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In Reply to: RE: Spinning Ray Brown posted by SteveBrown on January 13, 2017 at 15:46:49
You can get better sound from using some feedback. One of the selling points of triodes is that they do not require feedback to be linear. However, UL pentodes do need feedback. You should at least try it. However, you may be digging the distortion from not having it.
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The Mind has No Firewall~ U.S. Army War College.
You seem to have made the assumption I didn't try it. I did. DIdn't like it as well as naked. Perhaps someplace in between would be good - which I have not experimented with. UL of course, gets close to triode.
Ok, just watch out for pleasing distortion. UL ain't triode.
I'll quote Frank VA. "You like distortion, you have bad taste." ;> D
Either it's HIFI or it's not. High fidelity, get it.
Everybody is entitled to their own opinions and, in their own "castles", do pretty much what they please. Just DON'T go calling distortion HIFI. It may be euphonic, but it's not high fidelity.
Eli D.
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Have Fun and Enjoy the Music
"Still Working the Problem"
The term "distortion" is a technical term.
Anything coming out of a circuit that wasn't part of the input signal feeding that circuit, no matter how pleasing it might be to you or anyone else, is distortion.
If, in your opinion, as long as it's pleasing to you then you don't care if it's distorted or not then just say so.
But you can't go around redefining the term "distortion" simply based on your own personal preference. It's inaccurate and it shows ignorance.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
To make this work, we must also question the level of distortion, below which, we have "fidelity". NO amplifier, even instrument amp, is perfect, so where is it "good enough"? And again, I'd say at the output levels I use, I'm guessing distortion is actually very low. In this amp I've also eliminated two prior pre-amp stages and thus, eliminated the distortion from them. Introducing feedback isn't something I'd see doing to help distortion at the end of the day. I might do it to improve damping factor, however.
If you are at or below 0.3% THD, when full power is produced, you definitely are good to go. Only folks with the very best hearing can(?) detect that, when the driving signal is a pure sine wave. Under 1% is probably OK, when a complex music waveform is present.
IM distortion is another matter, as it is much more irritating than harmonic distortion.
Eli D.
I attempted to measure output power and distortion (distortion on my old Leader LDM 171). I got 7.6w out just prior to clipping. And IF I used the distortion meter correctly, it was about 2.3% distortion. Of course that made me suspicious that I was doing something wrong. Power was too low, distortion was too low. I figured 7 to 8 watts was what I'd get with triode wired EL84's so I took a peak under her skirt and sure enough, sometime long ago I converted to triode. As for the distortion reading, I'm not at all sure I did that correctly, seems like you should put a test signal in both the amp and analyzer but the instructions don't say anything about that. I also don't know if the amp should be on a dummy load, but I assume so. Anyway, distortion is suspect to say the least.
"seems like you should put a test signal in both the amp and analyzer"
No, you just match the frequency with the distortion analyzer. You did it right.
You have to deduct the distortion of your generator to get an accurate number. It's complicated but the manual should walk you through it.
IMO 2.3% is high considering that it's a PP circuit and all the even ordered distortions should be canceling.
IMO 2.3%, made up of 3rd and 5th and (God forbid) 7th, will sound much worse than 2.3% of 2nd.
BTW My objection to kitch29's post was his redefinition of the term "distortion". To me he said that "as long as it sounds good to the listener then it can't be described as distorted".
That's just not true. What if the listener has no idea what the recording should sound like? Personal preference for a particular type of sound is not the same as low distortion.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Thanks. My signal generator is an old and cheap one, and I don't have a manual, but I'll take a look on line and see what I come up with.
The manual for the distortion analyzer should describe the mathematical process for accounting for the generator distortion.
Run the generator straight into the distortion analyzer to get your base line distortion and then work from there.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
nt
and linearity is not reasonable output impedance.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Precisely! The (IMO) inadequate damping factor will be problematic with many speakers. A few speakers may mate tolerably well.
Eli D.
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.
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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