M
Minion
Blow Me !!!
It"s there simply because I coppied the input part of the curcuit from a DOD Overdrive which has the 10k in that position....
thebigcheese
"Hi, I'm in Delaware."
Pictures and such posted! (in a different thread so I'm not taking up too much space here)
rayc
retroreprobate
My project was to replace the treble attenuators in my AR4x speakers. After waiting 2 months for the replacements, removing the grills, lifting the midrange speaker, cleaning off the speaker sealant, cutting the speaker free, removing the fibreglass fill (YUCK - that's stuff's awful) and cleaning out the residual fibers I realised that working with 2 hands through the m/range hole (there's no other way in without reducing the integrity of the box - the only way in is through the mr hole) to see what I am doing and replace/rewire a component that sticks out the back is beyond me. I'll have to take them to sparky.
At least I've cleared the way for him.
My DIY became SIY&TGAE (start it yourself and then get an expert).
Bottom!
At least I've cleared the way for him.
My DIY became SIY&TGAE (start it yourself and then get an expert).
Bottom!
dgatwood
is out. Leave a message.
Mic mods
My current project (the preamp work is on hold until I can get shielded inductors) is finishing my recap of mics. I got a bunch of tiny value silver mica caps (7-100pF) to replace the remaining ceramics.
Last thing I should do is adjust or add filters to open up the bass response in the cheap Chinese condensers and/or change the grooves in the body. If anybody has ever dealt with the Nady CM-90 or similar mics (MXL 603, etc.) and has any advice, I'd love to hear it.
My current project (the preamp work is on hold until I can get shielded inductors) is finishing my recap of mics. I got a bunch of tiny value silver mica caps (7-100pF) to replace the remaining ceramics.
Last thing I should do is adjust or add filters to open up the bass response in the cheap Chinese condensers and/or change the grooves in the body. If anybody has ever dealt with the Nady CM-90 or similar mics (MXL 603, etc.) and has any advice, I'd love to hear it.
Fishmed
Well-known member
I am glad I found this thread. I am in the process of re-racking my pair of Symetrix SX202. They have both been mod'ed by Jim Williams, but I hated the cases they were in and wanted to add some line out transformers. I am also adding four attenuators with Jensen transformers. Maybe this thread will get me to finish the project before the end of the month. I have the Attys side complete and will work on the pres more this weekend.
thebigcheese
"Hi, I'm in Delaware."
I'm looking to adopt a preamp project, but I don't even know where to begin... mshilarious, how are you managing $20/channel? I know your stuff is good, so you must be doing something right 
mshilarious
Banned
I'm looking to adopt a preamp project, but I don't even know where to begin... mshilarious, how are you managing $20/channel? I know your stuff is good, so you must be doing something right
Because I am cheating. No really, I inhabit the land between DIY and commercial gear. This is because I sell enough stuff to order anything I want in a PCB, and keep the per unit costs low. The same PCB I use to do a microphone I can use for a preamp, and it's less than 2 square inches, so the unit cost is very low.
But let's take a step back. What is a preamp? If you decide upon an IC preamp, the major cost is not the IC, nor the perfboard you can use to build it, but rather the power supply, the case, and the connectors. That's all the money right there. Since building a power supply for eight channels is no more expensive than for two channels, build an eight channel preamp and spread the cost around.
Here's a breakdown:
1u Mid-Atlantic economy case: $35, or $4 per channel
30VAC CT transformer, $28
misc. power supply parts, $10
OK, that's $9 per channel, just for power. Ouch.
But it's looking up!
Per channel:
Dual expensive opamp (one for differential input, one for variable gain, or just an instrumentation amp), $3
pot & knob, $3
XLR in, TRS out $3
small parts $1
PCB/perfboard $1
OK, there you go, $20 per channel. There are many variations within that basic framework, but until you go to tubes, input/output transformers, or something like a stepped gain switch rather than a pot, the cost will not change much.
Of course, you might want extras. Polarity switch, low cut, pad, phantom switch, metering . . . these all add to the cost, but don't really change the sound (OK, maybe the pad). But my life is simple. I can use the meters on my converters; I can leave phantom on all the time, polarity can be inverted in software, as well as low cut.
mshilarious
Banned
Hey, where are you in DC? I spend the best 33 years of my life in the DC area, before I bailed to a warmer climate 
thebigcheese
"Hi, I'm in Delaware."
Well, technically Northern Virginia, just outside of Fairfax, but I was in school at AU for the past couple years (just finished my degree in December and I'm still a little weirded out by that).Hey, where are you in DC? I spend the best 33 years of my life in the DC area, before I bailed to a warmer climate
What do you mean by IC preamp? I don't really know anything about them, sadly. I've just been using the ones built into my interface, but I'm getting an interface with less preamps and will be in the market for some decent preamps, mostly for vocals and guitars (I'll probably use the 4 on the Motu for drums). I saw some web sites with kits, but those are all quite expensive. I was hoping to save money by going the DIY route... I wouldn't mind spending around $100, but I don't know anything about it, so that might not be possible. I would want phantom power and at least an LED that tells me when I'm clipping (although I think my interface will also tell me that).
EDIT: While I don't know a lot about electronics and stuff now, I am looking to learn more about it. I love building things, which is the main reason I'd want to go the DIY route.
thebigcheese
"Hi, I'm in Delaware."
Another project I'd like to add to my list is the basic set of Billm mods for my Blues Jr. The only reason I'm hesitant to start on those is that I got it at GC and they were nice enough to throw in one of their one year repair plan dealies, so if I mod it and they notice, I'm out of luck. I know I couldn't get away with the presence knob, but I might be able to get away with the basic set. They seem like big, obvious changes if you know what's supposed to be there, though... bah.
mshilarious
Banned
What HS you go to?
IC = integrated circuit, meaning an opamp or instrumentation amp (which is really just three opamps) on a chip. The alternative is a discrete circuit, which can be either tube or solid-state, meaning transistors. Some people will only call ICs solid-state and say that a discrete transistor circuit is not solid-state. Those people are very confused. There are also discrete opamps--an opamp is a type of circuit, not a type of construction. Heck, you can build an opamp with tubes!
While all-tube circuits (meaning tube rectifiers, no semiconductors anywhere) are still common in guitar amps, they are very rare in mic amps (I don't know of one, but maybe there are still some out there). Purists consider a mic amp as a tube pre only if the audio path is all-tube. I personally don't care, they are all components; use them as you see fit, in any combination you like!
So your basic options are:
- IC (solid-state; transistors on a chip)
- discrete transistors
- tubes
- some combination of the above
IC & discrete transistor combinations are very common, but my cheap DIY pre is all IC in the audio path. You can buy really, really good ICs these days that are very cheap for what they do. My pre will use OPA2227 in a two-stage design.
mshilarious
Banned
DIY update!
I have completed two guitar cables; one TS, one TRS. I used the nicer Neutrik plugs and some Mogami 2552 cable I had laying around. The capacitance measured kinda high, but it doesn't really matter as these are only 10', and I don't really care anyway. I mean, I always go direct, so the cable capacitance just sheds some of the highs I would have lost in a woofer anyway.
I will not bore anyone with pictures. My camera is broken
I have completed two guitar cables; one TS, one TRS. I used the nicer Neutrik plugs and some Mogami 2552 cable I had laying around. The capacitance measured kinda high, but it doesn't really matter as these are only 10', and I don't really care anyway. I mean, I always go direct, so the cable capacitance just sheds some of the highs I would have lost in a woofer anyway.
I will not bore anyone with pictures. My camera is broken
Fishmed
Well-known member
Would you care to share your design? Or even better, create a DIY kit like you've done for your mic and sell it from your site or a "group buy"?
mshilarious
Banned
I think I did, on another thread here. I'll have a look and post a link later. No, I'm not really interested in offering a kit. First, it's a single-supply design, which most people would not want to trouble with. Second, the PCB is tiny (0.5" * 1.525"), and uses 0603 SMT components. Not too many people would want to work with that.
It looks like this--there is a separate board for the input, with the phantom supply resistors, switching, LEDs, input diodes, etc--this is just the two opamp stages with gain control, output, virtual ground, and power filtration. I also have a separate board that does my meter LED (although if you are running into a converter with a meter, just design your pre with headroom above 0dBFS and don't worry about that).
antichef
pornk rock
MSHilarious - have you considered adding input tranformers to your simple pres? I bought this "Gaines" single channel one from mrbowes, and it's extremely basic - 1 dual opamp and input transformer, and I really like it. It does have unbalanced output, which hasn't been an issue for me, but I suppose that's easy enough to fix.
mshilarious
Banned
Sure, everybody loves input transformers. I just drop them in place of the input caps. The input impedance of my pre is set high, and the opamp's current noise is low, so something like a 1:4 turns ratio is nice for extra low input noise.
thebigcheese
"Hi, I'm in Delaware."
Robinson. It sounds like I have a lot more research to do if I want to build a preamp... sigh.
Fishmed
Well-known member
mshilarious
Banned
Yeah, for some of them. I actually have six different PCBs for mics at the moment, which build something like eleven different mics. That's why there are some strange layout features on the board, like the output pads on the left (they line up with XLR pins), and the input pads on the right, which for the preamp are tucked underneath the pot (which uses the six big round pads).
I've been thinking about what a simple, cheap kit pre would look like. There are PCBs out there you can get that will do just about any opamp circuit you want, and really, a simple pre is just an opamp. But then feature creep sets in, everybody wants switchable this and switchable that, lots-o-LEDs, +48V phantom, and so forth, and the size and price goes up . . .
I think I could mod that board to simplify parts of the circuit that aren't used for the preamp, and stick the input board next to it which eliminates a fair amount of routing between boards and the power supply. That would make a 1" * 1.5" board, with room for a pot, a switch (probably toggle, they are cheap and easy to use), and a LED. Balanced input (transformer balanced if you like, just replace the input caps with the leads from a transformer), impedance balanced output.
I am struggling with the LED though--you can get three colors (green, yellow, red) out of a single dual green/red LED, but the required circuit to activate the LED in a useful fashion will take up a lot of space. An alternative is a single LED, and let it flicker according to signal strength. This is kinda why I shy away from LEDs, since my converter already solved that problem for me . . .
I think I would keep the design single-supply. That allows +9V battery operation, or any wall wart you want to use, or a simple onboard power supply. You could use a 24VAC transformer, not center tapped, and a voltage doubler for phantom. That's very simple. Or you can be like me, and just use the supply voltage for phantom.
The switch and pot would be chassis mount, so drill two (or three, with the LED) small holes, and the board is mounted. You would just need two more holes for the connectors on the back of the case . . . you could easily fit 8 channels in a 6" deep 1u case.
I still don't see a way to do it without SMT. If I increase the PCB size to allow thru-hole resistors, the cost goes up a LOT, like probably $15 vs. $5 for the PCB, plus the board would be long enough to require standoffs, which is more fabrication, and maybe the board is less suited for a portable, battery powered preamp (which is near to my heart for some reason, even though I would never use such a thing myself).
Oh well. Just thinking out loud here . . .
Fishmed
Well-known member
Update
I am now a little half way done with my project of the month.
Again, I am taking two (mod'ed) Symetrix SX202 mic pres and racking them into a 2 unit case. I am replacing the 1/4" outputs with XLR and adding output transformers. I have also added a trim pot between the pres and transformers for a little db reduction when needed. (I do mean a little). Since the SX202 units have a sum output, I am giving it an XLR out too, but it will not be going through any transformers. This will give me the option to record transformerless if I choose.
The other feature I am including on this box is four channels of transformers for other outboard gear that do not have their own transformers. These four channels have their own attenuators for limited db reduction.
I am now a little half way done with my project of the month.
Again, I am taking two (mod'ed) Symetrix SX202 mic pres and racking them into a 2 unit case. I am replacing the 1/4" outputs with XLR and adding output transformers. I have also added a trim pot between the pres and transformers for a little db reduction when needed. (I do mean a little). Since the SX202 units have a sum output, I am giving it an XLR out too, but it will not be going through any transformers. This will give me the option to record transformerless if I choose.
The other feature I am including on this box is four channels of transformers for other outboard gear that do not have their own transformers. These four channels have their own attenuators for limited db reduction.
