On Wed, 2012-12-12 at 09:09 -0800, Bill Putney wrote: > The 0 dBm spec is to make 1 mw into a 600 ohm load.
absolutely > in amplifiers > without feedback the amplifiers would only make that level reliably > when > the source and load impedances matched. Strongly disagree! Any number of pre solid-state amplifiers produced enough output to show 0dbm: Our standard GP amp [2 x ecc83 / 12aux7] delivered better than +10dbm at less than 0.1 % thd. Yes they had negative feedback, and bias correction as the tube aged. They ran for years without failure. If they started to sag they were picked up on the monthly checks. Source and load impedances need to be matched to get maximum transfer of energy. If you terminate a 600 ohm line at either one or both ends with some other impedance you introduce losses. So amps were built with 600 ohm output transformers, and plugged into lines which may be local studio lines, or telco lines and terminated by equipment that presented 600 ohms to the line. This gives maximum energy transfer. The critical issue was noise. The 0dbm level was the transmission level for phone lines, and the gear the telco used to relay over long distance ran at this level. Because of the clever stuff that jammed 50 calls on one carrier, and split our 10k audio over 3 circuits that 0dbm level had no margin. The noise floor was about -40 on a good day. The telco local lines had margin, you could feed them at +20 to get over line losses to the tx. Crosstalk was then the issue. Feeding them at 15 ohms made the line equalisation at the far end a lot easier. It also attenuated the crosstalk. It is true that "modern" equipment has all manner of impedance input and output, and this does cause level issues. These go un noticed because there is usually enough gain hanging around to solve the problem and noise is not such an issue. However impedance mismatch often produces a change in frequency response because input/output impedances are not always purely resistive. Try explaining that to someone who has spent rather a large amount of money on a sound installation in which the components don't match up, and the supplier/installer has no idea about the math of ohms law. All that stuff which the old guys drilled into us hasn't changed, and it's frustrating to see that people in the industry somehow believe it has. Sorry to grump but the mythology of audio transmission creates huge problems which I often get called to fix! Robert Jeffares Big Valley Radio Thames The Wireless Station Hawkes Bay _______________________________________________ Rivendell-dev mailing list Rivendell-dev@lists.rivendellaudio.org http://lists.rivendellaudio.org/mailman/listinfo/rivendell-dev
