use of series resistance on output for "tubelike" sound? - diyAudio
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Old 16th April 2007, 10:46 AM   #1
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Default use of series resistance on output for "tubelike" sound?

I've always liked tubes, but tube amps are too expensive for me at this point, and besides I like SS too. Recently I read that a good portion of the "tube sound" has to do with the lower damping factor and output impedances characteristic of tube amps and that one way to lower a solid state amp's output impedance is to insert some resistance on the speaker leads. Apparently Bob Carver used this technique to fool many experienced listeners into thinking they were hearing tube amps. While I had no pretentions of sounding "just like a tube amp", i figured SOMETHING would happen and I had to find out for myself....who knows, maybe it would slightly tubelike.

Some people seem pretty philosophically opposed to this, but i figured it couldn't hurt to try. I'd been told to try resistance anywhere from 0.5 to 2R as long as the resistors were rated for 10 watts or more...as luck would have it i had four 4R3 10w non-inductives laying around from a speaker zobel that never materialised so i just paralleled two of them per speaker to get ~2.2 ohms and incidentally a higher power rating.

I used my old Superscope receiver, already a relaxed and warm-sounding amp and an old pair of sealed Wharfedale 3 ways. As I expected the difference was noticeable but not huge and primarily affected the bass response, which sounded smoother and livelier...like it was less effort for the amp, you could say. this was good, but i found the midrange to be clearer and punchier, maybe even a bit more revealing since I found it easier to pick out sounds that were normally more easily picked out on headphones. I liked it although not surprisingly there is a slight reduction in sound output level, nothing i couldn't fix by turning the volume up a little more. I am not sure if i would call the sound more "tubelike" yet because i need to listen more. two non-audiophile housemates of mine immediately and seperately commented that it sounded clearer.

My question is, who else here used series resistance with their speakers? what are your thoughts on this technique, and drawbacks? I like this cheap mod and i'd like to build future amp projects with an in/out switch to control the resistance.
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Old 16th April 2007, 11:32 AM   #2
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Bright idea. You can make it even closer by using of tube preamp, see the image.
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File Type: gif tubepre.gif (39.4 KB, 371 views)
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Old 16th April 2007, 12:06 PM   #3
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JLH in his 75W BJT design used 0.22R WW resistor on the output.
Naim did the same in some of their amps.

The serial resistor dramatically decrease DF of amplifier.
Any other effect that comes to your minds?
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Old 16th April 2007, 12:44 PM   #4
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Protection, I see no other reason to use series resistor, unless one wants to worsen sound. It may help to broaden response of certain bass drivers (change of Qel), but is highly urecommendable for midrange and highs.
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Old 16th April 2007, 01:08 PM   #5
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In principle a wirewound resistor at the output does the same as a Thiel-network. But the serial resistor's inductivity is lower than Thiel's coil's. And paralel resistance is higher.
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Old 16th April 2007, 01:10 PM   #6
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Quote:
Originally posted by sixtek
.....The serial resistor dramatically decrease DF of amplifier.
Any other effect that comes to your minds?
The other effect is to boost the low bass response due to the impedance peak of the drive unit in its enclosure. The potential divider effect of the series resistor combined with the driver impedance peak will give a few dB of boost at the resonant frequency.
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Old 16th April 2007, 01:31 PM   #7
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Hmmm... Thanks, smart thought.
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Old 16th April 2007, 01:33 PM   #8
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Quote:
The potential divider effect of the series resistor combined with the driver impedance peak will give a few dB of boost at the resonant frequency.
I would say "a few dB" is a little exagerated.

Regards

Charles
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Old 16th April 2007, 01:38 PM   #9
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Depends on Qts of driver and box volume, for low Qts drivers you can get few dB. The question is why not to design the box properly, no resistor is needed then.
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Old 16th April 2007, 01:45 PM   #10
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Quote:
Originally posted by phase_accurate


I would say "a few dB" is a little exagerated.

Regards

Charles
Consider this example :-
Speaker with nominal impedance of 8 ohms, peaking up to 20 ohms at the fundamental resonance.

Using bikehorn's 2.2 ohms in series, we get an average attenuation across the band of 2.1dB. At resonance, the attenuation will be 0.9dB. So, the boost is 1.2dB in this case.

With lower impedance drivers, the effect will be greater of course.
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