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Old 23rd March 2010, 07:30 PM   #7511
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Hi Tinitus,
have you seen this thread?
May be the best way to use your renesas.

F5 meets Buzquito
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Old 23rd March 2010, 08:00 PM   #7512
tinitus is offline tinitus  Europe
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Thanks bobo, thats great
I did miss that one, inspiration from france, it seems
Need to study it carefully
Not sure, but could BA-2 actually also be the one, with different bias design
A power buffer with high gain preamp, maybe


Quote:
Originally Posted by m.massimo View Post

Then, when I turned it on the next day (from cold) I read immediately about 1500mV on both R11 and R12.
Being symmetric in both channels might tell something, just not to me, sorry

Well, its just to keep the focus on your problem
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Old 23rd March 2010, 08:36 PM   #7513
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Quote:
Originally Posted by tinitus View Post
Being symmetric in both channels might tell something, just not to me, sorry
not exactly symmetric, negative rail more than positive, an offset of about -200mV was generated.

Quote:
Originally Posted by tinitus View Post
Well, its just to keep the focus on your problem
Thanks
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Old 23rd March 2010, 09:37 PM   #7514
alazira is offline alazira  United States
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Quote:
Originally Posted by Zen Mod View Post
perfect opportunity to replace crystal ball with little mosfet testing setup

I found this cute little test. Only uses a DMM

This testing procedure is for use with a digital multimeter in the diode test-range with a minimum of 3.3 volt over d.u.t. (diode-under-test)

Connect the 'Source' of the MosFet to the meter's negative (-) lead.

1) Hold the MosFet by the case or the tab but don't touch the metal parts of the test probes with any of the other MosFet's terminals until needed.

2) First, touch the meter positive lead onto the MosFet's 'Gate'.

3) Now move the positive probe to the 'Drain'. You should get a 'low' reading. The MosFet's internal capacitance on the gate has now been charged up by the meter and the device is 'turned-on'.

4) With the meter positive still connected to the drain, touch a finger between source and gate (and drain if you like, it does not matter at this stage). The gate will be discharged through your finger and the meter reading should go high, indicating a non-conductive device.

Such a simple test is not 100% -- but is useful and usually adequate.
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Old 23rd March 2010, 10:41 PM   #7515
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Massimo,
it looks like a bad solder joint.
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Old 23rd March 2010, 10:42 PM   #7516
EUVL is offline EUVL  Europe
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> How closely matched were the devices you tested, and how were they matched? (static bias, or dynamic curve tracer?)

I took two random sample out of my measurements, and the Vgs values were stated in that post. They were measured dynamically using a functions generator to drive Vgs with 100Hz triangular wave (with variable DC offset) and Vds supplied by a lab supply, on heat sink, after 10 minutes steady state, i.e. exactly working conditions in the real circuit. 1000 measurement points per device.

I take my matching seriously.


Patrick

Last edited by EUVL; 23rd March 2010 at 10:44 PM.
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Old 24th March 2010, 02:38 AM   #7517
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Quote:
Originally Posted by m.massimo View Post
Hello everybody,
after reading all the thread, I started collecting parts to build a couple of F5 mono.
Last week I assembled the first (300VA 18-0-18, 27mFx3xrail, +24.1Vdc, -24.1Vdc, sk170BL, sj74BL, FQA12P20, FQA19N20C, mica and thermal paste, Peter Daniel board, no current and voltage limiter sections, resistor values as Pass scheme, alu hs 250x180x83mm).
First setup went smoothly, after a couple of hours, when bias was stabilized at 1.3A and offset at 0mV, I attached a speaker of no value and music flowed.
Another 2hrsx3 of testing and all was good, heatsink at 40C behind the mosfets, mosfets case at 50C. Then, when I turned it on the next day (from cold) I read immediately about 1500mV on both R11 and R12. I turned it off quickly, lowered pots to zero, did the setup again and I was able to get 600mV on R11-R12 stable with offset at 0mV.
Another couple of hours of continuing operation with no problems.
Next turning on from cold, today, same story. 1500mv on R11 and R12. I lowered again the pots, but now everytime voltage on R11 and R12 reach about 62mV, both increase abruptly to around 1500mV.
No parts burned, no smoke or something odd. I checked the passive parts, they are ok.
In the meantime I was testing the first mono, I assembled the other one (it's exactly the same). It works perfectly since the beginning, It's clear there's something in the first one that doesn't work properly, but I've no idea than rebuild the circuit with new active parts. I'd like to understand what happened. Have you any suggestion?

I do not know, but I can guess... but let me ask first, does this happen on two channels the same way and amount?

But I think you said it was the first... what may be happening is that the two Mosfets you chose to use are on "opposite" ends of the usual Vgs spread. No matter if that is a cause or not, they seem to drift different amounts from cold. Since you bias them when hot, they are fine - but then when they are cold they move "back" a different amount, thus the large offset.

IF this is what is going on you've got two main choices:
- make a simple test jig to determine the Vgs of the devices you have and try to put the ones that are closest to each other on the same board (or get some more devices and test and pick a better set of pairs). Actually the amp itself is sort of a test jig... you can set up a meter on the gate drive line and measure the voltage that way. (also you can test now what the Vgs is cold for 0 offset vs hot and see what the difference in absolute range is between P & Nch)

- or use the circuit suggested earlier or one like it to servo control the DC offset. In that case the static values would be chosen so that the cold bias setting causes the amp to have limited offset.

This is assuming that something else isn't a wrong value or miswired or otherwise malfunctioning...

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Old 24th March 2010, 02:40 AM   #7518
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Quote:
Originally Posted by EUVL View Post
> How closely matched were the devices you tested, and how were they matched? (static bias, or dynamic curve tracer?)

I took two random sample out of my measurements, and the Vgs values were stated in that post. They were measured dynamically using a functions generator to drive Vgs with 100Hz triangular wave (with variable DC offset) and Vds supplied by a lab supply, on heat sink, after 10 minutes steady state, i.e. exactly working conditions in the real circuit. 1000 measurement points per device.

I take my matching seriously.


Patrick
Ya.

Ok, so then how did you record the graph?

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Old 24th March 2010, 05:33 AM   #7519
EUVL is offline EUVL  Europe
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2 channel, 16-bit Digital oscilloscope.

You can of course also use a 24-bit sound card if you can take care of the intrinsic AC-coupling problem.

We also have a curve tracer from Locky_z which we are modifying with improvements for long-term stability.
But that will take a bit longer with too many projects running in parallel.

P.
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Old 24th March 2010, 03:58 PM   #7520
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Don't forget that the JFETs have their own set of thermal issues. I don't know how they are oriented on Peter's boards...I've gotten the amplifier to work within spec with GR and BL devices, but I am pretty relaxed on the layout of my boards, preferring not to crowd devices together. In fact, for all of my F5's I have Caddock's mounted to the heat sinks! You may also have the 0.47R source resistor or 2x100R feedback resistors cooking the JFETs.

As has been mentioned before, the Fairchild MOSFETs are NOT complementary devices. I use them in my F5's with the protection circuitry and have not had problems with offset (but patience is its own virtuous reward in setting up the bias and offset.)

With regard to matching devices -- I found out early on and it's probably on the F4 thread -- devices with matched Vgs from the same lot often exhibit the same or very close transconductance.
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