| bikehorn |
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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| PMA |
| Bright idea. You can make it even closer by using of tube preamp, see the image. |
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| sixtek |
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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| PMA |
| 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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| sixtek |
| 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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| cs |
| 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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| sixtek |
| Hmmm... Thanks, smart thought. |
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| phase_accurate |
| 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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| PMA |
| 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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| cs |
| 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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| sixtek |
| Am I right that with some speaker cables the serial 0.22R caused sonic problems with Naim's? |
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| cs |
| quote: | Originally posted by sixtek
Am I right that with some speaker cables the serial 0.22R caused sonic problems with Naim's? |
Naim amps were (and possibly still are !) notorious for not having Zobel networks on their outputs. This means they are very intolerant of capacitive loads, either due to the speaker crossover or the speaker cables. The 0.22R only provides a small amount of load isolation.
In a severe case, they can oscillate at MHz frequencies, and get very hot ! This is why Naim advocate only ever using their own NACA5 cables, which are low capacitance / high inductance. Dreadful design really !
PS. I own a Naim amp too !! |
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| Conrad Hoffman |
| There are lots of paths to where you want to go. My idea of clarity is separate amps for each driver, with high damping, connected to the drivers with short heavy gage wire. Anyway, remember that when you do something that changes one end of the spectrum, say the quality of the bass, you may hear changes elsewhere, say in the midrange, that have little to do with the change itself, but the way you hear. I know this because I can change an individual driver level and hear differences in a region where there has been absolutely no change. My brain obviously wasn't made by Tektronix or HP :smash: |
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| darkfenriz |
I've added a 3ohm resistor in series with speaker in my guitar amp.
The effect is definitely hearable, bass are muddier and heavier and hights not so bright, but the effect is probably small enough to really change the sound character of guitar amp and high enough to colorate much hi-fi reproduction. |
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| martin clark |
| quote: | | This is why Naim advocate only ever using their own NACA5 cables ... Dreadful design really ! |
I disagree; Naim's design decision was based on the observation that the cable could provide all the inductance required to guarantee stability, without a bulky and magnetically-susceptible inductor on the board. Hence the minimum cable length recommendation. Perfectly reasonable for many years before the marketing push elsewhere for high-dollar wire... which Naim still doesn't really acknowledge. In fact oscillation is only a problem with most capacitative cable types - Goetz et al.
The use of a series 0R22 ensures an absolute minimum of overshoot, even into capacitative loads (<10% on 8ohm +2uF), which is definitely something that can't be said for ill-considered 'bung-an-inductor-in' designs - there's a really good discussion of this in Doug Self's Amplifier Design book. In summary Self's comments are that the effect on DF is utterly negligable; and a principle reason the series resistor isn't more popular as an alternative is the loss of output for the marketing bods - it'll turn a 200W/4ohm amp into a 190W/4 ohm one. Again, actually insignificant... |
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| cs |
| quote: | Originally posted by martin clark
...Naim's design decision was based on the observation that the cable could provide all the inductance required to guarantee stability, without a bulky and magnetically-susceptible inductor on the board. Hence the minimum cable length recommendation.... |
Personally, I think that amplifiers should be designed to cope with any loudspeaker load, even highly capacitive ones, without oscillation. A properly designed Zobel network will achieve this. A 0.22R resistor will not !
I admit that I foolishly bought a Naim system, before finding what what was inside it. When I did investigate, I was appalled at the crude 1970's design lurking within. |
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| PMA |
| quote: | Originally posted by martin clark
I disagree; Naim's design decision was based on the observation that the cable could provide all the inductance required to guarantee stability, without a bulky and magnetically-susceptible inductor on the board. Hence the minimum cable length recommendation. Perfectly reasonable for many years before the marketing push elsewhere for high-dollar wire... which Naim still doesn't really acknowledge. In fact oscillation is only a problem with most capacitative cable types - Goetz et al.
The use of a series 0R22 ensures an absolute minimum of overshoot, even into capacitative loads (<10% on 8ohm +2uF), which is definitely something that can't be said for ill-considered 'bung-an-inductor-in' designs - there's a really good discussion of this in Doug Self's Amplifier Design book. In summary Self's comments are that the effect on DF is utterly negligable; and a principle reason the series resistor isn't more popular as an alternative is the loss of output for the marketing bods - it'll turn a 200W/4ohm amp into a 190W/4 ohm one. Again, actually insignificant... |
This funny, just marketing brain washing. Naim are masters in doing that. |
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| Sjef |
Part of the increased clarity could come from less rubbish getting back into the amp via the overall negative feedback (the main problem with using this kind of feedback).
An other explanation might be in the fact that not every loudspeaker likes to be tightly controlled by the amplifier. This is because the amp does not take control over the entire loudspeaker only over the voice coil, it has no control whatsoever over the way the cone reacts to the attached voice coil. When the cone likes to swing out but is forced to stop only at the place where cone and coil are glued together just imagine what's happening at that point, distortion galore. This happens in alot of speakers so the highest damping factor wich is technically possible to achieve doesn't always mean the best sound. |
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| bikehorn |
This has become an interesting little debate. I was not aware series resistance functioned at all like an output zobel network.
The results I've gotten using series R with my garage-sale collection of gear have impressed me enough to keep the resistance on my leads for a while. Unfortunately these speakers, dating back to the very early 70's are a little tired and have weak bass response below 65 hz(just a shot in the dark guess, not measured). I like the quality of the bass resulting from this experiment but the quantity produced by these boxes is not enough for me to tell if there was any increase or improvement in low bass, so i'm next going to try this with my friend's B&W DM220s to see how they react, also being a sealed design.
One reason I was interested in this trick is my interest in building a pair of horn cabinets using a pair of Fostex FE206 fullranges for my brother as a birthday gift Some people report that the subjective(or maybe objective?) bass response of these cabinets is weaker with solid state amps due to their higher damping factor whereas the cabinets were optimised for use with low-power tube amplifiers. Since I can't give myself a tube amp, I can't give my brother one either, so there is little choice but to try to simulate the damping factor of a tube amp using a nice SS design. |
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| DSP_Geek |
| quote: | Originally posted by bikehorn
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. |
Someone at Stereophile tried this a couple of years back with a 4 ohm resistor in series with a loudspeaker, and he was rather surprised to find his solid-state amplifier took on the "liquidity" previously ascribed to single-ended tube units. |
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| wilbur-x |
This maths may be a mouse-pee in the atlantic due to the series impedance in the loudspeakers low pass filter coils?
I owned a pair of Kef, they implemented a resistor in series before the coil in question, boosting the LF (same as weakening of magnet). Am I right? If not, I'm pleased to learn.. |
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| bikehorn |
| quote: | Originally posted by wilbur-x
This maths may be a mouse-pee in the atlantic due to the series impedance in the loudspeakers low pass filter coils?
I owned a pair of Kef, they implemented a resistor in series before the coil in question, boosting the LF (same as weakening of magnet). Am I right? If not, I'm pleased to learn.. |
In my case there is no LPF on the woofer or the midrange, they are both running wide open with only a cap on the tweeter. Was the resistor in your KEF speakers switchable? How did they perform? I was planning to make series resistance "optional" on my amp with a DPDT switch on the back. |
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| wilbur-x |
| The KEF RDM-2 performed well, the resistor was not switchable. (but, of course, the box were calculated in context with the resistor & x-o) |
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