Amps for TQWT-mkII and DTQWT-mkII speakers
Copyright 2013 © Troels Gravesen   Frequently up-dated with new experiences.

                                                                                                                                                                                                                                                                            FAQ on bi-amping

Over the last four decades I've had an endless list of amplifiers, mostly diy stuff based on designs published in magazines and mostly class AB solid state. Linsley Hood's 10 watt class A amp and Nelson Pass' 20 watt class A amps may ring a bell for those old enough. Next some Erno Borbely mos-fet amps were built before "The End" appeared in Danish "High Fidelity" magazine. This amp was developed into the LC Audio range of products and I built most of them. After that I started buying amplifiers and Luxman MQ70 (EL34 valves) was on the shelf for several years until someone brought in some 20 watt PSE 300B mono blocks and my road to aural nirvana took quite a turn as can be seen from my gear pages. More recently I've acquired some solid state amps again, the Jungson JA88D-09 and Hedo/Duro, low-feedback, partly class A amps to run my Jenzen speakers. Even my 40 watts SET 6C33 triodes cannot cope with 88-89 dB sensitivity speakers.

With regard to the DTQWT speaker, I wish I had a wider selection of amps to refer to but I've only had the opportunity to hear these speakers with a good handful of amps like my 8 watts 300B, 20 wpc Audio Mirror mono blocks, Gain Clone (used for CLIO measurements), Rotel RB981 solid state driven by my WOT line stage, 8 wpc 2A3 push-pull design (discontinued) from DiyHifiSupply Hong Kong, 15 wpc T16-OTL from Transcendent, 15 wpc 300B design from TriodeDick/Holland (built by DTQWT customer), 25 wpc amp from Jasmin (Sonata, EL156 valves) and most recently my JungSon JA88D. Amoung these I do not recommend the T16OTL, Rotel and Jasmin. BTW: I also tried a pair of (classic) 30 wpc EF86-ECC83-EL34 PP monoblocks designed by Claus Byrith (Denmark). Despite the use off serious global feed-back the sound was dynamic, smooth and easy - but transparency could have been better.
More recently, 2012, I've had visitors bringing in some PassLab designs and I have to say all of these worked extremely well with the DTQWTs.
Most recent acquired amp is the GlowMaster KT88 PP fully balanced amplifier, the best I've ever had.

Generally I prefer the valve amps over the solid state with the JungSon as an exception. What I would like to try are a few British amps like Croft, Exposure, Naim, Sugden (e.g. A21) and I'd also like to try some of the Nelson Pass' low-wattage amps to hear how they perform. All these to get more experience on solid state designs.

My default amp(s) for DTQWT are the (PSE) Audio Mirror 6C33 mono blocks and GlowMaster KT88. The 6C33s don't deliver the ultimate bass punch like the GlowMaster but they add a little extra warmth to my slightly tight sounding Beyma bass drivers I use. Due to room placement I have minimum room-gain and I rush to say that my DTQWTs in a smaller room and placed closer to the corners may sound better in the bass region from other amps with a more tight bass performance.
The listening room plays an integral part of any speaker set-up and will ultimately point back to the amp used, in particular when dealing with low-wattage valve amps with generally poorer bass control.

The TQWT and DTQWT speakers were designed to offer a speaker alternative to all those investing in low-wattage SET amps, but somewhat to my surprise most customers to these kits use solid state gear. The majority of solid state amps are 50-100 wpc push-pull class B amps and with all respect they may offer a decent compromise with regard to price and performance. To some DTQWT customers there may be more to extract from your source material by trying a variety of low-wattage valve/solid state amps where quality has priority over quantity. It's easy - and cheap - these days to make a 100-200 wpc solid state amp, but the general concept of push-pull class AB designs with global feedback has its shortcomings when it comes to sound quality. To my ears! I rush to say I haven't heard them all so I'm sure there are exceptions.
I've had visitors listening to my DTQWTs driven from my 300B and Audio Mirror amps and unanimously they said: "Well, I guess 300B is not for me..." Why? Because bass is more taught from the Audio Mirrors. This is my interpretation of their response despite the midrange from the 300B is the best of the two, but this is not what we generally favour. Kick-butt is more impressive and it takes an awful lot of time to evaluate the sound of the oboe, the flute, the piano, etc. from two different amps. We need the amps and the speakers for days for careful listening. Never judge an amp from 2 minutes listening like most people do.

As mentioned by Ilpo, the DTQWT can also play seriously loud with low distortion, thus an option for potent surround systems, something I hadn't anticipated when I made the speakers. Here some decent solid state gear may beat all the SET amps by a comfortable margin. Despite high efficiency we need quite a few watts to cope with 100-105 dB peak programme material. Pure math comes short here. It doesn't help calculating the watts needed to make 105 dB, we need serious headroom to cope with this kind of application.
That said, the DTQWT and DTQWT-12 are not made for 80-100 square meter living space if you fully want to load some 200-300 cubic metre volume. I have an increasing number of mails requesting speakers for this size rooms - and sorry, I don't have any, well maybe the OBL-11, because it has a high-pass filter to the midrange. For 80 square metre I would start with a 15" for bass and  build up from there. The QUATTRO + some 12-15 inch in a 80-160 litre cabinet + a 24 dB/LR electronic crossover at 140 Hz has been tried and is fully recommended for those with large rooms.

My default recommendation for an amp for the TQWT/DTQWT is a PP valve design like the Cayin A50T, A55T, A88T, etc. These amps usually delivering 20-25 wpc in triode mode and 35-45 watts in pentode mode.
There are numerous variants of these KT88/EL34 PP designs, like the Prima Luna and World Audio designs. Use google to find more - and not least countless diy designs on KT88/EL34/6650.
If you buy a commercial design, be prepared to spend some money on good coupling caps, i.e. super caps! This is of vital importance for making the best of tube amps. Buying directly from China may save you an awful lot money too, enough for good coupling caps, which you'll have to do i any case. No manufacturers deliver their amps with super caps, default is Solen FC caps and maybe Hovlands or some orange drops. The latter caps are no better than Solen FCs, only nice colours.

My ultimate recommendation is the GlowMaster KT88 PP fully balanced amplifier. This provides at least the level of transparency of my 6C33 mono blocks + the bass punch of my solid state amps. You will need a high-quality balanced line stage like the 6N6P + decent 24 bit high-res audio/min. 2kUSD phono cartridge to hear all what this amp can do. 

As you can see I've tried to avoid answering the question on whether the modded JungSon is a great amt for the DTQWT.....Hmm... give it 20-30 minutes to warm up and it will deliver a midrange transparency probably unbeatable for the price. Problem is that the JungSon could run a whole stack of DTQWTs and it's kind of overkill. If the choice is between Jungson and some really good sounding valve amps, it's really a choice between sound because the Jungson sounds distinctively different from (my) valve amps. I'm sure that if I had an audience in for evaluating the Audio Mirror amps and JungSon, the discussion would be on which amp is the most correct because the JungSon may actually be more correct, where the non-feedback SET amps may have something (warmth) the JungSon does not. If you want it all from one power amp, the GlowMaster is the answer.

Last, but not least: I have twice seen 8008 main driver's cone ripped apart from using 400 watt solid state amp - or lending the system to children's Saturday Night disco party. This from DTQWT mkII and DTQWT-12. Now, the DTQWT-12 was not made to play louder, only deeper. The main front driver still carries the full low-end signal and 95 dB sensitive speakers should simply not be fed 400 solid state watts. The voice coil was fully intact but the cone was just simply ripped apart. The other reason for this was that the speakers were supposed to energise more than 100 square metres rooms. This is equivalent to a smaller cinema space - and this is a completely different story. Before building speakers we need to analyse what kind of sound level we're aiming at, what is the room size - and volume, and what kind of amplifier are we going to use.

Any burned driver is a misused driver. 

This  VAIC brought in by a visitor did very well with the DTQWTs.

I've had the question on bi-amping the DTQWT and there are a number of considerations to address as listed below. Next I have the question on bi-amping plus use of an electronic crossover to substitute the bass drivers' low-pass filter.

First of all I hardly see the rationale for bi-amping a 95 dB system where there's no high-pass filter to the front drivers and only a simple 1st order filter - with an extremely low DCR series coil - to the bass drivers. The system as such requires little power compared to a modern low-efficiency speaker, but should you want to run a 2-4 watt SET on the front drivers, it may certainly benefit from not having to run the bass drivers as well, but this is not without problems.

We need to reiterate the situation as described in the DTQWT file: http://www.troelsgravesen.dk/JA8008_DTQWT.htm

"The DTQWT construction has an 8" front driver and two 10" rear mounted drivers, all connected with positive polarity. They're placed opposite to one another and the rear drivers are fed through a huge coil, thus only adding weight to the lower octaves. All drivers load a center horn and you may ask whether the large bass drivers won't have an impact on the front driver from sharing the same open space and the answer is yes - and this is intentional. Air goes where pressure is low, thus when the rear bass drivers move inwards, they will push air into the large center horn and by doing so they will suck air from the front horn and pull the membrane of the front driver. Obviously the front driver will do the same thing for the bass drivers. We have a situation similar to a push-push configuration of two opposite placed bass drivers in a vented cabinet.
The subjective experience from this is a cleaner midrange, distortion has been lowered at high SPLs and yes, we have more bass. In fact a lot more bass."

Based on experience, the amps running the front and rear drivers have to be the same. Using my Audio Mirror amps for the front drivers plus an e.g. Rotel power amp for the bass drivers didn't perform too well playing at loud levels. I guess that the amps have different phase characteristics in the bass region, thus jeopordize the synergy between front and rear drivers loading the same center horn. Amps being either AC or DC coupled may have an impact on this.

Bi-amping and electronic crossover to the bass: The bass crossover is a low slope 1st order filter and has a significant impact on upper bass/lower mid. Cutting the bass by e.g. 12-24 dB at e.g. 100 Hz will render upper bass/lower mid weakened.
In a situation like this, the front drivers would have to be run directly from the amp, bypassing the electronic crossover. Next the electronic crossover will have to mimic the transfer function of the passive crossover and this may take some experimentation. And again, the phase behavior of the different amps will still have to be taken into consideration.

All in all not that easy - and again: I hardly see the rationale given the system's sensitivity.