This David Berning designed preamp / headphone amp will be known to some of you. Has anyone had experience of using it as a preamp? The reason for asking is that a friend tried one as an alternative to his Pass XP32. He was using it to drive his Pass XA 250 monoblocks into a pair of Yamaha NS5000 speakers. The result two smelly XA250s (both switched off and only one returned to duty) and two blown woofers on his Yamaha's. He has heard of something similar happening with the ZOTL and a pair of Pass amps but it is hard (even though he probably switched the preamp on after the power amp) to imagine the Pass amps were at fault. By way of explanation the valve ZOTL operates quite differently from conventional preamps, solid state or valve. The ZOTL isn't actually an OTL. It has tiny air cored output transformers; tiny because they are dealing with radio frequencies of either 250khz (earlier models) or 500khz (later models including I suspect the ZOTL2.0). The voltage to current transfer (which is what conventional OTs do) is done at radio frequency in which the rf (250 or 500khz) is the 'carrier' for the audio output. The ZOTL uses two 6sn7s in push pull to provide the nearly 1 watt output at a relatively low output impedance of about 4 ohms. So whatever the problem - dc surge at switch on or rf interference from the preamp being placed too close to the power amps - it will be expensive to fix. Because the ZOTL has such a low output impedance it would be possible to use extra long interconnects but this doesn't resolve the problem. Has anyone heard of this before? I couldn't find anything on the net.

Wow ok. Blown woofers and stressed amp is severe. I’d put the LTA on the oscilloscope and check its output into a load that simulates the Pass. Do you know the Pass’ input impedance and capacitance? Check for both RF and DC, also any switch-on transients.

AMR-iFi R&D

Thanks Colin. I'm an enthusiastic amateur only and it is 400km down country so I won't be taking a look myself. The Pass 250.8 has an input impedance of 50k SE / 100k balanced but I don't know it's capacitance. It has a quoted low frequency response of 1.5 Hz and has 56 MOSFETS in the output stage so would take quite a bit to stress it. The LTA preamp has an unusually low output impedance of 2ohms as it is also a headphone amp. Just how the LTA by amplifying in rf and multiplexing (?) back to af might have a bearing on the problem I don't know. At least one reviewer (https://theaudiobeatnik.com/review-pass-labs-x250-8-stereo-amplifier/) has used the combination without reported issue. The Pass at least I fear may be in need of a holiday in the USA.

Can they have a tech check it over?
I remember in Nelson occasionally there were power surges big enough to cause damage. Hope there wasn’t something like that coincidentally happening at the same time.

AMR-iFi R&D

He will need an assessment by a registered technician for insurance purposes at the very least. I don't think there were any power surges. I don't know which of the two LTA power supply options (switching and analogue) were being used. Shouldn't happen in new equipment.