dBV, -10dV into 10k, etc.

[Synchrodyne]

Looking at the actual levels from the “line” outputs of domestic audio equipment, even in the early 1950s there was quite some range. In those days radio tuners were probably the main “line-level” source.

Firstly, taking a few British high-quality AM tuners from 1953-54, in the pre-FM period:

The Leak VS had quite a low output, namely 50 mV at 30% modulation. As I recall the radio inputs of Leak control units catered for this, as well as higher level outputs, for several generations afterwards.

The Rogers RD Junior was quoted at 1 V, presumably at 100% modulation, but not so stated. Unusually for its time, it had a low impedance output, because it used an NFB detector (infinite impedance type).

The Quad AM was 100 mV at 30% modulation. Quad adopted this as the norm for its subsequent radio tuners all the way through to the Quad FM66. Accordingly, the Quad 66 Control Unit had a 100 mV sensitivity radio input, even though the other line level inputs, including CD, had 300 mV sensitivity. Before the Quad II era, Quad had used a very low-level output for its tuners, somewhere around 10 to 20 mV, compatible with the gramophone input of its control units.

Others in the UK also adopted the 100 mV at 30% modulation norm, such as Pye for its Mozart HF108 FM tuner.

One reason for lowish output levels on high-quality AM tuners was that typically they had unbuffered outputs, and any material additional AC loading provided by the amplifier to which they were connected would upset the AC-to-DC load ratio, and cause audible distortion. Tapping down the output to a quarter or less of its ex-demodulator level via a resistive divider was one way of providing adequate isolation from the vagaries of amplifier input loads. This was less of a problem with tuners that had buffered outputs or used infinite impedance detectors, such as the Rogers RD Junior mentioned above. Even so, most needed to look into an impedance of 50k or 100k, or in some cases even higher.

Levels of around 100 mV or so were probably convenient for injecting into the second stage of control units.

In the era of FM valve tuners, a similar kind of range was found, although 100 mV at 30% modulation was probably at the bottom end. FM detectors were less sensitive to the effects of the additional AC loading from the amplifier input, but the de-emphasis could be affected if the loading was not in the specified range. Specified loads of 50k to 100k remained common.

Leak went to a variable cathode follower output of 1 V maximum, at 100% modulation for its initial Troughline model.

In the early solid-state era, buffered outputs became the norm, but output levels remained about the same. A skim through Hi Fi Year Book (HFYB) 1972 shows a general range from 100 mV at 30% modulation to 1 V at 100% modulation, with one or two above that. 1 V was itself quite common, I think more so than in the valve era, and was the maximum output level of landmark early “supertuners” such as the B&O Beomaster 5000 and Revox A76. 0.5 V at 100% modulation was also reasonably common. The Sugden R21/R51 was 0.5 V, as were the TV sound tuners from Lowther and Motion Electronics, both 1971 introductions.

There were one or two at 750 mV, but none yet at 775 mV. That appeared in HFYB 1974, with the Armstrong 600 series tuners. On these, the output was switchable between “high”, 0 dB (775 mV) 600R, and “low”, 100 mV at 30% modulation into 68k. Armstrong was catering for both the low and the higher ends of the range.

I suppose one could say that, taking 30% modulation as average programme, tuner outputs at the time mostly ranged between 300 mV (-10.5 dBV) and 100 mV (-20 dBV). In 1953-54 that range had been 300 mV (-10.5 dBV) to 50 mV (-26 dBV).

In the CD era, it appears that equipment makers typically chose tuner outputs somewhere in the range 0.7 to 1 V for 100% modulation to provide a reasonable volume match with CD players when fed into line inputs of the same sensitivity.

In the Quad 66 case, the tuner output was 100 mV at 30% modulation, and that became 300 mV at 30% modulation after passing through the radio input buffer/amplifier and to the input switching rail, to which the 300 mV CD input was switched directly.

Meridian used a 150 mV line input sensitivity for its 200 series components. In that case the tuner output was 775 mV at 100% modulation.

The Carver TX-11a tuner of the mid-1980s had a fixed output level of 700 mV at 100% modulation.

The 30% modulation number as representative of average programme level might have been in need of revision, at least for some broadcasts, by the 1980s or so. If we go to 40% modulation, to pick a round number, then with 775 mV at 100% modulation, this gives us 310 mV, or -10 dBV!

So, we could say that by the 1970s, typical hi-fi tuner outputs were moving to around -10 dBV, and most were probably at this level by early in the CD era. But even back in 1953-54, some also were around -10 dBV, although not suitable working into a 10k load.

This back-of-the-envelope, layman’s approach to also gets us into the -10 dBV ballpark, alongside whatever rigorous and searching professional work was done to support that standard. I am not quite sure whether that is reassuring or a case of – something must be wrong; it can’t be that easy.

Tape inputs and outputs add another dimension, particularly if one includes the DIN standard. But that can be for another post.

Cheers,