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F5 Turbo Builders Thread
F5 Turbo Builders Thread
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Old 1st May 2018, 06:31 PM   #5151
Hikari1 is offline Hikari1  United States
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Quote:
Originally Posted by 6L6 View Post
180mV is 0.004W into the woofer coil. Not enough to worry about.

The capacitor(s) on the tweeter and mid will block DC on those voicecoils.
True. But the fact that the DC offset changes so dramatically when input is grounded suggests bad things happening. It's not the existing DC offset I am worried about...it's that the amp is unstable.
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Old 1st May 2018, 06:48 PM   #5152
pchw is offline pchw  United States
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F5 Turbo Builders Thread
What if you connect the LSK pre but not power it up. Do you still see the same 180mV offset on that channel? If so, this will suggest the cause is from this particular channel of the amp. In which case, perhaps it is the layout of the board of that channel, or it is the components (pots??) etc.

If you have a variac, you can also lower the voltage to 24VDC to see the offset stays.
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Old 1st May 2018, 07:21 PM   #5153
Hikari1 is offline Hikari1  United States
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Originally Posted by Zen Mod View Post
dissconnect audio gnd from case , both channels

check with megaohmmeter conductivity between mosfet cases to heastsink

you can also check is there same issue with DC offset , without safety gnd ....... which is possible route from mosfets to audio gnd , in case of leakage through thermal transfer arrangement
Will do. Right now there is more than 10R resistance between mosfets and case. But I will confirm all is good by disconnecting PSU ground to case.
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Old 1st May 2018, 08:19 PM   #5154
Erlend Sæterdal is offline Erlend Sæterdal  Denmark
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F5 Turbo Builders Thread
Quote:
Originally Posted by Hikari1 View Post
Strange problem with my F5 turbo...I have been trying to sort it out in a different thread but I thought some of you here might have some ideas. Here's the other thread:

F5 dc offset issue

Amplifier is 2 pair of Toshiba sj201/sk1530, 31V rails, No cascode. No diodes. 100R gate stoppers. Sounds great, no signs of trouble.

A) DC offset changes dramatically when input shorted, goes from 0mv to -180mv when input shorted. Different preamps show different DC offset even though preamps are cap coupled. Only one channel exhibits this behavior.

B) Suspected Jfet leakage. Replaced jfets on misbehaving channel with other channels jets. Issue still exists.

C) Tried small cap across feedback resistor. No change.

What should I check next? I am really baffled here. I am a bit afraid to use the amp.
Mount another amp in the enclosure.
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Old 1st May 2018, 10:13 PM   #5155
Hikari1 is offline Hikari1  United States
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Originally Posted by Erlend Sæterdal View Post
Mount another amp in the enclosure.
If you are willing to send me the amp I am willing to try it.
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Old 2nd May 2018, 04:03 AM   #5156
Hikari1 is offline Hikari1  United States
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Quote:
Originally Posted by Zen Mod View Post
dissconnect audio gnd from case , both channels

check with megaohmmeter conductivity between mosfet cases to heastsink

you can also check is there same issue with DC offset , without safety gnd ....... which is possible route from mosfets to audio gnd , in case of leakage through thermal transfer arrangement
Just checked mosfets resistance to chassis with psu disconnected. No continuity. Iam pretty sure the mosfets are isolated well.

Iam stumped. Any other ideas?
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Old 2nd May 2018, 06:24 AM   #5157
Zen Mod is online now Zen Mod  Serbia
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F5 Turbo Builders Thread
which value of caps you tried across feedback resistors ?

of course , you mount two caps ,not just one ?
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Old 2nd May 2018, 05:21 PM   #5158
Hikari1 is offline Hikari1  United States
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100pf. I only used one. I guess it would be better if both jfets had cap compensation? I will try that next. Thanks Zen Mod.
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Old 2nd May 2018, 05:24 PM   #5159
Zen Mod is online now Zen Mod  Serbia
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well, NFB is having two paths

either compensate none or both, to have valid conclusions

besides , 100pF is too low in value

calculate an d use something in area of 100-150KHz for that test
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Old 18th May 2018, 07:56 AM   #5160
dewdrop is offline dewdrop  Singapore
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Originally Posted by Zen Mod View Post
BD139/140 , on this side of Pond
Many to choose options from the BD139 family, do I need the power dissipation to be 1.25W or 12.5W?
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