47R * 50mA = 2.5V
It can be a bit more or a bit less, depending on your output MOSFETs' Vgs characteristics and bias level you want to achieve.
Juice
Thanks Jacco,
As it turned out the juice was not enough with the GR 170/74s.
I switched to a pair of matched BL at 7.16 idss and everything is biasing up just fine now on the first channel. Yet to try the second channel.
The power supply is around 16.5VDC +/- with one channel biased to .51 Amps each output, (the bias keeps climbing, I keep turning it down.)
And thank you Fab,
I needed to remove the driver board in order to figure this out and I set it up like the schematic you posted.
Everything was wired correctly, just a juice issue!
Thanks Jacco,
As it turned out the juice was not enough with the GR 170/74s.
I switched to a pair of matched BL at 7.16 idss and everything is biasing up just fine now on the first channel. Yet to try the second channel.
The power supply is around 16.5VDC +/- with one channel biased to .51 Amps each output, (the bias keeps climbing, I keep turning it down.)
And thank you Fab,
I needed to remove the driver board in order to figure this out and I set it up like the schematic you posted.
Everything was wired correctly, just a juice issue!
Well after some testing I had an oscillation problem, so I inserted a 470pF across the 150 ohm resistors. That took care of the problem.
I also have .040 vdc on the output of the right channel of my QB9 so I added input caps to both channels.
The bias is stable with the cover off, cover on it goes up and everything gets hot! I have the bias set at .5 amp per device with it warmed up, cover off, it sounds great! I may cut some larger cooling holes in the cover, although don't know if enough holes could be cut to make a difference.
Thank you Juma (and Nelson Pass) for a great sounding amp to utilize the chassis and output of the Hafler DH200.
I will post these findings in the DH200 thread also, as that is where this journey started.
Rush
I also have .040 vdc on the output of the right channel of my QB9 so I added input caps to both channels.
The bias is stable with the cover off, cover on it goes up and everything gets hot! I have the bias set at .5 amp per device with it warmed up, cover off, it sounds great! I may cut some larger cooling holes in the cover, although don't know if enough holes could be cut to make a difference.
Thank you Juma (and Nelson Pass) for a great sounding amp to utilize the chassis and output of the Hafler DH200.
I will post these findings in the DH200 thread also, as that is where this journey started.
Rush
Hi Rush,
I'm glad you made it work
A couple of things to consider:
1. At +/-19V DC PS voltage it doesn't make sense to have such a high bias (0.5A per output MOSFET = 2A per channel) unless you use 4 Ohm speakers. For 8 Ohm speakers you'll stay in class A through whole power range (Vout_max=16V_peak) with 1A bias (250mA per output MOSFET).
2. Make sure to drill enough holes in the amp's bottom and cover plate. Good air circulation helps a lot.
Enjoy your amp
I'm glad you made it work
A couple of things to consider:
1. At +/-19V DC PS voltage it doesn't make sense to have such a high bias (0.5A per output MOSFET = 2A per channel) unless you use 4 Ohm speakers. For 8 Ohm speakers you'll stay in class A through whole power range (Vout_max=16V_peak) with 1A bias (250mA per output MOSFET).
2. Make sure to drill enough holes in the amp's bottom and cover plate. Good air circulation helps a lot.
Enjoy your amp
Hi Rush,
A couple of things to consider:
1. At +/-19V DC PS voltage it doesn't make sense to have such a high bias (0.5A per output MOSFET = 2A per channel) unless you use 4 Ohm speakers. For 8 Ohm speakers you'll stay in class A through whole power range (Vout_max=16V_peak) with 1A bias (250mA per output MOSFET)./QUOTE]
But it sounds so good at 2 amps! Who needs a cover?
how high were your standards in 1982 ?
same as today, just couldn't quite make it
basicly because of speaker Xover issues
Xovers were the major problem
ofcourse anything else was blamed
Prettiest and best sounding ones i've heard this year are the AMT140 monaurals from France, Class A up to 3.5 amps out, rather expensive though.
Best bang for the buck are the XA100.5, haven't heard the 160.5 yet.
(valve power amps are out, anything over 75W will do, don't need no power stations. but i fancy balanced, narrows it down a bit)
Best bang for the buck are the XA100.5, haven't heard the 160.5 yet.
(valve power amps are out, anything over 75W will do, don't need no power stations. but i fancy balanced, narrows it down a bit)