oscillation problem

I don't have a digital camera here (yes...I am indeed very poor:D )
But my girlfriend has got one, so on monday I'll ask her for the camera so I can give you a nice pic of my wiring.
I can tell you now already that leads are kept as short as possible.
But,the picture will make things clearer to you.


This is the first time I've heard about testing the open loop gain?!
A quick and dirty calculation learns that this value is very high!
So how and why should one try to measure that?


R3/C3 and R4/C4 are indeed frequency compensation for the differential stage.
I do not think they form any kind of a problem.


So, would it be BETTER to make the DC-feedback unity?

I don't like the DC-offset thingy around P1 either.
It still leaves my output with 600mV DC.
But then again... a healthy amp shouln'd have something like 2V DC offset to be trimmed away by the offset circuit ion the first place right?


Tomorrow I'll be hapy to post my power sypply schematics, maby even my PCB-layout.


Taking out the frequency compensations (as posted by amplifierguru ) doesn't realy change a thing!
In fact, the amp sounds more crispy, more clean, or clear :)
So, I don't think there's an oscillation problem with my amp...but still..I'm learning every day from this amplifier :)


Tonight I'm gonna change the emittor resistors in the differential inputstage into 1k and see what happens.

I once again want to state that the output stage now used is IRFP240/IRFP9240 and nog IGBT anymore (one broke and they are just extremely expelcive roudn here...'bout €27 a piece:smash: )


Once again, thanks for your time and brainpower!

Bart
 
Barre said:

This is the first time I've heard about testing the open loop gain?!
A quick and dirty calculation learns that this value is very high!
So how and why should one try to measure that?


R3/C3 and R4/C4 are indeed frequency compensation for the differential stage.
I do not think they form any kind of a problem.


So, would it be BETTER to make the DC-feedback unity?

I don't like the DC-offset thingy around P1 either.
It still leaves my output with 600mV DC.
But then again... a healthy amp shouln'd have something like 2V DC offset to be trimmed away by the offset circuit ion the first place right?


Open loop gain can be tested by placing a differential amplifier accross +/- differential inputs (input and feedback) and measuring it's response compared to the input voltage. The output of the differential amplifier will be down the number of dB of the open loop gain. -90dB of response= 90dB of open loop gain. Douglas Self outlines this in his Audio Amplifier Handbook.

If the open loop gain is above 1 and the phase shift of the system moves through 180 degrees the amplifier will not be stable. If R3/R4 and C3/C4 do not compensate the amplifier adequately stability problems will ensue.

As far as the DC gain, I can see no reason why you wouldn't be able to reduce the systems gain at DC to 1 to reduce your offset. I have to wonder if the DC coupled amplifier was done for a marketing advantage to tout response to DC.


Barre said:


Tomorrow I'll be hapy to post my power sypply schematics, maby even my PCB-layout.


Taking out the frequency compensations (as posted by amplifierguru ) doesn't realy change a thing!
In fact, the amp sounds more crispy, more clean, or clear :)
So, I don't think there's an oscillation problem with my amp...but still..I'm learning every day from this amplifier :)



Oh, I got you confused with the original thread starter. I gotta read these things more carefully.

Scott
 
Yes Barre,

You should go ahead with the test I originally said. Yes the RC series networks are frequency compensation but they could well be part of the problem. I wanted to dispense with them and apply large local feedback to reduce the first stage gain, hence the 1K's. The Vas should be fine without Ccomp and running from the higher source Z without the RC's.

I am trying to determine if your oscillation is in the diff'l and Vas OR the CFP! First to localise it.

We need to make it better or worse - an effect! That may aid in homeing in on it. It seems this design has a flaw.

Bear with me.

Cheers,
Greg
 
Barre,

I meant for you to make the changes I suggested ALL TOGETHER.

You need to remove C5 as well and make the 4 emitter resistors 1K. All one step. Then see if you have any oscillations. If you can post a picture of it. It may be in the CFP. If so you need to put small (size) film C's from supply pin of the MOSFETS to GND as close as possible. Use 100nf or thereabouts.

P.S. If it doesn't exist with the two RC's removed and C5 off then you can try reducing the four 1K's to, say, 330. Then 100. Leave C5 off.

Good Luck

Greg
 
To amplifierguru:

Oké I'll try the changer all together.

Thing is...I don't have the tools (generator, oscilloscope and lab power supply) at home to measure oscillations and school is out for summer (as in the song) right now.

I'll think of a way to measure my output signal with my computer...
but please be patient as this my take a while to get it right you know!

Just one more thingy...CFP = what exactly? Common Follower...something? Just a guess
(damn all those terms are different in all languages :p)
 
Leggendo tutti questo commenti mi è venuta voglia di riprendere il mio vecchio amplificatore a hexfet della Elektron da 60W pubblicato in italia su Progetto Elektor n.8 - 1994 e non come gli altri che lo hanno visto con la rivista Elektor n. 12 1993. Nel mio caso ho avuto problemi in una sola delle 2 schede perche' l'altra funzionava perfettamente per cui non ho mai capito il motivo. Ma adesso ho deciso di riesumare il progetto e fare funzionare anche l'atra scheda seguendo alcuni suggerimeti che ho letto in quqesto sito. Scusate potete tradurlo voi con google.
 
Just another Moderator
Joined 2003
Paid Member
:cop: ^ translation for above post
Reading all the comments I had the wish to resume my old amp hexfet the Elektron 60W published in Italy on Project Elektor 8 - 1994, and not like the others who have seen the Elektor magazine n. 12 1993. In my case I had problems in one of the two cards' cause the other one worked perfectly so I never understood why. But now I've decided to resurrect the project and run even the atra card by following some suggerimeti I read in quqesto site. Sorry you can translate it with google.

Manlio please also include the google translation in your post if you wish to post in your native language. Rule 10.


Manlio prega di includere anche la traduzione di Google nel tuo post, se si desidera inserire nella vostra lingua madre. Regola 10.
 
sorry,
Reading all the comments I had the wish to resume my old amp hexfet the Elektron 60W published in Italy on Project Elektor 8 - 1994, and not like the others who have seen the Elektor magazine n. 12 1993. In my case I had problems in one of the two cards' cause the other one worked perfectly so I never understood why. But now I've decided to resurrect the project and to also work the atra card by following some suggerimeti I've read on this site.
 
Chesus...I have one of these amps.
Quite nicely built by someone.
Dual mono, soft start, lots of caps (unfortunately Samwha)

My problem is that if I adjust BIAS to 100mA it will swing between like 50 and 150mA at a frequency of 0.1Hz or so.
Also every now and then one channel will go into protect (relay just opens)

Is the DC offset adjustment done really weirdly?
I am no expert in this particular matter, but connecting rails straight to the 1M offset trimmer, is that a good idea?

If I touch any ground, output, rail or emitter resistor with my multimeter I immediately hear a faint pop and relay opens. This can happen to both channels, but one is much more sensitive.

When I got the amp it was ungrounded. But I added a grounded mains cable and now everything is tied to the same ground (usually not a good idea, but here it works as before. Without hum.

Amp sounds awesome though...but yes it is a pain in the sitting organ.
Someone buy it from me, please! :)
 

Attachments

  • 2018-03-02 19.47.45.jpg
    2018-03-02 19.47.45.jpg
    702 KB · Views: 146