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Old 2nd February 2009, 11:37 AM   #2641
tms0425 is offline tms0425  United States
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Default Re: Re: Finished my F5

Quote:
Originally posted by dhole
Nice work on the amp.What is ther mesh made from on the top of your amp ? Looks like a good option for ventilation and economy.
dave


I just bought a .080" perforated steel plate ($8 for 12x14" piece) from a local Metal Supermarket store. I used it on the F4's and F5 and just a few holes or gaps in the base promote nice convection. I leave a gap between the face plate and base to let air in and it's a nice safe spot for openings in the case. It isn't really stiff enough to set anything on top of, but I'd never do that anyway for these amps so it's not a problem.

Jose',

I haven't had enough listening to compare to F4's yet, but my first reaction is the F5 has amazing transparency to small details yet doesn't beat you over the head with them. Not so much a toobey sort of sound, so for some a little syrup (peanut butter?) might be needed to feel better. Overall a very nice amp for such a small investment

Tom
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Old 2nd February 2009, 11:56 AM   #2642
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Well guys, my F5 still ain't right. I thought I had it fairly ok, was reading .5V across R11, had 25mV on the outputs, but I connected a set of inputs (just plugged them in) and the bias went down (had the meter still connected). Then I hooked up a speaker (old) and theres a high pitch whine and a good level of hiss through it.

I wonder is that oscillation? I do have the mosfets mounting on wires, but I have a 47r right on pin1 as advised back up the thread.

Fran
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Old 2nd February 2009, 12:56 PM   #2643
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Quote:
I haven't had enough listening to compare to F4's yet, but my first reaction is the F5 has amazing transparency to small details yet doesn't beat you over the head with them. Not so much a toobey sort of sound, so for some a little syrup (peanut butter?) might be needed to feel better. Overall a very nice amp for such a small investment
Ok thanks a lot Tom. I´m waiting for the F5's parts.
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Old 2nd February 2009, 01:02 PM   #2644
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by woodturner-fran
Well guys, my F5 still ain't right. I thought I had it fairly ok, was reading .5V across R11, had 25mV on the outputs, but I connected a set of inputs (just plugged them in) and the bias went down (had the meter still connected). Then I hooked up a speaker (old) and theres a high pitch whine and a good level of hiss through it.

I wonder is that oscillation? I do have the mosfets mounting on wires, but I have a 47r right on pin1 as advised back up the thread.

Fran

is it source - connected to F5 - offset free ?

did you set offset with shorted inputs ( that's must ) ?
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Old 2nd February 2009, 01:12 PM   #2645
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Thanks Zen,


no, I did not short the inputs. Will do that and try. No source was actually connected, all I did was plug in the interconnects, other end not actually connected at all.


So just to rehash.... both pots at zero. I turn up P1 until I see say, 50mV across R11. Then adjust P2 to bring offset at speaker terminals back to zero. Then bring up P1 again, allow to settle, adjust P2 again.

I do have that OK, don't I? So if I went for say, 50mV, then 150mV and so on that would be ok wouldn't it?


Fran
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Old 2nd February 2009, 01:22 PM   #2646
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by woodturner-fran
Thanks Zen,


no, I did not short the inputs. Will do that and try. No source was actually connected, all I did was plug in the interconnects, other end not actually connected at all.


So just to rehash.... both pots at zero. I turn up P1 until I see say, 50mV across R11. Then adjust P2 to bring offset at speaker terminals back to zero. Then bring up P1 again, allow to settle, adjust P2 again.

I do have that OK, don't I? So if I went for say, 50mV, then 150mV and so on that would be ok wouldn't it?


Fran

Fran - it's completelly irrelevant with which pot you'll start - both have same function - biasing one side ;
when both sides are biased ( roughly in ballpark of wanted Iq ) same , then you'll have zero offset - as result of equal currents through halves .

anyway - seems that you didn't do your homework .....

name of the file on Papa's First Watt is "F5-om_sm-080527.pdf "

partial text is :



Quote:
Initial Adjustment
Before applying power to the amplifi er, you will want to set the values of P1 and P2 to their minimum. Verify this with an
ohmmeter. When it comes times to “fi re” up the amp fi rst time, if you have a Variac, use it, fusing the AC line to the amplifi er
with a 1A fast blow fuse. Turn the Variac up slowly, and if you haven’t popped the fuse, then go ahead and confi rm the rail
voltages to the channels.
Each channel does not need to be attached to a load in order to adjust it. If the only load you have is the loudspeaker, I
would advise against using it during adjustment. For each channel you will be adjusting P1 and P2 alternately in order to
achieve 0 volts DC at the output and .59 volts across R11 and R12. Each time you adjust P1 you will probably have to go
back and adjust P2 again, and so I recommend adjusting the pots in half-measures, alternately setting the pots half-way to
their voltage goals and measuring the DC values. Unless there is something very wrong, when the output is at 0 V DC, the
values across R11 and R12 will be equal.
In spite of the thermal compensation in the circuit, you should assume that there will be drift as the heat sink temperature
rises, and you will need to readjust the values over the course of an hour or two. Usually it is best to start out bias
adjustment low, at maybe 0.4 mV across R11 and R12 until the amp is warmed up a bit.
You should be able to get the output DC offset down to 10 mV or so, and I would consider 50 mV the highest acceptable
fi gure for this amplifi er when warmed up. After the amplifi er has been operated for a few weeks, it is a good idea to check
and adjust the offset again after the parts have been burned in.
btw - having connected open cable on any gain stage is begging for trouble , especially when stage is "suspectible" ..... ie. with high input impedance ....
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Old 2nd February 2009, 01:51 PM   #2647
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I know that text well.... have it right beside me at the bench.

Didn't know about the shorted inputs.. then this is really the first SS amp I've built - I've only assembled a mini aleph before that someone else had soldered together. Tube work and phonostages though I have done.

I'm normally better than this....honest! I thought it would be easier than this!

I do some more later and report back.

fran
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Old 2nd February 2009, 02:18 PM   #2648
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I did all my adjustements with nothing connected to the amp and inputs not shorted, in my setup it didn't make any difference.

I started with trimmers set halfway, that produced approx 1V drop on source resistors.

If you install trimmers according to pcb footprint, turning screws clockwise will reduce the bias.

Initially, I had 3 voltmeters conected to all 3 test points, but later only two: one on either source resistor the other on the output.

Set the offset to zero first by using either of the trimmers. After that, turn both trimmers same way, either clockwise or counterclockwise to lower the bias or to increase it.
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Old 2nd February 2009, 02:25 PM   #2649
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Ok, my experience so far is that both are at zero... you need to bring both up a few turns to get any reading. I think what I'll do is start all the way clockwise (ie zero) and then turn both back up say 5 turns. I think the bourns pots are 20 turn AFAIK. See what I get and report back.

Fran
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Old 2nd February 2009, 02:57 PM   #2650
ichiban is offline ichiban  United States
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Default F5 oscillation

Quote:
Originally posted by woodturner-fran
Then I hooked up a speaker (old) and theres a high pitch whine and a good level of hiss through it.

I wonder is that oscillation? I do have the mosfets mounting on wires, but I have a 47r right on pin1 as advised back up the thread.
Fran
I think it's in Nelsons' article that he states, input wires should not be
near the outputs or feedback will cause oscillation. Make sure internal wiring conforms to that and short the inputs to bias.

edit:::
A caveat is in order here – this is a very wide band amplifi er with a high input impedance. In order to prevent the output
voltage from bleeding back to the input at very high frequencies (thus making a fi ne power oscillator), keep the input and
output cables separate, and don’t externally connect the speaker ground to the input ground. Good ground shielding on the
input cables is important, and caution is called for in using Litz and other specially low inductance / high capacitance cables.
I have not seen a specifi c example of a problem, but historically it is to be expected when an amplifi er’s bandwidth exceeds
200 KHz. If the amp makes funny noises, runs extra hot, or blows fuses, this might be an indicator of such an issue.:::
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