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Professor smith 1st September 2008 11:07 PM

preamp
 
Hi,

I am looking to build a very simple buffer with volume control for my hifi.

My questions are:

How important is it to filter the >20khz harmonics coming out of the dac from the cd player?
They are inaudible but the question is, what effect will it have on my amplifier?

Lukasz Fikus (http://www.lampizator.eu/) takes the output straight from the Iout or Vout without filtering. Is this wise?

What's the level of ultrasonics in normal cd players? How far does it extend and to what degree?

Finally, what design should I use? I thought about this one due to its simplicity given that I am only a beginner:

will it work if I put a volum pot in front?

http://www.lampizator.eu/LAMPIZATOR/...CDout_JFET.pdf

Many thanks,

John

flg 1st September 2008 11:51 PM

http://www.diyaudio.com/forums/showt...&pagenumber=54
:D
You could just jump to post#1338
:D

hifimaker 1st September 2008 11:53 PM

Hi John,

If you are using a commercial CD player, it should have a filter already part of it's circuitry. Without a filter, the tweeter is forced to try and resolve all the high frequency sound or noise you cannot hear. Sometimes this can be damaging, or at least force your tweeters to work in a frequency range that is without benefit to the human hearing range, when they should focus their efforts on what a person can hear.

Nelson just published a buffer circuit as part of his First Watt products. He has graciously provided us with the theory of operation. He will be offering boards soon on his www.passdiy.com website. B1 Buffer

Here's Nelson's B1 article and schematic published on Enjoy the Music - B1 Article

I'm sure others with more knowledge can help further.

-David

Nelson Pass 2nd September 2008 01:13 AM

Some sort of filter is probably still a good idea. At the very least,
I suggest a single pole at 25 KHz. You can implement this in a B1
with a simple RC network at the input. The JFETs can handle the
remaining noise, and if you want an additional pole, you can hang
a cap to ground at the output of the B1.

Somebody here will give you part values if you need them.

:cool:

Professor smith 2nd September 2008 11:39 AM

Hi Nelson,

can I ask what you mean by ''JFETs can handle the
remaining noise''?

I don't want to allow the noise through the amp and speakers, will a first order passive filter provide sufficient attenuation? I suspect it may only be a slight reduction and a slow roll off at that.

regarding the b1 buffer is it good to increase the input resistance by using say a 50kohm pot than the 20kohm one in your design?

And do the capacitors etc have to be special audio types? I don't know whether I'll be able to get hold of them esaily.

thanks
John

hifimaker 2nd September 2008 03:57 PM

A single pole filter (first order) filter is sufficient. There are more filtering schemes available with different components, like opamps. A simple resistor and capacitor pair is enough for what you'd like to accomplish, especially for a beginner. Starting with the basics and enhancing the circuit is a great learning opportunity.

Some basic filter types are Butterworth, Chebyshev, and elliptic filters. The filter cam be configured as high-pass, low-pass, band-stop, band-pass, etc. Search wikipedia.org for overviews of the various filter types.

Wikipedia has a good overview of low-pass filters, both active and passive: http://en.wikipedia.org/wiki/Low-pass_filter

Audio circuitry caps are as basic or exotic as you like. There certain instances where an exotic cap can make a difference, but don't worry about that at this point. Usualy the gains are nominal as compared to the cost. You can aways change caps to see if they make a difference later after the circuit is known good, and operational. Nelson recommends Panasonic electrolytics for example. The are cost effective and have very high performance. Save the Solen's and BlackGates for experimentation later on.

If the 50K pot is the value you have in your parts drawer, then give it a shot. Otherwise, I'd stick to the design and experiment from there.

-David

Professor smith 2nd September 2008 09:28 PM

Quote:

Originally posted by Nelson Pass
Some sort of filter is probably still a good idea. At the very least,
I suggest a single pole at 25 KHz. You can implement this in a B1
with a simple RC network at the input. The JFETs can handle the
remaining noise, and if you want an additional pole, you can hang
a cap to ground at the output of the B1.

Somebody here will give you part values if you need them.

:cool:

yes please, please provide me the values

metalman 2nd September 2008 10:30 PM

Quote:

Originally posted by Professor smith
Hi Nelson,

can I ask what you mean by ''JFETs can handle the
remaining noise''?

I don't want to allow the noise through the amp and speakers, will a first order passive filter provide sufficient attenuation? I suspect it may only be a slight reduction and a slow roll off at that.

regarding the b1 buffer is it good to increase the input resistance by using say a 50kohm pot than the 20kohm one in your design?

And do the capacitors etc have to be special audio types? I don't know whether I'll be able to get hold of them esaily.

thanks
John

The JFET's have a small input capacitance so that, in combination with the 1st order RC filter added to the input, makes a quasi-second order filter. This rate of roll-off should be sufficient.

You do have to be careful not to raise the resistance of the pot too high, but going from 20k to 50k won;t present any problems.

On capacitors, opinions will vary (greatly) about the effect of capacitor quality, but as long as you use something like a polypropylene metallized film cap and not an electrolytic it will be fine. Nelson himself uses good ol' run of the mill Panasonic caps in several of his designs.

Cheers, Terry


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