Got this amp thinking it’s one of Hitachi’s well-regarded MOSFETs power amps. Upon opening her up it was clear amp is not a MOSFET: the output stage is a quasi-complementary STK 4044V chip with two Darlington pairs but without any FETs.
This doesn’t mean it should sound bad; quasi-complementary discrete output stages were used in Marantz 22xx and some other amps of this era such as Sansui 5000 that sounded excellent.
The STK Darlington chips are high gain so it’s not surprisingly the amp at my bench test showed 153 WPC before clipping, both channels driven. The transformer in this thing is really huge, apparently capable of supplying enough juice.
The Hitachi was hooked to Nakamichi CA-7A preamp and Vandersteen 2CE sign speakers and it didn’t sound that bad; rather, it has a sonic signature of its own. Specifically, the lower midrange appears to be elevated, leading to slightly “boomy” bass. The soundstage is good but not as wide as my main power amp, the HK Cyt 12 MOSFET-modded 65 WPC. The upper midrange and high frequencies are there but again not as clear as Cyt 12. In this regard, sound is more “tubelike” that the MOSFET amp but lacking the “liquid midrange” tube amps have. Thus, the HMA-120 AV sound reminds me that of the unrestored Marantz 22XX and tube amps i.e. those with old caps and out of spec resistors.
So I decided to improve the Hitachi first by recapping this babe, replacing wires going from speaker switch to terminals and replacing terminals themselves to high-quality 5-ways. Another thing I will try is to convert the amp to DC-coupled by removing input electrolytic cap (C701, 4.7 uF/50V) and another one in the negative feedback loop which I believe is C703 (100uF/6.3V) These mods are described here: http://www.diyaudio.com/forums/chip-amps/3500-stk-chips-2.html; look for posts of rljones esp. #5 and #20.
What do you guys think? Any advices on the subject or on STK amps in general are welcomed.
This doesn’t mean it should sound bad; quasi-complementary discrete output stages were used in Marantz 22xx and some other amps of this era such as Sansui 5000 that sounded excellent.
The STK Darlington chips are high gain so it’s not surprisingly the amp at my bench test showed 153 WPC before clipping, both channels driven. The transformer in this thing is really huge, apparently capable of supplying enough juice.
The Hitachi was hooked to Nakamichi CA-7A preamp and Vandersteen 2CE sign speakers and it didn’t sound that bad; rather, it has a sonic signature of its own. Specifically, the lower midrange appears to be elevated, leading to slightly “boomy” bass. The soundstage is good but not as wide as my main power amp, the HK Cyt 12 MOSFET-modded 65 WPC. The upper midrange and high frequencies are there but again not as clear as Cyt 12. In this regard, sound is more “tubelike” that the MOSFET amp but lacking the “liquid midrange” tube amps have. Thus, the HMA-120 AV sound reminds me that of the unrestored Marantz 22XX and tube amps i.e. those with old caps and out of spec resistors.
So I decided to improve the Hitachi first by recapping this babe, replacing wires going from speaker switch to terminals and replacing terminals themselves to high-quality 5-ways. Another thing I will try is to convert the amp to DC-coupled by removing input electrolytic cap (C701, 4.7 uF/50V) and another one in the negative feedback loop which I believe is C703 (100uF/6.3V) These mods are described here: http://www.diyaudio.com/forums/chip-amps/3500-stk-chips-2.html; look for posts of rljones esp. #5 and #20.
What do you guys think? Any advices on the subject or on STK amps in general are welcomed.
Attachments
Last edited:
