2. amps-preamps
  3. 124321-369

Speltz Zero Autoformer with Tenor OTL 75

Hi,
I have a Tenor OTL 75 amp. It sounds great but there are times where you really miss having more power/damping to control the sound better. Has anyone tried the Zero Autoformer by Paul Speltz ? My primary concern is, is it just a band-aid like a make-shift solution or is it really effective ? Is there any loss of resolution or speed ?
pani
atmasphere's avatar
atmasphere
11,967 posts
 
It works quite well if you are trying to drive a lower impedance with your amplifiers. There is no loss of speed- the ZEROs have bandwidth to about 2 MHz, which is more bandwidth than most amps made.

People ask me if the ZEROs obliterate the OTL-ness of our amps and IMO they do not- this is because the ZERO is a problem solver- that problem being that the amp is being asked to drive a lower impedance with which its not particularly comfortable. The result will be more impact, lower distortion and more power. If the ZERO is being used with a speaker that the amp already likes then there is no point IMO/IME. You use it when you want to drive a lower impedance.

FWIW, Paul Speltz has comments from manufacturers of solid state amps essentially telling him that their amps sound better through the ZEROs driving a 4 ohm load rather than driving direct- even though their amps have no trouble doubling power into 4 ohms. This is likely because all amps, tube, solid state or class D, have lower distortion when driving higher impedances.

We have a good number of customers running our M-60s with Magnaplanars. The ZEROs make that happen with good success.
don_c55
1,025 posts
 
OTLs with external transformers, are not really better than tube amps with transformers, into lower impedance speakers. IMO!

What is the OTL advantage, over most tube amps, when using most non electrostatic speakers???? NONE!
bdp24's avatar
bdp24
9,979 posts
 

I’m sure Ralph will comment (though he already offered information on the difference between the Zero Autoformer and the output transformers in tube amps just above---did you miss it?), but may I say that when you say "non electrostatic speakers" you are assuming ESL’s are a high impedance load. Yes, the original Quad (57) and other 50’s designs were nominally 16 ohm (though very, very low at some frequencies), but modern ones sure aren’t. The current Quads are at most an 8 ohm load, other brands often 4 ohm. So they too, though being electrostatic speakers, benefit from the Zero.

As to whether an OTL does or does not have any advantage with non-ESL speakers, there is more to that question that simply the matter of the presence or absence of an output transformer in any given amplifier. The Atma-Sphere amps have many other elements unique to their design than that.

More to discover