chrissugar
Well-known member
As most of you know, I'm talking about the topology of Graeme Cohen that was published in AES 2106. I don't remember if the document was ever posted so here is a link from where you can download it:
http://www.seventhcirclestudios.com/SCA/C84/docs/2202.pdf
For a while I started gathering information about this topology and it's various implementations like the Millenni@ transformerless pre, Fred's jfet version:
http://www.forsselltech.com/JFETMP1.PDF
and Tim Ryan's version:
http://www.seventhcirclestudios.com/SCA/C84/docs/c84_sch.pdf
Interesting is that for a long time I had the same idea as Tim to add a Jung buffer that can drive anything. Although my idea is to put it outside the feedback loop like I saw in many audiphile preamps where the output buffer is after the global feedback.
I have some questions. In the original paper and in Tim's circuit there is no capacitor in the gain set circuit, but in Fred's version there is a 3900microfarad capacitor. My question is: is this capacitor necessary or not. It has a similar role like the ones used in the gain circuit of instrumentation amplifier type mic pres? It works like a high pass filter to remove DC or it is present only to avoid some DC on the gain pot?
My next question is in the Millenni@ manual ( http://www.mil-media.com/pdf/hv3c-manual.pdf ) they say the headroom is 32dBU but at the PSU voltages inside I have no idea how is that possible.
At +/-24V the maximum output peak-peak voltage is lower than 48V in ballanced mode, and that would be not more than 26dBU. Anyone has an idea? It is a mistake in the specs?
Another aspect is what kind of EMI/RFI protection would be suitable for this project? I'm more inclined to build a variation on Fred's version because of the Jfet inputs that are less problematic with EMI/RFI. Any thoughts?
chrissugar
http://www.seventhcirclestudios.com/SCA/C84/docs/2202.pdf
For a while I started gathering information about this topology and it's various implementations like the Millenni@ transformerless pre, Fred's jfet version:
http://www.forsselltech.com/JFETMP1.PDF
and Tim Ryan's version:
http://www.seventhcirclestudios.com/SCA/C84/docs/c84_sch.pdf
Interesting is that for a long time I had the same idea as Tim to add a Jung buffer that can drive anything. Although my idea is to put it outside the feedback loop like I saw in many audiphile preamps where the output buffer is after the global feedback.
I have some questions. In the original paper and in Tim's circuit there is no capacitor in the gain set circuit, but in Fred's version there is a 3900microfarad capacitor. My question is: is this capacitor necessary or not. It has a similar role like the ones used in the gain circuit of instrumentation amplifier type mic pres? It works like a high pass filter to remove DC or it is present only to avoid some DC on the gain pot?
My next question is in the Millenni@ manual ( http://www.mil-media.com/pdf/hv3c-manual.pdf ) they say the headroom is 32dBU but at the PSU voltages inside I have no idea how is that possible.
At +/-24V the maximum output peak-peak voltage is lower than 48V in ballanced mode, and that would be not more than 26dBU. Anyone has an idea? It is a mistake in the specs?
Another aspect is what kind of EMI/RFI protection would be suitable for this project? I'm more inclined to build a variation on Fred's version because of the Jfet inputs that are less problematic with EMI/RFI. Any thoughts?
chrissugar