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Old 19th June 2017, 09:18 AM   #4751
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F5 Turbo Builders Thread
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Old 22nd June 2017, 09:40 PM   #4752
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I've just got my mono block F5t amps wired up and am in the testing phase. I am unable to get past about 230mv of bias using some store bought 74/170. They are the b grade but are at the extreme low end of the range.
I am measuring just less than 6ma of current and am unable to get enough voltage across the 1k/5kpot combination to get past 230mv. The bias is slightly over compensated thermally by the thermistors. It actually goes down a bit as the amp warms up. This may be because I am at the high end of the adjustment range on the 5k pot. which makes the voltage across the 1k more sensitive to changes in the thermistor value.
I am thinking of changing out the 1k for 1.2k. Any other suggestions are welcome.
I am also getting offset drifts of maybe 40mv from cold to hot which seems to be fairly common. This is much trickier to adjust than my Aleph J which locked onto it's bias setting and maintains a low offset as it warms up.

Last edited by Bfpca; 22nd June 2017 at 09:43 PM.
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Old 22nd June 2017, 10:26 PM   #4753
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Originally Posted by Bfpca View Post
......
I am thinking of changing out the 1k for 1.2k. ......
good for start
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Old 22nd June 2017, 11:01 PM   #4754
laverda is offline laverda  United Kingdom
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Originally Posted by Bfpca View Post
I've just got my mono block F5t amps wired up and am in the testing phase. I am unable to get past about 230mv of bias using some store bought 74/170. They are the b grade but are at the extreme low end of the range.
I am measuring just less than 6ma of current and am unable to get enough voltage across the 1k/5kpot combination to get past 230mv. The bias is slightly over compensated thermally by the thermistors. It actually goes down a bit as the amp warms up. This may be because I am at the high end of the adjustment range on the 5k pot. which makes the voltage across the 1k more sensitive to changes in the thermistor value.
I am thinking of changing out the 1k for 1.2k. Any other suggestions are welcome.
I am also getting offset drifts of maybe 40mv from cold to hot which seems to be fairly common. This is much trickier to adjust than my Aleph J which locked onto it's bias setting and maintains a low offset as it warms up.
I had the same and ended up with 2.2k/5k pot as that was all I had knocking around which worked well for me. About 10 turns on the 20 turns pot for 250mv bias.

Offset, you should be able to get within just a few mv, even 0mv for a while. Mine runs less than 5mv after about 30-45mins. I ignore the cold mv reading ...
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Old 22nd June 2017, 11:37 PM   #4755
Bfpca is offline Bfpca  Canada
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After I get the bias up to my intended target of 250mv -(2 amps total) I'll start to adjust for low offset. My chassis are not that large, being 150mm high and 400 deep, but they have 10mm thick sinks with a total depth of 50mm versus 8mm and 40 total for the store chassis. At 225mv -.45amps/fet with a 48v supply the hottest temp I can find with an IR thermometer is 110f. The sinks and transistors are not what I would subjectively call hot after 45 minutes but just warm. I'm thinking .5amps/fet is doable , maybe a little more. I am not using the diodes, I just don't think they are worth the trouble for the loads I am going to drive. With 4 sets of outputs/channel the diodes don't get you much extra output with 4-8ohm speakers.
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Old 23rd June 2017, 04:14 AM   #4756
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Originally Posted by Bfpca View Post
I am also getting offset drifts of maybe 40mv from cold to hot which seems to be fairly common. This is much trickier to adjust than my Aleph J which locked onto it's bias setting and maintains a low offset as it warms up.
I get an offset drift of about 400mV from cold to hot. So I still quite don't understand from where it is coming....
Maybe i should change R11 / R12 resistors to some other value... But in which direction to go, more or less?
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Old 23rd June 2017, 08:54 AM   #4757
AndrewT is offline AndrewT  Scotland
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Originally Posted by Bfpca View Post
After I get the bias up to my intended target of 250mv -(2 amps total) I'll start to adjust for low offset. My chassis are not that large, being 150mm high and 400 deep, but they have 10mm thick sinks with a total depth of 50mm versus 8mm and 40 total for the store chassis. At 225mv -.45amps/fet with a 48v supply the hottest temp I can find with an IR thermometer is 110f. The sinks and transistors are not what I would subjectively call hot after 45 minutes but just warm. I'm thinking .5amps/fet is doable , maybe a little more. I am not using the diodes, I just don't think they are worth the trouble for the loads I am going to drive. With 4 sets of outputs/channel the diodes don't get you much extra output with 4-8ohm speakers.
100degreeF (43degreesC) is quite cool for a ClassA amplifier.
That leaves much leaway to go higher.

The bias voltage required to turn on the outputs is the Vgs of the two outputs.
That bias voltage comes from the Drain Loads on the input stage.
The Drain Loads are a parallel combination of resistors and variable resistors.
The current through those Drain Loads is less than Idss of the jFETs.
If you use 7mA jFETs and they pass 6mA in the circuit, then the Bias voltage will be 0.006A * Drain Load resistance in ohms.
i.e. a Drain Load of 500r will allow a bias voltage of 3Vbias.
If that is too low to turn on the output, then change the drain load, or change the jFETs.
Going from 1000r to 1200r will increase the available bias voltage. Try it, but I think you will need to go higher. Maybe 1300r, or even 1500r.
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Old 23rd June 2017, 10:49 AM   #4758
Bfpca is offline Bfpca  Canada
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Originally Posted by daniii88 View Post
I get an offset drift of about 400mV from cold to hot. So I still quite don't understand from where it is coming....
Maybe i should change R11 / R12 resistors to some other value... But in which direction to go, more or less?
Quote:
Originally Posted by AndrewT View Post
100degreeF (43degreesC) is quite cool for a ClassA amplifier.
That leaves much leaway to go higher.

The bias voltage required to turn on the outputs is the Vgs of the two outputs.
That bias voltage comes from the Drain Loads on the input stage.
The Drain Loads are a parallel combination of resistors and variable resistors.
The current through those Drain Loads is less than Idss of the jFETs.
If you use 7mA jFETs and they pass 6mA in the circuit, then the Bias voltage will be 0.006A * Drain Load resistance in ohms.
i.e. a Drain Load of 500r will allow a bias voltage of 3Vbias.
If that is too low to turn on the output, then change the drain load, or change the jFETs.
Going from 1000r to 1200r will increase the available bias voltage. Try it, but I think you will need to go higher. Maybe 1300r, or even 1500r.
Andrew - I was going to go to 1200 because I had 4 of them in stock but I found I still had 4 1500r left over from my Aleph J build so I'll swap them in today and re adjust bias.

Danii88- that's a lot of bias drift. Thinking about this drift I wonder if some of it is coming from the thermistor temperature compensation.
I made sure I had consistent mounting orientation and contact with the MOSFET on all the thermistors. Check yours for alignment. It's easy for them to get bumped and bent out of place. The VGS of the N and P sides of the output stage are offset a bit. I am wondering if this causes the temp compensation to be offset as well and that as the amp warms up there is a drift in offset due to this. I haven't reasoned this through yet but it seems like a possible source of offset drift. If so we may be able to reduce the drift by adjusting the series resistor in the temp compensation circuit.

Last edited by Bfpca; 23rd June 2017 at 10:56 AM.
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Old 24th June 2017, 12:27 AM   #4759
Bfpca is offline Bfpca  Canada
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F5 Turbo Builders Thread
Changed r5/6 to 1.5k and I can now bias at 250mv.
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Old 24th June 2017, 08:00 AM   #4760
AndrewT is offline AndrewT  Scotland
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Originally Posted by Bfpca View Post
Changed r5/6 to 1.5k and I can now bias at 250mv.
is it able to go to 260mVbias? or to 270mVbias? or to 280mVbias?
I would turn it up as high as 300mVbias for a minute or so, just to see how far it will go. The sinks will take a while to catch up.
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