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In Reply to: Re: Haven't tried it but I've a theory that ... posted by Naz on February 23, 2007 at 05:33:34:
Hello>>Partly agree but the use of NFB to lower distortion can give rise to an undesirable harmonic distortion structure and a correspondingly detrimental effect on the sound. 2ndH may end up better than triode but not high order distortion, particularly odd order.
yes, this is possible, but not probably. I think, NFB will reduce all harmonics because linearization of the entrance characteristic
>>.. and it's easy to design a low distortion driver anyway
and it is much easier to design this for less needed output voltage to drive a pentode with an 1/5 or 1/10 needed grid swing ;-)
>> but I haven't tried it with a pentode driver
than do it, I heard the difference, Darling with triode driver without NFB and Pentode driver with NFB.
>>There are many issues here. 1) Actually, a high OP impedance only helps smooth frequency response through its inefficiency, ie higher attenuation of the mid band rather than by delivering more power into the high impedance parts of the spectrum. 2) The poor damping factor can cause bass problems 3) A high OP impedance (poor DF) implies a poor load line and therefore high distortion. 4) Even if high OP impedance was desirable then it can just as easily be achieved by loading a triode OP stage more heavily, the bonus being greater power and reduced turns ratio in the OP tranny.
to 1: no, pentodes with high impedance are more efficient than triodes with low impedance, because of the small not controlable residual voltage. And you are wrong with higher attenuation of the mid band. Higher OP impedance behaves like current source, and higher load resistance have a higher power consumtion. For nominal load impedance in the mid band you get nominal output power, and the rising impedance at the ends of the frequency range makes another distribution of output power.
Triodes with low impedance are like voltage sources, at rising load impedance the current will be smaller and power is reduced.to 2: yes, this can be a problem ;-)
to 3: not necessarily, a loudspeaker is a magnetic transducer, and magnetic forces result from the current, which flows by the voice coil. And pentodes with current source characteristic delivers current..
to 4: no, it is not so easy. Example 300B with Ri 700 Ohms. To get low distortions ra should be around 3,5k. transformed to 8 ohm load, without any losses this gives a output impedance of 700/(3500/8), around 1,6 Ohms and DF=5. You can not wound an transformer for DF>1. For DF=1 you get only ra 700 Ohms, this gives much power with much distortion, but it is still not a current source.
A pentode like EL12 works also on 3,5k Ra, I can use a transformer with the same impedance ratio, but plate resistance of the EL12 is 25kohms, I have an output impedance of 25000/(3500/8), around 57 Ohms, DF = 0,14 (without NFB). NFB reduces output impedance, but not so much, DF will be ever smaller 1.I think, this is a reason because EL84 sounds very good on fullrange speakers.
regards, gerd
( sorry for my english, it is very difficult to interpolate between my memory and babelfish ;-)
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- Re: Haven't tried it but I've a theory that ... - Gerd 23:12:29 02/27/07 (0)
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