F5 power amplifier

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U

Uunderhill

Member
Joined 2008
Neutrality said:
Thought the F5 current limiter was there to prevent the FETs from selfdestruction in case of shorts and other fault conditions. A fuse in the power rails would do the same thing, cut of the power once you reach a certain current level.
Click to expand...

Interesting comments about the current limiting.

I've come to the same conclusion - but was going to ask others about it.

The voltage drop across the 0.47 ohm resistor starts to become quite significant at higher currents.

The only fuses I had handy, to take a measurement, were fast blow 6 Amps.
With 2 Amps flowing though one of these fuses, the voltage drop was 0.060v.
So the resistance of a normal fuse is around 0.03 ohms.

Also, high current has to flow through the power MOSFET's, before the current limiting circuitry can react.
So the total time for the current limiting circuit to react would be in the order of 30 to 50 ns.

Where as a fuse will react as soon as there is a need for high current.
 
nigelwright7557

nigelwright7557

Member
Joined 2008
murtonpikesystems.co.uk
Lerg said:
That said, I don't see the advantage of using a fuse which will take more time to react instead of the nicely designed protection that is already built into the stock circuit.
Click to expand...

In my experience fuses just stop fires they dont protect output transistors because they are far too slow.
On top of this the smoothing capacitors on their own will have enough charge to blow up the output transistors should a fault occurr.
 
A

AndrewT

R.I.P.
Joined 2004
Hi,
fuses are a long term limit.
IV protection can be short term limit or short term+medium term limit depending on the delay built into the protection locus.

I do not see either of them as exclusive to the other.

I see short term limiting and long term limiting as complementary to each other.

The short term should be designed to protect the amplifier and it's semiconductors.
The long term limit should be designed to protect the load and cables, etc.
This same long term limit can also give some Safety advantage.
 
N

Nikon1975

Member
Joined 2009
6L6 said:
NO TRUER WORDS HAVE EVER BEEN SPOKEN!!!





R11 needs to change to get the bias value more to the center of the pot. With the stock value of R11, you might run out of turns on the bias pot before getting to the bias you want.

Some people think the Toshiba FETs sound better than the other manufacturers. Other people think the inverse is true...



I haven't the foggiest. Hopefully someone else can point you in the right direction.
Click to expand...

Thanks for the tips.

What I found out digging in the thread is the following:

1) The N and P semiconductors are not perfectly complementary in various extent. Toshiba mosfet manage maybe to match n and P better than others.

2) Toshiba mosfet are not direct replacement of the IR component, so values of some resistors have to be modified.

3) It is possible to compensate the difference between the N and P devices (both Jfet and Mosfet) using slightly different value for the resistors. This lead to the use of the additional 5 ohm resistor on one jfet and to using 0R22 and 0R18 resistors on the mosfet (value for the toshiba)

4) There are various technique to trim the n and p mosfet to behave as close as possible.

5) I couldn't find anyone that did and A/B listening comparison. On the measurements there are measurable improvements and it is possible to modify the distorsion spectrum, but at a level that, to quote Morgan Jones, "will challenge the measurment equipment of the author"

Basically I have a perfectly working and sounding amp. Exactly like in the article (just a bit more caps). As eventually I would like to have two balanced monoblock, I am trying to understand if it's worth to go the way the balanced thread is going or, as I hoped, I can just make a second copy of the amp I have and use them bridged.

By the way, is 50 C an acceptable temperature inside the case ? My case has no ventilation. Should I start making holes ?

Thanks,

Davide
 
B

budwiser

Member
Joined 2004
So Nikon1975, did you use the Toshiba mosfets and use different resistor values or did you build based on parts list in article using the Toshiba mosfets. Will I have a problem if I dont make these adjustments?

Thanks.




Nikon1975 said:
Thanks for the tips.

What I found out digging in the thread is the following:

1) The N and P semiconductors are not perfectly complementary in various extent. Toshiba mosfet manage maybe to match n and P better than others.

2) Toshiba mosfet are not direct replacement of the IR component, so values of some resistors have to be modified.

3) It is possible to compensate the difference between the N and P devices (both Jfet and Mosfet) using slightly different value for the resistors. This lead to the use of the additional 5 ohm resistor on one jfet and to using 0R22 and 0R18 resistors on the mosfet (value for the toshiba)

4) There are various technique to trim the n and p mosfet to behave as close as possible.

5) I couldn't find anyone that did and A/B listening comparison. On the measurements there are measurable improvements and it is possible to modify the distorsion spectrum, but at a level that, to quote Morgan Jones, "will challenge the measurment equipment of the author"

Basically I have a perfectly working and sounding amp. Exactly like in the article (just a bit more caps). As eventually I would like to have two balanced monoblock, I am trying to understand if it's worth to go the way the balanced thread is going or, as I hoped, I can just make a second copy of the amp I have and use them bridged.

By the way, is 50 C an acceptable temperature inside the case ? My case has no ventilation. Should I start making holes ?

Thanks,

Davide
Click to expand...
 
bobodioulasso

bobodioulasso

Member
Joined 2008
Nikon,
Going balanced using the stock F5 is ok but, depending on the impedance of your speakers and the class A power you aim at, you have to calculate which bias you need and how much dissipation will result.
Do not forget that a balanced amp sees half the Z of the speaker. (Same with lower dips)
The question is: are your heatsinks big enough?
May be you'll need to use double outputs.
Or do you only want little class A power and high class AB one?
 
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smartx21

smartx21

Member
Joined 2010
Last edited:
N

Nikon1975

Member
Joined 2009
bobodioulasso said:
Nikon,
Going balanced using the stock F5 is ok but, depending on the impedance of your speakers and the class A power you aim at, you have to calculate which bias you need and how much dissipation will result.
Do not forget that a balanced amp sees half the Z of the speaker. (Same with lower dips)
The question is: are your heatsinks big enough?
May be you'll need to use double outputs.
Or do you only want little class A power and high class AB one?
Click to expand...

I understood that. My speaker are 4 ohm, so I am not in a good position.
What bothers me is that with the F5-X that they are cooking, I cannot use the transformer I already have. But I think I will wait at least the new cviller v2 boards and compare one channel with 24 V balanced with one or two mosfet.

I am using the conrad heatsink 150*350 mm and, having monoblocks for balanced, I will be using four of them in total.

For the time being I assembled the amp exactly like in the article. No experience with Toshiba yet.


D.
 
alazira

alazira

Member
Joined 2007
Toshiba Mosfet 2sj201 & 2sk1530

Nikon1975 said:
Can someone point me out to the discussion on:
1) Toshiba MOSFET and why to change R11 etc
2) Degeneration trick
Click to expand...

I plan on building a SE Toshiba mosfet with the degeneration trick while waiting for the Balanced version. Here are some notes:

1) IF single ended (+/-24V rail) then R11, R12 should be 0R47.
And you should replace P1, P2 with fixed resistors after trimming.
Voltage across R3, R4 more like 3V.
Reference post 9493 http://www.diyaudio.com/forums/pass-labs/121228-f5-power-amplifier-950.html#post2409557 and post 6503


2) For those who wish to organise their own JFETs, the 2SK170's should ideally have an Idss of 6~7mA.
Going lower with Idss would increase the amount of negative feedback proportionally, and vice versa.
If you use the 5R degeneration on the 2SJ74, as I recommended, then you can calculate the Idss of the 2SJ74's using the following equation :

Idss (2SJ74) = Id * (1 + Rs.Yfs)
where Id = Idss of the 2SK170 in this case; Rs =5.1R; Yfs = 0.033 S for 2SJ74 at that bias range
Reference post 548 in the F5X thread http://www.diyaudio.com/forums/pass-labs/172770-balanced-f5-question-55.html#post2364264

Some sample calculations:
2SK170=7mA
6mA(1+5.1*0.033)=6.0mA(1.1683)=7.01mA for 2SJ74
6.5mA(1+5.1*0.033)=6.5mA(1.1683)=7.59mA for 2SJ74
6.75mA(1+5.1*0.033)=7.89mA
7mA(1+5.1*.033)= 7mA(1.1683)= 8.178mA for 2SJ74
7.5mA(1.1683)=8.76mA for 2SJ74
8mA(1.1683)=9.35mA for 2SJ74

R2=15ohm and corresponding feedback 75ohm
R1=10ohm and 50ohm feedback
 
A

anilva

Member
Joined 2004
alazira said:
R2=15ohm and corresponding feedback 75ohm
R1=10ohm and 50ohm feedback
Click to expand...

You are right in your research. They are the right ones. I followed the same ones and built a F5 successfully with Toshibas. However, what I marked above is not required to be done and you can stick to the stock circuit if you use the 5 ohms degeneration resistance in the source pin of 2sj74. You do not need to make changes to R2, R1 and feedback etc.

This is my understanding and I am not a guru. Worked for me perfectly.

Cheers.
 
N

NYCOne

Member
Joined 2010
Are saying use 10 Ohms at R1 and R2 add the 5 ohm resistor at the SJ74 Source and leave the feedback resistors at 50 ohms for each side of the PP circuit?

anilva said:
You are right in your research. They are the right ones. I followed the same ones and built a F5 successfully with Toshibas. However, what I marked above is not required to be done and you can stick to the stock circuit if you use the 5 ohms degeneration resistance in the source pin of 2sj74. You do not need to make changes to R2, R1 and feedback etc.

This is my understanding and I am not a guru. Worked for me perfectly.

Cheers.
Click to expand...
 
smartx21

smartx21

Member
Joined 2010
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