RFSiesta
Well-known member
Hi,
This is my second thread on groupDIY, trying to build a 10-ch dedicated Keyboard buffer with balanced outputs.
Our studio is two-level, two control rooms and all rooms will be interconnected. Cable length is an issue, and while I'm used to record keyboards directly through line inputs or sometimes even to microphone preamps for transformer colouring (only devices with low enough output Z), we decided we need an impedance converter with 0, 6 or 12 dB gain (maybe switchable, maybe w/ locut as well) and an electronically balanced output for being able to drive cable lengths in excess of 15m without loss.
The device(s) will be included in our patchbay system(s), so rather small footprint is important. I want to have it all on one euro-PCB (160*100mm) including supply & in/out screw clamps and an external PSU for +-15V.
I'm planning a JFET input stage, still not sure if IC or discrete, zener protection diodes on inputs and 4004 on outputs, and a balancing line driver like the THAT1646.
Everything looked neat using ONLY the line driver, except the input impedance is 5kHz, which is a bit low, especially for some older instruments. There are other products on the market, like the SSM2142 with Z=10k, but maybe it's better to add a simple impedance converter to the input to increase impedance to 20-100k or something and get higher gain as well.
Since almost all of the circuits I gOoOrgled are designed for hi Z instrument outputs rather than actual keyboards, I would like to know if there's a typical approach to this without the use of transformers (space and cost issue).
I'm trying a simple version with a opa134 in an impedance converter configuration, according to it's datasheet (with feedback v. divider set for +6dB), since I truly like the relaxed sound of these opamps in the return section of my trident 80, where we exchanged the TL071 for OPAs in a 100%fb impedance converter. 134/137 seem to be known to be hassle-free regarding RF issues, so this seems like a simple solution for good quality. TL071 sound OK anywhere in our desk, but in full feedback they actually *suck* IMO...
I will post schematics as soon as possible!
Richard
This is my second thread on groupDIY, trying to build a 10-ch dedicated Keyboard buffer with balanced outputs.
Our studio is two-level, two control rooms and all rooms will be interconnected. Cable length is an issue, and while I'm used to record keyboards directly through line inputs or sometimes even to microphone preamps for transformer colouring (only devices with low enough output Z), we decided we need an impedance converter with 0, 6 or 12 dB gain (maybe switchable, maybe w/ locut as well) and an electronically balanced output for being able to drive cable lengths in excess of 15m without loss.
The device(s) will be included in our patchbay system(s), so rather small footprint is important. I want to have it all on one euro-PCB (160*100mm) including supply & in/out screw clamps and an external PSU for +-15V.
I'm planning a JFET input stage, still not sure if IC or discrete, zener protection diodes on inputs and 4004 on outputs, and a balancing line driver like the THAT1646.
Everything looked neat using ONLY the line driver, except the input impedance is 5kHz, which is a bit low, especially for some older instruments. There are other products on the market, like the SSM2142 with Z=10k, but maybe it's better to add a simple impedance converter to the input to increase impedance to 20-100k or something and get higher gain as well.
Since almost all of the circuits I gOoOrgled are designed for hi Z instrument outputs rather than actual keyboards, I would like to know if there's a typical approach to this without the use of transformers (space and cost issue).
I'm trying a simple version with a opa134 in an impedance converter configuration, according to it's datasheet (with feedback v. divider set for +6dB), since I truly like the relaxed sound of these opamps in the return section of my trident 80, where we exchanged the TL071 for OPAs in a 100%fb impedance converter. 134/137 seem to be known to be hassle-free regarding RF issues, so this seems like a simple solution for good quality. TL071 sound OK anywhere in our desk, but in full feedback they actually *suck* IMO...
I will post schematics as soon as possible!
Richard