F5Turbo Illustrated Build Guide

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Zen Mod

Zen Mod

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Joined 2003
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if palm rule permits , why not
 

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Kahuna123

Member
Joined 2016
wow. I am running around .52 V. First time I ever heard my Bamberg's really play. New soundstage. I do have a slight 60 hz going on. I can probably correct that tomorrow. Heard some bass on dark side of the moon I never heard before. Much more punch. You can tell this amp although 50W has a lot of headroom. Now, how to adjust P3??
 
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Blitz

Member
Joined 2002
I am in the process now to re-bias the whole thing after now having installed the bridge into the earth/ground.

What I did in the past:
- shorten the input to ground
- Measured the bias over the source resistor of one NPC-resistor
- Measured the DC-offset on the loudspeaker output.

And than play the p1-p2-game.

Is that correct ?

I am asking as I now read: "Place voltmeters across TP of output board, one on N-channel, one on P channel."

I am not sure what to do with this as both are never equal in bias, but both go up or down proportionally. I the target to have the same bias on both, n and p or what shall be measured ?
 
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Blitz

Member
Joined 2002
OK, here is my update:

I have include a large choke into the PSU, but still have 33.5V on the rails and no cascode (yet). The bridge with the npc is included now between earth and ground. I have dialed the P1 and P2 back to zero and startedthe biasing with shorted input.

The result is abolutely the same. I can dial the amp nicely into few mV DC with grounded input, but when I open Input, I have a DC of 2.6V.

I tried to bias the amp with open input as well: First DC of 170mV, a few turns and it goes fast toward 4-5V DC and can be dialed back max to 2.6V as well.

Ok, I give up and will now implement the cascode next. seems to be my last chance.
 
alexkosha

alexkosha

Member
Joined 2013
I'm not a guru, but it took me 5 min to adjust each of my monoblocks. Have you followed procedure found in the fist post with lightbulb and all following steps? Bunch of people here can help you here with that. What chokes did you use on PSU? Do they have enough current capability? Does your P3 is in mid?


Sent from my iPhone using Tapatalk
 
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Sangram

Sangram

Moderator
Joined 2002
Paid Member
Blitz said:
So, the voltage across your n channel source resistors and your p channel is exactly the same as you balance the, with p3 ?
Click to expand...

The chase to get exactly the same current through N and P is overambitious and its importance overstated. There's lot of hand-wringing when you see a few mV of difference between N and P readings taken across source resistors. a 5mV difference across a 0.5 ohm resistor translates to a difference of 2.5mA. It's too insignificant a number and merits insignificant amount of discussion. However, since you spent a few posts asking -

For a start, you can expect differences in measurement that are proportional to the differences between source resistor values and the MOSFETs, when measuring a single device in an array.

Second, the bias (and offset) varies with temperature and drifts with time and ambient conditions, and is different for each device based on the state of the die (which you cannot monitor or control). The thermistors also have an effect, and this means that the bias reading is illustrative.

You could measure across the supply resistors if you want an overall reading, but this includes the input stage currents. Thankfully these are constant and quite predictable at idle, so you can add it to the target bias figure to work out the number you need to chase. Since you have no resistors in the supply, this is not possible.

Finally, the amp behaves a little differently between cold and hot states. You could set it once, but a small change in environmental conditions imposes a proportional change in the operating point. You should not worry about small differences. A variation of 25mA is 1.5% of overall bias, assuming 2A bias. It is around 1.5%, and is not worth worrying about. Most of the time the differences are much smaller.

Finally, the best THD figures are arrived when P3 is balanced using a distortion meter. In such case the readings across source resistors means squat.

You need to watch the offset and get the bias as close as possible between the 2 channels. Unlike Class AB amps, the operating stability of the amp and its characteristics remain similar over fractional changes in bias and balance. After that, you should put it on a distortion test, trim P3 and readjust for minimal offset. The effort should be to get as good matching between left and right in terms of H2/H3 balance, and good offset stability at thermal equilibrium.

Offset is the difference between top and bottom half. At 0.00mV offset, the current will be exactly equal but this is not possible to reach or maintain in practice. Fractional current differences exist (and show up in the supply resistors that connect the returns between the cap banks) between N and P sides at all times.

A well-adjusted F5/T will start at slightly higher bias till it warms up, and show some movement of offset as the two halves of the amp work 'against' each other, till temperature stability is obtained. Once there, it will keep moving but in fractions, that are pretty close to the resolution limit of most multimeters in the 200mV range setting.
 
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Manolo47

Member
Joined 2006
TH1 AND TH2

Studying the F5T documents I have a question about these. In the build guide these seem to appear with a heatsink and there are 4 of them.

And in the BOM, it says to install only one per board????

These do not look like anything in the google images being square and in google all seem circular???

Can someone clarify?

Thanks
 
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