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arnoldc 1st February 2004 02:09 PM

Differential amp, Long tail, Split load - what difference in sonics?
assuming i use the same tubes and i drive the same power tubes in PP topology, is one sonically better than the other?

or is one more stable, has more drive, etc.

how to choose one over the other?


Jax 1st February 2004 02:44 PM

I prefer the longtailed pair as splitter direct coupled to the input amplifier. If I have a negative supply available I like to connect the tail to it, either a resistor or a constant current source. Drive is reasonable.

I have also used the concertina (split load), again direct connected to the input tube. The drawback is the absence of gain and unequal output impedances but with "easy" output tubes it sounds good anyway.

The third I have used is the paraphase splitter, simple and can have a lot of gain but the drawback is the second tube amplifying the first tube's distortion.

If you need drive, consider an LTP followed by cathode followers. These can be direct connected if you have a lot of supply voltage available.

Some use interstage transformers for phase splitting. I have not used those myself but I see no reason why they can't be good.

Just my 2 coins.

mjd_tech 1st February 2004 04:24 PM

General Rule of Thumb:

Split Load:
Best used for directly driving EL84, 6V6 or similar output tubes that don't require a lot of drive voltage.

Long tail:
Best used for driving EL34, 6L6, 6550 KT66, etc.

Williamson amp. Used between a split load and the output tubes

Notable Exceptions:
Dynaco ST-70. Split Load directly driving EL34s.
Vox AC30, Fender Deluxe Reverb guitar amps: Long Tail driving EL84 and 6V6,

My opinions:
Guitar amps: Long tail, all output tubes. Best overdrive characteristics. By varying the "tail" resistor, an amazing variety of tones can be achieved. I like about 6-10 db negative feedback.

Hi-Fi amps: The objectives are different than guitar amps and generally more negative feedback is used. To me, this tends to mask the sonic signature of the phase splitter. Many classic push pull hi-fi amps use a ridiculous amount of negative feedback to get the THD numbers down. I like about 12-15 db NFB. I would go with the above rule of thumb for choosing a phase splitter. Also consider the much reviled paraphase inverter circuit. Although it has lousy technical specs it can sound pretty darn good. I can't comment on transformer phase splitters. Too expensive for me.

The more NFB you use, the harder it is to make it stable. If you are willing to give up a few 10ths of a percent THD, life becomes easier. Use less gain and less NFB. Then you need less fussing around with compensation caps.
The Williamson circuit is notorious for being on the verge of instability dues to the number of gain stages and the 20db NFB.
The Mullard 5-20 circuit is stable but is rather sterile to many ears and can be improved greatly (in my opinion) by the less gain, less NFB formula, usually by triode wiring the EF86, and using 12AT7 or 12AU7 phase splitter.

Mike D.

arnoldc 1st February 2004 11:23 PM

jax, mike,

very enlightening. thank you for the replies!

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