Preamp Project - variable impedance, etc.

Hello All.

My friend and I want to build some mic pres, and are trying to figure out what kind of design to go after. My friend is pretty new to electronics, I have a good bit of experience, but more with repair than DIY.

I think we're looking for something solid-state for simplicity, but with some character. We're thinking transformers is the way to go, but our budget is fairly restricted.

I'm also interested in possibly having a variable impedance input.

Which designs would you suggest?
Thanks!

I'm very green when it comes to DIY, but I have had a chance to use a lot of nice discrete pres over the years.

Take a look at the JLM stuff-- in particular the Baby Animal might be great for starting out.

I'm starting with Spectra Sonics 110's and API 312's (very slowly I might add,) but those came about because I got good deals on one component or another. So unless you already have something to rack, make sure you check out JLM-- Joe is really helpful too.

http://www.jlmaudio.com/Baby_Animal_Mic_Pre.htm

There's also a build thread going right now.

[quote author="unheardof"]
I'm also interested in possibly having a variable impedance input.
[/quote]

you can do this with any pre, you just need to use an input transformer that has several taps on the primary of the input transformer. Cinemag makes a few, you can use a 10468 with different input impedances, the sowter input transformer for the am16 has a few taps, there's a few options.

dave

How much more would a multi-tap transformer cost than a single-tap? Remember, we are on a budget here.

I read in this thread on the subject where xvlk mentions using parallel negative feedback with a resistor.

I understand the concept of negative feedback, like with an opamp. How would this work with a transformer balanced input? Would it be feeding to the mic input from the output of some inverting gain stage furthur down the line?

Any other preamp project suggestions?

If budget is such a concern, I don't know if you want something with xfmrs...

Well I didn't say we had NO budget. Something in the price range of that Baby Animal preamp might work (which from what I can tell uses only an input transformer). I didn't see any prices on the Cinemag page, but I would imagine multi-tap transformers would cost significantly more (please let me know if you have any prices on these).

Assuming we did go transformerless, which designs would you suggest? I've already got an electronically balanced pre (Oram Octasonic), so I was hoping for something with a little more color, but still a clear sound. Just something different and musical.

There's the green pre and fabio's fetboy.

[quote author="unheardof"]solid-state for simplicity[/quote]

There are many dead-simple, great sounding tube circuits. Depending on your recording format (tape or digital), you can get away with only an input transformer. NewYorkDave's One-Bottle preamp is very simple and every possible question about it has been answered at least twice. :razz: Then there's the Hamptone Fet pre (aka FET-Boy), boards are available, or use the neat little perfboard layout someone posted, cheap and definitely has character.

Lulu the space cat's super simple 12au7 pre: CLICK

Talk of the usual suspects, Green Pre, et al: CLICK

More discussion of options, One-Bottle, one chip, one transistor: CLICK

I still haven't gotten around to testing different secondary loading and zobel networks on my input transformers. Yeesh.

You can also do variable input impedance (in one direction, down) without a multi-tapped transformer; just hang resistors in parallel with the input. See the article in the current Recording (yes, I did write it).

Peace,
Paul

And do a bit of work yourself , i.e. search & research
cause we don't know your budget but got our own anyway ,
so it doesn't turn into a best pre under 100.00 thread

But loads of good tech assit & ideas once you have homework ,

regards Greg

[quote author="unheardof"]I think we're looking for something solid-state for simplicity[/quote]

HA! That's an oxymoron if I ever saw one! :green:

It can't get any simpler than a 312, IMO... And they sound the balls, and you can experiment with different op-amps and transformers...

Call Cinemag up for prices. They're nice, helpful folks on the phone.

Good luck! :thumb:

Peace,
Al.

Actually you can do variable input inpedance with a pot, or switched resistors. If you use a transformer that stays fairly flat when you change the load on the secondary, all you need to do is make that load resistor switchable or even a fully variable by using a pot.

Its cheaper and you dont get a big level change like you do when you switch another winding in, making comparisons easier.

Our (JLM) 1:4 can do it, it will stay pretty flat even with 100k (instead of 10k) on the secondary. Also I think the Lundahls will work, 1538 and 1578, but dont quote me on that. There will be others too Im sure.

Welcome to DIY, may your stay be long!


M@

Hey, I appreciate the help, lots of great ideas.

I totally understand about doing my homework, I've been the knowledgeable forum member on other forums when the N00bs come around asking the same questions, so I know what it's like. The problem is I don't have the time to live and breathe this stuff like most of you, even though I would love to.

I even looked through the Meta-Meta, but even that was daunting as there are still pages and pages of threads to wade through. I realize this is a simple question with a million different answers, I was just looking for a few simple and proven recommended projects, and that's what I got.

Thanks for your help, and I'll let you know what we decide to go with.

A suggestion for the site:
- It might be nice to have a separate "newbie" section where people can ask dumb questions without worrying about clogging up the other forums. I appreciate your patience. :thumb: I'll be around.

Do you want variable impedance to accommodate specific mics, or to adjust the tone of the pre-amp? Because while the results will differ, I think you could just get by EQ?ing at mix for now instead?whereas having a nice discrete pre will up your game right away all other things being equal.

On the one hand, I really like variable impedance on the UA 2-610-- but it's really not a must have in my book and would never stop me from reaching for the Quad Eight's or Am-16's etc. becuase they just plain sound better on certain sources.

Just one reporter's opinion mind you..

what kind of budget are you working with? that is really going to determine the kind of advice that you'll be able to apply. If you have experience with repair it should probably be self evident but its my experience that DIY is not going to save you anything compared to purchasing some items from a manufacturer. you'll get to make something that is waaay better than a commercial item, but the odds of it being waaay cheaper are stacked highly against you. I havent used any but several folks here speak highly of edcor transformers which can be had inexpensively.

dave

We are mainly doing it just to do it, for fun. But I was hoping to save a little money along the way. Unless you are buying dirt-cheap made in China audio gear, you are usually paying a premium for the name on the front. The components inside are for the most part cheap, unless you decide to go with the audiophile grade stuff. I don't intend to spend much on the cosmetics, and I plan to use salvaged materials wherever possible. So in the end, I think I should come out with a better-sounding piece of gear than I would have got for the same money.

But I see your point. You are building your own piece of gear, so you put the best stuff possible in there, in the end you end up spending way more than a commercial unit might cost.

But we'll see how much this ends up costing, you may prove your point!:sad: I would love to see, when people post gear that they build, how much it cost them.

You asked about the budget, I mentioned it's about the range of the Baby Animal pre. We don't have a set budget in mind, but we're both married and don't have a lot of disposable income.

On the impedance issue, I'll probably go with the pot option, just for cost and simplicity. I'm mainly interested in it for tonal coloration. I think it's interesting to see what the same mic will produce when you vary the impedance. Yeah, you could do some of the same things with EQ, but then you get into phase distortion, etc. I've always been more a fan of getting the sound you want at the mic rather than fixing it in the mix, and this sounds like just another interesting tool to have at your disposal. Disclaimer, I've never used a pre with variable impedance, so I don't know exactly what it will sound like. Just interested in the idea.

[quote author="unheardof"]But I was hoping to save a little money along the way.[/quote]

For the most part, you'll find that this is not going to be the case.

Not trying to be a downer, just thought you should know.

man, if you can hear phase distortion I'll give you a dollar. I think too many people label just a shitty sounding EQ as "phase distortion".

In my experience impedance matching is impedance matching is impedance matching. It becomes a little more apparent in a mic compared to a line source as you then amplify the mic a whole bunch, amplifying the mismatch when present, but there isnt an EQ out there that I have used that is going to mimick what happens when you have a proper load. To my ears its a question of size and efficiency in the circuit much more than a tonal thing like an eq would be. Theres a ddefinite difference between letting the bass into the circuit as opposed to adding bass eq to a circuit. you'll also find that with some mics and amps are picky about the impedance while others are completely not. I dont know if this is too general, but with the mics I use, it seems like fet mics arent entirely picky about the input impedance so long as its close enough, I would imagine that this is probably because of the fet reflecting somewhat of a constant impedance on the capsule. Ribbon mics on the other hand are wildly effected by the input impedance of the preamp as most are just a piece of foil stuck between a magnet hooked up to a transformer. Im definitley speaking outside of my authority on this subject but thats an observation Ive consistently made over the years. I think its probably fair to suggest that some of my pre's that dont sound so good with a given mic are probably just mismatched for the load the mic wants to see.

anyhow, after you build your project reread this thread as you'll have a much better perspective on what things cost. you still havent posted a price. I would make up your mind about what you want to spend before you begin to think about what you want to build. you can build a 312 type circuit for a reasonable amount of money but a discrete opamp is gonna gost you at least $50 where as a chip IC can be had for $2. If you want to build a discrete circuit without having to pony up for a potted opamp, take a look at the old rca solid state stuff, the ba31/ba72 are awesome as a color and are only 4 transistors in the whole circuit, you can build the whole thing on breadboard in one night. Theres so many options its ridiculous but I would come up with a price you need to stay under and work back from there. Some projects you can spend $200 on just transformers alone and some projects specifically need those $200 transformers...

dave

[quote author="soundguy"]...but there isnt an EQ out there that I have used that is going to mimick what happens when you have a proper load. To my ears its a question of size and efficiency in the circuit much more than a tonal thing like an eq would be. Theres a ddefinite difference between letting the bass into the circuit as opposed to adding bass eq to a circuit.[/quote]

I totally agree, and this is what I'm hoping for with a variable impedance.

Tomorrow my friend and I are going to get together and figure it all out, source the parts, and get a cost estimate. At this point I'm pretty sure we'll be doing some variant of the Baby Animal.

If the goal is to properly match some low impedance mics you have (50 or 150ohms out) then you will need a transformer with multiple primaries. Thats the only way to do it properly and will give you the best noise results.

Do you have mics like this or are they all modern 600ohm mics? If they are and you just want to change the load to get some tonal variation then changing the load resistor on the secondary can work really well. The one provisor, like I said, is that you must you a transformer that stays pretty flat as you change the load on it. Like Dave said, the effect will be more pronounced on ribbons and dynamic mics and less on condensors.

It might be an idea to build the pre first however, and then fit the variable impedance parts after youve got it working and have had a listen to it. On the baby animal, all youd have to do is desolder the 10k load resistor and fit either a switch with several values or a (correct me if Im wrong here) 100k linear pot with say, 5k in series with it. This will let you sweep the input impedance from about 300ohms to over 6k, which should yeild a lot of tonal change from any mic. Actually it would be interesting to try a super low Z mic like an M149 with the input set to 300ohms.

Hmmmm youve got me thinking now.. :idea:
:thumb:

You can do this with almost any pre though, but you have choose the right transformer.



M@