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Old 5th August 2005, 07:39 AM   #11
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Mr. Giesberts amps are always prone to instability problems whether he is using hexfets from IRF or Igbt's from Toshiba.

Whereas this forum is full of great designs for the diyers to built in easy and accessible way...

K a n w a r
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Old 5th August 2005, 07:54 AM   #12
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Elektor carried two versions of this amp. One was called '60 watt HEXFET amplifier' and the other was called '90 watt IGBT amplifier'. The PCB for both amps are the same and that is why you will find a larger pitch pad for the output devices even with the 60 watt amp.

This is one amp that I have found almost impossible to stabilise. I did have one running for quite a while. It sounds pretty laid back and relaxed. The resolution is very good. But it lacks a sense of liveliness or 'bite'. I have four of these boards completely assembled and unstable. I wonder if it is worth pursuing this since there are too many amps out there that will outperform this one in terms of stability and sound quality.
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Old 5th August 2005, 09:44 AM   #13
ilimzn is offline ilimzn  Croatia
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If I was trying to 'fix' this design, I would probably do the follwing:
1) Use IRFP devices even for 60W (you do want these to survive the fuses melting in the event of problems)
2) Put the fuses into the D line of the MOSFETs, not S.
3) Reroute power supply and ground for the output stage - the fuses in the D lines would aid to thie by acting as large jumpers, freeing space to get the ground close to the S lines for proper decoupling using a pair of noninductive foil caps in parallel with 100uF or so electros.
4) Possibly insert low ohm NON INDUCTIVE source resistors for the MOSFETs, on the order of 0.15 ohms. Usually I try to avoid these in a one output transistor pair amp, especially as MOSFETs do not like inductive resistors in the source, but in this case they may alow better control of the idle current and less problems with oscilation since getting this at the expense of the gain is not a problem in a CFP configuration, and neither is the small voltage drop a problem (~1V).
5) Include proper gate protect / overcurrent limiter zeners.

Since this would need a redesign of the PCB, it would be a good oportunity to get rid of the rather large amount of jumpers. Experience has tought me that ground plane PCBs are a more than worthwhile investment - no need tor fancy through hole metalisation, this makes producing the PCB using DIY techniques quite easy, you just don't etch the ground plane side, and then 'blank' off the parts of it around the holes that are not connected to the ground plane with a large drill bit. THIS design I believe would benefit substantially from such a PCB.
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Old 6th August 2005, 04:24 PM   #14
Zimo is offline Zimo  Finland
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HEY EVERYONE!!!

You know what?

My amp works now like a beast!

I did the component changes like ilimzn told me to do.. (thank you very much of that!)

So I made these modifications to get it work:

R25 and R28 from 15ohm TO 22ohm

R26 and R29 from 150hm TO 120ohm

I move the places of that jumper and R26 nearby IRF9540N

C5 from 470pF styro TO 332pF styro 1% (a suggestion of MikeB)

I put 100nF capasitors beside C11 and C12 (10000ĩF)

I let those C3 and C4 to be 2.2nF (originals should be 2.7nF)

After those modifications we measured the bias voltage and the lowest value was about 1.8V.

We adjusted bias current to 100mA and used multimeter 10A line to avoid the effect of multimeter own fuse, and measured it with the 2.5A fuse like ilimzn advice said (we were not sure what should be the bias current cause the amp is using fets and in the quideline with IGBT it was 200mA after 30min.)

Then we let it to warm up about 30 min and checked and adjusted the DC offset, it was only about 0..4mV in both of channels! (with shorted line in)

Then we took and oscilloscope and an audiogenerator...

At 1kHz we couldn see no distortion, but when oscillossoce was in 10mV section we could see some distortion... so the circuit is a little bit tricky one and really choosy with components!

My friend who have repaired many amps at his job, wanted to test the circuit a little bit more.

So we tested how its frequency responce says at frequencies like from 20kHz to 60kHz. What we saw was unbelievable! It can handle frequencies to 60kHz like nothing! Also with squaremode of audiogenerator, it didnīt smooth the edges of square much even at 60kHz!

And last we took a resistive 4.7 ohm load (no inductive reactance).
With left channel it gave about 100W power at max point, and with other channel 90W. So we thought that the differences between the channels would cause by differences between bias currents...

Im now very satisfied of that amplifier!

We were going to add those zeners across gate-source. But didnīt know which direction is right

To Samuel Jayaraj!

If you have four of those completely assembled I would advice you to check what components YOU have countervailed!!!

Maybe that little oscillation in my amp now is from those fuses which have a little bit inductive construction! I will change them and maybe put them before the power rail comes to PCB...

But now Im going to listen it some and put circuits against DC.

See ya

Simo
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Old 7th August 2005, 12:24 AM   #15
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by Zimo
HEY EVERYONE!!!
You know what?
My amp works now like a beast!
Excellent, I'm happy to read about it!

Quote:
So we tested how its frequency responce says at frequencies like from 20kHz to 60kHz. What we saw was unbelievable! It can handle frequencies to 60kHz like nothing! Also with squaremode of audiogenerator, it didnīt smooth the edges of square much even at 60kHz!
Keep in mind that amplitude and load will change this somewhat, but even so, from the schematic it is a fairly fast amp. It would actually be prudent to limit it on the high end with an input filter, to prevent RF pickup generating intermodulation products with the audio signal. It already has a filter made out of R1 and C2, but it's actual cut-off depends on the source impedance. A signal generator usually has 50, 75 or 600 ohms output which is a low impedance and will result in a high cut-off frequency. In a regular application, depending on what you use to drive the amp, it may be lower. If you are using a low impedance source, you may want to increase somewhat R1 and C2.

Quote:
We were going to add those zeners across gate-source. But didnīt know which direction is right
A of zener1 to G of T12
K of zener1 to S of T12

A of zener2 to S of T13
K of zener2 to G of T13
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Old 8th August 2005, 06:19 PM   #16
Zimo is offline Zimo  Finland
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Hmm!

Maybe I was too close to success

I add those 6.8V protectionzeners to the finalstage fets today and listened maybe an hour... Then I shut it down and turn it on couple of times. When loudspeaker-relays were powered I heard awfull buzzing from left loudspeaker

Shut it down as fast I could but I have lost the game anyway!

I canīt get any explanation to this, but I thought - would those zeners cause this.

I didnīt check bias current after adding those zeners

Have to check it from the other channel to avoid that it doesnīt burn...

Maybe I just buy next those IRFP models if those would be better and more steady. And if I add IRFP should I again change some other parts or should only bias adjustment take care of that...

And maybe I should take still those fuses off from that source line and just put it before rails come to PCB or donīt put those anywhere...

This isnīt building an amp anymore - this is more like art or nuclear physics
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Old 8th August 2005, 09:01 PM   #17
ilimzn is offline ilimzn  Croatia
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Zeners should not present a problem. You may still need to increase the gate resistors some (I used a similar CFP with gain approach with IRFP240/9140 and put 68 ohms in the gate line).
You may also have instability into a reactive load - when you next do some tests, it would be a good idea to look up some of the 'standard' reactive loads used for amp testing, and test with that. Pure resistive loads nearly never match a real speaker (one exception are some of the magnepans but even they have a resonant peak). You may want to look at using those 2.7nF caps instead of the 2.2 that you replaced them with, though to be honest, I don't think it will make a huge difference, you may just get across the 'marginal' limit of stability but I myself prefer rock stable
Regarding IRFP9140/240, they are more similar to the original IRF parts, just much better complementaries. You need about 3-3.5V Vgs to get 10-200mA range of bias current. It should still be reachable with the component changes you made, if not, you change them back to what they were for the IRF540/9540.

Regarding fuses, use fast fuses with a straight wire inside and quality fuse holders. The layout of the amp is a bit odd, it could have been better, but debugging these sort of issues is way beyond text-only messages

Regarding art, well, building amps is an art
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Old 5th October 2005, 11:54 PM   #18
Zimo is offline Zimo  Finland
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Itīs been a while since last "report".

I have now changed my fets to IRFP240/9140 models. And I have done some changes also to fuses. I took off the fuseholders which were in Source line and put jumpers there. Now the fuseholders are between the cables which comes to main boards. This sould make outputstage more stability and (hope so) reliable.

So in this solution I have now one 10 000ĩF capasitor after fuse and ofcourse I have to change fuserating. I decided to try with 6.3 amp slow ones first. I donīt know is that a serious crime to leave one big cap after fuse... ok. It may cause a problem if fet shorts, then the whole capasity of that 10mF cap is going to discharge to fet.

I havenīt got those 2.7nF caps yet, but I will. I have also finished DC protection circuits. I decided to do a protection led to show when DC protection is blocking. I had to use a SN7432N 2 input OR-gate to get 2 channels to one LED.

Iīll inform you how it will make this time when I get my few components. Thanks for everyone... I couldnīt make it alone
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Old 6th October 2005, 01:27 AM   #19
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Hi Zimo,

Notwithstanding all the suggestions made to date, partticularly those from Ilimzn, the circuit has a flaw!

R10, R14 need to be linked out for the compensation on the first stage load to work!

Cheers,
greg
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Old 7th October 2005, 12:59 PM   #20
Zimo is offline Zimo  Finland
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Ok. So youīre suggesting that those two resistors are unnecessary or what do you mean with "need to be linked out.."? (sorry that I didnīt understand that). Well I have no exact clue why those are there, but I quess taking those off would make the differential pairs whole different. Could you please tell me more about what does those two resistors do there and what does it change if I took them off or connect them otherwise.. (sounds so radical )

However I know that R3, C3, R4, C4 limits differential amps maximum operating frequency to 6.5MHz and now I have maybe 10MHz rating there because of wrong values of C3, C4. Iīm just asking because I think Giesberts have not put anything there without a reason.

BTW have you much expierience of this circuit? If have I would like to share it with you

I appreciate your answers. Thanks

Simo
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