Do I need a sut?

150 watts into 8 ohms corresponds to about 34.6 volts. 24 db less than that is about 2.18 volts. 2.18 volts divided by 0.05 mv is a voltage multiplication of 43,600 times, which is about 93 db.

So to be able to drive the amp to full power when the cartridge provides its rated output you need a total of 93 db of gain between the cartridge and the input to the power amp. For that to be accomplished with the MM section of the phono stage providing 48 db and the line stage preamp providing 14 db, the SUT would have to provide 31 db (or more). Exactly mid-way between the value Hdm suggested and the value Folkfreak suggested :-)

Regards,
-- Al

Regarding the adequacy of 76 db of SUT + phono stage gain, that is 7 db more than the OP presently has. For a typical rotary volume control, which when viewed as a clock dial can be adjusted between limits of say 6:30 or 7 o’clock at min and 5 or 5:30 o’clock at max, and is used in the upper half of its range, a 7 db difference probably corresponds to a bit more than 45 degrees of rotation. For example, see the dial calibrations shown in this photo, although the photo is of an MFA passive (not active) preamp.

So **if** the volume control on the OP’s Herron preamp has similar rotational limits (approximately 6:30 or 7 o’clock and 5 or 5:30 o’clock or so) he should be able to approximately judge the effects on noise level of 7 db of additional gain by reducing the volume control setting by a bit more than 45 degrees, and seeing how that change in setting affects noise levels when no music is playing.

Also, I agree with Folkfreak that it in general it would be wise to assure that the combination of cartridge output rating, SUT gain, phono stage gain, and line stage gain is adequate to drive the power amp to full power. Even if the most power that is ever used is say half of the amp’s capability, that would only be 3 db less than full power. So if full power can’t be obtained even with the volume control at max (which is the position at which the preamp will provide its rated gain), 3 db less than full power may not be attainable at any volume control setting either. And regarding the KAB calculator, it can be important to note that the calculations of phono stage gain which it provides are based on boosting the cartridge’s rated output to only 325 mv.

Regards,
-- Al

Thanks, Itsjustme!

To clarify regarding the Herron VTPH-2 specifically, the input stage it provides for LOMC cartridges utilizes a FET (Field Effect Transistor). Consequently, as I mentioned earlier, it has an input impedance that for practical purposes is essentially an infinite number of ohms, but it provides connectors on its rear panel for purposes of connecting loading plugs. 47K loading plugs are supplied as standard with each unit, and Keith Herron can supply plugs in any other values that may be desired. He recommends that with many and perhaps most cartridges the VTPH-2 will provide best results with no loading whatsoever, and I and some other members here have found that to be true with our particular cartridges. Even though, for example, my AT-ART9 has a load resistance recommendation from the manufacturer of "100 ohms minimum."

As you may have seen in the past, Ralph (Atmasphere) has stated in a number of threads here that the need for relatively low impedance loading, where necessary, is mainly due to sensitivity of the particular phono stage to RF frequencies, which can be introduced into it at significant amplitudes as a result of the resonance that is formed at RF frequencies by the interaction of the inductance of LOMC cartridges and the load capacitance that is applied to them. Apparently the VTPH-2 can handle RF frequencies in a more graceful manner than many other phono stages, meaning that such frequencies will have little if any effect on audible frequencies.

So as I had mentioned interposing a SUT between an LOMC cartridge and the LOMC input of the VTPH-2 can be handled in a reasonable manner from the standpoints of cartridge loading and transformer loading. But the concern that would arise in this particular case would be the possibility of a voltage overload in the phono stage and/or in whatever preamp circuitry may be "ahead" of the volume control.

Best regards,
-- Al

Thanks, John (Jmcgrogan2).

Jeff, as the others have indicated SUTs are almost always designed to be loaded with 47K, and if improperly loaded can exhibit ringing or other undesirable effects. Since the MC input circuit of the VTPH-2 has a near infinite input impedance in itself, while providing connectors for external loading plugs, you could presumably load the transformer properly by connecting the 47K loading plugs Keith provides with each unit. With a 24 db SUT that would result in the cartridge seeing a load of about 187 ohms, not too far from the 10 to 100 ohm range that appears to be recommended for it.

***HOWEVER,*** 69 db + 24 db = 93 db, which would boost the cartridge’s 0.05 mv output under the standard test conditions to 2.23 volts, and the peaks of some recordings may result in levels as much as several times higher than that amount. Which in turn could very possibly overload the phono stage and/or the input circuit of the preamp. Keith could best advise on that possibility, of course.

So pending Keith’s advice my suspicion is that if you want to stay with the MC-2000 the way to go would be what was suggested by the others, namely using a higher gain SUT specifically designed for use with such a low output cartridge, connected into the Herron’s MM input.

One further point: Note that the capacitive loading recommendation for the T-2000 SUT, as shown in the datasheet Folkfreak provided, is 100 pf. That corresponds exactly to the specified input capacitance of the VTPH-2’s MM input, but the capacitance of the cable connecting the SUT to the VTPH-2 would add to that. So you would want to keep that cable as short as possible, and preferably choose a cable type having low capacitance per unit length. Capacitance on the secondary side of a high gain SUT can be an important consideration, because it will appear on the primary side (as seen by the cartridge), **multiplied** by the square of the SUT’s turns ratio, which for a 35 db SUT is very high.

BTW, I too use the wonderful VTPH-2, in its 64 db version, with an Audio Technica ART9 cartridge rated at 0.5 mv, with no loading plugs, and with great results!

Good luck. Regards,
-- Al