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

Post Subject: Define really quietPosted by floobydust on: 5/5/2009

Paul, et al,

I tend to agree that many DHTs can have a fair amount of AC hum output from an AC filament. In general, I've not found any tricks (like humbucking chokes) to make any significant difference. Ideally, a perfectly balanced AC filament "should" cancel all hum from appearing at the output. Realistically, it doesn't (or very rarely) happen. Why? Well, the cause needs to be examined.... take the old 45 triode as an example. It's a very simple mechanical arrangement. A coated filament in a "M" shape, with a spiraled grid assembly and a box plate. The coated filament uses rare earth materials to enhance electron emission at lower wire temperatures, i.e., much higher efficiency. IMHO, there are some reasons why hum cancellation is simply too complex to balance out completely.

1- the coated filament can not be coated with such accuracy to ensure perfect (equal) electron emission across the entire length of the filament wire.

2- the spiraled grid wire assembly makes a symmetrical flow gate from the filament wire impossible.

3- the plate (anode) is generally coated as well which renders it imperfect to electron absorbtion.

4- mechanical alignment can not be controlled precisely enough to allow such a perfect symmetrical electron flow.

When you look at the output hum on a scope, it is rarely if ever, a 60Hz (50Hz for other countries) sinewave. The "sometimes" complex looking hum component is the result of the above. Having an adjustable balance control does help, but only with larger offset problems. You still have some complex hum component that can not be balanced out.

So, in keeping with the post subject, how does one define "really quiet"? I've read many a post (other forums) where some folks consider less than 1mv as being quiet. Some have even gone as far as to state that while the output hum is clearly audible without a musical signal, they are still impressed with the low-level detail and such when playing music! I have a very simple viewpoint on this. If you can hear any hum and/or noise from your "system" without a signal, then you have a distorted output in the form of intermodulation distortion via the audible output hum being injected into the output signal. As a result, ALL playback levels are distorted with lower level signals being affected more.

If you do some (signal-to-noise) calculations based on 1-watt output (assume 8 ohms) you have a voltage ratio of 2.83Vrms to 0.001Vrms = 2830:1 which is about 69 dB. The more sensitive your speakers are, the greater the audible output noise will be (assuming a reasonably quiet listening room). I generally consider an acceptable S/N ratio of 80 dB (referenced to 1-watt) which equates to about 283 microvolts output noise. This can be a difficult figure to acheive and is more a result of the specific tube versus the (circuit) design.

I have numerous (45) triodes which easily meet or exceed the S/N requirement with some specific examples exceeding 90 dB and others barely able to meet 70 dB in the same circuit. This makes circuit design difficult and really does lean towards getting a larger quantity of triodes and testing each to ensure quiet operation. This is why many folks opt for a DC filament supply.... usually simple and generally knocks out the audble hum but can introduce other artifacts based on many variables.

Regards, KM

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