BIG IRON - the 200W monoblock monsters!

Not long ago I was contacted by David Counsell of dc10audio to create an amplifier for his company to sell along with his speakers. We collaborated and came up with a few ideas, the first of which was a variation of my 66-100 amplifier using a higher plate voltage and KT-120 output tubes, higher grade capacitors and wiring and a different color scheme. A higher VA rating required an all new power transformer, and a driver stage with more current capacity for those big tubes. This was successfully completed last spring and sold to one of his clients, who is very happy with the amplifier.

At 120 watts / channel this amp took control of the big Martin Logan CLX's at my buddies shop Reference Media in Everett, WA. The sound was big, open, tight and nuanced.

Fast forward a couple of months. David and I agreed to do the Rocky Mountain Audio Fest together and he wanted a super amp, a statement product to go with his speakers- a pair of 200 watt monoblock amplifiers. With the show only a few months into the future and knowing the lead time for such things as custom wound transformers and all new casework, I was  just a bit nervous at this proposition. After some consideration I determined that I could do the project if the 66-100 casework could be used. This would require a modification of the design since the stereo amp has 3 transformers and the monoblocks would have only 2 per unit. The solution was to create a custom aluminum top plate to cover the mounting holes for the stereo version, while at the same time dressing up the appearance of the amplifier and stiffening the chassis to accommodate the larger transformers.  

The first thing I needed to do was to order the custom output transformers so that I could build up a prototype and prove the design. Since my usual supplier doesn't make a power transformer above 350VA or an output transformer above 100W I had to find another vendor. After some internet searching and sending quote inquiries, I found a company that would wind the output tranny's. The power transformer was covered since I had already found a local commercial supplier that could build what I needed for the 68-120 project. I just had to specify a larger core size and higher VA rating to match the new output transformers.

So it all came together, the BIG IRON arrived and I was able to build one mono amplifier prototype that easily made 200W into 4 ohms with 4 6550's. (It would make 240W with KT-120's but 200 watts is enough power for most people and the 6550's sound better anyway, at least in this design.) The next step was to create a parts list based on that design; specifying the best parts for their respective applications while trying to insure they won't be discontinued can be a challenge. Over the years of doing this I've had to scramble to find replacement parts for those which had been discontinued more than once.

David wanted to incorporate some high-end wire and capacitors for this build so I used silver / teflon for input wiring, silver plated copper / teflon wires for the output transformer secondary leads. All other circuit wiring is point to point on the component level and signal carrying interconnect wires are silver plate copper / FEP. The capacitors ended up being the Jupiter beeswax / paper line and took a few weeks to arrive. These I ended up bypassing with some small value polypropylene film and foil caps, which improved the focus and tightened up the bass somewhat. We also used some really nice rhodium plated binding posts that cost a fortune. Here's a shot of the interior wiring during construction.

This design is merely a single channel of the 66-100 on steroids. Instead of 2 6550's producing 100 watts there are 4 6550's pounding out 200. The driver stage took some consideration- the drive requirements would be a challenge especially at high frequencies where the miller effect of the power tetrodes will cause distortion and attenuation. At first I explored a differential pair with cathode followers but thought the layout would be too complicated. I like to keep things simple so I just paralleled the two triode halves in each driver tube and tied both composite tubes together at the cathode thus creating a 'super-tube diff pair' (I could use that in marketing..). So essentially each parallel triode pair drives two paralleled 6550's for each half of the output tranny's primary. Got it? It worked like a champ! Of course this amp retains all the features found in the 66-100 like cooling fan and individual bias pots for each output tube and a built in meter for setting bias, and design features like local feedback in the output stage and optimized concertina phase splitter. Look at the huge PSU reservoir caps and the smoothing choke, also found in the 66-100.

So as Murphy would have it, what works great in prototype has issues in the final build. I spent a considerable amount of time chasing down frequency response aberrations, ringing and such. I experimented with the feedback level (very little loop feedback), compensation caps in the driver stage, replacing those beeswax caps with poly's, etc. Then I noticed that the response was nice and flat when the amp was open on the bench, but when the cover was grounded to the rest of the chassis the response had a rise in the high frequencies. Very strange. Turns out that the output transformer's core doesn't like to be grounded but there's no way around that! I ended up using a simple zobel network across the primary.

Here's the frequency response and THD plots at 200W into 4 ohms. The THD measurement shown is 1.6% @ 1kHz. Note the flat distortion spectrum above 40Hz. Below 40Hz the distortion rises due to transformer core saturation. This is pretty good for full output! But who listens to music at 200W?

 Here's the plot at 25W - .5% @ 1kHz.

Notice much less low frequency distortion in this plot. 25W is pretty much the most average power you need but having 200W in reserve gives you much more headroom and potential dynamic range.

Here's the 3W plot - .3% @ 1kHz. This is where most of your music lives. And this is why these amps perform so well even on high-sensitivity speakers.

So you're wondering how it all sounded? Very much like the 66-100 but more power, a little more liquidity and finesse. I attribute this to the upgraded wiring and caps, and of course the output iron is different as well. All of these things add up and contribute to the overall presentation that an amplifier will give. But the fact that it's so similar to the 66-100 is due to the circuit topology and layout itself being mostly the same. Hard to improve on an already proven design!

David was thrilled at the show and we had very good response to the sound in our room but that's another story..

These were built exclusively for dc10audio and carry their branding. They are available directly from their website: http://dc10audio.com/. They make some very nice high-sensitivity speakers, which happen to mate very well with ESA products~

Here's a shot of our setup at RMAF:
The speakers are very interesting - the tweeter horn doubles as the reflex vent for the woofer, and they incorporate a tone-wood resonator panel that damps internal vibrations while creating a passive radiator effect. The sound is immediate and refined and the overall bass response is incredible. You really have to hear it to understand - they don't sound like your typical high-end speaker, they sound like music. Lovely.

If you look closely you will see the ESA 66-001p Vacuum Tube Reference preamp. A new preamp was planned for the show but we ran out of time to get it all built and dialed in so the ESA unit was used to very good effect. Stay tuned for another post on the development of the new preamp, which uses circuit boards instead of point to point in an effort to cut down on labor.

Happy Listening!

Prototype Headphone Amp continued..

It has been quite a while since my last post on the prototype headphone amp, seen above. I built this up for the folks at Reference Media for a possible product launch that never happened, sadly. So, it's been sitting on the shelf for about a year now and I decided it was time to do something with it!

While visiting my brother earlier this month I noted how he is SO into music but really doesn't have anything decent for play back. Because of his situation he listens mostly with headphones so I decided to build him a handy dandy headphone amplifier. But, I needed a box or case to put it in, and to make a few decisions on functionality.

First, the functionality aspect of the design. The basic circuit can be adapted to serve many functions being set up as two gain stages separated by a volume control. I decided to make it more flexible than a straight amplifier by providing three inputs, a selector switch, defeat-able line outs for driving an amp (or powered speakers), a power switch and pilot light. A little less functionality than the 66-001 preamp; no balance control or mono switch, and less input options. With that figured out I could go shopping for parts, scavenging whatever I could from my spare parts collection (ever expanding I might add).

For the casework, I knew that the power transformers would be on-board (unlike the 66-001 series of products which have external regulated supplies), so I would need to find a box big enough to house everything. We have a local PC recycle place that has all sorts of discarded electronics for relatively cheap (though lately they have put ridiculous prices on some real junk) where I found an old Avid 888, an 8 channel 48K Pro-Tools interface from the '90's for $15.00. Perfect! Just need to drill a few holes, fill in a few more and do a custom faceplate.

Every project I do has a phase where I just sit and stare at it for a while, trying to figure out where everything will go. I ended up arranging it in a logical fashion; power transformer on the left side of the chassis because that's where the IEC AC inlet filter and fuse are mounted, PCB and output transformers to the right, with the OPT's in the rear near the output jacks. Next I had to fill the holes in the rear panel - 24 XLR sized holes that is! I happened to have an 18 gauge painted steel panel left over from something. It would need to be cut to size with holes for RCA jacks drilled into it. A few minutes with my angle grinder w/ cutoff wheel and drill press took care of that. I laid out the hole pattern for the circuit board, drilled and installed it with 4-40 screws and nuts, using nits as standoffs as well.

So far everything has been sourced from my spare parts but I needed some switches, knobs and jacks. That would come later, but for purposes of testing I wired in temporary jacks and fired it up. Having decided to change the tubes from ECC99's to the more common 12au7a's meant that some circuit parameters would need to be tweaked to gain best performance. Knowing that the max level it will probably ever see is 2Vrms from a CD player or DAC (ridiculous high level IMO) I ran some sweeps at that level to gauge the distortion. Not so good. Looks like the input stage would need more headroom, or higher plate voltage. No problem, I removed the PSU dropping resistor for that stage and wired in a 5K pot. This way I could adjust the pot, monitor the plate voltage and do sweeps to check distortion at different settings, keeping the plate and cathode resistors constant. It turned out that the best setting increased the plate by about 25V; the 4.7K dropping resistor was replaced by a 1.8K resistor. As you can see from the final sweeps the distortion is below 1% across most of the audio bandwidth, and is dominated by 2nd and 3rd harmonics, the higher order stuff down in the noise floor.The increased distortion at low frequencies is due to the output transformer saturating but at low frequencies our ear is less sensitive to distortion. Higher up the distortion remains fairly constant with frequency which is a good thing.

The distortion readings shown are at 1kHz. The top trace is the sweep and shows the frequency response, the black trace is THD, the rest of the traces are 2nd, 3rd, and higher harmonics (see legend at bottom of graph). The right channel shows slightly more distortion; this is due to differences in the tubes, which are never matched perfectly and are the biggest variable in designing tube circuits (modern passive components are pretty stable and precise in value). But the overall distortion in the right channel is at .321% THD, well below the theoretical 1% threshold of audibility.

Shown above is the frequency response. Not bad for a transformer coupled tube amp! The transformers are actually 70-volt speaker transformers. These work great for tube driven line stages because of their high impedance ratio, and you have your choice of impedance taps too. Though they're only designed for about 50Hz to 15kHz in their intended application, the fact that they're not being driven more than about 1/2 watt into headphones pushes their bandwidth to full spectrum. They have the added benefit of very low output impedance- in this circuit about 10 ohms; great for even low impedance 32 ohm 'phones which are the most common these days. And into higher input impedance's typical of an amplifier (10K and above) they perform even better, able to drive long interconnects with no HF roll-off. Plus, in my opinion, transformer coupled circuits have a richer, more harmonic and punchy sound.

So now everything is finished, tested and tweaked, ready for the faceplate. Below is an image of what that will look like, it's about a week out as of today.

This turned out to be a fun little project! And hopefully fun for my brother, too. Here's the final look with the faceplate~