Yes it's the Hiraga 20w class A. There are no adjustable components on the circuit boards.
Yes it's the Hiraga 20w class A. There are no adjustable components on the circuit boards.
Technics SP10 mk2
Jan Allaerts MC 1 Boron mk1 cart
Miyajima Shilabe cart
Hashimoto HM-X SUT
Siggwan (gimballed not unipivot) Cocobola 12"
Aurorasound Vida LCR Phonostage
The Truth linestage
Dave Slagle Autoformer Volume Controller
Cary 805c SET amps
Audio Note ANe-SPX speakers
Townshend Isolda speaker cables
Cardas Golden Presence interconnects
Location: Seaford UK
Posts: 1,861
I'm Dennis.
Connecting the speakers will make very little difference to the measured voltage because power amps are designed to be as near a zero O/P impedance as possible.
I empathise with your concern because 15 years ago I bought an ATC SIA150, which had about 180mV DC on its O/Ps also, but the calculation gives a very low DC dissipation wattage, of (0.18 X 0.18)/8, about 4mW.
I doubt that ATC would mess up their reputation with a faulty design. (Though usually, say with Quad, it is a couple of mV.)
Right now I have the speakers disconnected and am reluctant to hook them back up grrrrr
Technics SP10 mk2
Jan Allaerts MC 1 Boron mk1 cart
Miyajima Shilabe cart
Hashimoto HM-X SUT
Siggwan (gimballed not unipivot) Cocobola 12"
Aurorasound Vida LCR Phonostage
The Truth linestage
Dave Slagle Autoformer Volume Controller
Cary 805c SET amps
Audio Note ANe-SPX speakers
Townshend Isolda speaker cables
Cardas Golden Presence interconnects
Location: Seaford UK
Posts: 1,861
I'm Dennis.
I think you will be OK for the reasons above, and 4mW is tiny compared with the programme material, the equivalent of running a 90dB/W sensitivity speaker producing about 66dB spl of programme.
If worried, connect a couple of 10,000uF electrolytic caps back to back (i.e their polarities in opposite directions) in series with the speaker. This will remove almost all DC apart from a tiny leakage current inherent to the caps (in the context of a loudspeaker, for all intents and purposes no current).
If you get capacitors rated to the rail voltage of the amp then you will also get some level of protection against the amp going DC (this isn't 100% bulletproof protection in the long term but can't hurt)
But in reality this level of DC (in the non fault condition) will dissipate a tiny amount of power in the voice coil, actually listening to music will produce more.
Last edited by Radford Revival; 26-10-2017 at 11:07.
Radford Revival
www.radfordrevival.co.uk
As far as I'm aware with a conventional-ish solid state design this shouldn't cause any issues. Lots of single rail amps use a large electrolytic on their output as part of the design, and in many cases they are even inside the the amp's feedback loop. The capacitor will have a small ESR (10s of milliOhms) but not enough to worry about.
Weirder topologies like circlotron amplifiers probably woudn't work too well but they're pretty uncommon. They depend on a DC path through the loudspeaker to bias their output devices, and the opposing bias current from each half cancels out.
Radford Revival
www.radfordrevival.co.uk
Update - I realised that there is an adjustable pot on each circuit board so tried adjusting them - short lived success - got them both down to 45mv even after 15 minutes warming. Switched off and switched on again - now 170, 110. Adjusted again, meter reading dipping into - 25 so too much and adjusted again to around 60mv. Same thing after switching off and back on again around 180, 210 - how can they drift (both) so much and so quickly. They were measuring 300mv before I started messing!
Technics SP10 mk2
Jan Allaerts MC 1 Boron mk1 cart
Miyajima Shilabe cart
Hashimoto HM-X SUT
Siggwan (gimballed not unipivot) Cocobola 12"
Aurorasound Vida LCR Phonostage
The Truth linestage
Dave Slagle Autoformer Volume Controller
Cary 805c SET amps
Audio Note ANe-SPX speakers
Townshend Isolda speaker cables
Cardas Golden Presence interconnects
It sounds like it isn't the best thermal design.
The transistor characteristics will change from cold to operating temperature.
It is perfectly OK to have a small negative offset, it doesn't have to be positive, just small in actual value.
Probably better to adjust to say -25mV at operating temp, you will probably find that it will be less than 100mV (positive) when cold.
Thanks. I'm really unhappy and gutted that I've touched it. I think the pots are a bit flaky but then they're over 30 years old! If the readings would stay consistent (both when warm and cold) between switch on's I would be happier. I'm not a techie and should have known it wouldn't be so simple as using a multimeter and adjusting a bloody plot.
Technics SP10 mk2
Jan Allaerts MC 1 Boron mk1 cart
Miyajima Shilabe cart
Hashimoto HM-X SUT
Siggwan (gimballed not unipivot) Cocobola 12"
Aurorasound Vida LCR Phonostage
The Truth linestage
Dave Slagle Autoformer Volume Controller
Cary 805c SET amps
Audio Note ANe-SPX speakers
Townshend Isolda speaker cables
Cardas Golden Presence interconnects