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High Output Preamp for First Watt SIT-3?

I'm new to the audiophile world and recently purchased a First Watt SIT-3 with Magnepan .7s.  I have a Mytek Brooklyn Bridge for streaming, though I prefer to play vinyl with my Mofi Ultradeck.  The Mytek serves as the phono stage (69dB gain for MM?), and I don't use a preamp;  the Brooklyn is connected directly to the First Watt. 

I understand that the SIT-3 is only 11.5 dB of gain, and that's extremely low compared to most amps.  I like most everything about the system except for the fact that I have the Mytek maxed out volume-wise many days.  Certain LPs are cut quieter, and I wish I had more power.  There are times streaming where the Maggies aren't getting to their sweet spot.  I realize the Maggies are a lower sensitivity speaker. 

What should I look for in a preamp to fully drive the SIT-3?  Eventually, I'd like to replace the phono stage instead of relying on the Brooklyn.  Is there a tube preamp featuring a phono stage with sufficient output to drive my amp in the $4k range?  Otherwise, I'm eyeing the Modwright PH 9.0 and a linestage preamp, realizing that this will probably be costlier.  I'm looking for a warm sound, not analytical but engaging. 

Any help is greatly appreciated.  I'm a working musician with a basic understanding of related electronics but little experience in this world. 

murphbass

Showing 6 responses by almarg

almarg's avatar
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almarg
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While it won’t matter in the case of @joeinid ’s 107 db speakers it’s probably worth mentioning that the SIT-3 won’t be able to provide much power into an 18 ohm or other very high impedance, regardless of whether or not a resistor is connected in parallel with the speaker.

For example, based on the amp’s 18 watt/8 ohm and 30 watt/4 ohm ratings it is probably capable of supplying something like 22 watts into the 6.42 ohm load he mentioned, corresponding to a 10 ohm resistor paralleled with his 18 ohm speaker. In that case, though, most of the 22 watts would be consumed by the resistor, and it can be calculated that only about 8 watts would be delivered to the speaker.

It’s a different story altogether in the case of @charles1dad ’s Coincident Frankensteins, which are optimized for high impedance speakers and I’m guessing would not need a resistor to provide good sonic results and close to rated power even with an 18 ohm load.

Best regards,
-- Al

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almarg
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Here is an analysis of approximately how much of the amp’s power capability can and can’t be utilized in this situation when playing LPs, based on the cartridge’s rated output and the gain structure of the system:

Stereophile measured the gain of the SIT-3 as being 11.3 db, which corresponds to a voltage multiplication of 3.67x. For an amp having that gain to provide an output of 30 watts into 4 ohms, which corresponds to 10.95 volts, an input of 10.95/3.67 = 2.98 volts is required.

Mytek’s description of the Brooklyn Bridge states that “Brooklyn Bridge is identical to Brooklyn DAC+ in terms of audio circuits and performance but has the addition of a built-in Network Streamer.”

Stereophile’s review of the Brooklyn DAC+ states that “the DAC+’s feature set, which includes MQA and a built-in moving-magnet/moving-coil phono stage, is largely the same as the original Brooklyn’s.”

Therefore it appears likely that for MM phono cartridges the Brooklyn Bridge provides gain that is similar to what Stereophile measured for the Brooklyn DAC, namely 55.6 db. That corresponds to a voltage multiplication of 603x.

As stated above Murph’s cartridge has a rated output under the standard test conditions of 3 mv. A gain of 55.6 db will boost that to 0.003 x 603 = 1.81 volts, **if the volume control is at max.** That of course falls well short of the 2.98 volt input the amp requires to be driven to full power.

Since for a given load impedance power is proportional to voltage squared, the percentage of the amp’s 30 watt/4 ohm rating that is being used when the cartridge is supplying its rated output, and the volume control is at max, is:

(1.81/2.98) x (1.81/2.98) = 0.37 = 37% (!)

The corresponding power supplied by the amp when the cartridge is supplying its rated output is 0.37 x 30 watts = 11.1 watts (!!). Meaning that almost 2/3 of the amp’s power capability can’t be used for many recordings without additional gain being provided.

The peaks of various recordings can of course be either somewhat greater than or somewhat less than the standard LP test conditions. But this analysis appears consistent with Murph’s observation that there isn’t enough gain in the setup to support LPs which aren’t “hot.”

And given his earlier statement that he is “on the cusp of where I want to be” it seems to me that replacing the speaker or the amp is not necessarily called for, despite the mismatch in power capability. If neither of those things is done, what is called for is providing additional gain between the Brooklyn Bridge and the amp. 12 db or so should be adequate. The simple one-tube line stage I linked to earlier provides a gain of 22 db.

Best regards,

--Al


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almarg
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+1 Charles. I would also like to know what phono cartridge is being used. With that information and the information that has already been provided a determination can be made as to approximately how much of the amp's power capability is not being utilized on many recordings, with the volume control set at max.

Best regards,
-- Al
 
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almarg
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Hi Charles,

The reason for the discrepancy in power capability that you cited relates to the amount of distortion the ratings are based on.

JA's measurements in Stereophile indicated that:

Into 4 ohms (fig.5), the SIT-3 delivered 16.8Wpc (9.24dBW) at 1% THD+N, and 36Wpc (9.55dBW) at 3%.

And in the graph at the bottom of page 10 of the manual for the SIT-3 it can be seen that the amp can provide 30 watts with distortion of about 2.3%. The distortion being "dominated by second harmonic with a “negative phase”, the signature that we have come to prefer with the previous SIT amplifiers." Which would seem to be a relatively benign distortion characteristic.

Best regards,
-- Al
   
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almarg
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Hi Jose ( @jg2077 ),

No, I don’t see a Speltz autoformer as being helpful in this situation. As you most likely realize it is mainly useful where it is desirable for an amp to be presented with a higher load impedance than the speaker would present by itself.  But in this case, where maximizing the amp’s power capability is particularly important, the maximum power capability of the amp into an 8 ohm load is considerably less (18 watts) than its capability into the Maggie’s 4 ohm impedance (30 watts). Also, Maggies present a flatter and therefore more amplifier-friendly impedance curve than many and probably most other speakers having 4 ohm nominal impedances.

The thinking behind my earlier post was motivated by the OP’s statement that ...

I’m at the cusp of where I want to be. Many hotter LPs and digital files sound fantastic. I can coax a bit more streaming a lower level file with Fidelia or my UAD interface.

... coupled with my suspicion that depending on the particular cartridge he is using he may very well not be able to utilize a considerable fraction of the 30 watt capability of the amp when playing many LPs, even with the volume control at max. If so, inserting some additional gain in the signal path, such as with the simple one-tube line stage I cited, may result in performance that is satisfactory for his purposes.

Best regards,

-- Al

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almarg
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Excellent comments by the others, and I certainly agree that the pairing of the SIT-3 with the Maggies is not ideal from a power capability standpoint.

Regarding the original question, however, depending on what phono cartridge you are using, and assuming (as I suspect) that the gains of the Brooklyn Bridge are similar to those of the Brooklyn DAC (as measured by Stereophile), based on my calculations it appears that for many recordings on LP you may be falling several db short of being able to drive the SIT-3 to its 30 watt/4 ohm power capability, even with the volume control at max. (Although you certainly should be able to do that when using the Brooklyn Bridge as a DAC). Meaning that when playing many LPs you may not be able to drive the amp to output levels of more than say 10 or 15 watts, depending on the rated output of your particular cartridge. I could be more specific if you let us know the make and model of your cartridge.

In which case a simple and inexpensive solution may simply be to insert something like an Antique Sound Lab Line Two DT ($450) between the BB and the amp. I used its predecessor model (the Line One) for a while some years ago while my main preamp was down, with fine sonic results. And I see no technical incompatibilities that it would present with your equipment, in terms of impedances, gain, etc.

Good luck. Regards,
-- Al

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