F5 power amplifier

[sangram]

@jacco - reason is, if I increase the bias, I may not get additional power output if the amplifier has already reached its voltage swing limits. However if there is some swing to spare, I could probably get a little more output. I have 8 ohm speakers that are a gentle load, but very very low efficiency. Let's just say each watt counts

[EUVL]

Sorry Jacco, the ones we bought a year or more ago in our group buy were fqA19n20/fqA12p20, which are the ones I was asking for.

Groeten,
Patrick

[EUVL]

Someone asked me about an unbalanced F5 with Toshiba MOSFETs.

Well, I have not built one, and I am honoured that you would ask me instead of asking Nelson.
But if I were to build one (unbalanced), I would do the following :

1. Stick to 0R47 source resistors
2. Keep the drain resistors of the JFETs the same as the original circuit.
3. Reduce the JFET bias (by using an Idss of say 7mA) to account for the lower Vgs of the Toshibas (around 2.xxV for 1.3A).
4. If you want to play the gain balancing game, then use 5 ohm more on the source of the 2SJ74s, and scale UP the feedback resistor for the negative rail half according (so that closed loop gain for top and bottom halves are the same).


Hope it's clear,
Patrick

[tinitus]

F5 is specifically designed to drive relatively efficient 8ohm speakers, but takes on any kind of load, thus 2ohm stable

Honestly, I will only use my F5 during the cold winter time
Wouldnt dream of using it on hot summer days
Like the tube guys with heavy 845 PP amps
They usually have what they call "summer amps"

Here we have a season fore just about everything

[labjr]

Does anyone know where to get the Fairchild parts. I noticed Tech-diy supplies FQA19N20C instead of the FQA19N20. Is there a difference? Thanks

[a.wayne]

Originally Posted by Nelson Pass

I don't have any evidence that midrange magic is a
function of matching. Perfect matching reduces 2nd
harmonic. Those SE tube designs with the coveted
midrange are supposed to have more than their share of
2nd harmonic.

Originally Posted by AndrewT

the whole amplifier will run 15Cdegrees hotter when your day time temperatures are @ 38degC than when it is set up for daytime temperatures of 23degC.

If the Ts=50degC at set up then it will run @ 65degC when Ta=38degC.
From there you will find that Tc and Tjmax will both be 15Cdegrees hotter as well.

The thermistors will trim back the hot running current slightly, that is why they are there, but they cannot temperature compensate for big changes in Ta.

At what temp will the thermistors start to reduce current ? Is it possible just shut the amplifier PS down instead of any current restrictions ?

[juma]

Originally Posted by a.wayne

...At what temp will the thermistors start to reduce current ?

They do that all the time - whenever the ambient temperature changes, they change their resistance, when rasistance falls the Vgs of MOSFETs gets lower and Id through MOSFETs decreases. Google NTC thermistors - some thermistors are PTC but we don't want them here

Originally Posted by a.wayne

...Is it possible just shut the amplifier PS down instead of any current restrictions ?

Yes it is (with thermo-switch), but why would you want to do that? F5 has built-in over-current protection.

[a.wayne]

It just appears to me that the thermistors would have an effect on the sound quality , if so can they be left out ?

[MEGA-amp]

Yes, you can leave both current protection and temp-comp out. Without temp-comp it takes longer to properly adjust. Some say that without the extra "stuff", that it sounds better, I wouldn't know, I built it this way and it sounded great.

[labjr]

Originally Posted by Nelson Pass

I don't have any evidence that midrange magic is a
function of matching. Perfect matching reduces 2nd
harmonic. Those SE tube designs with the coveted
midrange are supposed to have more than their share of
2nd harmonic.

Hi Nelson,

Is there any disadvantage to using multiple sets of outputs?

Thanks