How do I convert db into volts?

[magic]

A lot of audio people are really sloppy about the difference between voltage ratios and power ratios. They will compare a voltage at a high impedance point early in an amp, and a voltage at the low impedance output of the amp, and use the ratio of those voltage to come up with a bogus dB gain figure that completely ignores the actual power ratio between those 2 points. Furthermore they usually get really rude when you point out the error.

Using dB for voltage ratios and counting 20dB per decade with no regard for actual power and impedance is a very widespread perversion.
Check out just about any opamp datasheet if in doubt.

[Vovk_Z]

For TS:
I guess, dBm makes sense only for radio application, so it makes sense for standart 50 R or 75 R impedance (but not 8 R).
If there is a talk about 8 R impedance then that's about audio staff, so we talk about dBu, dBV, etc.
'dBm' into 8 R is confusing, but may be that's ok for audio, IDK.

[coppice]

I guess, dBm makes sense only for radio application, so it makes sense for standart 50 R or 75 R impedance (but not 8 R).
If there is a talk about 8 R impedance then that's about audio staff, so we talk about dBu, dBV, etc.
'dBm' into 8 R is confusing, but may be that's ok for audio, IDK.

dBm is dB relative to 1mW. So, it turns a ratio measurement into an absolute one. Its widely used in many fields, especially telecoms. Almost every signal in the analogue telephone system is measured in dBm.

[TimFox]

There are many popular reference levels for dB measurements:  Wikipedia has a long list of accepted terms.
Simply, "dBm" is a power measure, with reference value 1 mW, and the impedance must be specified if you are measuring voltage.  Common impedances are 50 ohms (for RF) and 600 ohms (for audio), which are quite different.
However, "dBu" is a voltage measure, where the reference is the voltage (0.775 V rms) across a 600 ohm resistor with 1 mW of power, but the measurement is strictly voltage and independent of the impedance.
It is often used to specify signal transformers or measurements in audio amplifiers.
Similarly, "dBV" is a voltage measure, where the reference is 1 V rms, independent of impedance.

[coppice]

There are many popular reference levels for dB measurements:  Wikipedia has a long list of accepted terms.
Simply, "dBm" is a power measure, with reference value 1 mW, and the impedance must be specified if you are measuring voltage.  Common impedances are 50 ohms (for RF) and 600 ohms (for audio), which are quite different.
However, "dBu" is a voltage measure, where the reference is the voltage (0.775 V rms) across a 600 ohm resistor with 1 mW of power, but the measurement is strictly voltage and independent of the impedance.
It is often used to specify signal transformers or measurements in audio amplifiers.
Similarly, "dBV" is a voltage measure, where the reference is 1 V rms, independent of impedance.

Note that those voltage measures are fine for comparing multiple measurements at different times at the same point in the system, but are meaningless for comparing two different points in the system where the impedance may differ.

[TimFox]

There are many popular reference levels for dB measurements:  Wikipedia has a long list of accepted terms.
Simply, "dBm" is a power measure, with reference value 1 mW, and the impedance must be specified if you are measuring voltage.  Common impedances are 50 ohms (for RF) and 600 ohms (for audio), which are quite different.
However, "dBu" is a voltage measure, where the reference is the voltage (0.775 V rms) across a 600 ohm resistor with 1 mW of power, but the measurement is strictly voltage and independent of the impedance.
It is often used to specify signal transformers or measurements in audio amplifiers.
Similarly, "dBV" is a voltage measure, where the reference is 1 V rms, independent of impedance.

Note that those voltage measures are fine for comparing multiple measurements at different times at the same point in the system, but are meaningless for comparing two different points in the system where the impedance may differ.

No, the voltage-referenced measures (dBV, dBu, etc.) are also used to compare voltages at different points in the same circuit, such as a two-stage voltage amplifier, where the impedance and actual power are not relevant.
Pedantically, dB means a ratio of powers, but so long as one specifies voltage as a reference the usual formula (with 20) is useful for voltage ratios.
However, when comparing monetary units (e.g., USD), one uses 10 since money is power.

[coppice]

No, the voltage-referenced measures (dBV, dBu, etc.) are also incorrectly used to compare voltages at different points in the same circuit, such as a two-stage voltage amplifier, where the impedance and actual power are not relevant.
Used correctly, dB means a ratio of powers, but so long as one specifies voltage as a reference the usual formula (with 20) is used by the dumb and sloppy for voltage ratios.

There. I fixed that for you.

[Tation]

In spite of what some people here want, dBm is also used to measure voltages, not power, independently of the impedance.

See the dial, graduated as dBm, of the AC voltmeter below.

I'm sorry that this is an abuse of the definition of dBm, but it is what it is: in some applications dBm measures voltages and 0 dBm means 0.775 V.

[coppice]

In spite of what some people here want, dBm is also used to measure voltages, not power, independently of the impedance.

See the dial, graduated as dBm, of the AC voltmeter below.

I'm sorry that this is an abuse of the definition of dBm, but it is what it is: in some applications dBm measures voltages and 0 dBm means 0.775 V.

Those meters are not abusing anything. When you look at their specs, and often the corner of the scale plate in the actual instrument, they say something to the effect that this scale is for measurement into a 600 ohm load.

[Tation]

This scale measures volts, and the reference is in the dial. If you put 0.775 V on any resistor, and measure with this voltmeter, it will say 0 dBm.

So you think that the author of the adjustment procedure posted by the OP used a voltmeter with a dBm dial, similar to this, to measure upon 8 Ohm and then corrected the measure to take into account the impedance mismatch? Or maybe he used a bolometer to, really, measure power 

  come on, here on Earth nobody will do that. 0 dBm, when measuring voltages, means 0.775 V, period.

[DimitriP]

dB-any to Volts   
https://www.extron.com/calculators/db-to-volts/

DBM into any Impedance you want:   
https://coretechgroup.com/dbm-calculator/


...and here is what caused all this in the firstplace : https://keith-snook.info/quad-405-bbc-am8-16.html   

[TimFox]

This scale measures volts, and the reference is in the dial. If you put 0.775 V on any resistor, and measure with this voltmeter, it will say 0 dBm.

So you think that the author of the adjustment procedure posted by the OP used a voltmeter with a dBm dial, similar to this, to measure upon 8 Ohm and then corrected the measure to take into account the impedance mismatch? Or maybe he used a bolometer to, really, measure power 

  come on, here on Earth nobody will do that. 0 dBm, when measuring voltages, means 0.775 V, period.

That's why the expression "dBu" exists, to measure a voltage with respect to 0.775 V.
That is equal to "dBm" only when the impedance is 600 ohms.  "dBm" is also popular for 50 ohms, but the zero is shifted.

[TimFox]

No, the voltage-referenced measures (dBV, dBu, etc.) are also incorrectly used to compare voltages at different points in the same circuit, such as a two-stage voltage amplifier, where the impedance and actual power are not relevant.
Used correctly, dB means a ratio of powers, but so long as one specifies voltage as a reference the usual formula (with 20) is used by the dumb and sloppy for voltage ratios.

There. I fixed that for you.

No.  When "dBV" and "dBu" are stated, the reference is only a voltage and independent of impedance.
The voltage-ratio use of dB is a generalization past the original definition in terms of power ratios.

[TimFox]

No, the voltage-referenced measures (dBV, dBu, etc.) are also incorrectly used to compare voltages at different points in the same circuit, such as a two-stage voltage amplifier, where the impedance and actual power are not relevant.
Used correctly, dB means a ratio of powers, but so long as one specifies voltage as a reference the usual formula (with 20) is used by the dumb and sloppy for voltage ratios.

There. I fixed that for you.

No.  When "dBV" and "dBu" are stated, the reference is only a voltage and independent of impedance.
The voltage-ratio use of dB is a generalization past the original definition in terms of power ratios.
It is neither dumb nor sloppy.