Krill - The little amp that might...

This new thread is a split from the thread at:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=132367

I'm doing this to eliminate the 90% or so off-topic posts and to get a better to feel for any interest in this project.

This schematic was drawn by the same program I use for simulations. It simulates very much like it measures in the real world so I am reasonably certain that it is correct. No doubt there will be some problems found that I can correct later.

I both simulated and built this with the parts shown. You may substitute other brands of transistor and it should work well. Use what is available to you. Please use a decent power supply. I use a 120 V to 24 V times 2 at amp 4 transformer and 20,000 uF of filter cap per rail per channel.

Please pay attention to the odd values of R6 and R12. In the real world R6 will be a 330 ohm parallel with a 5K trim pot. I prefer to use multi-turn pots for the precision they give. R12 will be a 33K with a 10K pot in parallel. When the amp is first powered up adjust R12 to give 25 to 30 milliamps of current through the two output devices. R6 will set the DC offset. Run the Amp into an 8 ohm load at 10W for approximately 30 minutes and then readjust both resisters. Remember the adjustments are made with no load on the amplifier. If you have a distortion meter, choose an appropriate output power (I like to use 10W) and adjust R12 for minimum distortion.

This basic and simple amplifier sounds very good just as drawn. If there is enough interest I will show improvements to the design as well as higher powered units.

Here is the schematic with all voltages shown.

You're correct that the voltage amplifier is nothing special. I stated that in the thread that I referenced. I also stated that you could use any voltage gain stage that you prefer. The only thing I'm claiming here is the design of the output stage. I did design the voltage gain stage also but I realize that this is in the public domain and can hardly be considered mine.

Yes, both amplifiers do share the same supply although a different one for each channel. I'd mentioned improvements I would show if there is sufficient interest. A separate power supply for the front end would be obvious. In my production units I used a voltage doubler with a regulated supply for the voltage gain stage.

The amplifier just as presented has exceptionally good sound and very low noise. I wanted to present the amp in it's simplest possible working form. We can make things complicated and more expensive later.

Originally posted by analog_sa Very cool. A single cap/transformer coupled triode would make a great hybrid amp.

Click to expand...

If you use a capacitor coupled input you will need bias for the input pair. Use a 1 Meg 10 turn trim pot from each rail to input. Set both to maximum value. Look at your output DC offset. If it is positive, adjust the trimmer to the negative supply. If negative - well, you get the picture. I have included an example.

I hope you try it. Let us know how it sounds.

Re: Re: Krill - The little amp that might...

juma said:

I suppose it's R5 in CCS for input LTP, right ?

Click to expand...

That is correct. Thank you for pointing out that error.

lineup said:

Steve.
What is the approximate input impedance of the follower?
Take the version you posted.
Some guess, or some measured value.

Click to expand...

Very high. I don't know exactly. It is high enough to never have cause problems so I never measured. With the 1uF coupling cap and the 1Meg bias/offset resistors shown the -3dB point is under 1Hz.

Mine seem to be somewhat different.


**********
*SRC=2SC3298B;QSC3298B;BJTs NPN;Power;200V 1.5A
*SYM=POWBJTN
.SUBCKT QSC3298B 1 2 3
* TERMINALS: C B E
* 200 Volt 1.5 Amp SiNPN Power Transistor 12-03-1991
Q1 1 2 3 QPWR .67
Q2 1 4 3 QPWR .33
RBS 2 4 9.5
.MODEL QPWR NPN (IS=4.4P NF=1 BF=240 VAF=254 IKF=.9 ISE=475P NE=2
+ BR=4 NR=1 VAR=20 IKR=1.2 RE=5.6 RB=40 RBM=4 IRB=55.556U RC=1
+ CJE=50.7P VJE=.6 MJE=.3 CJC=32.9P VJC=.22 MJC=.2 TF=1.6N TR=61.2N)
+ XTB=1.5 PTF=120 XTF=1 ITF=9.6)
.ENDS
**********
*SRC=2SA1306B;QSA1306B;BJTs PNP;Power;200V 1.5A
*SYM=POWBJTP
.SUBCKT QSA1306B 1 2 3
* TERMINALS: C B E
* 200 Volt 1.5 Amp SiPNP Power Transistor 12-03-1991
Q1 1 2 3 QPWR .67
Q2 1 4 3 QPWR .33
RBS 2 4 9.5
.MODEL QPWR PNP (IS=4.4P NF=1 BF=162 VAF=254 IKF=.8 ISE=475P NE=2
+ BR=4 NR=1 VAR=20 IKR=1.2 RE=5.6 RB=40 RBM=4 IRB=55.556U RC=1
+ CJE=111P VJE=.6 MJE=.3 CJC=72.5P VJC=.22 MJC=.2 TF=1.6N TR=61.2N)
+ XTB=1.5 PTF=120 XTF=1 ITF=9.6)
.ENDS

Here it is with the regulated VGA supply. The supply itself is not shown. I think most people here can provide their own. There seem to be different preferences so use one you like.