Inca Tech "the Claymore"
- or: "double edged" (in German: "a mixed blessing")
completely refurbished, 3 years warranty*
*explanations see warranty disclaimer
as required, at least:
- all electrolytic capacitors and trimmers
- disassembling cleaning and sealing of all controls
- only for use with at least 8 Ohms speaker impedance
- black textured powder coated housing with gold printed black glossy front.
- Phono MM/MC, switches at the bottom
- balance trimpot at the bottom
- 2 line inputs
- tape loop with monitor switch
- mute switch
- headphone 6.3mm socket at the back (not switching)
- 4 mm speaker sockets
midi size with no nonsense front design
everything necessary at the back
My double edged Claymore story
When I had my first Incatech contact, I was already used to a lot of British hifi stuff. But I had never seen this one before or even heard of it. It was a Claymore email inquiry from Austria with some pictures attached, that showed nasty third party changes. It was hard to distinguish which parts were ment for what purpose without having the amp on my own desk, so it was sent here and I checked more elaborately.
Beside a defective power stage there were several ugly changes visible. The were all reversed, I found the schematics on the web so I was able to analyse the output stage and find a replacement solution for the obsolete Hitachi power MOSFETs. Having worked through every detail and developed a BIAS adjustment method without the missing test points, I changed from Hitachi to International Rectifier and replaced the output stage power devices by the far better available IRFP240/9240 types, which are often used for Audio applications. They have a slightly higher gate source cut off voltage, so some other parts (e.g. the BIAS limiting 6,8V zener diode across the trim-pot) had to be adopted. At least I set up an idling current equivalent to about 2W dissipation per channel and tested the amp. I was absolutely aware from the very beginning, that the cooling concept was only good for a very concentrated design of the power stage, but not for draining much heat. I thought I had chosen a low enough value and tested the amp. With my 8 Ohm BluesLine Beat floor standing speakers everything seemed right and the amp returned to the customer.
That first revision ended up in the bad news that at the customers 4 Ohm speakers the amp had collapsed with a big bang. After its return and repair I experimented with the BIAS setting and finally took about half the value formerly chosen. What I also learned: the original mains as well as the speaker fuses had too high values for the dangerous bad cooled power amp design. If I remember right, they were reduced to 1,6A slow blow (mains) and for the speakers 3,15A fast blow. Additionally I amended a maximum current to the power devices by limiting their gate source voltage using zener diodes.
current limiting by zener diodes
I tested the amp another week and became more and more attracted by its sonic capabilities during this test period, even more because slowly the new capacitors burned in. Now I knew that amp had a serious design problem with the cooling of its power devices (a detailed explanation follows below), but on the other hand I was absolutely astonished, how this output stage, very close to the Hitachi 2SJ50/2SK135 application note design, behaved. It had at least some clues on it that were really beating my all time value for price favourite, the Exposure X (XV).
reduced fuse rating helps reducing damage in failure situations
Buying downgraded amplifiers
The amp returned to its owner and I actually never heard of how things developed there. And I decided to buy some of this stuff and succeeded very soon in buying three used Claymores at ebay UK. At least two of them were still in "good working condition", so I could e.g. compare my replacement power transistor with the genuine Hitachi types.
The first impression was: much worse than the customers Claymore, that amp sounded inferior to what I remembered by far. Some harsh overtones, something unstable in the treble was very disappointing, soundstage was just a bad carbon copy of the one I had before. So I opened it and saw the amp was a later version than the one I formerly worked on. This one was an issue 6 board, the customer one's was an issue 3. But why this degrade?
Issue 6 issues
The first hint was the square wave response of the line preamp. Overshoot and ringing until your ears bleed...
TL072 OP-amps usually are unity gain stable, but these ones still were built in very close to the edge of their phase margin - almost oscillating in the ultrasonic range. No idea how the designer could fail that way and even more how he could totally overlook and/or ignore the obviously declined result afterwards (I wonder which would be the worst).
Or was just the initial design by Colin Wonfor and the Incatech company applied changes later?
This problem was easy to solve, some small values of styroflex capacitors and a switch to OPA2134 OPs did the trick without a total redesign. They provide a smooth frequency and phase response with even more sonic detail.
But then I checked the output stage and was also not very content with it. Some minor changes made obviously by far more crossover distortion, visible at the oscilloscope and clearly audible.
the line flat amp after removing overshoot and ringing
At this point I stopped my investigations for a long time, until a customer became interested in the Claymore - so I tried to work out a conception how to come back to issue 3 sonically and work around the cooling problems. The output stage was changed as much as possible back to the issue 3 version, I changed the fuse values, recapped the amp and tested it. I came quite close to what the issue 3 did, so after some days of testing it went to the customer. There it was running just a very short time until it blazed off. The output stage obviously failed at his 6 Ohms impedance speakers.
first attempt - the paralleled zener feeding resistors (bottom left) are added that way to provide the higher BIAS of the BB OP amps and can be removed for other choices
Insuperable bad cooling design
After having the Claymore here again I was even more convinced, that this special amp was designed for high impedance speakers of at least 8 Ohms, the high power supply voltage allows no good efficiency at all for lower values. A period of developing new ideas for avoiding the power stage sudden death followed, the first attempt was a stricter thermal negative feedback by replacing the BIAS trimpot by a transistor in the usual adjustable zener diode base circuitry. This seemed to work and allowed for a higher setting and less crossover distortion - which I was still not really content with.
No problem with my 8 Ohm speakers appeared, but after some experience with another repeatedly thermal blasting output stage of the ION obelisk I made some tests with resistor load that proved the following: from a certain BIAS base current on there was a possible thermal point of no return. My first solution for that was to add a 80°C thermal breaker switch, that was put into the current source line of the power stages long tail input pair. That was to switch off the whole output stage totally in the case of a too hot heat conductor. But that still didn't address the real problem of sudden death by a sudden build up of heat. Its thermal reaction was too slow to prevent a final thermal breakdown of the MOSFETs. The heat flow was too low, the thermal resistance of the aluminium brackets far too high and the back-to-back-mounting position of the MOSFETs provided far too much mutual heating. So from a certain current-heat relation on heat increased current and the other way round. That was exclusively dependent from the only value one can influence without changing the whole mechanical design: BIAS setup.
temperature switches added
As the thermal feedback was far too slow to stop the effect, I removed the added transistors and returned to the original scheme. As two output devices were blown before, I also had changed to a similar type (IRFP140/9140) that had less maximum current - a high maximum current in this case was useless anyway. But they had a slightly better thermal figure for the purpose and above all a higher maximum chip temperature (175°C instead of 150°C), which really improved reliability. By the way: the supposed PTC characteristic of MOSFET devices was not helping in this design anyway, mainly because this is a behaviour that appears only at high temperature where everything was already too late. The same effects are impossible on a real good heatsink.
In the end I set a low BIAS with no escalation tendency and left the overheat switches mounted to switch off all signal at really dangerous aluminium temperatures beyond 80°C.
the switches remained, the feedback is removed
Output stage frequency compensation
During that investigation another issue was solved: the crossover distortion - or better: the VAS and crossover distortion - it became obvious that there were asymmetrical portions also in this distortion. A comparison of the given schematics with many other versions of the same Hitachi design showed, that the majority used a symmetrical compensation for both branches of the current mirror voltage amplifier stage. Not so Mr. Wonfor - only the non inverting branch had a Miller capacitor. Adding slightly more capacity and a second capacitor of the same value at the other branch revealed a miracle: as soon as you do it properly, it works...
...after this and some minor compensation corrections distortion wasn't visible on the scope any more. Needless to say it also almost vanished for the ear, that improvement was much better than anything a higher but dangerous BIAS could ever gain.
Still the amp was ill-fated, since when I sent it back to the interested customer I soon got the message, that one channel of the amp switches off after some time, but returns to function later. He also described, that the bottom became very hot (which it didn't with my speakers). That convinced me even more, that a Claymore is dedicated to at least 8 Ohms impedance. The customer liked the sonic results but didn't want an amp, that switched off regularly one speaker.
the transformer must have made a jump to the side, see the damage above
After the amp returned, I found out, that there might have been more reasons for that behaviour, than I had thought: it was badly damaged by transport, maybe mostly on the way back, but that was something I could not distinguish any more. The volume control was smashed to parts, many components around the transformer an the balance pot were broken or sheared off, all nylon board holders were into pieces...
victims of a parcel throwing competition?
I repaired it again and tested it thoroughly - now it meets the issue3 sound quality with minimal differences and shows not the slightest problem with my speakers. One can play it loud and very loud, it has about 70 to 80 Watts RMS per channel into 8Ohms. Still it's impossible to handle high currents for a long time with it without overheating the cooling angles, it simply cannot long term dissipate the loss wattage belonging to the power supply's and output stage design.
My last tests with thermal switches mounted showed: you need some five minutes of full level signal into 8 Ohms per channel to reach the switch off temperature - but only with the cover mounted, without it you can run the amp that way infinitely under that conditions. Both channels switched off independently wit only 3 seconds difference and returned with only 10 seconds difference after cooling down a bit. At 4 Ohms that switch off behaviour will come much earlier in real life usage, because not only all currents double at the same volume, but also the output voltage has the worse relation to the remaining supply voltage. In fact the Claymore's dissipation at 4 Ohms is rather about 4 times higher than for the same sound pressure level at 8 Ohms with a similar speaker efficiency - thus overheating almost immediately at average levels. This again is by its crazy design, just put it on a average efficient 8 Ohm speaker and everything is fine.
On the other hand side dynamic music at a well chosen speaker makes a lot of fun with this amp, very good, deep and detailed soundstage and very little mask effects with astonishing liveliness - the only thing I can imagine better is a total removal of MOSFETs harmonics characteristic, they are a point you still can perceive a little bit and that make some special colouring. After some time of getting used with it, that may get out of focus for many, the level is low enough not to disturb the enjoyment of that otherwise really well sounding amp.
new volume and balance pot, board holders, zobel capacitors...