Hi,
I've been reading this forum for quite a while now, and decided to register for some time ago. This is actually my first post. My questions are:
1- Is there any significant advantage of using pot's to attenuate output rather than using the pot in feedback loop?
Also, I'm going to use this chip pa as pa section for my guitar amp but since I don't like how they distort, I'm planning to limit preamp gain so that even moderately aggressive playing wouldn't drive the chip into clipping.
2- Should I increase gain of preamp and lower the gain of pa? As this would help with THD figures?
I have some explanations in my mind but I can't be sure. Thanks in advance!
I've been reading this forum for quite a while now, and decided to register for some time ago. This is actually my first post. My questions are:
1- Is there any significant advantage of using pot's to attenuate output rather than using the pot in feedback loop?
Also, I'm going to use this chip pa as pa section for my guitar amp but since I don't like how they distort, I'm planning to limit preamp gain so that even moderately aggressive playing wouldn't drive the chip into clipping.
2- Should I increase gain of preamp and lower the gain of pa? As this would help with THD figures?
I have some explanations in my mind but I can't be sure. Thanks in advance!
Welcome to the forums 
Using a pot in the feedback loop is possible for small signal stages using 'unity gain stable' opamps. That effectively creates an 'active volume control' that can attenuate all the way down to zero output. It can also be configured to have overall gain.
Power amplifiers can not use that approach because most power amps (chip or discrete) are not 'unity gain stable' and always need to run with a fairly high gain. If you look at data sheets for common chip amps (LM3886, TDA2050 etc) they always quote the minimum closed loop gain (gain with feedback) that the device is stable at. Go below that and it will burst into oscillation and be unstable.
Using a pot in the feedback loop is possible for small signal stages using 'unity gain stable' opamps. That effectively creates an 'active volume control' that can attenuate all the way down to zero output. It can also be configured to have overall gain.
Power amplifiers can not use that approach because most power amps (chip or discrete) are not 'unity gain stable' and always need to run with a fairly high gain. If you look at data sheets for common chip amps (LM3886, TDA2050 etc) they always quote the minimum closed loop gain (gain with feedback) that the device is stable at. Go below that and it will burst into oscillation and be unstable.
Active volume control:
Active Volume Control for Professional Audio - TIPD136 - TI Tool Folder
Active Volume Control for Professional Audio - TIPD136 - TI Tool Folder
I think this should move to instruments, as there will be a lot more relevant replies specially wrt gainstaging and where/how you want to clip.
A string of diodes across the chipamp inputs will introduce soft clipping at high amplitudes, you may also be able to use a JFET across the input to achieve a more sophisticated anti-clip/soft limiter effect. The J112 is an excellent candidate here and is very cheap.
Pots in the feedback loop are possible but only within the minimum gain specified by the chip. You didn't say which chip we are talking about but 20-25dB is about the minimum for most of the bunch. You will gain nothing in terms of THD at lower gain settings (at least not for an instrument amplifier).
A string of diodes across the chipamp inputs will introduce soft clipping at high amplitudes, you may also be able to use a JFET across the input to achieve a more sophisticated anti-clip/soft limiter effect. The J112 is an excellent candidate here and is very cheap.
Pots in the feedback loop are possible but only within the minimum gain specified by the chip. You didn't say which chip we are talking about but 20-25dB is about the minimum for most of the bunch. You will gain nothing in terms of THD at lower gain settings (at least not for an instrument amplifier).
Thanks that cleared things up. I actually read those but didn't grasp the meaning it seems. As for active control, I'm pretty sure I'll have hard time getting those chips, even if I could, I wouldn't buy them since they probably cost as much as my preamp We utilize some distortion even when we play "clean" but the harmonic content and cracking of a chip is never the same. Thanks again.
We utilize some distortion even when we play "clean" but the harmonic content and cracking of a chip is never the same. Thanks again.
The active volume control will work with most unity gain stable opamps such as the NE5532 etc.
The simplest version uses a single opamp:
Headphone Amplifiers
The simplest version uses a single opamp:
Headphone Amplifiers
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