F5 power amplifier

[h_a]

Can I do the tuning at 10 W ?

Sure you can. The amp is then optimized for 10W instead for 1W. No big deal anyway.

Hannes

PS: I wouldn't go to higher frequencies, you only run into bandwidth problems.

 

[Nikon1975]

I think that after reading and re-reading here and in the balanced thread I have a clear idea of what direction to go to play a bit with the circuit.

I'm very happy with my amplifier, this is just to learn something.

It's clear that the goal is to have the two parts of the p-p as close as possible. What is NOT perfectly clear to me (if this is correct) is why it does not make sense to twick the first stage only and not the second one.

Namely, I am talking about the addition of the 5 ohm resistor at the source of the jfet.

Thanks,

Davide

 

[bobodioulasso]

The toshibas , when matched, have nice overlaping curves.
The trick is for obtaining the same result from the jfets

 

[Marra]

Help. I connected up my F5; one channel; following AndrewT's guide lines using a bulb tester. The neon lights up and I have +- 24.2V onthe power supply but - 0.5V on the output! Any pointers would be much appreciated.
Thanks Keith

 

[prooptiki]

Hi,
If you are planning on doing the 5ohm degeneration trick you need:
1) N-P matched mosfets for each channel.
2) 2SJ74 with an Idss about 1mA more than 2SK170
3) either add 5ohm resistor between the 2SJ74 source pin and the feedback node, or replace R with 15ohm and feedback of 75ohm.

All three steps are required.

For Toshiba mosfets gate resistor?

Thanks

 

[Zen Mod]

@ Marra
bias it properly

 

[Marra]

@ Marra
bias it properly

Thanks Zen. So if I bias it up as per Nelsons article the -0.5V on the output will zero itself. The amp currently has the trimpots at 0 ohms.

 

[Zen Mod]

if you can't find in thread short how to bias guide , I think that we are going to explain again

one DVM across any source resistor , one DVM from output to gnd

iteratively fiddling with two trimpots , you'll get both zero offset and bias

 

[Marra]

Thanks. I fully understand how to set the bias; just hadn't realised that with the trim pots at zero and no current running that there would be 0.5V on the speaker terminals.
I will connect up the second channel and bias things up this weekend.
Thanks again.

 

[AndrewT]

Help. I connected up my F5; one channel; following AndrewT's guide lines using a bulb tester. The neon lights up and I have +- 24.2V onthe power supply but - 0.5V on the output! Any pointers would be much appreciated.
Thanks Keith

if everything is OK and nothing is overheating and the fuses did not blow, then it's time to plug your amplifier direct to the mains.
Then follow the bias procedure.

 

[EUVL]

> What is NOT perfectly clear to me (if this is correct) is why it does not make sense to twick the first stage only and not the second one.

Because the 2SK1530 / 2SJ201 are perfectly complementary, and the 2SK170/ 2SJ74 are not.
So the latter need some help from the 5.1R degeneration resistor.

All explained before.


Patrick

 

[alazira]

For Toshiba mosfets gate resistor?

Thanks

Hi prooptiki,
The 5ohm trick is for the 2SJ74 jFet source, not the Toshiba mosfet gate.

 

[prooptiki]

Hi prooptiki,
The 5ohm trick is for the 2SJ74 jFet source, not the Toshiba mosfet gate.

I am talking for Toshibas mosfets resistor gate (47R or 100R?)
In addition the source resistor can be dicreased to 0.22R without any changes?

Thanks

 

[JimM]

F5 heat sink

Heat sink USA has a sink with 2in fins and 10in by 7 in which they claim E=C\W\3in of 0.80, so I'm assuming the 10 by 7in sink would be around 0.4, if E is a linear function to size. Has any one used one of these sinks for a single channel F5 and do you think it is sufficient. Any help is appreciated.
Thanks

 

[labjr]

Heat sink USA has a sink with 2in fins and 10in by 7 in which they claim E=C\W\3in of 0.80, so I'm assuming the 10 by 7in sink would be around 0.4, if E is a linear function to size. Has any one used one of these sinks for a single channel F5 and do you think it is sufficient. Any help is appreciated.
Thanks

I saw a post where someone used a 8 in piece of the 10 in profile and thought it ran rather hot. I think you need about a 10 in x 10 in piece. The F5 article mentions a 6 x 8 piece per output device. That's about 96 Sq in. per channel.

 

[JimM]

Thanks I need to read the article better before I ask for help, thanks again.

 

[alazira]

I am talking for Toshibas mosfets resistor gate (47R or 100R?)
In addition the source resistor can be dicreased to 0.22R without any changes?

Thanks

Hi,
The Mosfet gate resistors (R13 & R14) are there to prevent parasitic oscillations (like R9). 47 ohm or more is okay, although I'd rather stick to 47R or 50R instead of 100R. Too low a value runs some risk, but you could always test the lower boundary by using a trimpot and later replacing it with a set value.

I believe TeaBag made one with 0.22R. You can read about it in his blog.

 

[Nelson Pass]

You do get a little more bandwidth with 50 ohms. I
suppose you can try 22, but at some value it will oscillate,
and this will depend on several variables, so you will want
to retain some margin (say 2:1) above the resistance value
that does allow oscillation.

 

[6L6]

In the F5, is there an advantage or disadvantage to either more PSU capacitance and/or a bigger transformer?

 

[labjr]

In the F5, is there an advantage or disadvantage to either more PSU capacitance and/or a bigger transformer?

I'm not sure going crazy with either will make a large difference. I don't think Peter Daniel heard much if any difference.

http://www.diyaudio.com/forums/pass-labs/155599-f5-listening-impressions-discussion-30.html