Reverseengineering an HK_Ottala
Power amps are something very close to my music heart.
I have quite a lot of them around here, Zen in different versions, Accuphase, own designs etc, and my favourite for the latest few years, an ML11, the last power amp Tom Collangelo designed, a magical little amp that can drive anything and equiped with quite a lot of power despite its modest sepcs.
As all Mark Levinson amps it has a certain sonic signature, leaning towards the darker kind of voice, never bright but with brutal transientresponse and attack when delivered such.
A while ago, rather long while actually, i traded a pair of JBL HF drivers and horns for an interesting amp, as i if i needed more of these.
Utterly sophisticated design, B&O has something to learn here,.
But to the reason i wanted it. This amp is designed by Matti Ottala during his time as chief engineer at Harman Kardon.
The information i´ve got says it was designed as a prototype for the Citation XX project and the same story/rumour says 3 or 5 of this amp was built at HK workshop.
The amp is among the best i´ve ever heard with an extreme lightness combined with unbelievable power and transientresponse , a "sharp" voice but still not bright, drives anything i´ve loaded it with the same ease.
One major flaw, as an extremely wide bandwith amp it is very prone to amplify and distribute to speakers any wire or airborne signals in the vicinity.
So, i will dismante this amp, primarily for replacing wiring and also check/replace caps and find the right place to insert lp filtering to
If there is interest in recreating this fantastic amp, it actually is just that, among the best i´ve ever listened to, ill start a thread reengineering the amp and se what we can end up with.
I have been a fan of the Otala designed KH amps since I bought a HK 750 integrated in the late '70s early 80s. It was a wonderful sounding amp.
I for one would love to see you put the circuit for this up for the consideration of the diy fraternity.
More pics please, lots more - there definitely will be some wiring layout tricks which make-up the design and amp's sound/performance.
[QUOTE][ do go ahead/QUOTE]
Will do that then.
The physical construction of your amp is very similar to several Krell and Nelson Pass Threshold designs that include:
1) Separate PCB attached to heatsink for the high heat output transistors and Re resistors.
2) Separate PCB on heat-isolating riser posts attached to the output PCB. Heat kills! Avoid putting input devices close to Hot Outputs. Some riser posts can serve as electrical connections. e.g. a mechanical star ground path.
2b) Using ThermalTrak outputs with integrated temperature sensing diodes reduces the chance of oscilation with a separate driver PCB over a transistor Vbe multiplier. The Leach Amp document discusses this.
2c) Output stages seldom change. Using a separate input stage PCB simplifies testing new driver circuit ideas.
2d) Dr. Keith Johnson (Go Stanford Cardinals) of Spectral favors putting the driver PCB at 90 degrees from the output power PCB to minimize the coupling of magnetic fields from the high current outputs from crossing into the low noise inputs. Just sketch the magnetic field lines from the high current outpu bus traces and keep the input circuits at a safe distance. Spectral uses a metal can over the JFET intput circuits for further mag shield and temperature stabilaztion.
3) Output power supply capacitors directly next to output terminals in order to optimize common star ground and feedback with speaker R-L-C complex impedance load.
4) The Magtech construction(jpeg) uses very large PCBs and wastes some heatsink length for a clean physical construction plus a simple case construction. Power Capacitors are under the PCB attached to output terminals. I think 4-ounce copper PCBs or soldering copper bus bars to the PCB would be necessary to carry the high currents.
Hope the pictures add to your thread discussion.
:-) It is not nasty to build a 3-way amplifier into one case :-) Bass+mid+tweet
The Magtech amplifier look very beautiful. It's very powerful, BW = DC ~ 100KHz, SR = 500V/uS, 500W/8R... do you have schematic/service manual?
The orderly look of these fine amps are nice and thats one reason i´m dismantling it, to tidy up and wire it with, at least, some concern over interference issues, also replace caps as it getting on in years.
The interior actually looks just like my workshop, a bit like if i did the latest tidy up with a handgrenade.
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