Increasing gain for your vinyl front end when pre-amp doesn't quite have enough juice

A minor correction to something I said in the first of my posts dated 4-27-2019. When I said:

Since the power amp only provides balanced inputs, it will interpret 0.567 volts provided to it in single-ended form as 0.567/2 = 0.28 volts.

Usually the gains and sensitivities of balanced inputs of a power amp are specified based on the difference in voltage between the two signals on the XLR connector, rather than on the voltage of each of the two signals. And usually the output power that would occur in response to a single-ended 0.567 volt signal provided to a balanced input would be the same as if it were a balanced pair of signals having a difference of 0.567 volts (i.e., each signal being 0.28 volts). A look at a schematic I found for the particular amp adds confidence to that conclusion.

The rest of my post remains as stated, except that the reference to "less than 1/4th of the voltage required to drive the amp to full power ... with the volume control set at max" becomes "less than 1/2 ...." as a result of that correction.

Regards,
-- Al

@uberwaltz,

I'm sure there are online calculators that can be found which when either number is entered will provide the other number. But I do that calculation manually, using a hand-held scientific calculator.

As you may be aware the ratio of two voltages, expressed in db, is:

db = 20 x log(V1/V2)

where "log" is the base 10 logarithm.

So to convert in the opposite direction, from db to the corresponding voltage ratio, I divide the number of db by 20, and then raise the number 10 to "the power of" that result. (For example, 10 raised to the power of 2 = 10 squared = 10 x 10 = 100).

In this case:

42/20 = 2.1

10^2.1 = 126 (rounded off to three digits).

Best regards,

-- Al
 

@yogiboy The ">10 volt" maximum output spec of the Saga means that it is capable of putting out more than 10 volts before it will clip, when its active mode is being used. (In passive mode there is presumably no practical limit to its maximum output voltage). The output voltage it will provide at any given time, though, **if its volume control is at max,** will equal the input voltage provided to it increased by its gain. And in this case the gain of 0 db means that the output voltage will equal the input voltage.

I’ve seen several instances here in the past in which very large preamp output voltage specs (e.g., 10 or 20 volts or so) have misled people into thinking that is what the preamp actually outputs, rather than representing maximum capability before clipping.

Looking at the specific numbers in this case, the 42 db gain setting that was originally used on the phono stage (which is typical of gain settings that are used with moving magnet cartridges) corresponds to a voltage multiplication of 126 times. When the cartridge is putting out the 4.5 mv it is rated to put out under the standard test conditions the 42 db gain setting will result in an input to the preamp (and an output from the preamp if its volume control is at max) of 126 x 4.5 mv = 0.567 volts. Since the power amp only provides balanced inputs, it will interpret 0.567 volts provided to it in single-ended form as 0.567/2 = 0.28 volts. That is less than 1/4th of the voltage required to drive the amp to full power, **and will only occur with the volume control set at max** if the cartridge is putting out 4.5 mv. (Although the peaks of some recordings can cause the cartridge to significantly exceed the 4.5 mv it is rated to provide under the standard test conditions).

The bottom line here, as I see it, is that phono stages are generally designed with the expectation that they will be used with line stages providing significant gain. And in this case the line stage provides the same gain as a passive preamp, namely none, which will result in exactly the issue the OP described with many and perhaps most cartridge/phono stage combinations. So IMO the root cause of the problem should be fixed, before other changes to the system are considered.  That said, though, it does seem possible that the 48 db setting of the phono stage could prove to be satisfactory.

@noromance Thanks for your comment.

Best regards,

-- Al

After looking over the specs and descriptions of the equipment, it appears to me that the root cause of the problem is that the Saga does not provide any gain, even when it is used in its active mode. (Or putting it more technically, its gain is 0 db). And the fact that the power amp is being driven with unbalanced signals, even though it only provides balanced inputs (its 1/4 inch TRS inputs are balanced, as are its XLR inputs), is most likely costing you 6 db on top of that. (The spec that is provided for unbalanced input impedance probably misleadingly refers to the impedance per leg of its balanced inputs).

If the good suggestion that was made of trying the 48 db gain setting of the phono stage does not prove to be satisfactory, rather than band-aiding the problem with a device providing additional gain I would consider fixing the root cause of the problem, by replacing the preamp with something else.

BTW, for everyone’s information the reference to "8W" in the power amp’s sensitivity spec ("1.23 VRMS @ 8W") is undoubtedly someone’s mis-translation of "8 Ω." Based on the specified gain of 34 db, an input of 1.23 volts will drive the amp to slightly more than its rated maximum power (450 watts into 8 ohms). If an input of 1.23 volts would only drive the amp to an output of 8 watts, more than 9 volts would be needed to drive it to full power, which would be ridiculous. And the gain spec would be wrong as well.

Good luck. Regards,

-- Al