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 17th February 2013, 10:27 AM #21 Jempie   diyAudio Member   Join Date: Jan 2008 Fahey, thanks see not mutch capacitors in this design. must i use the volume on the input or output? and if i want to use the KT603? how did you calculate this
 17th February 2013, 11:31 AM #22 Mooly   diyAudio Moderator     Join Date: Sep 2007 A single rail design will always have coupling caps. No design changes are needed for different transistors (although the input coupling cap seems a bit on the small side. I'd increase that by a factor of 20 and use a 1uf)
JMFahey
diyAudio Member

Join Date: Mar 2009
Location: Buenos Aires - Argentina
Quote:
 (although the input coupling cap seems a bit on the small side. I'd increase that by a factor of 20 and use a 1uf)
Sorry, I intended to write 0.47uF, which would have a cutoff frequency below 10 Hz.
1uF is fine, of course.

Yes, I normally use electrolytics for values 0.47uF and above.
No cork sniffing here.
At most, I sniff soft drink crown caps and can tabs

Quote:
 how did you calculate this
Easy, Electronics 101 schoolbook example, but with more realistic values than those usually shown there.

Nice midrange value.
Not too high to unduly increase output impedance, not too low to increase consumption or lower *input* impedance
The design is a compromise if we use *just* 1 transistor, so don't expect miracles.

2) Gain = R2/R3 so for 10X, R3=470r

3) choose collector current so collector sits at around 1/2 +V
In this case:
I choose +V around 25V (within the OP design limits) so Vc around 12V .

Note: my calculations typically use "Engineering" precision, which says that it's useless (except in a few special situations) to work with more than 2 or 3 significant digits, when parts often have 5 or 10%tolerance, and transistors a 3:1 gain spread, so I usually round to nearest voltage and parts values to nearest "standard series" value.

Back to calculation:

4) We have 12V (as I said, I'm always rounding values here) across R2 so Ic=2.5mA
So Emitter sits at 2.5mA x 470 r above ground .
Ve=1.2V

5) we need to bias the base.
It will sit at (0.65V + Ve) above ground= 1.9V

6) the base will need a biasing current of:
2.5mA/Hfe

I choose a cheap general purpose widely available transistor which I personally buy in bags of 500 or cases of 1000 (or "machine gun strips" of 1000) : BC547C

I also stock its brother, BC557C and use them **everywhere** .

Don't suggest you stock that many, but any DIY should buy at least 20 of each (for peanuts) and have them available.
VERY useful.

I use BC547B or C .
The letter indicates Hfe , 250 or 500 respectively.
If I design considering Hfe around 300, both work .

Your KT603 has a very poor and inconsistent HFe of "10 to 80" , not useful here.
The preamp stage would have *very* low impedance, require larger input cap, etc.
You save nothing.
As I see them , they are only suitable to, say, drive a Led or something similar.

Back to biasing.
The base current will be 2.5mA/300 (Hfe) = 8uA

A standard textbook compromise is to choose current through the biasing divider to be 10X the actual needed base current, so as to minimize its influence.
Fine, it works, but it needlessly lowers input impedance.
School typically designs something that works in the Lab and simulates well.
Fine with me.
But I design commercially so stuff will always be used connected to "something else" , so I'd find raising input impedance a bit a real advantage.

So I choose biasing divider current "only" 4X base current.
Which is a safe bet because the transistor I chose may err having *higher* than normal Hfe (I chose B or C )

Since we will *still* have the actual base current (Ib) flowing through it, they will add, so we have 4 x Ib + Ib = 5 Ib = 5x8uA = 40uA.

So: R2=22V/40uA=550000 ohms.
So a 560K resistor would be fine, but since I know I rounded values a few times along the way, I choose the next lowest value (meaning I allow for a slightly higher current) resistor value.
Which would be 470K .
So R5=470K.
Yes, some Simulator package might give a value of, say, 528K or 493K or whatever, but let's go back to the real world.

7) R6 is what's needed to put around 1.9V at Q1's base, one of the original conditions.
Of course I choose a "standard" value close enough.

8) Input impedance:
You will find a "reflected impedance" (maybe there's a better name for it) of Hfe x R3 = 300 x 470r = 140K
Which is in parallel with R6, so the actual *stage* input impedance will be around 40K .

9) input capacitor: whatever's needed to reach "low enough" (your choice) considering the input impedance.

Being a MI amp designer and builder, I usually cut below, say, 60Hz in Guitar amps and below 30 Hz for bass amps, to avoid unnecessary "mud", but 10 Hz is reasonable in the Music Reproduction world.

Won't get into discussions as to what *really* means Hi Fi, be my guest to set your own preferences.

10) I got more detailed than usual, because I wanted to be very clear, and at the same time explain why "Real/Commercial" World design often takes so many compromises.
It's a matter of competitiveness and diminishing returns.
Getting a little extra performance spending 10X the \$\$\$ is a Commercial suicide.
The enthusiast or DIY dos not have those constraints.
Good luck .
And .... buy a few "good" transistors and leave those KT603 for less demanding uses .
Good luck again.

moschfet
diyAudio Member

Join Date: Jul 2008
Location: Hannover
This is a simulated example with a Mosfet.
Attached Images
 pre.jpg (156.7 KB, 383 views) pre2.jpg (249.2 KB, 376 views)
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 28th April 2013, 08:30 PM #25 Jempie   diyAudio Member   Join Date: Jan 2008 Hi jmfahe, i have an 2 way speakersystem with 89 db sensetivity and use an amplifier with Input Impedance >10K Ohms Gain +27dB (1.7V sensitivity) how to fine tune your design? what happen if i use fi an V-cap lower then 1 uF 0,1 uf and what does the C2 do in sound
mjona
diyAudio Member

Join Date: Jul 2007
Location: Upper Hutt
Quote:
 Originally Posted by Jempie Im looking for an single ended solid state preamp only with one transistor design. I have looked everywere to see if there is an design with only one transistor use in an line preamp. does it exist
There is an example by Linsley-Hood in the modular pre-amp design intended for use with his Simple Class A amplifier on Rod Elliots Sound Pages website. It is a lengthy article and the example is shown in figure 10 towards the end. It will work from a range of power supply voltage.

see
The Class-A Amplifier Site - Modular Pre-Amplifier Design

You could also look up Paul Kembles pages, A Paul Kemble web page - index to 'sound' webpages. there are a few examples there notably in the Cambridge Audio P series amplifier preamp stages. There is plenty of other interesting material on this and Rod Elliots pages.

Michael J

JMFahey
diyAudio Member

Join Date: Mar 2009
Location: Buenos Aires - Argentina
Quote:
 Originally Posted by Jempie Hi jmfahe, i have an 2 way speakersystem with 89 db sensetivity and use an amplifier with Input Impedance >10K Ohms Gain +27dB (1.7V sensitivity) how to fine tune your design? what happen if i use fi an V-cap lower then 1 uF 0,1 uf and what does the C2 do in sound
This preamp has 10X gain.
If your power amp has 1.7V input sensitivity, using this preamp ahead of it means that it cam be fully driven by 170,V audio.

Practical use?:

the typical generic MP3Player/Pendrive fed from 1 AA battery has an earphone output of about 200mV, so it can now drive your amp to full power.
If you have a weak signal such as this one, put a 25K or 50K Volume pot, preverably audio taper, between preamp and power amp.

If you have a strong output CD player or some other line powered signal source , with up to 2 V signal, you will need the same volume control at the preamp input, or it will be overdriven even if you "lower the volume".

C2 is there because you have 12/15V DC at the transistor collector and you do not want it at the output.

The input cap defines the low frequency cutoff.
.47uF give you around 10 Hz cutoff; .1uF will give around 50 Hz (which may be acceptable with cheap speakers) and so on.

AndrewT
diyAudio Member

Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Originally Posted by Jempie ......... with 89 db sensetivity and use an amplifier with Input Impedance >10K Ohms Gain +27dB (1.7V sensitivity)........
that's a maximum of 180W into 8ohms speaker, or 360W into 4ohms speaker.
Adding a 10x (+20dB) preamp in front of that, giving a sensitivity of 170mVac for 180W into 8ohms is unusual.
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regards Andrew T.

Last edited by AndrewT; 29th April 2013 at 03:05 PM.

 10th May 2013, 12:15 PM #29 Jempie   diyAudio Member   Join Date: Jan 2008 Fahey, i have try the one with BC547b, the sound is mellow less drive how can i change that
 10th May 2013, 12:20 PM #30 JMFahey   diyAudio Member     Join Date: Mar 2009 Location: Buenos Aires - Argentina Please post the schematic of what you actually built and how you connected it. Also what signal source you used. And what speakers too. If you used some powerful woofer but no tweeters sound *will* be dull.

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