How do you think about my amp?

[newbiediy]

Hi there...

I'm a newbie both in this awesome forum and in electronics. I need your opinion about my homemade amp.
It is basically a Little Rebel with jfet input buffer for better "clean" sound (not "true" clean as there is always some amount of distortion).

A "demo" video: http://www.youtube.com/watch?v=3BO4j-4KXXs (sorry about the bad playing). 

Any comment or suggestion will be VERY appreciated.

[J M Fahey]

Very good.
Congratulations.

[erikb1971]

Nice! What speaker did you use?

[newbiediy]

I use a cheap 6" 8ohm 20W speaker. Its brand is ACR. I think this is the spec: http://www.acrspeaker.com/productdetail.php?categ=K12&idproduct=12

Not very ideal for guitar speaker, but it is better sounding and cheaper than woofer type speaker I tried earlier. Any suggestion on the speaker type to improve sound? Please don't suggest Celestion, Eminence, or other branded guitar speaker. They are waaaaayyy too expensive for me. 5" - 6" is preferred because I made my box based on that sizes.

One more question. I added this jfet input buffer in front of the Little Rebel schematic: http://www.muzique.com/lab/buffers.htm
At first I used the basic common drain schematic (the 1st one). Then I tried to improve it (as suggested by the 2nd schematic) by connecting R1 (1M) to 4.5V instead of ground. I got better clean sound indeed...
Then I tried connecting R1 to ground and 4.5V back and forth via a switch to compare the sound. Not for long I got a bonus feature on my amp: FM RADIO RECEIVER!!! 
And then I noticed when I connected R1 to 4.5V, both 386 (I use MIK 386) felt very hot. So I decided to connect R1 to ground.
And the question is: What was wrong? I have no electronic background and this is my 3rd project after the infamous Little Gem and Ruby. Any good (not too) technical explanation would be very appreciated.

[phatt]

Hi Newbiediy,
                   Great effort, sounds good, and yes Your speaker choice is just fine  :dbtu:
You are correct, woofers don't work for guitar amps.

Can't help with the bias of fet might be the way you wired it all up but if it works without getting seriously hot it likely ok that way.

Looks like the original site has lost it's cyberspace?
Here is the pdf for those interested.
Phil.

[newbiediy]

Thank you all for the replies. This forum really rocks!! TEN THUMBS UP!!! :dbtu:  :dbtu:  :dbtu:  :dbtu:  :dbtu:

I am planning to add Condor cab sim (http://www.runoffgroove.com/condor.html) to my amp. Obviously my amp's treble content needs to be cut away. :trouble

Where should I add it?

My amp's current layout:
jfet input buffer --> preamp --> MIK 386 power amp --> headphone out --> speaker out
(Little Rebel has send-return feature and I implemented it in my amp)

Which one is correct?
jfet input buffer --> preamp --> Condor --> MIK 386 power amp --> headphone out --> speaker out
OR
jfet input buffer --> preamp --> MIK 386 power amp --> Condor --> headphone out --> speaker out
OR
Should I use it like a stompbox (between guitar and amp)?

Is the jfet amplifier stage in Condor really needed? Can I eliminate it and use the schematic starting for 47n cap? My problem is I can't find J201 / MPF102 / 2N5457. The only jfet I can find is 2N3819 and I had a very disappointing experience with it when I built Thor. Very faaaar away from high gain. Or should I leave it intact (and try it with 2N3819)?

If I don't need Condor's volume control, can I just eliminate it or should I replace it with a 10K resistor to ground?

(Oops, sorry if I ask too many questions...)

Regards,

newbiediy

[joecool85]

I would say run it like a pedal (between guitar and input on amp).

[J M Fahey]

I think your amp is fine, it might sound trebly now because you use a very light 6" speaker in a half shoebox size cabinet.
And yet is sounds quite good.
Hook it to a 12" speaker in a normal sized cabinet and you'll be surprised !!!
The condor sim is meant for recording.
If you take the speaker out and just pad it down to mixer console line in level, it will be way too buzzy, *there* the Speaker simulator does its job best.
Build it, and put it in your chassis, feeding an extra jack called "Compensated Line Out".
You´ll hear your 6" speaker only as a monitor, but the recording will sound like if you used an expensive 12" guitar speaker. Cool.

[newbiediy]

Thanks for the quick reply.

I thought I can "simulate" a 4x12" cabinet on a 6" speaker using Condor. My amp has a nice bass end (at least to suit my taste). But it also has *harsh* treble frequencies which you can't hear on the video (thanks to the poor quality of the mic, lol ).

I am dreaming of a small 6" practice amp with a frequency response similar to 4x12". Is it actually achievable?

[erikb1971]

what about this driver:

http://jukebox-revival.eu/ch-mod-6-15.html

[joecool85]

what about this driver:

http://jukebox-revival.eu/ch-mod-6-15.html

The Jensen Mod series is quite highly regarded around here.  Though I haven't used one myself, I understand they are a great budget guitar speaker.

[J M Fahey]

I thought I can "simulate" a 4x12" cabinet on a 6" speaker using Condor.
.....................
I am dreaming of a small 6" practice amp with a frequency response similar to 4x12". Is it actually achievable?

Sorry, but not.
It's a clear Physics problem.
Speakers are pistons moving air.
Just calculate how much more air can 4 x 12" pistons, travelling 1/2" move, than a single 6" one, moving 1/8" .
Now, the Condor Cab sim can simulate it *in a recording* , where you can copy its frequency response but you are moving no air at all.
Not bad.

On the other side, we have seen the "Smokey" 1/2W battery powered amplifier driving a real 4x12" with impressive results.
*Big* amplifier sound.
http://youtu.be/1KSWa7N1xGY

[newbiediy]

Sorry for the late reply. I've been busy lately.
I've watched the video and wow... that was driven by a 1/2 watt amp(??!!!) OK, I get the message. :dbtu:
I guess size does matter.
I've read many good words about Jensen speakers. :dbtu: But there is budget problem here.
My build is only about 25 EUR total, so an 18 EUR speaker will be considered next time for a bigger and better project. Thanks for the suggestion.

[rocklander]

bit of a dredge sorry. just wondering how you added the headphone out to the little rebel?
thinking about etching one of these this weekend but keen to add headphone jack. thanks for any advice (schematic?) you can offer.

[newbiediy]

I'm using the schematic at the FAQ section of runoffgroove.com. Sorry I can't give you direct link, I'm replying from my cellphone. It's very simple, really. Connect a 10 ohms resistor from jack's ground to speaker out negative.
But instead of 10 ohms, I use 220 ohms/2w because it's still too loud for me. Works for me, but I don't know if it's technically correct.
Use plastic socket. It needs to be floating. Grounding it will make VERY loud hum.
You can wire it so when you insert headphone jack, it will break the connection with the speaker.
Don't expect too high. This is the simplest way. The sound will be a bit dull. A cab sim will do better.

Hopefully this helps.