input caps - time to ask the experts - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 10th August 2002, 06:04 AM   #16
diyAudio Member
 
Join Date: Jun 2002
Um yeah ignore that last post.

ESR (equivilent series resistance) has nothing to do with it.
  Reply With Quote
Old 10th August 2002, 08:03 AM   #17
diyAudio Member
 
JensRasmussen's Avatar
 
Join Date: Jul 2002
Location: Denmark - Jutland
Send a message via ICQ to JensRasmussen Send a message via MSN to JensRasmussen
Default ESR

The ESR parameter has do with the performance of the cap when cgarging/discharging it quickly. It has nothing to do with the inductance.

BTW the inductance is often not noticable under 50 khz when one sweep the impeedence vs freq. so unless you are building a monster amp with wery high specs, i would not worry about the input cap at first.

Another matter is the cap in the feedback system, I would try and find a low leakage NP electrolytic cap instead of the specified one in tha project 3 A

\Jens
  Reply With Quote
Old 10th August 2002, 08:25 AM   #18
diyAudio Member
 
ashok's Avatar
 
Join Date: Jun 2002
Location: 3RS
Default Capacitor voltage

Most circuits , tubed or transistor , have their inputs at almost zero volts . Most sources also have their outputs at close to zero volts dc. This would mean that you could use a low voltage capacitor and 50 volts seems to be Ok especially if you use large ones like 10 uF at the input. Personally if I use 1uF or lower I use 250 volt or 400 volt types . Larger than 1uF I use 63 or 100 volt types. The only exception is if the capacitor is connected to points with high dc voltages. In that case I use at least a 50 percent margin or more for the voltage rating.
For those who want to know :
With a 10uF capacitor and 10 Kilo ohm input impedance you will have a -3db point at 1.6 Hz. You can find the -3db point for other values by simple division of these figures.
Eg. For a 1uF capacitor and 22 K ohms input impedance

= 1.6Hz X (10uFx10Kohm)/ ( 1uFx22K ohms)
= 7.2 Hz.
Note : the text in the equation is only explanatory .
__________________
AM
  Reply With Quote
Old 10th August 2002, 08:27 AM   #19
diyAudio Member
 
Join Date: Jun 2002
The only low-leakage electrolytic caps I could find all had 20% tolerance. Is that okay? It seems a bit high, seeing as you can get high precision e-caps with respectably tight tolerances. I guess a 100uF film cap would be out of the question...

I've got another question.. since I beefed up my input cap to 12uF (as opposed to 4.7uF), does that mean I have to lower R2 (goes from input, after the input cap, to ground)? And adjust C2, which goes in parallel with R2?
  Reply With Quote
Old 10th August 2002, 07:14 PM   #20
diyAudio Member
 
ashok's Avatar
 
Join Date: Jun 2002
Location: 3RS
Default Input capacitor

Altaic,
I am assuming C2 is a low value cap. Keep the product of C2 and R2 same.
That is C2xR2=CnewxRnew.
You can determine the value of the input capacitor as explained in my previous post. You do not have to go any lower than -3db at 10 Hz or so. Depending on the powersupply and other factors a low roll off could cause problems if your source has plenty of very low frequency content.
__________________
AM
  Reply With Quote
Old 10th August 2002, 09:13 PM   #21
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Quote:
Originally posted by Altaic
The only low-leakage electrolytic caps I could find all had 20% tolerance. Is that okay? It seems a bit high, seeing as you can get high precision e-caps with respectably tight tolerances. I guess a 100uF film cap would be out of the question...
My opinon about low leakage is that this parameter is most of time totally irrelavant in audio circuits as long as the leakage current is "normal".

One trick though to reduce it: Apply DC voltage at the rated voltage or a little bit higher (you must measure the leakage current so you don't distroy the cap) for a day or a week or longer if you have the time. You get much better results and you get even better if you use the capacitor at half of it's voltage rating. When you apply voltage you build up the oxide at the aluminium foil. The oxide is the isolation. Aluminium (= positive electrode), oxide at the aluminium (= isolation), and "geggamojja" as we say in swedish (=negative electrode along with the aluminium can).
__________________
/Per-Anders (my first name) or P-A as my friends call me
  Reply With Quote
Old 14th August 2002, 01:29 PM   #22
diyAudio Member
 
Join Date: Aug 2002
Location: france
Default Re: input caps - time to ask the experts

Quote:
Originally posted by Altaic
I've looked high and low for info on input coupling caps (google, diyaudio, etc.). I'm doing Elliott's (ESP) 3A project (the cap in question is C1, just to make clear), and the best info I could find was right there on the linked construction guide.

Well, this is what I gathered: It should be "large" between 1uF and 10uF, polypropylene or polysterene, although polyester is okay. So I'm thinking, input doesn't need to be more than a few volts, right? I mean on the official page, it said the bootstrap cap (C7) has to be 35V, but the rest of the polarized ones can be whatever is on hand. That is a bit non-specific, no? I guess if I knew what I was doing I'd know what is sensable. So I browse around the construction guide, and contrary to his own recommendations, Mr. Thornblade used a 2uF 400V oil/paper cap. So I have been looking through cap catalogs and such, and have found some 400VDC (275VAC) metalized polypropylene caps for ~$13 each.

But, I have a hunch it doesn't need to be more than a few volts, so if I get a reasonable cap, say 50VDC, I can get a better poly-film/foil cap for much less $$$.

So, what I'm asking is: what is the optimum voltage for an input coupling cap? And I'm guessing a 10uF input cap would be best.

Hmm, now that I'm thinking about it, it doesn't seem to give the recomended voltages for the nonpolarized caps either. I'd guess they should be >=35V (rail voltage), but I might be wrong.
if you use a capacitors with big voltage you will have a very low ESR (equivalent serial resistor) and that is very good!
the oil/paper capacitors are the best for audio traject signal!!!
(sorry for my english i am french)
  Reply With Quote
Old 14th August 2002, 01:47 PM   #23
diyAudio Member
 
Join Date: May 2002
Location: Cambridge, MA
Default myth of the bad electrolytic

Okay:

Really is there somthing wrong in using a large modern electrolytic as a DC blocking cap? With a low -3dB point the cap's impedence is miniscule in the audio frequency range, compared to the resistive element. I would worry about the cap quality say in a crossover design when the impedence is high right smack in the middle of the audio frequency range, but when your -3dB point is somewhere down at 1 Hz, then it should not matter. Should it?

As for high frequency effects, the inductive effects don't start to become signficant until well past 20kHz (as mentioned by Jens). In my experience not until past 100kHz and even then the impedence change is still very small.

Of course there is the issue of listening tests. But on technical merit, are there really problems with *modern* electrolytics for DC blocking?

That said, in my last DIY amp, I used polypro DC blocking caps.

-willryu
  Reply With Quote
Old 14th August 2002, 03:15 PM   #24
diyAudio Member
 
Join Date: Feb 2002
Location: Germany
Default not a myth

The typical resistive part is < 1 k (e.g. 18K over 1 k = gain of 19). Typical ESR of small electrolytics is 3-10 R and has substantial variation with voltage etc. Assume 1 R variation, this will give you approximately 0.1% nonlinearity in the transfer function.

Besides, voltages will be around zero with both signs occuring. A polar cap will be reverse polarized, with nasty effects on impedance. A bipolar cap will have some crossover behavior.

Last not least, there is the issue of dielectric absorption which is suspected to smear impulses.

Eric
  Reply With Quote
Old 14th August 2002, 03:32 PM   #25
diyAudio Member
 
Join Date: May 2002
Location: Cambridge, MA
Quote:
The typical resistive part is < 1 k (e.g. 18K over 1 k = gain of 19).
?

If we are talking "typical" amplifiers, the first stage usually doesn't have voltage gain, such as the ESP 3a (first stage is differential pair) that started off this thread.

So, the resistive part can be much higher than 1k. A simple DC blocking RC can be say, 22 uF cap with a 10kOhm resistor. Assuming a high input impedence input stage, that gives an distortion figure of <0.01% (using your ESR values).

-willryu
  Reply With Quote
Old 14th August 2002, 03:48 PM   #26
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Quote:
Originally posted by willryu

?

If we are talking "typical" amplifiers, the first stage usually doesn't have voltage gain, such as the ESP 3a (first stage is differential pair) that started off this thread.

So, the resistive part can be much higher than 1k. A simple DC blocking RC can be say, 22 uF cap with a 10kOhm resistor. Assuming a high input impedence input stage, that gives an distortion figure of <0.01% (using your ESR values).

-willryu
Sorry but I must correct you, the amp has indeed gain in the first stage (approx. 16). 99.9% of all amps has gain of at least 10 in the first stage. The cut frequency is given by pull-down resistor and the input impedance of the stage in particular case. If the stage has feedback the impedance is "high", no worries.

Personally I thrive to avoid electrolythics as much as possible, feels better with plastic and they also last longer.
__________________
/Per-Anders (my first name) or P-A as my friends call me
  Reply With Quote
Old 14th August 2002, 04:03 PM   #27
tiroth is offline tiroth  United States
diyAudio Member
 
Join Date: Dec 2001
Location: Pittsburgh, PA, USA
As Eric alluded, performance about the zero crossing is the real killer for polar (or "bipolar") caps. Small absolute nonlinearities are pretty awful distortion-wise with low-level signals.

Even .01% distortion at the input stage is bad when the amp as a whole has less distortion than that. Whether .1% or .0025% /really/ matters is eminently debatable, but it seems a bad idea to throw performance away by using an electrolytic.
  Reply With Quote
Old 14th August 2002, 04:05 PM   #28
diyAudio Member
 
JensRasmussen's Avatar
 
Join Date: Jul 2002
Location: Denmark - Jutland
Send a message via ICQ to JensRasmussen Send a message via MSN to JensRasmussen
Default Gain

Sorry but I must correct you, the amp has indeed gain in the first stage but it's approx. -40 db

99.9 % of what amps ?

Most amps I have seen have a VAS to make the voltage gain. Normally this is the second of three stages

Input -> Voltage amp stage (VAS) -> Output

\Jens
  Reply With Quote
Old 14th August 2002, 04:18 PM   #29
diyAudio Member
 
Join Date: May 2002
Location: Cambridge, MA
Peranders, I'll reply to your email in reverse order :

Quote:
Personally I thrive to avoid electrolythics as much as possible, feels better with plastic and they also last longer.
I also avoid electrolytics when I can, but I have always wondered the *technical* reasons why we should avoid electrolytics for *blocking DC*when the impedence of the Cap is way out of the audio band.

Quote:
The cut frequency is given by pull-down resistor and the input impedance of the stage in particular case. If the stage has feedback the impedance is "high", no worries.
So I think you are agreeing with me here. Distortion in the audio band (20 - 20kHz) due to having blocking electrolytic will be very low. Less than 0.01% for figures quoted above.

Quote:
Sorry but I must correct you, the amp has indeed gain in the first stage (approx. 16). 99.9% of all amps has gain of at least 10 in the first stage.
No problem with correcting me! I mispoke (and misremembered). ESP3a does have gain in the input stage. But I will argue that your 99.9% figure is slightly off . Many 3 stage solidstate amps have all the voltage gain in the second stage. First stage is typically a differential pair with no voltage gain.

-willryu
  Reply With Quote
Old 14th August 2002, 04:20 PM   #30
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Default Re: Gain

Quote:
Originally posted by JensRasmussen
Sorry but I must correct you, the amp has indeed gain in the first stage but it's approx. -40 db
\Jens
Have I got I wrong: 1 mA in diff stage gives gain of 40/2 (at 25 deg C) per kohms collector res (diff stage, you get half the gain)

35 /12k = 2.92 mA -> 1.45mA as collector current

Gain (1.458 * 40 * 0.56)/2 = 16.33 => 24 dB
__________________
/Per-Anders (my first name) or P-A as my friends call me
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Sikorel caps break-in time. beppe61 Power Supplies 2 24th July 2006 07:27 AM
Burn in time for Solen Caps? adamt Multi-Way 8 30th June 2006 10:15 PM
Input Caps on TDA7264 ThSpeakerDude88 Chip Amps 1 4th August 2005 03:44 AM
First Time Amp Builder in Need of Help from Experts! Creelove Solid State 9 30th June 2004 05:12 PM
Open baffle drivers configuration: time to ask the experts Peter Daniel Multi-Way 28 31st October 2002 06:54 PM


New To Site? Need Help?

All times are GMT. The time now is 12:12 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2