741 op.amp works, tl 071 is oscillating?

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zoranaudio

zoranaudio

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Joined 2008
megajocke said:
It depends on a lot of other things in the circuit, I'd just try with another value and see what happens. I'd guess it needs to have a lower value but I'm not sure.

You could try it and see if it gets less stable or more stable.
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wg_ski said:



That cap is the key to getting this stable. I've made many many many of these to work - including kilowatters. In addition to the 90 degrees thru the opamp and the 90 thru the VAS, you get a few extra degrees through the outputs. Normally you can ignore them, but not here. Tap the lead compensation cap from the output of the VAS, NOT the output of the amp. And the value will need to be determined experimentally, unless you do a Bode analysis of the circuit (in which case you can nail it the first time).
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Ok, I will try to fix this problem , and will play a little with C10, lowering the value perhaps desolder it and with higher value, this cap is ceramic, is that the best type for this purpose?
 
W

wg_ski

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Joined 2007
zoranaudio said:


Ok, I will try to fix this problem , and will play a little with C10, lowering the value perhaps desolder it and with higher value, this cap is ceramic, is that the best type for this purpose?
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I use silver micas for these and the Miller caps. The ESR can be a bit high on micas but it doesn't matter here. Class 1 ceramics are fine, but Class 2 or 3 are absolutely NOT. Trouble is, many times you don't know which you have.
 
zoranaudio

zoranaudio

Member
Joined 2008
megajocke said:
It depends on a lot of other things in the circuit, I'd just try with another value and see what happens. I'd guess it needs to have a lower value but I'm not sure.

You could try it and see if it gets less stable or more stable.
Click to expand...


wg_ski said:


I use silver micas for these and the Miller caps. The ESR can be a bit high on micas but it doesn't matter here. Class 1 ceramics are fine, but Class 2 or 3 are absolutely NOT. Trouble is, many times you don't know which you have.
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DEAR FRIENDS, YOU DONT'T KNOW HOW MUCH I AM HAPPY
NOW THIS IS WORKING, YES, YES, YES!!!!
SORRY TO BE NERVOUS IN THE PREVIOUS POST, SO WHAT HAPPENED:
FIRST I TRIED WITHOUT THAT CAP C10, WHEN I TURN ON THE AMP, I NOTICED A SMOKE COMING OUT FROM THE R25+L1(OUTPUT HF SUPRESSOR) AND TURNED OFF.
AFTER I TRIED WITH HIGHER VALUES OF THE C10 TILL 1000 PF BUT NOTHING CHANGED!!!
FEW MINUTES AGO, A BIT MORE RELAXED COME BACK TO THE AMP AND PUT 18 PF FOR THE C10 AND AMP IS PLAYING MUSIC AT ANY POSITION OF THE TRIM. POT FOR BIASING!!!!!!!!
NOW HOW CAN I BE SURE THAT 18 PF IS THE RIGHT VALUE FOR MY AMP??????
AND HOW TO MEASURE IF , PERHAPS SOME OSCILLATIONS ARE ABOVE THE AUDIBLE FREQ. ABOVE 20 KHz??????
MY BIGGEST THANKS TO MEGAJOCKE AND WG_SKI, YOU HELPED ME, AND WILL WAIT FOR YOU IF SOMETHING I CAN CHANGE IN ORDER TO GET THE MAXIMUM FROM THIS AMPLIFIER!
 
W

wg_ski

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Joined 2007
If it were still oscillating, you'd have either distortion or smoke. The fact that you have neither suggests it's stable now.

To get the max out of the amp, go as low as you can on that cap without peaking in the closed loop response. If you can't measure that, use your ears. Above-band peaking will cause it to sound very "detailed" or "bright" initially, but cause fatigue from extended listening. My guesses say this will happen when you get down around 6-8pF. It may start oscillating before than anyway.
 
zoranaudio

zoranaudio

Member
Joined 2008
wg_ski said:
If it were still oscillating, you'd have either distortion or smoke. The fact that you have neither suggests it's stable now.

To get the max out of the amp, go as low as you can on that cap without peaking in the closed loop response. If you can't measure that, use your ears. Above-band peaking will cause it to sound very "detailed" or "bright" initially, but cause fatigue from extended listening. My guesses say this will happen when you get down around 6-8pF. It may start oscillating before than anyway.
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Dear friend!
I would like first to thank You for Your help.
No, the sound now is clear and detailed, when the amp. was oscillating, i can hear "bird song when the drums hit"
I will stay at this value of c10=18pf.
Now I must to check the bias of the power tr. .
Also I would try with changing the TL 071 with NE5534, to see if the sound is better?
Do you think that NE5534 will work now ?
 
W

wg_ski

Member
Joined 2007
zoranaudio said:
All was my mistake!
The c10 was "681" cap that means 680 pf.
I solder 60 pf in place of c10 cos don't have 68pf, even amp was working with 18pf, so nothing's wrong with this scheme.
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And "680" could mean either 680pF or 68, further adding to the confusion. If you're ever in the market for a new DMM, get one with a capacitance function.
 
W

wg_ski

Member
Joined 2007
zoranaudio said:

Do you think that NE5534 will work now ?
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Personally, I've never liked the way the 5532/5534 sound in that application. The Bi-FETs (TL071, etc) just sound better. My fave is actually the LF412 (dual, same as LF353 with better DC spec). If you want lower residual noise than the TL071, go to something like an OPA604 (which is also FET input).
 
zoranaudio

zoranaudio

Member
Joined 2008
wg_ski said:



And "680" could mean either 680pF or 68, further adding to the confusion. If you're ever in the market for a new DMM, get one with a capacitance function.
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Hi wg_ski!
No there is no need for the Digital multimeter, cos I learn to read resistors and capacitors when I was 10 years old, but mistake can be also made!!!!
About one year ago when I constructed the PCB Didn't noticed that, also customer mistake!!!
To be all more complicated that cap. was so little green ceramic about 3mm of diameter, and I can not read because the heatsink of BD 469 or 470 was so high and that cap is located 2-3 mm close to the heat sink.
Byt yesterday when I experiment with the value I just desoldered one leg of the cap, and when I saw that the amp. works with 18 pf cap, I remove old cap completely to put a new cap in the place, AND THAN i SEE WHAT STUPID MISTAKE I MADE, 1 & A HALF YEAR AGO.
ANYHOW THANK YOU FOR THE HELP, TO POINT ME TO THE RIGHT COORDINATES TO FIND MY STUPID MISTAKE.
THIS SCHEMATIK IS O.K AND NOW I AM ENJOING THE MUSIC OF THIS NICE AND STRONG AMPLIFIER.
CHEERS ALL!
 
zoranaudio

zoranaudio

Member
Joined 2008
wg_ski said:



Personally, I've never liked the way the 5532/5534 sound in that application. The Bi-FETs (TL071, etc) just sound better. My fave is actually the LF412 (dual, same as LF353 with better DC spec). If you want lower residual noise than the TL071, go to something like an OPA604 (which is also FET input).
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Yes I just call my brother in Skopje to buy me opa 604 and will try with this op. amp, thank you again my friend!
CHEER'S!





 
zoranaudio

zoranaudio

Member
Joined 2008
Hi!
Anyone have just two capacitors "elna silmic' 4,7mfd to 10 mfd for C1 input capacitor of this amp.
I am searching 2 years here and no chance to find this caps or similar.
I will pay for this "elna silmic" or better coupling caps, if someone can send me!
I need just two for this amp.
I can not open this amp every day to test caps, I'd like to put just good caps and close the box.
Any input, please
 
A

AndrewT

R.I.P.
Joined 2004
Hi,
use a film cap.
Either a metal foil/plastic film or metallised plastic film.

All will perform better than any electrolytic.

If you want cheap and small then use MKT/PES/polyester/Mylar they are all fairly similar.

If you have the space and the money then go for Polypropylene. Warning:
a PP 10uF foil is enormous, about 40mm diam by 60mm long.
 
zoranaudio

zoranaudio

Member
Joined 2008
I put bipolars 4,7 mfd now because I don't have exactly same type of film caps, they are different manufacturer, and I am a bit scarry because one of the channels is close to the toroid even I have a metal shield but film caps I know that are tending to pick up some EMI.
This amp. sounds great, I compare this amp with my old MARANTZ PM80 and I think The Marantz is still more open at the higher mids and trebles , I love this sound of PM 80, very fine.
I measure the offset of my home made amp, and is 0 milivolts on one ch., and 1 mV on the other.
I adjust bias to 25 mV measured from point "8" to point "9" or point 9 and point 10 on the emitter resistors of the power transistors, after 10 minutes turn on without load at 220 volts main voltage, cos the voltage is very unstable and that affected bias too much.
I put LF356N cos I think sound better than TL 071 which is too sharp i think, and LF 356N is clear not dry but have some fat bass i think.
I have no audible hum when put my ears 2 cm to the bass drivers, also just a little noise on the midrange and tweeter but when you go about 10 cm to the tweeter and 1 cm to the midrange driver.
Before I solve my mistake I have bad resistance to EMI , RFI , I got problems when i turn on my fluorescent lights!!!, now that is old story, and now i have to check resistance when I operating on the ham radio with my old YAESU FT 101 E to see how is resistance to RFI for example 14 MHz, , I got problems also before with this.
PS:I tryed to listen music with bypassed c1 , input cap of the amp. and before this amp I have "preamp" also it is a phase invertor for bridge mode, achieved with TL 074, and there is no cap at the output, just a film caps of 0.47 and 1 mfd at the input (balanced XLR or RCA).
I measured 2,9 miliVolts at the output of the pre amp. and i bypassed C1 input cap on the power amp. also I got the same offset at the speaker output!!! which is not too high.
I test this to see what is the difference without C1 input cap, and the sound was clearer and the bass was more cleaner too and better definition .
IS IT DANGEROUS TO WORK THIS MODE WITHOUT C1 INPUT AMPLIFIER CAP???

 
A

AndrewT

R.I.P.
Joined 2004
Hi,
if you decide to DC couple your amplifier, then as a minimum I recommend a DC detect circuit that triggers an output protection, relay, or FET switch, or supply rails crowbars.

It would be nice to add a DC servo to maintain low output offset during all normal operating conditions and to also fit an input mute triggered by that same DC detect circuit.
 
zoranaudio

zoranaudio

Member
Joined 2008
megajocke said:
Very nice amp chassis!

Is that frame made of steel that is welded at the corners? What is the bottom plate made of?
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Hi megajocke, First I would like to thank You for the help, because you were the first which point me to "C10", ....my fatal reading mistake ...680 pf 10 times higher value that original!!!
Yes this chassis is my personal made!!!
I weld steal L profiles 2cmx2cm 2 mm thickness with simply electric welding with normal electrodes, but i think what about corrosion, and I been on the local factory from EMO- Ohrid which made towers for el. distribution and all steal chassis is hot zinc plated, as they said warranty is about 30 years but outside, not in door use!!!!
Bottom of the chassis is from alluminium, and front but I have not more thick alu. for the front panel just 3 mm, Back side is also from steel profile.
All this weights 42 KG, as I measured .
Just big Alu heatsink taken from Siemens soft motor start and copper "U" profiles " weights about 12 kG!!!!, toroid the same!!!!

I am still looking to find "ELNA SILMIC"caps cos The Marantz also use
them and I love Marantz sound.
Also looking to find some "BB" OPA 604 for this amp, but here it is not available, nobody buys that!!!












http://www.metacafe.com/watch/2283055/audio_input_selector_by_z37hwx/

http://www.metacafe.com/watch/2257228/z37hwx/
 
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