F5 Turbo Builders Thread

Just checking in another F 5 mono bloc build, Hello will post some pictures when I can, just got what I thought was all the parts together, however after looking at one of My chasis I ordered it has not had the UMS milling done, odd I ordered an option for both boxes and only one box was UMS milled, and DIY store hasn't given a reply back to Me, so kind of dead in the water, also the order took over a month to get to Me, so sooner or later, looks more like later I'll post some pics....
 
I have finished my F5T v2 build with a regulated PSU, but have a question regarding DC offset drift...

As I set the offset to -70mV in the 5U Deluxe chassis with open casing and close it, the DC offset rises to +70mV as it gets hotter inside the casing. The NTCs are touching the Transistors through a washer like in the pictures from 6L6 member. Bias over 0,5 Ohm resistors is 300mV at 0mV offset and goes down to approx. 270mV at +70mV offset.

Is this behavior normal? what is the offset span normally? How are your values?
The adjustment is quite difficult with such a behavior...

Thanks!
 
As I set the offset to -70mV in the 5U Deluxe chassis with open casing and close it, the DC offset rises to +70mV as it gets hotter inside the casing. The NTCs are touching the Transistors through a washer like in the pictures from 6L6 member. Bias over 0,5 Ohm resistors is 300mV at 0mV offset and goes down to approx. 270mV at +70mV offset.

Is this behavior normal? what is the offset span normally? How are your values?
The adjustment is quite difficult with such a behavior...

Thanks!

I would suggest this is not normal. You should be able to maintain
1.0mv or less 'steady state' offset.



This was after 4 hours warm up. 1/2 turn on each pot at a time until 300mv of bias and 1.0mv or less of offset.
 
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How much does your DC offset change when you lift up the lid?

3-7mv.

There is a 'nack' to over adjust the offset with the lid off so that when you put the lid on it comes back to 0mv. Takes a bit of time but its part of the joy..ha

I was surprised after about a month of daily use I checked the offset again on the monos. This was ok, but the bias on one amp was out. Originally I set bias to 300mv but now it was 225mv. I waited another 3 hours to let the amp warm up to heat soak......the bias was now 250mv!! I readjusted to 300mv. which was a very small movement on the pots. The other mono amp was OK.
 
upgrade resistor

Hi
I would like to upgrade the resistances of my f5 turbo to give a greater point of warmth to the amplifier. I want to change to TX2575 'naked' foil audio resistor which have given me very good results in other projects.

Due to the price I will not change them all, but those that are more determinant or are in the passage of the audio signal. At first I thought of changing:

R13, R14, R15, R16, R5, R6

Do I have enough or lack any resistance to make it worth the change?

Thank you!

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.
 
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.
 
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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.

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 ...
 
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.
 
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?
 
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.
 
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?

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.
 
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