Little Big Amp - page 2 - diyAudio
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Homeskillet's House
of Audio Electric Oddities


The circuits shown here are most certainly not appropriate for general use. I build them for my own enjoyment and curiosity. They usually are dismantled in a few days and the parts are used for something else.

The schematics and designs here are all bench designed using a basic circuit idea and then trial and error to arrive at specific values. They may have problems.

They are connected to a variac with a fuse and current meter. They are turned on and off by slowly turning the variac up and down. They are not subject to any of the rigors of consumer use and they are not intended to be.

I always wear eye protection and turn the power off by turning down the variac. Nothing is subjected to hard switching or a quick turn on. I also work with one hand behind my back if the circuit is to be approached while powered on.

I also continually check to see if the power is truly off as I work on a circuit by verifying the variac is off and the voltmeter and current meter reads zero. It is easy to be interrupted and resume working on a circuit to find that you haven't powered it down so checking continually as one works helps prevent this.

I also power the unit down and let it drain before applying test leads or making any change whatsoever. This part is really a big deal.

If I do post anything that is a working design suitable for general use I'll make a note of it and probably make a big deal of it.
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Little Big Amp - page 2

Posted 5th August 2013 at 04:34 AM by homeskillet
Updated 5th August 2013 at 06:49 AM by homeskillet (added a bit of plate to plate feedback)

Continued after some listening and more tinkering from here:
http://www.diyaudio.com/forums/blogs...e-big-amp.html

It seemed lacking in the high end so I added a bit more capacitance to the bypass cap on the cathode resistor on V1.

This picked the high end up nicely. I then hooked the speaker up to the 16 ohm tap and used the 8 ohm tap as the feedback center tap. This resulted in a bit more output and I like the sound just fine.

The volume seems to increase after it's turned on a while. Also, it needs a B+ fuse in the event that V1 doesn't draw then V2 will have no bias and will draw tons of current and then will dump tons of current into it's cathode resistor.

The schematic posted here is updated with voltages and values done to the best of my ability and it sounds very nice. It's not a finished design but great for experimenting.

I will be setting this aside as my curiosity is leading me elsewhere. I'll give it a listen for an evening or two though as my reward for a bit of building and head-scratching.

Here's some youtube videos of it running:

0 to 40 kHz sweep:
Simplified Loftin White with feedback from transformer secondary 0 to 40 kHz sweep - YouTube

Playing solo piano:
Simplified Loftin White with feedback from transformer secondary playing solo piano - YouTube

Playing Obscura:
Simplified Loftin White with feedback from transformer secondary playing Obscura - YouTube

Solo piano is such a brutal test for a simple design. This amp does the best of any of my designs so far. Maybe I've made it complicated enough to play solo piano.

Thanks for looking. -Fred

p.s. About an hour after posting I was giving it a listen and I felt the sound was a bit dry or hard sounding. There was an element missing from the sound of the plate to plate feedback version, so I decided to introduce a little and see what happens. I added an old wire wound resistor I had on hand. The level went down slightly on the oscilloscope so I know it's doing it's thing. The plate to plate feedback seems to give a slinky sound to the high frequencies which is what I was hoping it would in the first place.
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