Dorrough meters, what they actually measure

Following @dotneck335 question in https://www.tapeheads.net/threads/fluxivity-for-newbies-how-to-select-a-test-tape.83694/

I think these counters deserve their own thread so as not to hijack jdcolombo's and to avoid information being scattered in different topics that wouldn't necessarily be directly related to Dorrough's meters.

I am a bit confused as to exactly WHAT the Dorrough meter measures. In the descriptions I have read, the Dorroughs meter is described as reading AVERAGE and PEAK levels simultaneously. NOT RMS. This is a distinct difference.
In terms of a SINE wave: 1.414 volts PEAK = 1 volt RMS = 0.9 volts AVERAGE.
If we call 1 volt RMS equal to ZERO db, then PEAK = + 3db and AVERAGE = -1 db. So that's ~ a 4db difference between peak and average.
If you present your Dorroughs meter with a 1-volt RMS signal (as measured by a quality AC voltmeter), what does the Dorroughs meter display?

Click to expand...

According to my knowledge, the "averaging" is close to the traditional VU meters with a slightly modified ballistic while peak metering is PPM type II.

Wrt you question, if you feed the meter with -10dBV or whatever you use as 0dB ref level, it will display 0dB (if calibrated for that) in the persistence zone of the bargraph that is shared with the PPM meter.
For RMS, they use a precision rectifier to convert from Vpp to DC with a BW ranging from 20Hz to 25kHz. Then, there are the peak detectors. I do not see any issue with this ad I can tell it works VERY well.

There are a lot of details in the US patent filled by Dorrough Electronics.

I've put the A-40 on the bench, I think pictures will tell all:

Continuous -10dBV 1kHz sine


Continuous -4dBV 1kHz sine


As you can see, the Leader needle and the Dorrough bargraph are on the same spots.


Then, the 'magic' of Dorrough meters ...

Burst -4dBv 1kHz sine



With the hope it clarifies

I was on another topic while getting the pictures. Read 0dBV instead of -10dBV, and +6 instead of -4 ...

See them in action: