Compact Portable Preamp. I'm having some trouble.

Acoustiks

Member
2010-11-11 4:05 am
Hello all. I've been browsing through many forums. By far, this one is the best! I did some searching on the forum for something similar, but I haven't quite found what I was looking for.

I'm a videographer who is always on the go, I shoot documentaries. I need to be able to set up and tear down quickly. The reason I am waiting a small preamp is because my inbuilt one in my video camera is terrible. I get quite a lot of hiss (white noise) at the 5k range and increases in volume as the frequencies get higher. I want to be able to kick down the gain in the camera as low as possible and have more input.

I would prefer to run this on a single 9volt, if possible. It needs to be very clear, no humming, buzzing, or noise (or at least keep the noise very low). I tend to compress my audio, so when that dynamic range is crushed, I really don't want that noise to come back. I'd love to have a headphone output also to monitor audio and maybe a primitive UV meter with some LED's or maybe a single LED to let me know when my audio is clipping.

I know some basic electronics and have plenty of components lying around the house. I'm just not at a point yet where I can calculate values and such (I was in algebra 1B in my senior HS year last year). I do know basic Ohm's law and stuff.

TLDR; I need a small, low noise, compact preamp with 2 headphone outs, output to the cameras audio input, 1/8inch mic input and an XLR input.

I would really appreciate some help with this. I know I'll learn a lot in the process. I really want to learn, ask questions, etc. Working with electronics is so fun!

Tomorrow I'm going to an electronic store, so maybe I can luck out and have some prototype to tweak and test from parts I get there. No rush though.

Is it possible to add phantom power using a 9volt bettery? Reason I'm asking is because I know microphone batteries tend to cause interference, is there anyway to clean up the signal? Some way to balance it?

Thank you! I'm really excited about this! :devily:
 

Atilla

Member
2008-07-20 4:16 pm
An opamp based preamp with a buffer for each headphone out sounds like what you need.

Digi1 has a simple opamp-based preamp board Here. That should be a good start.

You can power those with 2 9-volt batteries in different ways, or if you choose the right chips, you can use 1, with a rail splitter. For example see the Mini-cubed portable headphone amplifier at AMB labs. I haven't seen headphones that it can't drive properly yet (although I'm sure they exist, hehe).

I don't know the slightest thing about microphones however, so I can't help you there.
 

Acoustiks

Member
2010-11-11 4:05 am
An opamp based preamp with a buffer for each headphone out sounds like what you need.

Digi1 has a simple opamp-based preamp board Here. That should be a good start.

You can power those with 2 9-volt batteries in different ways, or if you choose the right chips, you can use 1, with a rail splitter. For example see the Mini-cubed portable headphone amplifier at AMB labs. I haven't seen headphones that it can't drive properly yet (although I'm sure they exist, hehe).

I don't know the slightest thing about microphones however, so I can't help you there.

Thank you for the reply! :D The UCP1 looks like it would do the job. I can just add on my own modifications. For now I just need it to do the job as a preamp.

I see that it runs off of 12 volts. Would it still operate on 9? If not I'll have to use like 8 AAA'a or a 9volt and 2 AAA's.
 
I would strongly recommend you use 2 x 9V batteries. This gives you more headroom and a nominal earth which saves you all sorts of trouble. You can also use a pair of voltage regulators to clean up the supply (LM317 and 337). No the supplies from batteries are not perfect, esp under load (which you will have a little of with the headphone amp.) See Walt Jung's Op amp Handbook where he has a few mic preamps. For an electret you will need a small phantom supply (very easy, you just put it on the other side of the input capacitor through a resistor from the supply rail). Start with an easy and cheap op amp like a TLO72 in dil sockets and replace them later on with something less noisy. 5532s are an immediate obvious next step, but you'll find plenty of advice on other more exotic amps here or hereabouts. Filter the i/p with something like a 100pF capacitor and an R to get it to around 60kHz. Put the LF cut of at around 1Hz (22uF and 10k approx) or less if it's worth it on the video input.
 

Acoustiks

Member
2010-11-11 4:05 am
I would strongly recommend you use 2 x 9V batteries. This gives you more headroom and a nominal earth which saves you all sorts of trouble. You can also use a pair of voltage regulators to clean up the supply (LM317 and 337). No the supplies from batteries are not perfect, esp under load (which you will have a little of with the headphone amp.) See Walt Jung's Op amp Handbook where he has a few mic preamps. For an electret you will need a small phantom supply (very easy, you just put it on the other side of the input capacitor through a resistor from the supply rail). Start with an easy and cheap op amp like a TLO72 in dil sockets and replace them later on with something less noisy. 5532s are an immediate obvious next step, but you'll find plenty of advice on other more exotic amps here or hereabouts. Filter the i/p with something like a 100pF capacitor and an R to get it to around 60kHz. Put the LF cut of at around 1Hz (22uF and 10k approx) or less if it's worth it on the video input.

When I get a chance, that book is on my order list. :D I'll looks up some circuits that base their construction around that amp. My microphones impedance is 200 Ohms.

Found this on a similar OP amp. http://www.zen22142.zen.co.uk/Circuits/Audio/lf071_mic.htm

I was searching (unrelated to the TL072) and came across this. Think it would do the job.

One thing I didn't mention. I only need 1 Channel for my headphone output and mic output to the video camera input. The microphone is mono.
 
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When I get a chance, that book is on my order list. :D I'll looks up some circuits that base their construction around that amp. My microphones impedance is 200 Ohms.

Found this on a similar OP amp. Op-Amp Microphone Preamp

I was searching (unrelated to the TL072) and came across this. Think it would do the job.

One thing I didn't mention. I only need 1 Channel for my headphone output and mic output to the video camera input. The microphone is mono.

You don't need to buy the book. It's on Walt's site and I think also on the Analog Devices site.

Either of those would work. I don't see much "high quality" about the TLO71 circuit, but if it says so ... Personally I would do a balanced input for a mic pre-amp - after all you may as well. Off hand I can't see what significant difference the impedance of your mic would have. We are trying to pick up signal, not transfer power. So you want the largest possible input impedance within the constraints of noise. The type of op amp will affect this value.

Incidentally, investing in some decent low capacitance shielded twisted pair mic cable will probably help the sound more than pi**ing about with different op amps.

CT
 

Acoustiks

Member
2010-11-11 4:05 am
You don't need to buy the book. It's on Walt's site and I think also on the Analog Devices site.

Either of those would work. I don't see much "high quality" about the TLO71 circuit, but if it says so ... Personally I would do a balanced input for a mic pre-amp - after all you may as well. Off hand I can't see what significant difference the impedance of your mic would have. We are trying to pick up signal, not transfer power. So you want the largest possible input impedance within the constraints of noise. The type of op amp will affect this value.

Incidentally, investing in some decent low capacitance shielded twisted pair mic cable will probably help the sound more than pi**ing about with different op amps.

CT

Sorry if I may have not explained in detail enough. My video camera is a Canon HV20. The hiis from the camera's built in amp is a common problem, its not my cables. The camera itself does not have an XLR in. Only an imbalanced input. I use two mikes. Rode Video Mic and a UEM-81 Condenser (got it for free).

Yes my goal is to gain signal strength.

Do you have any OP Amp suggestions that are under the $10 range?
--
Thank you all so far for the help. I really appreciate it. :D
 
The 134s are a good choice. I'd think you were mad if you spent more than $10 on op amp on a beginners project like this.

You can still have balanced inputs to your pre-amp and have this go out as an unbalanced output to your camera. This really would help keep noise down.

It occurs to me that maybe the simplest thing you could do - where all the work is pretty much done for you - is use an INA217. Or an SSM2143 which I seem to remember is virtually identical (but it gives you two sets of resources to read). Read the documents and you'll see what's going on. Then just add another op amp to vary the level. Put a 100 ohm R and a large C (100u or 220u) on the output before your RCA socket.

Choice of op amp for this buffery bit is personal but you could do worse than a 5534 single (which are better than the duals, but you may not know how to use it to its best). This is where the 134 wins out being both good and easy to use. What you are looking for is low noise. The TI website is excellent for this. If pushing the boat out you could think about the opa211 or the newer version which is the opa1611. This seems to be pretty well liked in audio circles.
 

Acoustiks

Member
2010-11-11 4:05 am
The 134s are a good choice. I'd think you were mad if you spent more than $10 on op amp on a beginners project like this.

You can still have balanced inputs to your pre-amp and have this go out as an unbalanced output to your camera. This really would help keep noise down.

It occurs to me that maybe the simplest thing you could do - where all the work is pretty much done for you - is use an INA217. Or an SSM2143 which I seem to remember is virtually identical (but it gives you two sets of resources to read). Read the documents and you'll see what's going on. Then just add another op amp to vary the level. Put a 100 ohm R and a large C (100u or 220u) on the output before your RCA socket.

Choice of op amp for this buffery bit is personal but you could do worse than a 5534 single (which are better than the duals, but you may not know how to use it to its best). This is where the 134 wins out being both good and easy to use. What you are looking for is low noise. The TI website is excellent for this. If pushing the boat out you could think about the opa211 or the newer version which is the opa1611. This seems to be pretty well liked in audio circles.

The project may be because I'm a beginner with electronics. But, I'm not with audio in general. I've been a music producer for 5 years now. I have no job or anything. That is why it has to stay cheap. :/

I built the TL071 circuit, it will do for now. I'm having one issue though. The 100u capacitor seems to just short the circuit. Maybe I'm doing it wrong.

I have a Lm386 lying around too. I can use that for my headphones. Would it cause any interference with the other IC?
 
As long as you include the power supply bypass caps as the datasheets of those chips recommend, you shouldn't have any issues.

Which 100u cap are you referring to? Can you post a schematic of what you've done? That should help people give feedback.


Sorry, my apologies. It could be something wrong on my part. It's the 100u capacitor that goes between the V+ and the ground. When I use the capacitor, it shorts.

Maybe something on my end? I'm sure its there to clean up the input from the battery? I'm getting this high pitched ring. Very annoying.

[/img]http://www.zen22142.zen.co.uk/Circuits/Audio/tl071_mic.png[/img]

Other than that, it performs better than expected. I'm going to make a better circuit very soon.
 

Atilla

Member
2008-07-20 4:16 pm
Okay, that all seems correct in the schematic. The cap should go between + and -, close to the chip. It's there to provide stability at high transients.

Make sure the polarity is right and that the voltage rating of the cap is higher than your supply voltage. I know it's a noobish thing to say but we've all done that mistake before.

If all seems ok, check the datasheet of your opamp and see what supply bypass is recommended. A smaller value may be more appropriate in your scenario - a polyester or ceramic one. Also with short leads between the battery and the opamp, with this opamp you might even get away without having a bypass.
 

Acoustiks

Member
2010-11-11 4:05 am
Okay, that all seems correct in the schematic. The cap should go between + and -, close to the chip. It's there to provide stability at high transients.

Make sure the polarity is right and that the voltage rating of the cap is higher than your supply voltage. I know it's a noobish thing to say but we've all done that mistake before.

If all seems ok, check the datasheet of your opamp and see what supply bypass is recommended. A smaller value may be more appropriate in your scenario - a polyester or ceramic one. Also with short leads between the battery and the opamp, with this opamp you might even get away without having a bypass.


Fixed the issue. Now ran into another one... Input from a media player works fine. But using input form my mics and I get nothing... WUT
 
Your mic problem is probably because they are electret mics. These need a small voltage to drive them. It's best to find the spec for them (or they could have batteries that are flat) and then drop that voltage onto the mic side of 10uF input capacitor through a potential divider in the kilo-ohm region.

Now you have got something that works can I suggest you do some reading to get yourself something a bit better.

Best wishes

CT