F5 Power up bias problem - help please!

LucasAdamson

Member
2009-02-20 10:31 pm
Helping my friend power up his F5 today, we ran into an immediate problem.

He has Peter Daniels tiny boards, that I meticulously populated myself, with the full circuit, and the original resistor values from the earliest schematic publication.


The power supply was tested prior to power up, and was reading 26.2v & -26.1v, but once connected to the amp board, for power up, was reading 19.2v & -19.2v.


The Problem:

On power up, across R11 & R12 values were reading 1.8v DC on both channels. Both P1s and P2s were set until clicking fully anti-clockwise. We did not test the speaker outputs yet, as we only have 2 multimeters, and we already had a power down problem. Searching the boards etc, yielded nothing but scratched heads. Can anybody help?

Many thanks
Lucas
 
depending of orientation of trimpots , clockwise or anti-clockwise means nothing

they need to be shorted ( minimal value ) as starting position for power up

turn them fully to other side , confirm with ohm-meter are they in proper position

there are several tutorials for F5 powering up and setting procedure
 

LucasAdamson

Member
2009-02-20 10:31 pm
Thanks ZM.

I have several tutorials. They all make the presumption that on first power up, ideal conditions of 0.0v are achievable with trimpots at starting position. I didn't get further than that before powering down, with a tiny whiff of smoke. With 1.8v on both channels, I should be at bias of about 3.6A which is too high, and we have probably blown something already. The question is, what has blown, and why can't I power up with the bias down to the normal level? Any ideas?
 
When the trim pots are set correctly for first start up the F5 does not draw significant current.
In this state the amplifier perfectly suits start up using a mains bulb tester.
If the bulb goes off, then you know the circuit is not drawing excessive current.
You can now measure voltages around thew circuit and PSU.
Once you KNOW that everything is OK, power down and restart direct from the mains.
NOW and only now are you ready to start biasing the output stage of the F5.

However, if in that "test" state the bulb lights up you know that something is not right.
You can measure the PSU voltages. They will be very low, maybe around +-2Vdc to +-8Vdc.
You can measure currents around the circuit to find what components are drawing current. From this you can ask questions or work out what is set/wired incorrectly.

How many times does this need to be repeated?
 
Thanks ZM.

I have several tutorials. They all make the presumption that on first power up, ideal conditions of 0.0v are achievable with trimpots at starting position. I didn't get further than that before powering down, with a tiny whiff of smoke. With 1.8v on both channels, I should be at bias of about 3.6A which is too high, and we have probably blown something already. The question is, what has blown, and why can't I power up with the bias down to the normal level? Any ideas?

easiest - check Jfets and mosfets out of circuit , with adequate test

from where you got input Jfets ?
 

LucasAdamson

Member
2009-02-20 10:31 pm
Well, it seems with the standard orientation on DIYAudio boards (CViller) the correct starting position is fully anti-clockwise, BUT on Peter Daniel's boards, with trimmers fitted as per the printing on the PCB, the correct starting position is FULLY CLOCKWISE. Thanks for the comments and guidance. Problem SOLVED.
 

lorienblack

Member
2009-09-28 5:47 pm
Update - with pots turned in opposite direction we have ZERO Mv at start up :) I wonder if you have any thoughts regarding which component might have produced the smoke and would you recommend changing the mosfets? I confess I have a nagging doubt that I may have reduced the SQ by briefly frying something?

Thank you once more.
 

Russellc

Member
Paid Member
2003-03-06 12:59 am
midwest
Usually its the resistors that you are measuring bias from...are they a little off color? If not fully roasted they are liable to still work fine. BUT...listen to what others here are saying, I was lucky on my fireups, and fortunately have a variac. I assume you didnt use variac or lightbulb, so when the full on bias hit it could have damaged the Mosfets and other parts, hopefully you shut down quick enough.

Russellc
 
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LucasAdamson

Member
2009-02-20 10:31 pm
Thanks Russell,

By the way, I note that you used 220R feedback resistors in place of the 100R nominal ones, to slightly boost the gain, and that you also removed the current limiting portion of the circuit. This is also what I intend to do on my build (which is currently queued behind my DAC completion). How did that work out for you?
 

Russellc

Member
Paid Member
2003-03-06 12:59 am
midwest
Thanks Russell,

By the way, I note that you used 220R feedback resistors in place of the 100R nominal ones, to slightly boost the gain, and that you also removed the current limiting portion of the circuit. This is also what I intend to do on my build (which is currently queued behind my DAC completion). How did that work out for you?

That amp still needs a case to put it in and the power supply caps. Have all other parts and sinks. The F-5 I have up and running is a standard F-5, dual mono. Got side tracked with the F-5 turbo group build, but have every intention of finishing the no limiting/220 ohm feedback. Some say not much difference but more gain, I think that was Buzz, sorry if I am remembering wrong, I saw one report that they thought it messed with the bass, but they were having other problems. Nelson and Zenmod suggested it, how could you go wrong I figure. I could always change it back I guess.

Russellc
 

LucasAdamson

Member
2009-02-20 10:31 pm
That amp still needs a case to put it in and the power supply caps. Have all other parts and sinks. The F-5 I have up and running is a standard F-5, dual mono. Got side tracked with the F-5 turbo group build, but have every intention of finishing the no limiting/220 ohm feedback. Some say not much difference but more gain, I think that was Buzz, sorry if I am remembering wrong, I saw one report that they thought it messed with the bass, but they were having other problems. Nelson and Zenmod suggested it, how could you go wrong I figure. I could always change it back I guess.

Russellc

Didn't Nelson suggest a bit more than that? I remember this particular advice he gave, to somebody wanting to use carbon resistors instead of metal film, in order to soften the unyielding hardness of the amp. Papa Pass suggested 2x 3w 200r (=100r 6w) for R5/R8 in place of 2x 3w 100r (=50r 6w) plus:

22r in place of 10r for R1/R2...plus:

1 Ohm fo R11 & R12 in place of 0.47 Ohm.

Don't ask me why...the result was found very pleasing to the poster, who commented:

"Yep, Yep, amp runs a little cooler(perhaps 8-10 degree farenheit) and the "tight fist" now has a velvet glove! I found the original spec a bit dry and etched in sound relative to my Dynaco st70 - accurate and detailed but lacking a bit of body or warmth. The new feedback seems to be definitely a step in the right direction."

Having said this, my friend Lorian has made the same complaint initially, only to retract it after burn in, which opened up both bass and warmth, so best wait before implementing resistor value changes. Mr Pass sure knows what he's doing!
 

LucasAdamson

Member
2009-02-20 10:31 pm
For the record, I have no desire to use carbon anything in my F5.

But when faced with a builder that suggested it to "warm up" the sound of the amp, NP suggested instead a few (metal film) resistor changes that are interesting, and not merely 100r to 220r R5/R8, but also the other changes I posted. I wonder if changing only R5?R8 could cause any issues....
 

Russellc

Member
Paid Member
2003-03-06 12:59 am
midwest
For the record, I have no desire to use carbon anything in my F5.

But when faced with a builder that suggested it to "warm up" the sound of the amp, NP suggested instead a few (metal film) resistor changes that are interesting, and not merely 100r to 220r R5/R8, but also the other changes I posted. I wonder if changing only R5?R8 could cause any issues....

Yes, remember taking notes about Nelson's suggestions to the poster wanting to warm up his F-5 while reading this gigantic thread when I initially built. There were several resistors changed up, I didnt follow that route as some others felt it might be a bit much "warming up." My amp never struck me as needing "warmed up" (maybe because of the IRF outputs?) just a little more gain.

Russellc
 

AudioSan

Member
2009-02-12 7:47 pm
For the record, I have no desire to use carbon anything in my F5.

But when faced with a builder that suggested it to "warm up" the sound of the amp, NP suggested instead a few (metal film) resistor changes that are interesting, and not merely 100r to 220r R5/R8, but also the other changes I posted. I wonder if changing only R5?R8 could cause any issues....

look at F5 turbo. there is no changes other then R5-R8. this will raise the gain from 15.5db to 22db.
regarding carbon. i use allmost only carbon composit in my F5(exept feedback resistors and source resistors). and i like it.