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Old 25th June 2002, 09:42 AM   #11
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<b>What do you use for current source? Mosfet? I am using LM317 since I am lazy.</b>

In the front end, I use the one out of the TPCB using a 2N 5459. Drain to junction of gain set resistors. Gate to gnd via 1M. Source to -24V via 1k2. Gate to -24V via 1M//1uF. The 1k2 looks too high to me to get 13mA (3C is 19mA), so I'll use a multiturn to adjust it in circuit. I'll also probably need to parallel the 2N5459 to keep dissipation under control and within the Id max.

The one in the line stage, I'm going to tweak the same basic circuit.

I'd planned to use the LM317 in the superregs, but Allen told me not to recently, as there was a big improvement by substituing an appropriate resistor. My PSU has some big chokes in it, and only plastic caps, except for the one next to the rectifier.

LM317s are poxy things. I have a whole box of them, but I think I'll even do the current regs in the heaters by mosfets, rather than use them. I think they'll add tons of noise in the RIAA if you use one there.
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Old 28th June 2002, 11:27 PM   #12
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Default it's not easy

I tried building a studio-grade pre-amp as you described (fully balanced). It was not easy.

My design was split into two parts: line-level stage using x-coupled cascode circuit, direct-coupled to cathode followers. This was based on Alan Kimmel's balanced circuit which I found online. Used 3 6189 tubes in each channel.
The microphone section was ambitious: cascaded diff. amps (2 stages) using 6DJ8. I had custom mic transformers wound by Sowter here in England. ALso a custom power transformer (cheap).

Two high-tension regulated power supplies and two DC regulated heater supplies. I fit it all into a 3U rack case but it weighed a ton.

In short: the mic section worked but was quite noisy thanks to the crappiest 6DJ8's I've ever had (shame on me for being cheap). The line-level section was outstanding. But the power supply was temperamental and overly complex. I built sub-chassis for the tubes and ran wires onto perf. boards with the components, which was not ideal.

Basically I wasted a lot of money and I still have the parts, but I dismantled much of it and plan to re-build. I'm now half way through my degree and know how to proceed a lot better (this project was after high school but before university). I am going for a FET/triode hybrid, and burr-brown op-amps at the output. I just don't have time now unfortunately.


The moral of the story is: do your homework, and start with something simple/low sensitivity that is sure to work. A mic preamp isn't anything trivial. And put the power supply (supplies...) in a separate chassis and connect with a multi-core cable. Microphony and noise are very hard to get rid of at mic levels, even in a balanced circuit. op-amp outputs are ideal because you really need the low-Z drive to the recording equipment...



Edit: don't use the 317 for current sinks in sensitive circuits. for a power amp it might be OK but in my design I used cascode BJT circuit. FET could be even better though. As I said, read as much as possible before proceeding.
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Old 15th July 2002, 04:53 PM   #13
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For some DIY recording studio projects - compressor, eq and so on - check GYRAF's diy-page:


http://www.gyraf.dk/gyraf2/gy_pd/gy_pd.htm


Jakob Erland
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