pstamler
Well-known member
Hi folks:
I'm new to the forum, so let me introduce myself. I'm Paul J. Stamler; I write articles for Recording magazine and audioXpress (ex-Audio Amateur). I've done a good deal of recording, mostly of acoustic music, and I also play in a couple of bands, which perform for dance groups. I should be practicing guitar at the moment.
Here's my query. I've published two mic pre designs so far; one was a tubed design which ran in audioXpress, using a JT110KE input Xformer, with active duty handled by the two sections of a 6SN7 tube. It sounds very nice.
The other, which also sounds very nice, was an IC-based design. It also used a JT110KE tranny, with gain handled by a Burr-Brown OPA604 opamp (first stage) and a Linear Technologies LT1028a opamp (second stage). It had both balanced (-10dBV) and unbalanced (+4dBu) outputs and a simple EQ circuit, all based around OPA604/2604 chips; the power supply was quite complex, with a brute-force raw supply (external), pre-regulator card (LM317/337-based), and on-card regulators using opamps (normally NE5534s) and pass transistors -- basically the Sulzer regulator circuit, as published in TAA years ago. All parts are high-quality; typically there are only one or two coupling capacitors in the direct circuit (one between the slider of the level control and the second stage, the other between the first stage and the control -- that one's not there when you're set to flat bass response.) The second stage has a servo. All caps are polystyrene or polypropylene. All ICs use current-source biasing to force their output stages into single-ended Class-A operation.
Here's the question. I've had requests for a smaller, less expensive version of this preamp, one where builders could put up to 8 channels in a 2RU cabinet, for use in project studios and remote rigs where 8 channels of low-coloration preamp would be welcome. I've done the design work and come up with what I think is reasonable. I've deleted the EQs, made bass rolloff simpler (either 100Hz or flat), added phantom switching for the individual channels. The biggest changes are two. I've simplified the supply; regulators are still on-card, but they're now LM317/337 (with TL783 for the phantom), with no pre-regulator. And I've added the option of using an NE5534a for the second gain stage, after the level control, which actually provides the lion's share of gain in this design. (One can choose a 5534a for the first stage too, but one then sacrifices the possibility of running it direct-coupled when the bass rolloff is switched out.) Using an NE5534a for the second stage saves about $80 in an eight-channel board. Parts mostly come from Digi-Key and Allied Electronics.
Projected cost for the unit as designed would be about $150 per channel, not including the cost of the circuit board, plus about $100 for the shared power supply, again not including the cost of the board, or the box to put it all in. Two channels would cost $400 plus boards and boxes, four channels would cost $700 plus B&B, eight channels $1300 plus B&B. (One can remove features and save money; for example, you could eliminate the balanced outs, or unbalanced ones, and save a few bucks.) This isn't a Mackie-priced box by any means, but neither is it up in Gordon territory. Price is roughly comparable to a Sytek box for four inputs, or somewhat less than two Syteks for eight inputs, but this is a transformer-input box, which many people prefer, and although spartan, it has a few options the Sytek doesn't, like unbalanced outs and bass rolloff. Not to mention gain that goes lower, which is something that bothers me about the Syteks.
So...would anyone here want to build something like this? I'd be selling the article to, presumably, audioXpress, and I'd sell circuit boards, the cost of which would massively depend on how many orders I got -- you know all about circuit board setup costs.
Any interest? This doesn't pretend to be a world-beater by any means, but it's a very clean, uncolored design that has made a *lot* of good recordings for me (check out the Buckhannon Bros. CD for a good example), and it offers the possibility of packing enough channels into a small space to be useful, for not a huge amount of cash, and *way* better sound than, say, a Presonus, which seems to be the only commercial product of this nature (there are other 8-channel rigs out there, but they're all transformerless).
What think?
Peace,
Paul
I'm new to the forum, so let me introduce myself. I'm Paul J. Stamler; I write articles for Recording magazine and audioXpress (ex-Audio Amateur). I've done a good deal of recording, mostly of acoustic music, and I also play in a couple of bands, which perform for dance groups. I should be practicing guitar at the moment.
Here's my query. I've published two mic pre designs so far; one was a tubed design which ran in audioXpress, using a JT110KE input Xformer, with active duty handled by the two sections of a 6SN7 tube. It sounds very nice.
The other, which also sounds very nice, was an IC-based design. It also used a JT110KE tranny, with gain handled by a Burr-Brown OPA604 opamp (first stage) and a Linear Technologies LT1028a opamp (second stage). It had both balanced (-10dBV) and unbalanced (+4dBu) outputs and a simple EQ circuit, all based around OPA604/2604 chips; the power supply was quite complex, with a brute-force raw supply (external), pre-regulator card (LM317/337-based), and on-card regulators using opamps (normally NE5534s) and pass transistors -- basically the Sulzer regulator circuit, as published in TAA years ago. All parts are high-quality; typically there are only one or two coupling capacitors in the direct circuit (one between the slider of the level control and the second stage, the other between the first stage and the control -- that one's not there when you're set to flat bass response.) The second stage has a servo. All caps are polystyrene or polypropylene. All ICs use current-source biasing to force their output stages into single-ended Class-A operation.
Here's the question. I've had requests for a smaller, less expensive version of this preamp, one where builders could put up to 8 channels in a 2RU cabinet, for use in project studios and remote rigs where 8 channels of low-coloration preamp would be welcome. I've done the design work and come up with what I think is reasonable. I've deleted the EQs, made bass rolloff simpler (either 100Hz or flat), added phantom switching for the individual channels. The biggest changes are two. I've simplified the supply; regulators are still on-card, but they're now LM317/337 (with TL783 for the phantom), with no pre-regulator. And I've added the option of using an NE5534a for the second gain stage, after the level control, which actually provides the lion's share of gain in this design. (One can choose a 5534a for the first stage too, but one then sacrifices the possibility of running it direct-coupled when the bass rolloff is switched out.) Using an NE5534a for the second stage saves about $80 in an eight-channel board. Parts mostly come from Digi-Key and Allied Electronics.
Projected cost for the unit as designed would be about $150 per channel, not including the cost of the circuit board, plus about $100 for the shared power supply, again not including the cost of the board, or the box to put it all in. Two channels would cost $400 plus boards and boxes, four channels would cost $700 plus B&B, eight channels $1300 plus B&B. (One can remove features and save money; for example, you could eliminate the balanced outs, or unbalanced ones, and save a few bucks.) This isn't a Mackie-priced box by any means, but neither is it up in Gordon territory. Price is roughly comparable to a Sytek box for four inputs, or somewhat less than two Syteks for eight inputs, but this is a transformer-input box, which many people prefer, and although spartan, it has a few options the Sytek doesn't, like unbalanced outs and bass rolloff. Not to mention gain that goes lower, which is something that bothers me about the Syteks.
So...would anyone here want to build something like this? I'd be selling the article to, presumably, audioXpress, and I'd sell circuit boards, the cost of which would massively depend on how many orders I got -- you know all about circuit board setup costs.
Any interest? This doesn't pretend to be a world-beater by any means, but it's a very clean, uncolored design that has made a *lot* of good recordings for me (check out the Buckhannon Bros. CD for a good example), and it offers the possibility of packing enough channels into a small space to be useful, for not a huge amount of cash, and *way* better sound than, say, a Presonus, which seems to be the only commercial product of this nature (there are other 8-channel rigs out there, but they're all transformerless).
What think?
Peace,
Paul
