Hi guys. I have made pcb which made possible using four ad797 instead of one QUAD Opamp MC33078-D. What improvement in sound quality - great. But there is one problem. When I turn the volume pot close to miniumum there is a BUZZZZ sound in the speakers. When I touch the volume pot BUZZZING sound gets noticeably smaller. Volume pot is a metal one.
This really bothers me. AD797 gets hot/warm while working - but not that much so You can't touch it.
With 797 this amp sound the best and I need to o something with this.
Decoupling of 797 is made by two tantalum caps from -V and +V pins. Maybe to far away from the supply pins of the opamp?
Please help. Thnx.
This really bothers me. AD797 gets hot/warm while working - but not that much so You can't touch it.
With 797 this amp sound the best and I need to o something with this.
Decoupling of 797 is made by two tantalum caps from -V and +V pins. Maybe to far away from the supply pins of the opamp?
Please help. Thnx.
Attachments
Do not copy the MF preamp. It is a VERY bad design.
The chip is probably oscillating because at low volumes, the gain is too low for the circuit to be stable. Eventually the pot will go bad from DC across it's track.
See this page for more info: http://www.mhennessy1.f9.co.uk/mf_a1/technical.htm
Mark has also made a replacement preamp circuit for the A1 which is far better. He uses an NE5532 but you should be able to adapt it for 2xAD797 if you wish.
AD797 needs high frequency decoupling, try some good 100nF ceramics as close to the AD797 as possible - perhaps fit them beneath the pcb soldered to the pins directly.
The chip is probably oscillating because at low volumes, the gain is too low for the circuit to be stable. Eventually the pot will go bad from DC across it's track.
See this page for more info: http://www.mhennessy1.f9.co.uk/mf_a1/technical.htm
Mark has also made a replacement preamp circuit for the A1 which is far better. He uses an NE5532 but you should be able to adapt it for 2xAD797 if you wish.
AD797 needs high frequency decoupling, try some good 100nF ceramics as close to the AD797 as possible - perhaps fit them beneath the pcb soldered to the pins directly.
Yes this is true. The design od the preamp is no the best one - dc at pot. But after replacing old QUAD opamp the difference is HUGE. I really would like to stay with this design because I don't want to mess inside right now. I'll try to add 100nF as You mentioned. Is there a way to change gain at this circuit when pot is at minimum?
Not in the configuration used - if you tried to do that, you would find your volume control does not go to zero!
Another alternative might be to bypass it totally and wire the volume control up as an attenuator only. If you only use high output sources such as a cd player or a PC, this would work OK.
Another alternative might be to bypass it totally and wire the volume control up as an attenuator only. If you only use high output sources such as a cd player or a PC, this would work OK.
Thanks. I generaly use PC and CD. So It could do the job. In my expirience passive preamps always make sound thinner IMHO. That is why I want to keep it that way - by changing opamps I can change the sound in a way I want. Thanks again.
PS. Do you think that changing 47pF caps from ceramic to Silver Mica and resiostors in signal path to something more refined is a good ideaand worth a try?
PS. Do you think that changing 47pF caps from ceramic to Silver Mica and resiostors in signal path to something more refined is a good ideaand worth a try?
the so called "BUZZZZ sound" in the speakers is in most cases oscillation effect, often in the MHz aera. That must be check in any case with an oscilloscope. It is also to do by the pos. and negative supply rails for te OP AMP device. A second reason are residual dc components at the volume control pins. The so called "BUZZZZ sound" is audible in this case only during the turn at the volume control. Then it is more easy to find solution.
In this cases you must first replace the electrolytic capacitors between volume control output and power amplifier input (mostly between 1uF and 4,7uF) through unipolare MKS, MKC or MKH versions (e. g. Siemens or WIMA) and secondly you must choise an OP AMP with jFETs (by AD797 you can introduce additional jFETs).
To save time for troubleshooting I introduce by A1 and AVALON (A200) independend external power supplies for the OP-AMPs in the line- and phono stage. For connecting this devices the brand "binder" is a good solution - see
http://www.binder-connector.de/ximages/1385328_713810mm.pdf
I upload for you my back engeneered schematic from A1 - I think, the different to your A2 isn't very large by the line amplifier section.
In this cases you must first replace the electrolytic capacitors between volume control output and power amplifier input (mostly between 1uF and 4,7uF) through unipolare MKS, MKC or MKH versions (e. g. Siemens or WIMA) and secondly you must choise an OP AMP with jFETs (by AD797 you can introduce additional jFETs).
To save time for troubleshooting I introduce by A1 and AVALON (A200) independend external power supplies for the OP-AMPs in the line- and phono stage. For connecting this devices the brand "binder" is a good solution - see
http://www.binder-connector.de/ximages/1385328_713810mm.pdf
I upload for you my back engeneered schematic from A1 - I think, the different to your A2 isn't very large by the line amplifier section.
Thanks. The cap before the poweramp is already MKP right now. It seems like the AD797 at the output is not a way to go in here.
I'll try to find something as good as 797. Someone told me to change the resistor 4.7k after the first opamp to 1k to change gain.
Is is correct? If I change this resistor to 1k - does it change gain of the second opamp?
The other idea was to put in parrarel a cap 47p with the pot.
Confused.
I'll try to find something as good as 797. Someone told me to change the resistor 4.7k after the first opamp to 1k to change gain.
Is is correct? If I change this resistor to 1k - does it change gain of the second opamp?
The other idea was to put in parrarel a cap 47p with the pot.
Confused.
If you reduce the resistor behind the first stage to 1K, the max possible gain factor of follow stage is max. 50 instead 10 - I don't recommended this.
an another approach is to remove the complete line amp section (so that you have only passive preamp function). If you have done, you must use GND-ref. for volume control. Therefore you must connected the output of tape monitor switch to the input of volume control. The output of volume control goes to the input caps of power amp. The third connection of volume control goes to GND (for both channels). If the "BUZZZZ sound" is still present, then the reason is by the power amp section or you have a unwanted GND loop.
an another approach is to remove the complete line amp section (so that you have only passive preamp function). If you have done, you must use GND-ref. for volume control. Therefore you must connected the output of tape monitor switch to the input of volume control. The output of volume control goes to the input caps of power amp. The third connection of volume control goes to GND (for both channels). If the "BUZZZZ sound" is still present, then the reason is by the power amp section or you have a unwanted GND loop.
This opamp is not oscillating when I listen at normal levels - in fact i really very rarely goes with volume pot to minimum. So most of the time there is no oscillation. I can live with it. Details and overal quality of the sound have nothing to do with oscillation. It sounds so much better. Bas, treble, rich midrange. Great improvement.
If you make at first step the passive preamp modification as I describt above, you can clearly say, whether power amp or pre amp section is the reason for BUZZZING sound at very low level close to the zero position of your volume control.sviru said:
Additional you can compare the sound quality in this situation to that with AD797.
Now you can better make your decision what is to do as next step(dependent of the results, which you can then there observe)
Basically - the higher the voltage gain resp. more active gain stages in the signalpath, the higher the distortion and additional the risk of unwanted effects
If you make an voltage gain of 20 bis 50 in your line stage, you get a whole voltage gain of 1500 until 2500 - dependent of your voltage gain of power amplifier stage - that is aprox the same order.
The theoretical advantage is that low level signal sources (arround 50mVss) can already drive your amp to clipping zone. But normal line levels from today devices have at least arround 1Vss - 2Vss. So the theoretical advantage is clearly a disadvantage.
Please note additional, that your line stage gain faktor by musical fidelity's volume control topology (= inv. mode) is variable and not constant. i. e., by "zero position" of the volume control knob - gain factor goes down also to zero. AD797 is only stable by unity gain . Please ask Analog devices, what is to do for "zero gain" - conditions as it is present by many older Musical Fidelity devices.
Buzzing sound is coming from AD797. With original opamp everything is fine. With AD823 - everything is fine. In the AD datasheet they use compensation capacitor from pin 8 to output - 50pf to eliminate distortion. Maybe this could help.
I would like to stay with this preamp, I don't like the sound of passive preamps.
I would like to stay with this preamp, I don't like the sound of passive preamps.
i agree with Eva's suggestion.
i have a funny story about a friend who built a headphone amp with the old EL2008 high buffer buffer as an output stage.
when operating, the circuit was quite hot. my friend talked about how clean it sounded. i asked if he was concerned about the heat - he said no, it's operating in full class a.
i demonstrated the circuit was oscillating. after fixing that, he said "it doesn't sound so clean anymore.
mlloyd1
i have a funny story about a friend who built a headphone amp with the old EL2008 high buffer buffer as an output stage.
when operating, the circuit was quite hot. my friend talked about how clean it sounded. i asked if he was concerned about the heat - he said no, it's operating in full class a.
i demonstrated the circuit was oscillating. after fixing that, he said "it doesn't sound so clean anymore.
mlloyd1
Eva said:You are noticing the HF distortion produced by op-amp instability as more "detail"
If the oscillation goes away, that distortion will disappear too
if you are determined to stick with a feedback type volume control, you could do much worse than baxandall's arrangement that you can see employed in doug self's opamp based preamps.
also, i understand you really like the ad797. however, it is a bit finicky if you are not rigorous in attention to details. pma's suggestion to closely review the datasheet is right on. perhaps a less picky part may also provide the performance the seek: something from the new national parts line for example or maybe even the old opa627.
with the impedance levels you are using, the noise performance of the ad797 is likely not of value.
mlloyd1
also, i understand you really like the ad797. however, it is a bit finicky if you are not rigorous in attention to details. pma's suggestion to closely review the datasheet is right on. perhaps a less picky part may also provide the performance the seek: something from the new national parts line for example or maybe even the old opa627.
with the impedance levels you are using, the noise performance of the ad797 is likely not of value.
mlloyd1
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