An illustrated guide to building an F5

This is really great.. all we now need is someone with time an enthusiasm to put together some PDF files for Pass DIY builds. This would be a great example of how to do it and it would be even better if this thread were referenced from a more central point, say Pass DIY site? It need not mean that they support the build description, but it would give newbies a flying start and the thread could be used for updates.

Thanks for a great piece of work

George

this was first on the list when I just tried searching...
http://www.diyaudio.com/forums/pass-labs/188691-illustrated-guide-building-f5.html
 
Wierd f5 bias problem....... Hello, I have a problem with my f5, stopped working due to rectifier bridge which I fixed, and when I reconnected inadvertely reconnected negative to positive and vice versa, with some nice smoke. Blew out R 11 and 12 which instantly turned bright red before fuse blew.

To repair, I changed all transistors and output devices, as well as resistor r11 and 12. However, now, when I try to bias I get a wierd problem,

when I put meters on r 11 and r12 and turn the pots to change bias, when I change one pot, both meters move, such that when one meter is at .4v the other is at .6v or so on, no matter how I change adjustments and no matter what pot I adjust, voltage at both resistors 11 and 12 changes, with about a .2 v difference. one channel offset of 3mv and other channel offset of .65 volt and pots adjustment will not change this at all on this channel.

Even with one pot maxed out showing .4 v on R 11, I cannot get r 12 to follow. the differential stays the same and both r 11 and r12 voltages move in parallel when I adjust either pot, with approximately .2 volt differential. Frustratingly, it is the same for both channels with a little smaller differential on one channel.

Not sure what to do next. Checked orentation of all devices double checked and all good. Both output devices are in the correct positions.

Would this be a problem perhaps due to use of poorly matched devices.

perhaps this is due to bad resistor on the pcb which has no apparent damage.

Any ideas, thought appreciated.
 
no, on one channel I get offset of .6 v and other 3 mv, which stays steady throughout range of adjustment of both pots. For some reason offset does not change when pots are adjusted, only voltages across resistors 11 and 12.

I guess the problem is due to some of the resistors in the board are damaged, without showing apparent damage.

The drain of sj74 and 170 are R2 and r3, both 10R resistors so perhaps there was damage there?
 
A pair of diodes across the +ve to PG and -ve to PG eliminates the reversed PSU damage.
The bulb lights up full brightness and the worst case reverse voltage on your power amp semiconductors cannot exceed ~800mVdc.
It is also very worth while adding a pair of diodes +ve to speaker and -ve to speaker to protect from back emf from the speaker.
These 4 diodes cost a few pence (1n4002) and potentially save $$$s (£££s).
 
Another F5 is born.

But where is P3 ?
 

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thanks the thought Andrew, VR resistance changes when I adjust because power changes, it just that when I adjust one pot both R 11 and r12 change, it goes from zero to up to .6 v on six turns, and would go higher if I kept turning (25 turn vr). It is just that it is not possible to adjust the voltages of r11 and r12 to the same figures, when I turn either pot both voltages on these two resistors change almost exactly together, keeping the .2 volt differential.



If I remember previously biasing f5 when you changed one pot then only voltage at one resistor would change, not both resistors. Additionally, offset will not change no matter how I adjust either pot, it stays the same on one channel no matter how I turn.

When power is on the voltage changes on both r 11 and r12 when I adjust one pot, when it is too high on one, I move to the other and both go down together, with a differntial of about .2 v.

I have the diyaudio speaker protection board installed, so I assume speakers will be protected without diodes.

But no matter how I adjust the pots, it does not have an impact on offset, which on one channel is around .6 volts.


unless someone has an apparent view, I am at a loss, and assume that some of the resistors blew on the board and I cannot see the damage and are making this wierd adjustment

All I can think of doing is starting over, new boards and new parts. Any further ideas anyone before I start desoldering and ordering parts.
 

6L6

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John -

Once the transistors start conducting and the bias pots have some effect it will feel like one of the pots is controlling bias across both sides and the other is controlling the DC offset.

How much bias can you get with zero offset?

Check your PM.
 
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Thanks very much Jim, everything works real good now. Appreciate the thread and help and bit embarassed to not have figured it out myself.

Cheers, John

Dont feel bad, not unusual at all. I had my bias perfect but offset was sky high. I was ready to swing it around by the power cord and let it fly out the window I was so frustrated....all it needed was the pots twiddled with a little to zero offset. I was convinced it was seriously screwed up.:) It wont be so difficult next time!

Russellc
 
6L6, thank you very much for the fantastic tutorial. It's helped me get as far as I have - one operational F5 channel, biased to .58v with zero offset! :)

The F5 and a Boozhound phono stage this summer, were my first solid state projects, so I am a newbie.

So, on my other F5 channel...I seem to be in bias hell. I've searched, and read these forums for the past few weeks, and am now at the point of making a beseeching post.

Cviller Rev 2.0 boards, new DIYaudio PSU, Deluxe 4U.
V+ 23.8 V- 23.8

Q1 2SK170 BL (sourced from alweit)
Q2 2SJ74 GR ( " " )
Q3 IRFP9240 ( " " )
Q4 IRFP240 ( " " )

With P1 maxed out clockwise:
R11 = .03v
R3 (10k) = 3.6v

With P2 at 3 turns or less:
R4 (1.2k): 2.5v
R12 = .6v

Spkr terminal offset: .85v at the above levels

Q1 was originally a GR grade, and R3 would not come up past .02v even at 10k. Going to BL seemed to barely make a change (+.01v)

Since R4 at 2.2k was easy to get up to .59v, I've been hesitant to bring it up to the 10k that R3 is at, so I only added another 1k.


It appears that the P1/R3/R11 group has the exact opposite problem that P2/R4/R12 has.
- R3 has voltage that comes up when P1 turns, but very little develops across R11.
- R4 has low voltage, but P2 can drive R12 well above .59v.


I've triple checked that my MOSFETs are in the right spots.

I've searched for cold joints, to no avail. Triple checked all component values and board ins and outs.

Do I need to match R3 and R4 exactly, even with R4 being so sensitive to P2? What else should I be measuring in order to help target the problem, rather than swapping out more components and burning my traces?

Thanks very much in advance for any ideas, disciplinary lectures, or suggestions. :)
 
Getting there. After some more reading, I lifted R11 and R12, and found that R12 had burnt at some point, and had decided to become a 1M resistor... :eek:

I replaced R12, set R3 and R4 back to about 4.7k. P1/P2 dialed down to 0. Bias would not budge on startup, but was evenly very low on both sides. I replaced the 2SJ74(GR) on a hunch and on the next startup, R1 burned up (1w carbon film).

I am replacing R1 with a 2w, and backing R3/R4 off to 2.2k again.

If R1/R2 go up in smoke, do they take a JFET with them? R1 cooked 3 or 4 seconds at most before I shut down...