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.

Otala1.jpg



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.

Fabulous buildquality.

Otala2.jpg


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
avoid interference.

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.
 
Disabled Account
Joined 2006
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.

Otala1.jpg



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.

Fabulous buildquality.

Otala2.jpg


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
avoid interference.

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.

Would be great if you would reverse engineer, do go ahead. If this amp is related to the xx I might be able to help as I have some parts of schematics for it. The design is indeed very interesting, many claim this was and will still hold as one of the best audio amps ever, as scarce as hen teeth. I lack the outputstage circuit and maybe some minor details regarding the inputstage.
 
Power amps are something very close to my music heart.

Fabulous buildquality.


Hi Ingvar,

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
 

Attachments

  • Krell1.jpg
    Krell1.jpg
    60.5 KB · Views: 308
  • Krell2.jpg
    Krell2.jpg
    50.1 KB · Views: 219
  • Threshold.jpg
    Threshold.jpg
    67 KB · Views: 230
  • Magtech_inside.gif
    Magtech_inside.gif
    64.9 KB · Views: 209
Last edited:
Hope the pictures add to your thread discussion.

Everything, allmost, adds to my threads.
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.