The Story

Back in the throws of the pandemic I was feeling pretty isolated and was working hard on the damping system that would become the Atrium Damping System (ADS). I had an early version of the headphone that would become the Atrium, and was excited about it and had to show someone, so I sent it over to Steve Deckert, who I also wanted to reconnect with after not seeing him for a couple of years. After sharing what had been going on in our lives and the continuous difficulties of running small audio businesses we decided that since we love each others work so much why not collaborate! This was in early 2021, and here we are now after many iterations with the ZMF Reference OTL Amplifier, which is what I believe to be the finest production amp you can buy for high impedance dynamic driver headphones, like ZMF’s.

The Journey

Well - great things never seem to come easy, and what I thought would be a six month project ended up taking about 2.5 years. Here’s what happened; about two iterations of the amp in we realized this wasn’t going to be an “upgraded” CSP 3 - my favorite pre-amp of all time - but really it’s own thing. Steve decided to redo the power section, the size and numerical values of the caps, utilize the 25th anniversary technology and then some on the power circuit, and also add in a cathode bypass feature as well as utilize boutique parts on every part of the amp among other things. We decided that there’s a lot of “fine” OTLs out there, but not a lot that go all the way to reference grade and that’s what we were after.

Another year went by, and many versions came and went. we even went into production and made ten units before deciding to tear them down to redo the wiring to make the amp quieter as well as change the power tube from 6N1P to 6N6P. After all this the result is a perfectly quiet amp that can be driven perfectly from any source, DAP to monster DAC, .5V to 6V input with a clean, expansive, dimensional tube nature and deep quick bass, not one that is syrupy or slow.

We took our time to make it right, and I’m glad we did.

 
 
 

I’ve always wondered why not have a gain pre-amp circuit on an amp to perfect the sound instead of always having to utilize a pre-amp for the myriads of sources that exist. Well here it is, a high quality gain stage with Vishad mini pots so you can control each channel and make everything from an underpowered DAP to a large 6V source. You will always get perfectly clean sound from the ZMF reference OTL as each of the 4 RCA inputs has it’s own control pot.

See those two large blue capacitors coming out of the top of the ZMF Reference OTL? Those are Mallory Capacitors and they are huge! This extra expense was also spent on all the caps that are also on the underside of the amp - making sure that no part is tasked with close to it’s specification, making for an amp that purrs, even when going over 100 MPH.

With a 6N1P input, you can also utilize 6922, 6DJ8, ECC88, and then for more gain the 7DJ8 and EC189 type tubes. The malleability of an OTL is important to me, because tube rolling is all part of the fun in finding the “right” sound for your tastes! This allows for a lot of choices in just swapping out a single tube.

A Cathode by-pass switch is implemented just above the volume knob, this allows a slight adjustment in negative feedback by utilizing, or not, a cathode bypass capacitor to adjust the tube biasing. With the switch on, you get a slightly more refined and buttoned up sound, off, and we go from 1800 mV to 2000 mV and more lively unabashed output. Salt to taste.

If you haven’t tried utilizing a good pre-amp into your solid state or transformer powered tube amps, this is your chance! There’s no amp I’ve plugged the ZMF Reference OTL into that hasn’t become a beast. More finite control over volume, more weight, extension, liquidity and space is what it’s all about. Give it a try, you won’t want to go back. The sound is even quieter as the power stage of the ZMF Reference OTL filters out unwanted nasties.

My rectifier collection has gotten out of control since I’ve started tube rolling. Who thought that the rectifier could make such an impact on sound? Well - roll away! The ZMF reference OTL has an optimized power stage to allow for most 5V rectifiers to work beautifully, from the 274B, GZ32, GZ34, 5Y3, 80 or 280 and of course the 5U4G. I’ve tried them all, and they all are glorious.