F5 power amplifier

[Franz Gysi]

Have you 0.63A on both (240 and 9240)?, whats is your osffet voltage?.

Yes, I measure the same current on both devices.

The offset on the output is stable within a +-5mV range.

How precise are your resistors?

Do you use ONE DVM to measure both or do you use two DVMs?

Franz

[juma]

Originally Posted by Franz Gysi

... Any recommendations for different MOSFET's to reduce the needed voltage at the gate? ....

Toshiba's 2sk1529/2sj200.
They have much lower Vgs at Id=0.63A and they will certainly perform better with such a low Vds (your PS voltage is very low and standard HEXFETs' parasitic capacitacies are playing crazy at Vds=11.2V).
OTOH Toshiba MOSFETs have a bit lower Yfs which means a bit higher THD (that may not necesarilly be bad).
Anyhow, with such a low PS voltage I'd use k1529/j200.

[EUVL]

If you use 2SK1530/2SJ201, they have about the same Yfs as the IRFPs.
This is well proven, see my posts near the beginning.

I have used them at +/-12V rails without problems.
My balanced version uses +/-16V.
Again all published in this thread ages ago.

Only advantage of 2SK1529/2SJ201 is slightly lower capacitance, IMHO.


Patrick

[ramallo]

Originally Posted by Franz Gysi

Yes, I measure the same current on both devices.

The offset on the output is stable within a +-5mV range.

How precise are your resistors?

Do you use ONE DVM to measure both or do you use two DVMs?

Franz

Hello Franz,

The resistors are ok (I measured resistors before mount it), I have low bias on the 240's on both channels, maybe a not perfect matched 9240 vs 240.

I'm using 3x DVM (One at output for osffet)

Thanks

[Franz Gysi]

I try to resume the experience with the Mini F5 up to now:

After many different bias settings, I did not find a completely satisfying solution (the measuring equipment is not available at the moment).

The very first run with the Veravox 3S fullrange speakers, driven by the low voltage from a iPod, was the best sounding configuration.

Low volume level, due to the low effiecency, the low gain and the low input voltage. Idle current for this test was 1A.

+-11.5VDC seems defintely too low rail voltage to drive the IRFP9240/240 in a linear matter.

As I have some higher voltage trannies in my stock the next tries will probably be with higher voltage prior considering using different MOSFETs.

Ramallo:

Do you really think this is a major problem? It is within a few percent and could be caused by overall circuits tolerances.

Franz

[tinitus]

I thought dc offset was caused by the Jfets, and the ones to pair

[AndrewT]

there are only 4 active devices in the F5.
All are FETs.
Vgs of each FET can be anywhere in the voltage range specified by the manufacturer.
It's that enormous range of Vgs that results in seeing big differences in bias currents and even bigger differences in output offset.

Matching of half the devices will not improve the matching of the other half of the devices.

[Franz Gysi]

Todays configuration / settings of my four channel Mini F5:

- +/- 15.2VDC rail voltage
- 0.8 A bias (0.38V at R11/R12)



This trannies are very small but I have no other choice at the moment.

So I gave it a try and it works.

The trannies are 2x12V 80VA. The calculated DC load on the rectifier is now about 50VA, so I should not overload the trannies imho.

They get about 45 C after one hour.

The sound?

Some kind of better than before.

Give me time for my conclusion.

Franz

[AndrewT]

are you using a 12+12Vac 80VA transformer to power one or two channels of F5?

[Franz Gysi]

are you using a 12+12Vac 80VA transformer to power one or two channels of F5?

One transformer to drive two channels.

Hard on the limit, I know.

But I remember a story from Willi Studer: when he visited the laboratory, he touched the transformers.

Could he touch them more than five seconds, he judged the tranny to be overdimensioned.

I know, for Audio purposes, specially for power amplifiers, it is worth to overdimension trannies.

So, my trannies are experimental, I think. Maybe I switch to 15VAC trannies later (more VA...).

Soundwise, it seems that I am on the right track to increase the voltage for the Mini F5 version.

Before I can judge I have to complete the "full" F5 soon to drive the woofers (I will use a 500VA for the full version...).

My woofers are playing up to 860Hz, so it is not easy to judge right know the Mini F5, playing above 860Hz.

Franz