Particularly with the EI's, you need to be careful about pick-up because of their wide field. You will only get pick-up in parts of the circuit that are sensitive to this. This means inputs, and particularly inputs to your pre-amp. Also, pick-up is much more likely from high voltage cabling, i.e. mains.
Your best bet therefore is to keep the high voltage mains AC away from the signal circuitry. This means putting the transformers in a separate box. The low voltage AC that you take into your amp box (complete with anything this might have picked up) will be rectified and smoothed, hopefully lessening the effect of any uninvited guests.
So, if you have all your transformers together, there is more risk that your low voltage AC lines could pick up radiated interference from the EI, but this shouldn't matter, because:
1) Any pick-up will be rectified and smoothed
2) The low voltage AC is feeding your DC rails, and is not being amplified (unlike signal inputs)
3) Your 3886's have very good power supply rejection ratio.
12V for the soft-start can go anywhere because it isn't involved in the audio path (unless you are going to use this for your pre-amp: if you do, use the same transformer, have a local bridge and smoothing for the soft-start, then a bridge and smoothing [and regulation] remotely in your pre-amp to supply this).
Fans??? Please tell me these are temperature activated and for emergency overheating only. These can be just as bad, if not worse, than EI's for pick-up. My 10 3886's are on 2.5degC/W heatsinks (one each) and haven't got warm yet.
On the op-amp front, I be surprised if you noticed much difference between any of the better devices mentioned above - most of these are alot better than the multitude of lousy opamps that most recorded music passes through in the recording studio.
Jamie
Your best bet therefore is to keep the high voltage mains AC away from the signal circuitry. This means putting the transformers in a separate box. The low voltage AC that you take into your amp box (complete with anything this might have picked up) will be rectified and smoothed, hopefully lessening the effect of any uninvited guests.
So, if you have all your transformers together, there is more risk that your low voltage AC lines could pick up radiated interference from the EI, but this shouldn't matter, because:
1) Any pick-up will be rectified and smoothed
2) The low voltage AC is feeding your DC rails, and is not being amplified (unlike signal inputs)
3) Your 3886's have very good power supply rejection ratio.
12V for the soft-start can go anywhere because it isn't involved in the audio path (unless you are going to use this for your pre-amp: if you do, use the same transformer, have a local bridge and smoothing for the soft-start, then a bridge and smoothing [and regulation] remotely in your pre-amp to supply this).
Fans??? Please tell me these are temperature activated and for emergency overheating only. These can be just as bad, if not worse, than EI's for pick-up. My 10 3886's are on 2.5degC/W heatsinks (one each) and haven't got warm yet.
On the op-amp front, I be surprised if you noticed much difference between any of the better devices mentioned above - most of these are alot better than the multitude of lousy opamps that most recorded music passes through in the recording studio.
Jamie
AudioFreak
Obviously the bigger the heatsink the better. But the method of mounting can make a large difference too. I don't use washers on my system, and just have the thinnest scraping of heatsink compound, and assume that my heatsink is at the negative rail.
I guess if you are using these devices full range, this is important.
Jamie
Obviously the bigger the heatsink the better. But the method of mounting can make a large difference too. I don't use washers on my system, and just have the thinnest scraping of heatsink compound, and assume that my heatsink is at the negative rail.
I guess if you are using these devices full range, this is important.
Jamie
Well, I finished off the pre-amp board as well as the power suply for the pre-amp. Now all I need is a case, which I'll be putting together tomorrow or the day after... My question is, is there any way of testing the pre-amp without the power-amp? Could I pass the output of my pre-amp to the input of an integrated reciever (total crap) and see if there is any output (test volume etc, should make me happy enough, just want to test if everything was done right)?
Yes.JoeBob said:
By the way, I have combined my power amp power supply and preamp power supply into the same box. Is that what you were originally asking? It works fine. I keep the power supply outputs electrically isolated from one another for more flexibility (i.e. no common ground between their outputs).
The preamp and power amp are in two other boxes.
Well, I plugged in my pre-amp power suply, just to check everything was done right, before I tried playing music through it, and well, it's almost right... I'm using a plug-pack 16V AC 20VA transformer, and a full-wave voltage doubling circuit using the 7915 and 7815 voltage regulators. But the positive rail is +15V while the negative rail is -17V. I'm using the OPA2134 (or OPA2604, have both will see which is better, heard good things about both) in my preamp, will this be a problem? TI's speck sheat for the OPA2134 says up to +/- 18V, so I don't have too much voltage, but they're not the same, I'm really hoping this isn't a problem, but...
Hi,
Slightly unbalanced voltages shouldn't matter too much; the negative peaks will clip earlier but you shouldn't be running anywhere near clipping. The op-amp doesn't care that it's not quite referenced to the midpoint of the two supplies.
I'm curious why the -ve supply is -17V. Suggest you check everything over carefully. Also try some kind of load (a few LEDs and dropping resistors?) to make sure it's really regulating.
Slightly unbalanced voltages shouldn't matter too much; the negative peaks will clip earlier but you shouldn't be running anywhere near clipping. The op-amp doesn't care that it's not quite referenced to the midpoint of the two supplies.
I'm curious why the -ve supply is -17V. Suggest you check everything over carefully. Also try some kind of load (a few LEDs and dropping resistors?) to make sure it's really regulating.
I know it's strange, I've checked and rechecked, I'll try a load. One strange thing is as soon as I unplug the transformer, the - rail drops down to -15V (or would it be considered raises, hmm... oh well). So I'm rather confused, but if you don't think it'll be a problem, I guess that makes me feel a little better.
Two common reasons for voltage regulators giving a higher than expected voltage are firstly that the device is out of spec (try another device) or secondly that the device is unstable and is oscillating. For the latter, ensure that you have a minimum current draw in excess of 5mA at all times and that input and output bypass capacitors are are connected close to the regulator terminals. The 7915 usually needs at least a 2.2uF tant (25uF electrolytic) on the input terminal if the regulator is more than 3" (75mm) from the main reservoir caps and a 1uF tant (25uF electrolytic) on the output terminal (National Semiconductor datasheet figures).
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