I'm having a problem with my power amp. It is a design from Elektor mag called IGBT power amp. Its schemetic can be found below:
http://web.tiscali.it/audiofanatic3/Tipo/Stato_solido/pic_finaliSS/140W_IGBT_GT20D101_201.jpg
The problem is that there is a burst of oscillation on the peaks (both positive and negative) of the output. This only happens when the output level exceeds 30W (I measured with a 5 ohm resistance). The oscillation is observed on the powers rails and also slightly on the output itself.
Does anybody have a clue as to resolve this problem?
Usually I only play at 1-2 W level, at which the oscillation is not present. Would oscillation at these higher levels affect sound quality at listening levels?
regards Abo
http://web.tiscali.it/audiofanatic3/Tipo/Stato_solido/pic_finaliSS/140W_IGBT_GT20D101_201.jpg
The problem is that there is a burst of oscillation on the peaks (both positive and negative) of the output. This only happens when the output level exceeds 30W (I measured with a 5 ohm resistance). The oscillation is observed on the powers rails and also slightly on the output itself.
Does anybody have a clue as to resolve this problem?
Usually I only play at 1-2 W level, at which the oscillation is not present. Would oscillation at these higher levels affect sound quality at listening levels?
regards Abo
I have experience the problem myself with 2SK135/2SJ50 where only K135 oscillated above a certian current value. This is caused by the behavior of the capacitances across the mosfet. The capacitances are very unlinear, a drawback of mosfets 
You could increase the value of the gate resistors or insert ferrite beads. I suggest that you start to change the resistor values.
You could increase the value of the gate resistors or insert ferrite beads. I suggest that you start to change the resistor values.
I agree that there shouldn't be any oscillations whatsoever in amp amp. But that doesn't really answer my question.
Would oscillations at high power levels affect sound quality at lower levels at which the oscillation do not occur?
Strangely I don't get a hyperlink to the schematic to work. It can be found with a google search. Search for:
igbt monostato 140W.
This will yield 1 result. This is a bulleted list. The schematic is found by clicking the 10th bullet: 140 IGBT gt20D101/201.
Lex
Would oscillations at high power levels affect sound quality at lower levels at which the oscillation do not occur?
Strangely I don't get a hyperlink to the schematic to work. It can be found with a google search. Search for:
igbt monostato 140W.
This will yield 1 result. This is a bulleted list. The schematic is found by clicking the 10th bullet: 140 IGBT gt20D101/201.
Lex
I've had a situation as described where oscillation seemed to "flare" a little bit on peaks. (Around 400kHz) I cured it by making a new PCB where: A: the out put section was better isolated from the small signal sections and B: taking pains to minimize parrallel traces and keep those that remained farther away from each other.
While I was working on the new PCB, I went ahead and installed the first set and used them untill the new ones were ready. Although it was reasuring to have a new set of PCBs without the oscillation, I can not honestly say that I could percieve a difference. Maybe if I had conducted very precise ABX tests I might have heard something - maybe not. I conclude while that as a matter of principle non oscillation is is to be preferred, very small amounts or parasitic oscilliation don't seem to do any audible harm. The key to this may be that it needs to be limited to frequencies where the the amps gain has begun ti roll off significantly.
While I was working on the new PCB, I went ahead and installed the first set and used them untill the new ones were ready. Although it was reasuring to have a new set of PCBs without the oscillation, I can not honestly say that I could percieve a difference. Maybe if I had conducted very precise ABX tests I might have heard something - maybe not. I conclude while that as a matter of principle non oscillation is is to be preferred, very small amounts or parasitic oscilliation don't seem to do any audible harm. The key to this may be that it needs to be limited to frequencies where the the amps gain has begun ti roll off significantly.
there is an update on the design by elektor to prevent oscilating problems and actually the amp slightly got better alsow.
in dutch
-> Monteer parallel aan R31 een condensator van 27n ( aan de soldeerzijde )
-> Plaats tussen de collector van T8 en massa een weerstand van 20K
-> Plaats tussen de collector van T9 en massa een weerstand van 20K
-> vervang R20 door 1K8
-> vervang R17 en R18 door 390E
-> vervang R3 en R4 door 33E
Rudy / sry no time, if somebody would be so kind to translate it for the rest off the people here
in dutch
-> Monteer parallel aan R31 een condensator van 27n ( aan de soldeerzijde )
-> Plaats tussen de collector van T8 en massa een weerstand van 20K
-> Plaats tussen de collector van T9 en massa een weerstand van 20K
-> vervang R20 door 1K8
-> vervang R17 en R18 door 390E
-> vervang R3 en R4 door 33E
Rudy / sry no time, if somebody would be so kind to translate it for the rest off the people here
No he's not kiddin'!
I tried that very same mod. But it only made things worse. After changing the circuit, the amp was now really oscilating. I wasted a few evenings on that mod.
After I had changed everything back to how it was, I still had these parasitic oscillations. I think I need higher gate resistance. How much should I increase the values? Anyone?
Thanks
I tried that very same mod. But it only made things worse. After changing the circuit, the amp was now really oscilating. I wasted a few evenings on that mod.
After I had changed everything back to how it was, I still had these parasitic oscillations. I think I need higher gate resistance. How much should I increase the values? Anyone?
Thanks
okey, the things i had to do before mine worked, alsow i did not make this one, i made the hexfet ( originally ) version, but its basicly the same amp.
1. I made the changes according to elektor.
2. I changed the BD139/140 pairs into MJE340/350 pairs, they have a higher HFE and came from the same brand ( The BD's didn't ), and then i took the time to match these, witch is also mentioned in the article.
After this no problems annymore with the amp.
1. I made the changes according to elektor.
2. I changed the BD139/140 pairs into MJE340/350 pairs, they have a higher HFE and came from the same brand ( The BD's didn't ), and then i took the time to match these, witch is also mentioned in the article.
After this no problems annymore with the amp.
Seems like this amp is really a pain in the b*t.
When I put my speaker on the amp, one of the IGBT's draws all the current and the other one doesn't do anything anymore :s:s:s:s:s
So, one of my IGBT's died of course when I put some music on the amp (klasse A ingesteld?)
I changed the IGBT's for a pair of IRFP240 / IRFP9240 and it working....but still the same problem.
So for now I've just added a capacitor at the end, so that a speaker cannot infuence the DC of the outputstage.
Anyone who has any idea what to do about it?
The schematic is symetrical so...I'd prefer not to work with that capacitor you know!
Hope there are some smart dudes out here ;-)
Thx
Barre
When I put my speaker on the amp, one of the IGBT's draws all the current and the other one doesn't do anything anymore :s:s:s:s:s
So, one of my IGBT's died of course when I put some music on the amp (klasse A ingesteld?)
I changed the IGBT's for a pair of IRFP240 / IRFP9240 and it working....but still the same problem.
So for now I've just added a capacitor at the end, so that a speaker cannot infuence the DC of the outputstage.
Anyone who has any idea what to do about it?
The schematic is symetrical so...I'd prefer not to work with that capacitor you know!
Hope there are some smart dudes out here ;-)
Thx
Barre
Dominant pole capacitors?
Is it just me or are there no dominant pole capacitors in this circuit? T9 and T8 form the VAS stage right? There is nothing in those stages that I see that form a local feedback loop, increasing feedback with frequency and slugging the whole amplifier. Is that functionality supposed to be provided by C3 and C4?
Scott
Is it just me or are there no dominant pole capacitors in this circuit? T9 and T8 form the VAS stage right? There is nothing in those stages that I see that form a local feedback loop, increasing feedback with frequency and slugging the whole amplifier. Is that functionality supposed to be provided by C3 and C4?
Scott
To apmlifier guru:
I'll check that first thing tomorrow.
Now I still have one exam to finish.
You call is oscillation?
What you advise me to do is take away every capacitor that prevents oscillation and increasing some local feerbackresistors.
Did I get this right?
ToScottRHinson:
Sorry if I'm not familier yet with all english terms yet, but VAS = voltage amplification stage?
Yes, that's the only purposeT8 and T9 have I gues.
Also they form an active load in T7.
No, in this stage there isn't much feedback, just local feedback formed by R22 and R23 I guess.
Correct me If I'm wrong...I'm learning...but this is the only way to get to know things right?
another thing that comes up to me.
The AC feedback is the same as the DC-feedback in the overall feedback loop.
Wouldn't it be better if I added a capacitor between te lower side of R6 towards the ground .
In that case the DC amplification is set to 1 and ...I always lthought that could'n be a bad thing, keeps things stable,no?!
I really appreciate the time you spend to help me out (If you copare that to Belgium forums,lol) and hope we are in for a learnfull journey
;
Barre
I'll check that first thing tomorrow.
Now I still have one exam to finish.
You call is oscillation?
What you advise me to do is take away every capacitor that prevents oscillation and increasing some local feerbackresistors.
Did I get this right?
ToScottRHinson:
Sorry if I'm not familier yet with all english terms yet, but VAS = voltage amplification stage?
Yes, that's the only purposeT8 and T9 have I gues.
Also they form an active load in T7.
No, in this stage there isn't much feedback, just local feedback formed by R22 and R23 I guess.
Correct me If I'm wrong...I'm learning...but this is the only way to get to know things right?
another thing that comes up to me.
The AC feedback is the same as the DC-feedback in the overall feedback loop.
Wouldn't it be better if I added a capacitor between te lower side of R6 towards the ground .
In that case the DC amplification is set to 1 and ...I always lthought that could'n be a bad thing, keeps things stable,no?!
I really appreciate the time you spend to help me out (If you copare that to Belgium forums,lol) and hope we are in for a learnfull journey
;Barre
Barre said:
ToScottRHinson:
Sorry if I'm not familier yet with all english terms yet, but VAS = voltage amplification stage?
Yes, that's the only purposeT8 and T9 have I gues.
Also they form an active load in T7.
No, in this stage there isn't much feedback, just local feedback formed by R22 and R23 I guess.
Correct me If I'm wrong...I'm learning...but this is the only way to get to know things right?
another thing that comes up to me.
The AC feedback is the same as the DC-feedback in the overall feedback loop.
Wouldn't it be better if I added a capacitor between te lower side of R6 towards the ground .
In that case the DC amplification is set to 1 and ...I always lthought that could'n be a bad thing, keeps things stable,no?!
I really appreciate the time you spend to help me out (If you copare that to Belgium forums,lol) and hope we are in for a learnfull journey
Barre
Whoops, I apologize, I sometimes forget that different countries use different technical terms for things. Yes, VAS= Voltage Amplification Stage.
Yes you are correct the Amp has a DC feedback that is the same as it's AC feedback, and therefore the gain at DC is the same as AC. That's the reason for P1 and the associated networks...it's used to adjust DC offset. I don't particularly like that scenario myself since it can lead to offset drift with temperature.
I'm not sure of the function of R3, C3 and R4, C4 in this circuit, I'm not sure I've ever seen an amp circuit with those in there. If anyone has a quick explination I'd be all ears.
Without putting the whole circuit in spice to check open loop gain, I would think that placing a 47pF to 100pF cap from collector to base on both T8 and T9 would improve stability. Thats the typical value used for most amps. Because of the circuits differential nature slew rate should still be more than sufficient.
Also, when I was at University it was drilled into us that IGBT's have an inherent structure such that when they excede a certain current conduction that latch on permanently until power is externally removed. There is no protection circuitry in this design to prevent this condition.
Scott
One more thing. Can you post a schematic and picture of your chassis and power supply wiring? I have seen more than one amplifier that is quite stable on its own when wired correctly, but when the power supply leads are too long, not twisted, badly grounded...etc bursts into spurious oscillation in some of the weirdest possible ways.
Scott
Scott
