Review: David Berning Co MicroZOTL Tube amp

Category: Amplifiers

I purchased this amp in February 2002 direct from David Berning Co. At my time of purchase, it was only available by factory direct purchase.

This amp was designed to provide access to the Berning ZOTL form of amplification, at a reasonable price. It is intended for high efficiency applications, or bedroom/office systems, or as a headphone amplifier.

All controls are on the top of the unit. It is equipped with a Noble Potentiometer, so it can be used as a single input integrated amp. If you want to use a preamp, you can just turn the Noble pot up full, or bypass it with a soldered jumper. There is a locking headphone jack for its intended use as a headphone amplifier. It also has a power switch that is labled AC/DC which allows ON/OFF switching for either AC or DC power.

The rear of the unit has all of the input and output connectors. Single ended, RCA type input jacks and 5-way binding posts are provided for 1 stereo input and one pair of stereo speaker outputs. Very high quality connectors are used on this unit. Power enters via a removeable IEC connector which allows aftermarket power cords to be used. There is also a DC tip-and-ring connector for 12vdc battery power input.

The MicroZOTL is a small amplifier, about the size of a typical shoebox, and weighs about four pounds. It is very plain in appearance, with a simple bent metal two piece chassis and a glass window in front. Available in 3 colors: olive, red, and blue.

The unique Berning ZOTL circuit is showcased in the MicroZOTL in a Class A no feedback PP triode design. Using 12AT7 as input and 6SN7 as output tubes,to provide the breathtaking 1 watt output of this amp. The linearity of the 6SN7 is really apparent in the sound of this little beauty. You've probably never heard a preamp tube powering a speaker before. It is delicious, using the twin-triodes in each 6SN7 in push-pull within a single tube envelope. One output tube on each channel. This sound goes directly to the speaker with the absence of traditional output transformers. The impedance matching is done by the ZOTL circuit, which uses a 250kHz carrier frequency to piggyback the signal on. The carrier frequency does the impedance matching, and the signal is unaffected, and goes right out to the speaker. It has the best of both worlds. The impedance matching is handled as well as any transformer, and the signal is pure OTL. This circuit is patented by Berning and represents a true quantum leap in OTL design.

I first used this amp as an integrated, with a single source from my Sony 9000 SACD/CD player. It drove a set of single-driver Fostex speakers to reasonable listening levels, but could not get really loud, or handle full symphonic music. The 89db efficiency of the Fostex's was simply not enough for the low power output of the Berning. But, the sonic "goodness" was definitely there. The direct input from the Sony was a little harsh, so I got the preamp, and that was a better setup. This allowed me to then get a turntable rig and use 2 sources. Sound from the analog setup was a clear improvement over CD/SACD. But, the volume level was still too low. I needed more efficient speakers. I settled on the Lowthers, and built Voigt Pipe cabinets with mods. This brought the MicroZOTL into its own.
The sound was awesome, from top to bottom. This has to be heard to be believed. I can get just a few db from real symphonic listening levels with a 1 watt amp. Anyway, 100db is a pretty good SPL for most living rooms. If you stay in the near-field, it's loud. Great dynamics and speed, no grain, crystal clarity, super soundstage and imaging. The only thing is that with direct radiators, you can't get to the 106db level. With horns, no problem. This would be all the amp you'd need with Avant-Gardes or other 106db efficient horns. And for SET fans, this amp is tight right down to the bottom, with no "bloat" or loss of control. The highs soar right up to the sky, with no transformer induced rolloff. No need to sacrifice frequency extremes to get "golden midrange" with this amp.

This is probably the easiest tube amp in the world to live with. It is auto-biasing. Doesn't get hot. Tube life is estimated to be 10-20 years. Uses very little power, and I run mine on battery(2.1 amp draw at 12vdc), for ultra-clean power. It's warmed up after about 15 minutes. You can change tubes in under 5 minutes, when you buy your NOS, which I recommend. Comes with JAN Phillips, and Sovteks. It takes up very little space. Weighs next to nothing. Can put it just about anywhere. No microphonics problems. Plug-and-play. And for a measly $750, you have a killer sounding OTL.

If you can live with 100db or less in your living room, this amp(with Lowthers, or 106db with horns) will give you the best sound possible for anywhere near the price. I have heard higher power amps that cost $5k that would be embarrassed by my little MicroZOTL.

Highly recommended. Twl-ophile Class A component. Best buy.

Associated gear
MFA Magus Tube Preamp with Amperex Bugle Boy NOS Tubes, Cardas Jacks, Cardas Wiring, REL Capacitors, Wonder Capacitors

Sony DVP S9000ES CD/SACD/DVD player

Teres 135 Turntable (Clear Acrylic)

Origin Live Silver 250 Tonearm

Denon DL103 Cartridge

M.A. Cotter phono step-up transformer

Lowther EX3 Voigt Pipe modified

Bybee Ultra Quantum Speaker filters with Nordost SPM tails

DIY interconnects

DIY Speaker cables

DIY Room Lenses

DIY Bass Traps

DIY Wall Panels

DIY 12vdc Battery Power Supply

Similar products
Many tube amps, but none of this low power.
Thanks for the informative thread. I have been hearing great things about his designs.
This is going to sound silly to some, but most of the time I use my micro-zotl in my study to drive a pair of Spendor LS3/5a's, which have only 83dB efficiency. I admit freely that this combination does NOT get loud, or even close, but the sound is quite nice and I never have to worry about my kids blowing my speakers by cranking the volume. I also do not have to worry about damaging my hearing, which is especially sensitive to harm because I played in a rock band as a youngster. I use an Optimus CD3400 as my source, and since it runs off 6 volts DC, I can run the whole setup with a rechargeable, portable battery designed for jump starting cars (from Sears, $35). [I have also used the battery to jump start a friend's car.] It's a pretty cool hifi when I take it outside. I also use my micro-zotl for driving my Sennheiser HD600's. With that combination I can hear details that are inaudible on excellent speakers even after I know what to listen for. The Berning Micro-ZOTL is a wonderful little amp. For those of you trying to decide on color, I'm convinced the ones in the original blue sound the best. ;)
TWL, this is a beautifully written review. Much better than many "professional" reviews. My hat is off to you. Well done.
I, for one, really appreciate you sharing with us the unique nature of your entire system. It sounds wonderful! Is it possible to get this amp design in a more powerful configuration?
Happy listening,
Thanks, guys. I wanted to do a good job for you. I appreciate your support.
Patrick, there are higher power ZOTL Berning models. The Seigfried(8watts, 300B or 12 watts, 811) costs around $5k-$6k. The ZH270(70 watts) costs around $4500. I have a custom model SET ZOTL based on Type 45 tubes(2watts) being hand made for me by David Berning. It is not a production model, but a one-of-a-kind custom prototype designed for my particular need to get that last few db out of my system. It will also be battery powered.
Your review implies to me that besides that output power, any limitation or weakness of the Micro is not siginificant enough to include. Is that true? Would you recommend it to an A.G. owner?
Yes, ohlala, that's true. I think that the sonics of this amp are superb, and not at all inline with the price point. The low power is the only thing that might be considered a drawback for those with less efficient speakers. Otherwise, this is a fantastic amp with features like ZOTL that cannot be gotten from any other amp, except the expensive Bernings.
Well, something isn't adding up here. It's laughable to think that the Micro is a colorless amp, but maybe you do not consider its coloration a weakness. This is a 'review', though, and I believe it is necessary to include any deviation from nothingness as well as price-point comparisons. As we all (should) know, the best thing you can say about a component is nothing. I am thinking that maybe you have not heard it in enough situations to make that judgment. Is that possible?
Are you driving your single driver speaker with this amp? You may have answered this in the listing of your equipment but I didn't realize it. Lets see some more reviews, I like what you've done so far and I'm interested in your approach.
Ohlala, perhaps I was overcome a bit with the positive qualities of the MicroZOTL, and didn't mention any negatives. It is push-pull, so it doesn't have the ultimate midrange purity of some of the great single-ended designs. But it is really good midrange, nonetheless. And it does have the strong bottom end that would normally be associated with a good PP amp. I suppose that there could be a tiny amount of detail that could be improved upon with some Caddocks and Black Gates, and possibly the removal of the Noble pot. Potentially, eliminating the circuit board, and hand wiring the unit could yield a touch better sound. Now with only 1 watt, there is not the chest thumping bottom end that would require alot of power to move the air, but I don't really consider that a sonic deficiency, only a side effect of the low power output. With speakers that drop low in impedance, the sound would harden up some and lose some "life", so it is not the greatest match for 4 ohm speakers. With the stock JAN Philips 12AT7 tubes, there was definitely a glare that needed eliminating. The NOS Mullards I bought took care of that. The stock Sovtek 6SN7 output tubes were a bit "flat" and lifeless, so I put in some 1940s Sylvanias. This perked things up real nicely. So, it wasn't perfect, and still isn't. But, it is a very good sounding amp, considering its power limitations. With a few easy upgrades from the stock tubes, it reached an excellent level. You see, the ZOTL has a much shorter signal path than any other OTL made, so it can pass a better signal to the speaker, and drive the impedance of that speaker, better than any OTL. It can be equalled by good SETs for short signal path and impedance matching, but then they have the transformer difficulties to deal with(phase-shift,ring,saturation and bloat). If we combine OTL phase-coherence and frequency extension(due to no output tranny), with SET short signal path and impedance matching(due to having an output tranny), then we get the ZOTL. This combination makes the ZOTL circuit nearly unmatchable in the amp world today. Cost remains low because there are no very expensive output transformers to push the price up(as in SETs), and there are not rows-upon-rows of tubes required to match up the output impedance(as in OTLs). So the price/performance ratio is excellent. Even the entry level model like the MicroZOTL has sonics that would be expected in amps at $5k or more. Albeit with much less power. But efficient speaker designs, like the Lowther, reduce this apparent power deficiency to minimum. The result, in my system, is the sonics of a high priced amp, with the cost of a mid-fi or budget component. And I can still get 100db peaks of SPL. So now perhaps you can see why I am so thrilled with the purchase. Not because it is perfect, but because it is so close to perfect for a very low price. And if you configure the system properly, you can get good SPL results too. You'll never run Maggies with this amp, but if you can live with a high efficiency speaker system, it can give sound far beyond its price point with them.
Nrchy. Yes, Nate, I am driving the Lowther Voigt Pipe single-driver speakers with this amp. They are good for 100db max SPL on peaks with this amp.
Well, I have sold my little MicroZOTL, and gone on to the bigger Berning model. I did get an email from the member who bought it, and he loves it also. The main comment in his email was,"Wow! It sounds great!" I couldn't have said it better myself.
Thanks to all the previous folks who wrote such thoughtful reviews. I use my Micro ZOTL strictly as a headphone amp; source is an unmodified Sony 333ES SACD player; 'phones are Sennheiser 650s (the 600s before them), connected with Stefan Audio's Equinox cable. The sound is simply glorious; the best in my house. Detailed and involving, but also totally liquid; one of those rare products that's just crazy good.
Very Cool!
Hi Tom, now I have some horn speakers, Klipsch Heresy's MKI, I did a little Googling and came across your excellent review, small world!! I am going to have to give this one a try, based on your experiences, thanks for the review and to the others as well for their contributions.