Mic Pre Workshopping...

Hey Folks,

In an attempt to bring understand of outboard gear in the home studio and in true DIY spirit I'm hosting a free workshop at a local music store this weekend and putting together a power point presentation and will be dissecting mic preamps on a basic, informative level.

No mention of slew rates, compensation caps of feedback networks,a simple basic , easy to digest understanding of mic pres which I'm approaching from the level of an absolute beginner and have structured the proceedings as such and have the following topics.

What is a Mic Pre? (The Role of a Mic Pre Amp)
How does a Mic Pre Work? (Amplifying Sound (Op Amps), Levels Matching etc, Basic Circuits)
What is in a Mic Pre? (Transformers, Tubes, Solidstate (Discrete and IC, Class A/AB etc)
Using a Mic Pre? (Controls including Impedance, Pads, Polarity and +48V)
How to choose a Mic Pre (Colour, Sound, Mic + Pre Flavouring)

I'm trying not to get too technical and will be doing it in a block digram format, using the Universal Audio SOLO Tube and Discrete mic pres as my walk through, since they are readily available here and it is nice to show 2 pres that are build differently.

I have put together a diagram of an opamp, the basic schematic overview and detailed view of the an NE5532 amp as well as a Melcor DOA and have side by side photo to compare the 2 etc for the 'How does a mic pre work' part. But I am hitting a bit of block through in explaining why one over the other, or rather we people use DOA's and not just opamps/chips in designs.

Some basic thoughts are that the circuits can be customized to designers/clients needs offering a different flavour from generic opamps. I know that typically the slew rates of DOA far exceed that of chips, but how do i say this without the 'geek speak' and are there any other reasons I've left out?
Any help would be most appreciated and even comments about anything you think I haven't covered in the outline above.

Thanks for your time, and wish me luck!

Cheers

Matt

Designing opamps is basically about optimizing the parameters you need - at the cost of specs that you regard as less important. Chip design and setup for manufacturing is extremely expensive. So you need to come up with the compromize in specs that will appeal to the widest audience. And high-end audio is not a money-maker market, so you specify your 5534 to be "good enough" for audio, while still running on e.g. relatively low supply power and offsets - to make the wider audience happy.

On the other hand, with DOA, you get the specs that you need - if you trade in specs that are unimportant in the application you're designing it for.

Jakob E.

Remember that opamp spitting thread a while back - that brought stuff to life
http://www.groupdiy.com/index.php?topic=1440&start=0&postdays=0&postorder=asc&highlight=opamp+spit
There was also a old thread at recpro - now gone on history - although ask and somebody will have a copy somewhere on designers popping in Mark McQuillen pops to mind (ti ws supooosed to be a Mic Pre version of Harvey Gersts Big Mic Thread)

Thanks Guys,

Very helpful. to give you an idea of what I'm hitting for, here is my opening paragraph, let me know if you think it is TOO basic, or not clear enough, and or misleading.

What is a Mic Preamp?

A mic pre is an preamplifier that amplifies the 'weak' output signal from microphones by boosting the signal to a standard line level which can then captured by recording devices. This amplification process takes place by increasing the amount of GAIN within the amplifier. A good quality mic pre amp should be able to amplify the source without adding additional noise or hum of its own, which are tell tale signs of bad design or inferior quality parts.

Click to expand...

Cheers

Matt

An important aspect of a mic preamp is that it will work well over a very wide range of input signal level. That, and the high possible gain, are the central issues..

Jakob E.

Thanks Jakob,

I've updated the intro below taking into account your very valid points.

What is a Mic Preamp?

A mic pre is an preamplifier that amplifies the output signal from microphones by boosting the signal to standard line level which can then captured by recording devices. This amplification process takes place by increasing the amount of GAIN within the amplifier. A good quality mic pre amp should be able to work well over a wide range of input signals and provide you with a high gain output, amplifying the source without adding additional noise or hum of its own, which are tell tale signs of bad design or inferior quality parts.

Click to expand...

Thanks again.

Matt

I know that typically the slew rates of DOA far exceed that of chips, but how do i say this without the 'geek speak' and are there any other reasons I've left out?

Click to expand...

Be carefull--I don't think that DOAs tend to have higher slew-rates. Easy to understand reasons to go discrete are:
* the quality of the passive components is far higher (on a chip, they are basically abused semiconductor material)
* the power dissipation can be much higher which simplifies the design task for low distortion (e.g. you can use a class A output stage) and low noise
* each part of the amplifier can be chosen to fit specific requirements (e.g. the transistors at the input need low noise while those in the output stage can become medium power devices)

Samuel

Thanks Sam,

Very valid points, and need to keep things on track and easy to understand,
Here is what I have so far.

HOW DOES A MIC PRE WORK?

As stated the role of a mic pre is to amplify the input signal. This is achieved using GAIN which in Solidstate designs is created using what we call an Operational Amplifier. Operational Amplifiers (Op Amps) come in a variety of shapes, colours and sizes but essentially fall into 2 categories: Discrete or IC (Integrated circuits).

Op amps can be used for a variety of uses within a pre amp design from Gain Staging through to Balancing/Unbalancing Input and Output signals.

The Schematic symbol for an Op Amp looks like this

[Insert Op-Amp Pic]

Lets take a look at Discrete and IC Op Amps.

[Insert a pic of the Melcor 1731 and NE5532]

In this picture we see both types, to the left we have an old Melcor 1731 Discrete Op Amp (DOA), a circuit custom built for a specific audio application and to the right is an 'generic' IC opamp, this particular one is ubiquitous with audio design and is called the NE5532. This particular amp is a DUAL amp and is actually 2 independent circuits on a single chip.

THE PROS AND CONS OF DISCRETE VS IC OP AMPS.

Discrete Op Amps tend to be found in Higher End, Boutique Audio Equipment.

Pros

1.Each part of the amplifier can be chosen to fit specific requirements.
2.The quality of the passive components is far higher than that of the IC Semiconductor.

Cons

1.The cost increase in R&D and cost passive of components translates to a more expensive product.

IC designs are found is most budget mic pres and mixing desks.

Pros

1.Cheaper to build, resulting in a cheaper retail price.

Cons

1.Because of the cost of manufacturing a chip a compromise in specs has to be made that will appeal to the widest audience, which ends up being 'good enough' vs 'exceptional'

This is not to say that IC designs are 'inferior' to discrete designs, there are many great designs from companies like Focusrite and AMEK that rely on IC's in their designs for Gain staging, it is not only the part but how the designs use the chip in their designs that will dictate the performance and sound.

Click to expand...

Thanks again for all your help, I'll keep this going if you don't mind, and maybe when we are done I can compile the whole thing here, it could be a good 'Newbie' resource for the META's?

Cheers

Matt

Interesting point made by Cyril Jones made by in the visit last week
Although some ICs are supposed to be rated by academics as inferior for exampel take the TL072
This is actually a "honky" sound chip - who application in Rock n Roll is ideal but in broadcast and "clean" enviornments is not

His words paraphrased by me - I take "honky" to be excess mid range
ie very 1970s Les Paul and Marshall sound

I agree with Samuel. There are no inherent advantages of using a discrete component op amp over an integrated one. In fact, there are some disadvantages (or they never would have gone with ICs). Thermal tracking is probably the biggest factor. All the transistors on the die track each other. This contributes to excellent DC balance. Of course, the name "operational" says it all. The amplifiers are designed to perform mathematical operations so the DC reference is very important. In audio, this is a secondary consideration. Additions are made to integrated op amp circuits to optimize characteristics such as slew rate, gain bandwidth, null offset and other parameters. Gains in "expensive" ic op amps are far greater than can be realized with discretes. By expensive, I mean >$10 per amp, probably. For audio, they are way overkill.

Back in the good old days when I was designing mixers, we considered using discrete op amps for the principle reason that they could tolerate wide voltage swings and put out significant power over their IC brethren. They certainly didn't perform as well, dollar for dollar, as the ICs but we needed to drive those 600 ohm lines with enough swing to light a light bulb (praise be to the telephone company). At that time, discrete amps with a 5 MHz GBP were costing north of $100 ea.

I think the discretes are showing promise in audio because they are simple and there are limits to the use of internal negative feedback that the ICs use to maintain stability down in the "gain of 1" range. A discrete can be engineered to perform well in a given situation with a known range per gain block. And they can be easily built to light a light bulb.

Burdij,

Thanks for the input, it is all VERY valid and makes sense to me, But bear in mind talking to a bunch of guys who will be coming from the mentality of 'My soundcard has a build in mic pre, why should I use an External Pre?'.

I tried to make is clear by adding:

This is not to say that IC designs are 'inferior' to discrete designs, there are many great designs from companies like Focusrite and AMEK that rely on IC's in their designs for Gain staging, it is not only the part but how the designs use the chip in their designs that will dictate the performance and sound.

Click to expand...

Also the same guys maybe have not used a stand alone pre with a PAD, Polarity/Phase Switch, Phantom etc and trying to explain what they all do, by keeping it as simple as possible.

Cheers

Matt

I'd try and get an idea of what your target audience already know, in order to judge what content to include. Do they know what the term "line-level" means?

It might be cool to mention the actual factor that the mic pre amplifies the signal - eg. at 60dB the input signal is multiplied in amplitude by ***** times. Is it one million? I can't remember. Get out the calculator :razz:

I think its X1000?

Mantra:

X10 - 20db
X100 - 40db
X1000 - 60db

Correct!

I write articles for a couple of magazines. In my experience, it's best not to get too technical. It's very important to be systematic and, above all, not expect too much knowledge. And: Beware of riding your hobby horses.

I would begin with the source, explain the requirement for amplification including the fact that various mics may typically require vastly different amounts of gain. Then maybe transformerless vs. transformer coupled, tube vs. solid state. Discrete vs opamp or instrument amp. Phantom Power may be an interesting topic (also because many cheap pres are underpowered). People may ask questions such as "what's more important, mic or preamp?" or "I have a Mackie mixer, how much will this tube pre improve my sound?"
Noise may be an issue for some listeners. Depending on audience, you it may be necessary to mention some basics such as "Use XLRs for microphone connections. This TRS input is not for microphones but for line sources or instuments. Don't put anything between microphone and preamp, if you want to use a compressor, it must go behind the preamp."

I wouldn't write the whole thing down like a lecture. It's usually easier for people to follow, if you explain things to them in everyday language and enter into some kind of dialogue with the audience.

Anyway, just a few suggestions. As always, YMMV.

Hey Rossi,

Thanks for the input, you hit it on the head I think. People don't need the pictures and the lecture. I've done many lectures before without visual aid and they all were received well.

I think I need to approach it the way I normally do, informally, and do it in an open forum, so shoot off things to spark discussion and have people ask questions, meeting them where THEY are at and to give them the answers THEY need.

I also write for magazines as well, so my style is normally informal but filled with easy to understand information.

Thanks

Matt

Hi Matt, a fellow hack, at last! :grin: Yeah, I think the easy-to-understand-factor is the most important. If they need in-depth information, they'll buy a book or ask questions till you're blue in the face. A few simple pictures may help, but nothing too fancy. Those overblown multimedia presentations really don't do much for the audience. They look professional, and it's easy to see how much work you put into them, but for the audience it's like watching TV. The main problem for guys like us is to overcome our nerd knowledge in our particular niche. Sometimes I like to overhear conversations in music stores. That often puts things in perspective. "A phase switch - what is it good for?" A simple question, yet not that easy to explain in a manner that makes your listeners go, "ah, of course, that's why!"

Hey Matt,

Sounds like a fun workshop. Are you planning to do any demonstrations? Some people understand (and retain) information better if you can show them directly. Will you have access to a basic recorder in the store? A couple of different mics? If so, do a demo of the salient features of a mic pre.

You could quickly demonstrate the different gain requirements of dynamic vs. condensor mics (or ribbon if you have one) by recording something with both and showing the relative levels on the recorder and the gain settings required to get there. On the utility of the polarity switch--mic both sides of a drum and demonstrate the sound with and without polarity reversal on one of the mics. Why would you want a pad switch? Mic that snare drum with and without and show that the pre is clipping (or the input transformer is saturating or whatever is appropriate for your setup).

On the DOA vs. IC thing...that's pretty technical and complex. It's all about engineering tradeoffs (cost vs. performance, size vs. performance, breadth of application vs. performance, etc.) and system design. Transformers and tubes are similar issues. Perhaps the best thing to say is that each technology can sound different. There are no hard and fast rules.

Have fun!

A P


p.s. On DOAs vs

Hey Guys,

Thank you for your support, it is MUCH appreciated.

Yeah Rossi, we are akin

I write for a few music publications here and have done many workshops focusing on various aspects of the music industry from 'Running your own Record Label', to 'Surviving as an Independent Musician' and even focusing on product specific things. I did the launch of Props Reason 3.0 here in SA and a friend of mine was on the Sound design team and I got to give some input, which was fun.

The last workshop I did got a review here, which was fun to read, and rather humbling: http://www.mio.co.za/article.php?cat=&id=709 The workshops are always FREE and my way of putting back into the community.

My worry is exactly what you are talking about, overhearing some of the things I've heard in music stores is what made me want to put this all together. I've heard the weirdest things said by some of the staff and just thought it might be good to break it down in 'simplease' for the average guy to understand. I once had the head of sound at a store try sell a Firewire interface to a guy, which as actually USB...

I thought I'd try do a presentation and spend the last 2 days trying to make slides, or at least draw the images and collect photo's, but it driving me nuts and it is so foreign to the way I normally present, but for some unknown reason I thought it might be good... you guys have helped me see the light

AP, yeah I've already made sure they have all the gear on display and and plugged in for demo use, from the budget gear like the Joe Meek and Presonus, to the High End Universal Audio and Toft gear.

I think 'hand on' is the way to go and your suggestions are great, at least I know I can crack a snare drum and if anyone was sleeping... they won't be for long, Hah hah!

Thanks again guys

Matt

[quote author="matta"]My worry is exactly what you are talking about, overhearing some of the things I've heard in music stores is what made me want to put this all together. I've heard the weirdest things said by some of the staff [/quote]

So true. I almost always leave the music store giggling or scratching my head. Every piece of advice I've heard them give someone was followed by "Everybody uses these" or "<insert big artist name here> uses these." They are coming from the same perspective that creates signature artist stompboxes that supposedly recreate the exact sound of a particular recording. That recording was probably made with a one-of-a-kind guitar , vintage amp, string of modified/broken effects and a gorgeous-sounding room in a top notch studio, but surely the $50 pedal is a worthy substitute for all that. :roll:

What musicians often don't realize is that many of their favorite recordings were made with broken or just plain shitty equipment. They should just get out the 4 track and the sm57 and see what they can do with what they have. A little patience is bound to garner surprises.

Sorry for ranting. I'm just tired of the salesperson asking me what interface I use, which is invariably followed by "You need the DigiMaxLT, the mic pres in that are killer, dude." Um, no thanks.