Discrete mic pre project: proto PCBs have arrived.

[Harpo]

amazing work.

Just guessing, RFI cap C1 @ 470pF might be a typo. I'd expected about 47pF here. Why did you place these C1,C2,C3 caps after the common-mode choke and not as close as possible to the input XLR with ferrite beads @ XLR pin2/3 to keep Hf out?

Caps rating for C68,C70,C74,C75 @ 35V might get exceeded with assumed >=44V CT transformer.
Is heat the only reason behind using U4/U5 and U8/U9 instead of simple forward diodes as load isolators and getting away with U2/U3?

A jumper between R20/R111/R192 and servos out might come handy for trimming offset.

 

[jdbakker]

Just guessing, RFI cap C1 @ 470pF might be a typo. I'd expected about 47pF here. Why did you place these C1,C2,C3 caps after the common-mode choke and not as close as possible to the input XLR with ferrite beads @ XLR pin2/3 to keep Hf out?

I reckon 470p for C1 (and C2/C3) to be fine here. I know some pre's have lower capacitance at this point, but I prefer to be a bit more RF robust, especially given the wide bandwidth of the input stage.

Placing C1/C2/C3 after the CMC but before (or: near) R8 and C5/R31 helps reduce peaking in the RFI filter (usually caused by a pre input Z higher than the impedance of the RFI filter at resonance).

The one thing I should fix here is to move the P48 injection point to before the CMC.

Caps rating for C68,C70,C74,C75 @ 35V might get exceeded with assumed >=44V CT transformer.

Those caps should be OK, given that the load on the transformer is reasonably constant which reduces the effects of transformer regulation. Frankly I am more concerned that C88 et al should be upped from 63V to 100V types, even taking the drop in the RC filter resistors into account.

(Plus I have a big box of 470u/35V Pana FC caps...)

Is heat the only reason behind using U4/U5 and U8/U9 instead of simple forward diodes as load isolators and getting away with U2/U3?

I'm more concerned with possible interstage coupling, and LM317/337s are a cheap way to get a lot of in-band filtering.

A jumper between R20/R111/R192 and servos out might come handy for trimming offset.

What trimming procedure did you have in mind that would benefit from such a jumper? I would propose simple successive approximation (turn four full turns, wait 10 seconds, see if the offset has improved and correct accordingly, turn two full turns, wait 10 seconds...).

Thanks for your feedback,

JDB.

 

[volker]

Hi JDB,

how's your preamp coming along, any progress lately? I'm really curious to see the outcome!
Just thought it was time to bring this great project back to attention again...


[quote author=jdbakker]Sure; I especially like the newer low-rbb hi-hfe duals produced by Philips NXP and Zetex Diodes Inc.; I use quite a bunch of those in my DOA-in-progress (three dozen transistors and counting...). However, all of those come in SOT-457 (aka SOT-23-6, SOT-26 or SC-74) packages or smaller, with a lead pitch of 0.95mm or less, and I wouldn't want to force first time SMD solderers to deal with those. ATM my lower limit in parts size for this design is 0805 for passives and SOT-23 for transistors.[/quote]

Could you give me some recommendations for an input pair/array? The ones I could find looked too low hfe to me. And no rbb specs in sight.


Volker

 

[ruairioflaherty]

JDB,

As you're back here posting a little more, did anything come of this project?  Did you get to measure its performance or do some listening tests?

I'm gearing up to take on some SMD work soon and this project was in the back of my mind.

Cheers,
Ruairi

 

[jdbakker]

The boards have been sitting in a box in the garage since my last move.

I ran into 10MHz-oscillations in the first stage, and I got sidetracked building a transistor noise tester (did I ever post about that one here?) to sort and select input transistors for this and other microphone amps.

Then came the move, and a change in work environment.

I do intend to pick up where I left off, but work-wise that won't be happening until after the summer.

As for SMD-related projects, I have a headphone amp project that I use as a platform to teach soldering/troubleshooting skills to EE freshmen. I have considered starting a build thread here as well, if there's any interest. I have posted the schematic here earlier (http://groupdiy.com/index.php?topic=58861.msg748296#msg748296), the component placement for all but the de-pop circuit is here.

To be continued,

JDB.
[my current tour de force is designing and building a breadboard-based modular synthesizer kit. Only the VCO, LFO and noise generator are microcontroller-based, all other modules are analog. We'll need about 700 kits by September; I got work to buy an entry-level P&P-machine to stuff the boards. Tomorrow I get to test the most complex analog board, a (PWM) VCF/dual VCA combo on a 26x57mm PCB. Let's see whether it works as designed...]

 

[ruairioflaherty]

Thanks JDB,

Glad to hear the mic pre has not been completely abandoned.  I think I'll be turning my GML into cash soon and love the idea of a clean, low distortion pre.

The headphone amp looks interesting, will print and try to understand the schematic tonight.  How does it perform?  I just bought some Sennheiser HD650 this week and love the idea of a really good (not esoteric for its own sake) headphone amp to drive them.  Please do start a thread about it when you have time.

The Modular project sounds fascinating!  When you say 700 kits is this going to be a commercially available DIY project?

Cheers,
Ruairi