From Oakley Sound Sent Wed, Jan 17th 2018, 12:32
From Oakley Sound Sent Wed, Jan 17th 2018, 12:32
> First thing we want to do is to reduce the input impedance. This will reduce the noise as well. Yes, it reduces input resistance but at the expense of having to increase the coupling capacitor to maintain low frequency response. It's not a bad thing to reduce the input resistor to something like 220K and up the capacitor to a 220nF polyester. However, I would argue that replacing the cap with a 4.7uF electrolytic like in Don's modification isn't going to improve things. Noise is going to improve with Don's mod but only if you don't actually plug anything into the input. As soon as a synth is connected the output resistance of the synth is going to effectively short out the input resistance of the CE-300 and any thermal noise from that 1M resistor will be effectively silenced. The other problem I have with that particular mod is that the input signal now has the potential to shift the virtual 7.5V reference for the whole unit. Current travelling through that 10K input resistor, if big enough and low frequency enough, will disturb the reference voltage. I'm not sure what effect that will have on the sonics but it can't be good. > Next thing is the input gain This is OK I suppose but there is a pot right in front of the amplifier stage. The mod effectively has the same effect as turning the pot to half of what it would have been. This maybe useful if the input signal is really hot but I'd be tempted to pad the input before even the first op-amp. The CE-300 has only a +15V supply and a virtual 7.5V for reference. Thus all the op-amps can only swing by about 10V peak to peak, which is quite small for a pro-audio unit. Any modular synth directly connected to this unit will distort - modded or not. Lowering the resistors as in the mod does reduce any bias current noise, but 22K might be better overall than 10K to reduce the load on the op-amp and keep the input impedance high enough for C5 not to affect the audio band. > Now it's time to take care of the output stage. Required I guess because of the reduction in gain at the input. But notice that the gain has been increased by a factor of four while the input was reduced to half. I'd be tempted to leave it at two and make R105 and R108 both 100K. None of these mods will affect the sonic character of the unit - bar the possible problem with large low frequency inputs causing a shift in the reference voltage. Ignoring that bit all I can see they do is to have the same effect as simply turning the input level down a bit and increasing the gain required on the output. The comments further down that page that the unit now becomes a totally different unit are, I believe, misplaced. I've never heard a CE-300 so I can't comment on its sound. However, from the schematics I see no reason why it shouldn't sound as good as at least the Dim-C. But like Florian so eloquently put it - just go easy on the controls to get the subtle chorus that we all know and love. Tony www.oakleysound.com