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Dirty-South Blues Harp forum: wail on! >
AMP article in Electronic Design Engineering
AMP article in Electronic Design Engineering
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STME58
270 posts
Oct 24, 2012
4:30 PM
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http://electronicdesign.com/content/topic/tubes-versus-solid-state-audio-amps-the-last-word-or-house-of-fire-part-2-/catpath/analog-and-mixed-signal
The above link it to a 3 page article with references about tube vs solid state amps. I thought some of you electronic types might be interested and might not subscribe to Electronic Design. As an ME I don't follow it easily but some of you amp tinkerers might.
I copied the heart of the article below. It is interesting but does not settle the long running argument.
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The Drop-In Experiment
A while back, Richard Dunipace and I worked together on non-audio circuitry. But we shared a similar passion for sound and measuring the oddities that set amplifiers apart. I came up with a straightforward idea—replace the output tubes in a high-end, push-pull, triode-type tube amplifier with high-voltage MOSFETs. We studied and conferred at length, and Richard developed the necessary ballasting, biasing, level shifting, and local feedback circuitry to emulate a triode-connected 6L6 vacuum tube with 1200-V MOSFETs (Fig. 5).
What we found, which led to subsequent investigations on damping, passive components, transformers, inductors, and transducers, was that the MOSFET replacement sounded and measured identical to the tube within 1/2 dB. Further, the MOSFET version measured the same on THD plots, burst modes, square-wave response, and other dynamic testing. The output tube was not the reason that the amplifier sounded the way it did. Most of the tonality that we heard came from the pre-amplifier, the inverter stage that drove the output devices, the output transformer, and the transducer.
We later proved this a second time by taking horizontal output transistors (low-gain, high-voltage BJTs) and plugging them into the same application, with their own thermal compensation, level shifting, and local feedback. Again, the amplifier sounded and measured within 1/2 dB. We never got around to testing insulated-gate bipolar transistors (IGBTs). We concluded that the sound difference wasn’t dominated by the devices used. Hats off to Richard for working up these circuits, usually dead bug style, and painstakingly stabilizing and measuring the results.
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Greg Heumann
1825 posts
Oct 25, 2012
9:36 AM
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Although I agree that preamp distortion is probably the main source of the tone we crave, I'm willing to bet those guys were operating the amp within its "clean" range - i.e., like a Hi-Fi- NOT with power tubes oversaturated like in a guitar amp. It highly unlikely that the MOSFETS would clip the same way a power tube does.
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/Greg
Last Edited by on Oct 25, 2012 9:37 AM
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timeistight
877 posts
Oct 25, 2012
9:55 AM
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Ever since the invention of the transistor, engineers have been swearing they were better than tubes. Doesn't seem to have hurt the sales of tube amps to musicians, though.
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jimbo-G
143 posts
Oct 25, 2012
4:01 PM
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i done a presentation back in college about valves vs solid state. I believe that you will never be able to emulate the distortion curve of a valve using solid state components. Valves are just to smooth man!!
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STME58
272 posts
Oct 25, 2012
9:24 PM
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Greg, you make a good point about the likelihood of the amp being operated only in a clean range. There was a mention of the difference between an amp for live performance and an amp for accurate sound reproduction but it was not clear if the distortion properties were considered.
I was impressed that they said they could hear the difference in an apm caused by tying the input cable in a knot thus increasing the inductance. Has anyone here tried that and been able to notice the difference? Or is that a bit of an exaggeration?
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5F6H
1403 posts
Oct 26, 2012
3:49 AM
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It's been long accepted that SS amps can sound great within their rated W RMS (just look around at the average high end hi-fi, or the PA that puts your tube amp tone out to the FOH), there are even some that sound good for musical instruments (MI), even when slightly pushed/with drive channels etc (& some tube amps that sound awful). Brian May & Billy Gibbons have both recorded extensively with SS amps, no one seemed to notice.
However, in many MI applications, a tube amp operates in excess of it's clean rating, a 50W tube amp might make 60-70W of noise under drive. A 50W SS amp will drop off much more quickly, clean W RMS & max output are pretty much the same thing (for a given speaker load). Some speaker manufacturers have different W ratings for their products, depending on whether they are used for SS or tube amplification & this is why the rule of thumb with tube amps is to use speakers rated for twice their power output of the amp.
If you are looking for an early 50's tube amp/PA style tone, then it's going to be hard to replicate with SS (it will take some development, good ears & therefore, inevitably $...one of the advantages of transistors is the low price, this also follows on to cost of cab design etc).
But a tighter gigging tone, if the amp had a suitable, tweakable preamp section, should be achievable...as long as you are using a 100W SS amp to compare to a 50W tube amp, using a good sounding cab & speakers ...& prepared to only sell a handful of amps that cost much, much more than the other SS MI amps on the market! ;-)
Hmmm, even makes me wonder what a SS 200W, 4x10" (for example) power amp, with harp friendly cab & speakers, would sound like with a tube amp lined out as a tone generator...?
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