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In Reply to: RE: No feedback and UL transformer connection posted by Eli Duttman on January 14, 2017 at 19:21:02
Well.. the driver is kind of a cross between an El Cheapo and a Baby Huey, as for output you know, on my speakers the bass sounds really solid. I'm using 97db JBL 2226 - more of a mid-woofer, likely not much below 40hz but sure sounds solid to me. One thing I've found with these speakers is they need a bit of power - more than a 2A3, but not a lot more. Very enjoyable and my point in the original post wasn't to garner criticism, though I'm not surprised it did, but rather to encourage us to all sit down and listen to some MUSIC. I do have a vintage Leader distortion analyzer and maybe next week I'll put her on the bench but I can't believe at the 3 to 5 watts I'm asking that I'm getting much more than a couple % of distortion.
Follow Ups:
errr...not being critical( too much...LOL), just trying to take exception of the simplistic FB statement made by chip.
I built an open loop amp around the Dyna A441 outputs once. Sounded pretty good with the tertiary winding feeding the screens. Sounded even better with a fixed g2 voltage and the cathodes riding the screens. On a wide variety of speakers the CFB rigging was The Way To Go, but on a few, open loop U-L was a little bit better.
Then I hit upon the Plate-to-Grid implementation we called E-Linear. Not looked back, and built several full pentode amps of that topology.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
OK How about
"The use of global negative feedback does several things: it flattens and extends the frequency response, it reduces distortion generated in the stages encompassed by the feedback loop, and it reduces the effective output impedance of the amplifier, which increases the damping factor. All of these things affect the tone in some manner.
The flattened, extended frequency response obviously changes the tonal character by removing "humps" in the output stage response and producing more high and low end frequencies.
The distortion reduction makes the amp sound cleaner and more "hi-fi", up to the point of clipping, with less output-stage generated noise.
Perhaps the main difference for the "feel" in a negative feedback amplifier, as opposed to a non-negative feedback amplifier, is the increased damping factor produced by the negative feedback loop. The decreased effective output impedance causes the amp to react less to the speakers. A speaker impedance curve is far from flat; it rises very high at the resonant frequency, then falls to the nominal impedance around 1kHz, and again rises as the frequency increases. This changing "reactive" load causes the amp output level to change with frequency and changes in speaker impedance (a dynamic thing that changes as the speakers are driven harder). Global negative feedback generally reduces this greatly. This can be good or bad, depending upon what you are looking for. Negative feedback makes the amp sound "tighter", particularly in the low end, where the speaker resonant hump has the most effect on amplifier output."
http://www.aikenamps.com/index.php/what-is-negative-feedback
Hi Douglas, I remember this being discussed in the DIY forum a number of years ago - in fact found some posts I guess from you ca 2003. Wow, time flies. In the schematic I saw it used 6AU6's and EL34's. Might be fun to give it a try. I'm in kind of a mood to experiment. It's probably buried in that old post, but to resurrect it a bit, can you discuss the relationship between the coupling cap and the feedback resistor from EL34 G2 to plate of 6AU6? How are values chosen?
That FB resistor as you call it is just the plate load for the 6AU6. I'd rather select a signal pentode that has a higher plate/g2 current ratio...the 6AU6 is rather poor, though I have had them work very well. Using the power tube's screen tap just sets the ratio of plate signal delivered to the signal tube. Plate to grid FB, and with pentode OPT's it can be done with a voltage divider too...the screen tap is just convenient...:) The type EL84 turns out to be a very fine signal pentode.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
IIRC, the 6BQ5/EL84 was used, on occasion, as a video O/P device. Adequate and then some bandwidth there. ;> )
Eli D.
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