Is there aDIY mic pre equivalent of the Chip Amp?

  • Thread starter Thread starter santiu
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You are hilarious, now, aren't you? :D :p

Digikey was my second guess, I just thought there may have been somewhere else. Mouser says they're not expecting them until March of '09! :eek:

I will check out the THAT chip.

This project senior level? No. I agree, it is not. But I am minoring in Electronics and I am in a tight spot needing to graduate for a job and need to get this class over with... so my guitar amp is going to take too long (was going to build another P1X, but add tube rectification and whatever else I needed to change on my own to make my professors happy :) ), and I'm thinking this should be cheaper and a bit easier. We'll see what my professor says. :D

Brandon
 
NeveSSL said:
You are hilarious, now, aren't you? :D :p

Digikey was my second guess, I just thought there may have been somewhere else. Mouser says they're not expecting them until March of '09! :eek:

I will check out the THAT chip.

This project senior level? No. I agree, it is not. But I am minoring in Electronics and I am in a tight spot needing to graduate for a job and need to get this class over with... so my guitar amp is going to take too long (was going to build another P1X, but add tube rectification and whatever else I needed to change on my own to make my professors happy :) ), and I'm thinking this should be cheaper and a bit easier. We'll see what my professor says. :D

Brandon
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Well you could be sporting and build your own discrete opamp :p
 
gdavi1 said:
i was wondering about these diy preamps. do you need any expeirence in these or can you just jump in and make one of the cheaper seems to me easier ones?
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i haven't actually built my seventh circle stuff (have a buddy that's really good with all that sort of stuff that does it super cheap) but from looking at the kits, the main thing is being able to solder neatly.

oh, and following directions. that's also important.:cool:
 
Hey guys... slight change of direction here but I was hunting around for some info on building a pre for my ribbons. I don't want a full kit to build as I have recently acuired a shedload of parts eg. xlr connectors, gzillions of electronics components rack plates, project cases VU meters... the list goes on. The only thing I would need would be the PCBs and transformers. Is there a direction you could point me in and maybe hold my hand when it comes to understanding the instructions:o. My electronics experience is limited to swapping out parts and the like.
Any ideas??
 
I'd recommend JLM Audio. I've built a few channels of Baby Animal mic pre. Since I built those they've released a "dual" (as in dual opamp) version which will give you more gain for your ribbons. The outputs are electronically balanced but you can add output transformers for more gain still.

They're pretty flexible in what they'll provide you so just PCBs and trannies will be no problem. If there happened to be a couple of other parts you required I'm sure they'd sort you out.

The support is decent. If you get really stuck Joe will help you out if you email him. (Although the kits are really easy to build).

And of course most importantly of all they sound excellent.

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

I'd be happy to help out but do bear in mind my "postage stamp" comment a few posts back. :D

The only "instructions" you really need are for the wiring, the PCBs are silk screened so all the component values are marked on the board.

You sound like you're in the UK? That being the case I'd recommend OEP transformers from Canford.

http://www.canford.co.uk/
 
looks good, thanks.
This may sound a little stupid but is the output of these things line level or mic?
 
All preamps output at line level...If it output at mic level then the Preamp would be useless....

:D
 
Kevin Deschwazi said:
I'd recommend JLM Audio. I've built a few channels of Baby Animal mic pre. Since I built those they've released a "dual" (as in dual opamp) version which will give you more gain for your ribbons. The outputs are electronically balanced but you can add output transformers for more gain still.
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For a ribbon pre I'd certainly want two gain stages, and probably an input transformer more so than output. The critical specification is input noise, so I'd be wary of just throwing any parts you have on the board, and pay attention to the parts recommended for the kit.
 
mshilarious said:
For a ribbon pre I'd certainly want two gain stages, and probably an input transformer more so than output. The critical specification is input noise, so I'd be wary of just throwing any parts you have on the board, and pay attention to the parts recommended for the kit.
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Yeah they use an input transformer as well as the opamp(s). Like I said an output transformer can be added too for an additional 6dB or so of gain (and for "proper" balancing of the outputs, obviously).
 
Kevin Deschwazi said:
Yeah they use an input transformer as well as the opamp(s). Like I said an output transformer can be added too for an additional 6dB or so of gain (and for "proper" balancing of the outputs, obviously).
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OK then. I just get a little concerned when people focus solely on gain when considering a ribbon pre. Noise is far more important. If you have a low-noise pre, you can make up 6dB of gain in mixing without penalty; on the other hand, if one uses opamps only capable of -110dBV noise performance, no amount of gain is going to yield a satisfactory result.
 
mshilarious said:
OK then. I just get a little concerned when people focus solely on gain when considering a ribbon pre. Noise is far more important. If you have a low-noise pre, you can make up 6dB of gain in mixing without penalty; on the other hand, if one uses opamps only capable of -110dBV noise performance, no amount of gain is going to yield a satisfactory result.
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How do I know what is the noise performance of the opamp in dB, when the datasheet only states equivalent noise input voltage, in nV/sqrt(Hz), at 1khz?
 
kubeek said:
How do I know what is the noise performance of the opamp in dB, when the datasheet only states equivalent noise input voltage, in nV/sqrt(Hz), at 1khz?
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Well, if they don't have a noise graph (many don't), you have to assume the noise is flat as frequency increases through the audio range (a fairly safe assumption). That's not true with low-frequency noise, but we don't really care about that.

So, if we decide we care about a 20kHz bandwidth, that's nV*(20000^.5), or nV*141 (note that "/" means "per", not "divided by". I guess computer scientists and EEs don't get along!)

Then take that figure in the following equation to convert to dBV:

dBV = 20*log(nV*141)

So a typical figure of 8nV/Hz^.5 should yield -119dBV. That's unweighted noise; A-weighted would be around 3dB less. That's not the whole noise picture, but it's good enough for government work.

Now, what noise do you need for a ribbon mic? Well, let's presume you want to record a 94dBSPL source with a -54dBV/Pa ribbon mic ("/" means "per" here too). That's conveniently -54dBV signal, so just take the difference between that and your noise rating of -119dBV, that's a 65dB dynamic range, or stated another way, like using a condenser mic with a 29dB self-noise rating (note that is unweighted; more comparable to a 26dBA self-noise rating).

You can see why gain isn't too important with such a preamp--and that is a fairly typical opamp. To get lower noise, you really need to watch specs of the opamps you are using. Some of the popular instrumentation amps have much lower noise, so those are generally selected for ribbon pres.

Another approach is to put a transformer in front of the opamp, to get a few dB of gain before you hit the opamp. That doesn't quite get you dB for dB increase in noise--you lose a bit to thermal noise--but it's close. The downside there (beyond cost) is that you have now raised the minimum gain of the preamp, so to avoid clipping the preamp when using a condenser mic on a hot source, you probably need to add a switchable pad in front of the opamp (or make the transformer switchable).
 
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Thanks, that is quite helpful.
And what about a single transistor preamp before the opamp, like in the Green pre?
I guess that depends more on the topology and surrounding resitors, than only on the transistor itself.
 
My Name said:
jesus wept that a lot of stuff i don't understand....
Anyway I found this http://diy.usedforcomparison.com/arp/ looks prety useful. what do you guys think?
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Useful for what? It's useful for what it is designed to do. Listen, PRR is a real genius, but I hardly think he would say this is a general-purpose mic amp. It's a simple ribbon pre-preamp (there is a more involved, higher performance design floating around prodigy-pro, I think it used 5 or 6 transistors and generally a lot more parts). A problem with a design like this is it's not going to be real good at high gain/high input levels. But it doesn't need that for this purpose, it's designed to give a very small signal a small boost.

It would also need to run about 5-6mA, I think, not terribly efficient. I worry about the "fairly low self-noise" statement, as we have seen what you need is really low self-noise. Rode does an opamp-based unit in their D-plug, but that kinda falls on its face with respect to noise. Again, if you are feeding a converter, you don't need much gain. If you feed a ribbon into a mic preamp with dynamic range as discussed above, and say add a mere 40dB of gain, you've got a signal at -14dBV and noise at -79dBV. Your converter noise is waaaay less than that, so just makeup the rest of the gain digitally.

The Green pre I don't quite think is the same concept, but in either case an NPN transistor is typically a low noise device. Most fast opamps are FET input though, and FETs tend to be noisier. Why a FET? Higher input impedance is one reason. So you probably wouldn't select either design if you also wanted a DI (unless you designed a FET buffer stage just for the DI).
 
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