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In the Forum: Melquiades Amplifier
In the Thread: Single-stage Melquiades vs. DHT amps

Post Subject: SET usable power and making the most of itPosted by floobydust on: 2/14/2009

Well... tis for all reading/following this thread.

As Romy pointed out, having high gain is not the same as power. And as Adrian hinted at... the overall system paradigm is important. If you're only barely cracking the volume control on your amplifier or preamplifier then it should be obvious that you have more gain than you need. Personally I don't put volume controls on power amplifiers... I prefer ONE point of control, not multiple... but to each his/her own.

If you're designing and building your own equipment then managing the system as a whole becomes less of an issue. If you're picking piece parts (either built, kit or implementing somebody else's design) then you need to manage the various gain of each component and determine how to best integrate them into a system. As far as the gain in the amplifier goes, it's a moot point IF you manage the overall system gain properly. I've done two SET designs over the past few years... and they are quite different on gain, yet have very similar specifications. There are both technical and financial reasons for each design but both have been optimized for performance, not gain. Also, both take into account the OPT winding/phase and result in a non-inverting design.

A brief word on power.... the 45 triode in a proper design will deliver a solid 2- to 2.5-watts and can be flat from 20Hz to 50KHz (or more) depending on the design, components and how it's actually built. A 2A3 triode will deliver up to 4.5 watts in the same application... please note that this is ONLY a 3dB change (double the power). As such, it's not really surprising that you (Adrian) realized little benefit from the 2A3 over the 45... you didn't gain very much (power). Having speakers which have higher sensitivity is a real requirement for using low power amplifiers. And even then, the last 1 to 2 octaves will require some additional help for anything beyond moderate listening levels. There's no free lunch to acheiving SPL levels.... it's a match between the sensitivity of the speakers and the available output power, i.e., no magic involved.

If you plan on using 2- to 5-watt SET amplifiers, you need to mate them with speakers of adequate sensitivity AND which will mate well with a low powered SET, of which most (if not all) have no negative feedback and have low damping factor. This places a much greater demand on the type of speaker that will perform well with the amplifier. Personally, I find the sonic neutrality and open soundstage of a properly designed SET to be the dominant factor that makes me want to use one. The (technical) reasons are beyond the scope of this post however. The down side is two-fold: 1- The electrical and mechanical parameters of the speaker (impedance curve, self damping, moving mass, etc.) can wreak havoc with any SET design. 2- Low frequency performance will be limited regardless... as you walk down the scale of organ pipes you simply need more power and larger drivers to deliver those low pedal notes. Getting the last couple of octaves will require additional support and you're not going to manage it with low powered SETs.

As for modifying your Moondog amplifiers, I would recommend against it. While you can effectively reduce the overall distortion and gain a bit of additional power, it's not worth gutting the amplifier and start swapping out expensive parts. If you're not happy with the amount of power now, NO change will fix it as you simply can't gain enough power to deliver Wagner.... also what speakers are you driving with the MDs? You would likely be better off with a different (SET) amplifier which has enough power to meet your needs.

Regards, KM

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