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Hi Roger,A mate on AA suggested that I should ask you for an explanation of the following question I have:What factor in amp design gives amp A a greater instantaneous power-delivery capability (for, say, 1/100th of a second, for better transient delivery) than amp B - when both have the same power spec and the same DC rail voltage.I'd be much obliged if you can educate me on this matter.Regards,Andy
Its all in the power supply. It is sad that music power ratings were outlawed by the FTC many years ago because they were abused by dishonest ampmakers. That was a good system. Two amplifiers can have the same RMS rating but the one with the less tight power supply will have a higher burst rating. An amplifier with a regulated supply will have the same RMS and burst rating. Now man think a regulated supply is a good thing, however in a properly designed power amp it is not. Now here are some interesting numbers. My RM-10 is rated at 35 watts/ch RMS. it will do 45 music power. My Electostatic amp that drives my ESL and Acoustats can do 1200 Volt-Amps (similar to watts) peak but not nearly that RMS. Music demands high peaks, not high RMS. A conventional amp driving an Acoustat demands similar volt-amps which few can deliver. This is why direct drive is so important. The ESL amp can do it because the voltage is so high (5000) and thus .25 amps does it where .25 amps on an 8 ohm speaker is not even a watt.It is interesting to note that Dynaco made two very popular solid state amps called the Stereo 80 and Stereo 120. The amplifier circuits were identical. The only difference was that the higher power 120 (60 RMS/Ch) had a regulated power supply and the 80 did not. The music power of the two was identical as was the peak power. The 80 was less expensive, more reliable and a better amp.Hope this helps but not surprised if it raises more questions.
Thanks, Roger.I thought it must be all to do with the PS - so it's interesting that regulation is one aspect. However (as you predicted), sorry, your answer does raise another question! OK, you said: "the Dynaco Stereo 80 and Stereo 120 had identical amplifier circuits. The only difference was that the higher power 120 (60 RMS/Ch) had a regulated PS and the 80 did not. The music power of the two was identical as was the peak power."I'm assuming (because of its name) the Dynaco Stereo 80 was 40w RMS/ch? Yet its music power and peak power were the same as the 60w/ch amp?This is a conundrum, to me:1. I was told (by someone who answered a post I made on AA) that the DC rail voltage is the "limiting factor" in the output - whether it's 'RMS', 'music power' or 'peak power'.2. So if the Stereo 120 amp can produce 60w RMS ... it must have a higher DC rail voltage than the 40w Stereo 80?3. But if this is the case - as #1 says DC rail voltage limits output power (whether RMS or peak), the Stereo 60 must have a higher peak power than the Stereo 40?4. Yet you said the amps had the same music power & peak power??So I guess I have 2 Qs:1. Were the DC rail voltages different for the Stereo 80 & the Stereo 120?2. If the output wattage (into, say, 8 ohms) is determined by the DC rail voltage, how can an amp produce a peak power which is greater than RMS?Thanks for your time, Roger.Regards,Andy
You are correct that the rail voltage does determine the peak output power as long as the audio circuit is able to use all of it. In some cases there may be a bit more drop across the output transistors (or tube) but lets say for this situation the voltage drop (loss) is constant. Yes, the power specs made me curious after I had repaired one of each. Here's what David Haffler did (and had to do given the date of design) This is a unique example of two amplifiers sharing the same audio circuits but different power supplies. To my knowledge no one else ever did this, but then thats Hafler for you, a very clever man when it came to bang for the buck.Because he choose 2N3055 transistors which were about the only cost effective power transistors of the day he was limited to 70 volts total rails. This amp has a single supply and output coupling cap the way we do it now with dual rails wasn't invented yet. That came later. His circuit came essentially from the RCA transistor manual and I had built the same for my keyboard amplifier in 1967, so i knew it well by the time I had to fix the Dynaco. As I recall Harmon Kardon used the same circuit in their first solid state Citation 12 power amp. http://www.google.com/imgres?imgurl=http://i555.photobucket.com/albums/jj456/letstakeawalk42/Stereo/HarmanKardonCitation12004.jpg&imgrefurl=http://www.audiokarma.org/forums/showthread.php?t%3D256599&h=768&w=1024&sz=275&tbnid=Joc310ExMi9jTM:&tbnh=92&tbnw=122&zoom=1&usg=__UMfRhW52uI2vP92YOGRbltVDumg=&docid=H1Mnv7WwMBq8RM&hl=en&sa=X&ei=P-WYUaLuIo7m8gSAw4HYAQ&ved=0CDAQ9QEwAA&dur=395To get 40 watts RMS he needed 18 volts RMS (root of 40x8=17.9 volts) which is 25 volts peak. To get 60 watts RMS for the bigger amp he needed (root of 60x8 which is 22 volts RMS) which is 30.7 volts, call it 31. With the losses of a few volts that brought him close to a 70 volt supply (same as plus and minus 35 volt rails in a modern amp).Adding a few volts to 31x2 gets us dangerously close to 70 volts and the output transistors were on the edge but Hafler often did this. My 1968 Allied Radio catalog has 2 full pages of Dynaco amps, with the only solid state being the Stereo 120 at $159/$199 (kit/assembled). On the facing page the Stereo 70 is still its original $99/$129. In my 1972 Lafayette catalog Dynaco only has one page and all the tube amps are gone. The Stereo 120 is still the same price however the Stereo 80 appears at $119/159 perhaps replacing the tubed Stereo 70. Solid state had really taken over in those few short years. What he did was make an amp with a 65 volt power supply which un-regulated fell, as all do, to some lower voltage at full output and produced 40 watts per channel. Then he realized that if he made a regulated supply that was always at 65 volts he would always have 60 watts per channel. Playing music, the unregulated supply would not drop enough to matter so the 80 behaved like the 120. They only differed when asked to to RMS which we don't need for music. Those who paid the extra $40 bucks for the 120 got a less reliable amplifier (because the regulator was not so good) and could play it no louder than the 80. I would love to know the number of both built as this amp was a real bargain in its day. Like all Dynaco kits, the circuit boards came pre-built and tested. The builder then mounted all the hardware, output transistors, filter and output caps and soldered up a lot of hook-up wire from this to that. Cleverly, Hafler had the builder wind the output choke around the output capacitor. It really was a nice amplifier for the price, though a bit hard to fix.I think this answers question 1. As to question 2 the simple answer (contained above) is that in an unregulated supply amplifier the rail voltage is always higher for music than RMS. The RMS demand simply drags the rail down. Bose was very proud to state in the specs for their big amplifier for the 901s that came out in 1974 that the rail drop from idle to full power (400 watts/ch) was only 8%. You can Google BOSE 1801 amplifier or cut and paste the URL below. It had nice meters and an LED display too. https://www.google.com/search?q=bose+2200+amplifier&source=lnms&tbm=isch&sa=X&ei=weKYUayJIZDY9QS7ooCoDg&ved=0CAcQ_AUoAQ&biw=1366&bih=600#imgrc=0hPzBH-sIN1_ZM%3A%3Bbj-AOB0YfSGPPM%3Bhttp%253A%252F%252Fpages.videotron.com%252Famfm%252FBose1801openframe.jpg%3Bhttp%253A%252F%252Fpages.videotron.com%252Famfm%252Fvintage_audio.htm%3B440%3B380 This was better than most amps at the time which were 10% or worse. Hafler's regulation calculated from my story above would be 31- 25/31 volts or 19% due to its rather small power transformer. The Bose transformer was massive.
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