst capacitor dielectric will introduce unmeasurable distortion in the audio band if it is enormous enough...
Sorry, but this is wrong, surprisingly, capacitor linearity only depends on dielectric properties, capacitor geometry and frequency.

b/a ≈ [ε(ω)tan (δ)] / ω

Or even worse
b/a ≈ ε(ω)ESR C

Capacitor linearity decreases with capacitance, and has nothing to do with applied voltage, however we must be cautious to not exceed manufacturer specs, but some dielectrics have a favorable behavior with a slightly increased on temperature.

Click the image to open in full size.

Click the image to open in full size.

Things like this are strange, but possible.

Click the image to open in full size.

Seems that polyester capacitors doesn't know too well that capacitor distortion is minimized by minimizing the signal voltage across them.

Quote:
Originally Posted by popilin View Post
Of course, if you minimizes the signal voltage applied across the capacitor, you are minimizing the applied field E, then the dielectric hysteresis loop D=f(E) will be thin, the area inside it will be small too, and the device results quite linear.
I was wrong too!

Electric field E also depends on capacitor geometry, real world capacitors are complex beasts and it is very easy to believe on an audio myth, but a more deep analysis can clarify a little this matter.

BTW, more details here

ECC82/12AU7 Line Preamp
Attached Images
File Type: jpg Cap001.jpg (243.9 KB, 353 views)
File Type: png Cap002.png (77.2 KB, 342 views)
File Type: gif Cap003.gif (31.2 KB, 191 views)
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Old 29th June 2014, 08:13 AM   #127
disco is offline disco  Netherlands
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Johann,

In the past I reformed ancient electrolytic capacitors by applying HV through a high value resistor. The oxidation of the aluminium took time and electrical energy, one could monitor the proces with a current indicator.

You are probably talking about film caps for coupling audio stages but how is the similarity? My old caps present certainly a load to voltages at the edge of the working voltage, resulting in unlinear behaviour.
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Old 29th June 2014, 04:06 PM   #128
popilin is offline popilin  Argentina
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Just in case, I want to let perfectly clear that this is not another capacitor thread, its purpose is to analyze the behavior of volume pot, and it is unavoidable to talk about capacitors in order to see the whole picture, on the context of a valve preamplifier with a high value volume pot.

Quote:
Originally Posted by disco View Post
In the past I reformed ancient electrolytic capacitors by applying HV through a high value resistor. The oxidation of the aluminium took time and electrical energy, one could monitor the proces with a current indicator.
The oxidation process take only a part of total energy, electrolytic capacitors are leaky and this is another contributor to energy dissipated as heat.

Quote:
Originally Posted by disco View Post
You are probably talking about film caps for coupling audio stages but how is the similarity?
I'm talking about capacitors in general, a comparison between an MKP against an electrolytic

C1=100 nF 630 VDC (Solen SM010)

tan (δ1) ≈ 9 x 10⁻⁵, ε1 ≈ 2.1, d1 ≈ 6 μm, V1 = V, ω = 100 Hz

C2=10000 μF 10 VDC (Vishay 54103E3)

tan (δ2) ≈ 0.31, ε2 ≈ 8, d2 ≈ 0.015 μm, V2 ≈ V x 10⁻⁵, ω = 100 Hz

Then
a1 ≈ 250 a2

b1 ≈ 0.019 b2

b1/a1 ≈ 7.6 x 10⁻⁵ (b2/a2)

BTW, the electrolytic is horrible.

Quote:
Originally Posted by disco View Post
My old caps present certainly a load to voltages at the edge of the working voltage, resulting in unlinear behaviour.
Electrolytic capacitors are nonlinear "per se", but its behavior can be even worse due to, e.g. aging, voltage, temperature, etc.
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Last edited by popilin; 29th June 2014 at 04:35 PM.
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Old 29th June 2014, 06:59 PM   #129
popilin is offline popilin  Argentina
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Achilles heel of my original proposal of post#1 is that it needs more capacitors

Click the image to open in full size.

Even worse, roughly

C1 ≈ 10 C2

Not to mention C3, so, we must be very careful with capacitor selection.
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File Type: jpg ScreenShot001.JPG (102.4 KB, 100 views)
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