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Can I use a Integrated Amp in place of a Preamp?

 

New member
Username: Trico

Post Number: 1
Registered: Nov-10
I'm trying to help my daughter with a science project. Many cars are now using sound to deaden unwanted sound. We have 2 microphones that will be placed different distances from a sound source. We need to take the input from these 2 microphones and combine them into one and this we will feed to a set of earphones. If I understand this right we need to feed the 2 microphones to a preamp to combine them and this to an amp and then to the earphones. Reading into some of the post in this forum many are writing about integrated systems that do both. I have an old Fisher Integrated Component System Model # MC-3010. It has inputs for Phono-1 (magnetic), Phono-2 (ceramic), Aux and Rec out. Will this work for both the Preamp and Amp. It seems that we would have to be able to combine two input sources and this system is set up where only one source can be selected. Could one input source be used where one microphone is on the right speaker and the other on the left? The key to this experiment working is that each of the microphones will be picking up sound that will be out of phase by an adjustable amount (depending on where the microphones are placed in relation to the sound source) so the mixing has to be done in a matter that will not disrupt the true final single source of both of these inputs combined. If we do need to have a preamp of course we want to spend as little as possible but what I am seeing out there varies from very reasonable to crazy money. Any thoughts on what we may be looking for to make this work? Thanks for any help.
 

Gold Member
Username: Stu_pitt

Stamford, Connecticut USA

Post Number: 4092
Registered: May-05
I'm not really sure I follow what you're saying, but...

Don't put anything into a Phono input that isn't a turntable. There's voltages, eq'ing, etc. It'll sound horrible for the minute or so it'll output sound until it's ruined.

For the mics being in each channel, if the mic has an RCA output, you could plug one into the right aux, and one into the left aux.

I'm pretty sure the mic has either a headphone looking connector or an XLR connector. If either are the case, I doubt you'll be able to get 2 into one input. I doubt you could use either into your amp.

I'd try a musical instrument and/or pro audio store. Guitar center may have what you're looking for for cheap. The best bet is going into a store like this and asking the staff.
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 15540
Registered: May-04
.

An "integrated amplifier" contains both a "pre amp" amd a "power amp". In that you are correct. Feeding two microphones to your Fisher will first depend on the type of microphone you use. Dynamic mics - the cheapest variety - are designed to go directly into a pre amp's input though they typically have a termination in a 1/4" phone plug and not in a RCA as found on the Fisher. You'll probably need some adapters.

Most dynamic mics will provide some level of sound when used directly into an "aux' input though the output level is quite likely to be too low for what you are attempting. Condensor mics will be higher priced and require a "pre"-pre amp in front of the integrated's pre amp due to their extremely low voltage output. So, to make this work at all, you'll need to find dynamic microphones and then hope they provide sufficient output levels.

Next, you'll need to have microphones with a frequency response which extends deep enough into the bass to make the experiment work. Most inexpensive dynamic mics are not meant for studio grade response and will lack the bass extension required to for what you seem to be planning.


I'm not sure exactly what you're intending to do after you have the mics plugged into the pre amp though. Are you going to record the out of phase signals or, are you going to perform a "live" demonstration? You have no provision for "mixing" the two microphones' output with the Fisher. And, expecting the placement of the microphones to be your only method of progressively placing the two signals out of phase is not likely to work. Out of phase conditions are really only audibly noticeable on long signal waves such as deep bass. Smaller, shorter wavefronts in the upper frequenices are literally too short to be easily placed out of phase with each other for human detection. The length of a (long) bass wave can be as much 20-64'. First, you'll need to have that much room between the two mics to achieve an out of phase signal and then you'll have to take into account the reflected waves which exist in a normal room. Bass is omnidirectional and there will be reflected waves coming from every surface in the room. Remember, even at a somewhat high 40 Hz (well beyond the lower frequency limits of most low cost dynamic mics) there are 40 pulses per second and each of those wavefronts will arrive at a surface and be reflected randomly into the room. The chances of avoiding a reflected wave through placement of the mic alone is ... well, not good. This means should you account for the length of the wavefront by position, you'll still have reflected waves which arrive at the mic diaphragm not completely 180 degrees out of phase with the intial wave. I'm guessing you understand that only at the 180 degree out of phase mark will to wavefronts cancel each other. Anything greater or lesser than 180 degrees and you will still have some signal response.

To expect to achieve cancellation of sound in this set up is all but impossible and would only have a good chance in a completely anechoic chamber. Do you have access to such a room?



A more realistic approach IMO would be to use a small mixer with a continously variable phase control. This would allow you to cancel two single frequency sinewaves by progressively shifting the phase of one signal against the other. At this point you could use midrange signals though you will still run into the same problem of reflected signals arriving not far enough out of phase to provide complete cancellation should you rely on microphones. The only way to work around this is to use a signal generator which can input two discrete single sinewaves into the mixer's input and the two channels can be shifted in phase against one another.

If I understand what you are attempting, I think you need to reconsider your plan for several reasons. You might head to somewhere like Guitar Center or a pro audio shop and ask what they have to rent for the occasion. Otherwise, I'd say you've bitten off more than you can chew for a simple science experiment.


You might also want to investigate the concept of "standing waves" and see if that works better for your experiment.



.
 

Platinum Member
Username: Plymouth

Canada

Post Number: 15511
Registered: Jan-08
Greg

Yes you will obtain sound from the magnetic phono input but this input has a correction which will not give a good sound.
Phono ceramic input are high level input then does not work for a micro.

A old tape deck has 2 micro input!

You can also buy a good quality mixer for low cost like this one:
http://www.behringer.com/EN/Products/502.aspx

http://pro-audio.musiciansfriend.com/product/Behringer-Xenyx-502-Mixer?sku=63126 2
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