Re: BAN Build Thread
Posted: Thu Aug 12, 2010 8:56 am
No BC109C it just older. BC549C is modern equivalent.greenmanhumming wrote:BC109C is much more expensive, is there an advantage?
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No BC109C it just older. BC549C is modern equivalent.greenmanhumming wrote:BC109C is much more expensive, is there an advantage?
The LM394 is 2 x 50 parallel matched transistors so will usually drop the noise floor level 1 to 3dB more.greenmanhumming wrote:I have upgraded my BANs with LM394s, seem to be quieter , and a bit more solid sounding, maybe less sibilant, more emphasis on the low mids.should I ground the case of the LM394s?
Yes that is correct. The transformerless input BAN, SSL, Amek, Neve Mozart, THAT1510, SSM2017 etc will be 6dB down when not presented with audio signal on both phases of the balanced line.greenmanhumming wrote:I don't have a schematic for the BANs.
I have just plugged in 2 C414s identical in every way except one has an output transformer and the other doesn't - it has pin 3 grounded through a cap.
the 414 with the output transformer is significantly louder than the other on the BAN, but when plugged into a pre with a transformer input they are the same level.
Yes correct.am i right in thinking that a BAN is essentially 2 channels of amp independent of each other one for each side of the balanced signal?
It is only used by the preamp to do the noise canceling.so that in transformerless output one side goes unused? would this explain the level difference?
It depends on if the noise pickup pin3 has the same impedance (resistor to pin 1) as the audio output pin 2 from the microphone as to how good the signal to noise is. The fully floating transformer output will usually always be better signal to noise wise.greenmanhumming wrote:Is the signal to noise or other performance degraded by this effect? All my mics are transformerless except the ribbons and the one 414 mentioned and most only output signal one one pin.
Easy to do but will cost you twice the price of the BAN for a 24way attenuator and then you will be stuck with 2dB or 3dB steps which with most same type microphones whose output varies +/-3dB output level wise isn't really going to help. Like all our 75dB mic pres you need a stepped attenuator and a small range trim as nothing is matched in the real audio world when microphones and air distance are included.greenmanhumming wrote:I'm also getting frustrated with the cheap 10k reverse log pots, I'd like to replace them with a stepped attenuator, but reverse log attenuators are not to be found, I could build one, but getting the right resistor values could be tricky. how about a kit?
or can you use a linear attenuator with a resistor across it to make it log?
If working properly can only be due to the +/-1% resistors and +/-5% 10ohm and +/-10% 10k rev pots.greenmanhumming wrote:I seem to be having multiple problems with my BANs possibly since changing to LM394, and fitting IC sockets to the input transistor holes for easier swap out. but some of the problems may have been pre-existing.
* one channel is louder than the other, this is partly pot mismatch, but not entirely.
Sounds like a plated hole could have been open circuited while changing the transistor pair or the bandwidth limiting ceramics need to be a larger value for your configuration. Gain pot wires to long and not very tightly twisted. Also worth checking if it does it with the transformer output mic or only with the transformerless mics. As bad transformerless mics short pin 1 and pin 3 of the XLR together which is bad.* both channels seem to have high frequency oscillation, the louder channel has it in both + and - sides but it disappears if the gain is above 40% on the knob. The other (quieter) channel its lower level only in one polarity but doesn't disappear until the gain hits about 85%
Is your oscilloscope probe turned to x10 to minimize direct injected capacitance when measuring sensitive mic input stages. As often I see oscillation caused by the oscilloscope probe itself. Is it a pure frequency or noise?its not audible, but easily visible on the scope when I was looking for causes of the gain disparity.
The OPA2604AP running on 48v will always run hot as even at 15mA this is a lot of wattage in a small package.also the opa2604 chips seems to be hotter than i would expect (ranges from warm to too hot to touch for more than a couple of seconds), but it's hard to say which is hottest or under what conditions.
No not at all you just misunderstand how the pre works. It has 2 unbalanced gain amps that are then summed together to noise cancel with a third opamp and then a inverting unity opamp to make the -out pin 3. So you will always get the same signal on pin 3 inverted as +out pin 2.BTW when I input a signal generator on only one pin, I do get output from both polarities of the output, phase reversed correctly. this seem to contradict our conversation above.
You need to do testing with different mics and no mic to see if that changes the oscillation and read the frequency and level at every opamp output pin to narrow down the problems you are having and make sure the oscilloscope isn't actually causing the problem. If the mic pre is oscillating in the gain stage there will almost always be a point on the gain pot where a crack or thump sound happens as the mic pre goes into oscillation at that point.help! what should I check? I can't find anything incorrectly connected.