Hi, I've been playing around with the microphone input on my Creative SoundBlaster Live 24-bit card and a low impedence, unbalanced dynamic microphone designed for a radio tranciever. The input level is extremely low, and in my readings I have found that cheap condenser mics require bias voltage, and put out a much larger voltage signal. I also found that this particular soundcard outputs +5V for bias on a separate line from the input of the speaker, using all three pins of a standard stereo jack. I found one person who cleverly designed the circuit below to use the bias voltage to power a simple Class A amplifier to boost the signal of a dynamic mic to something the sound card will pick up:


(Picture Source)

I'm trying to understand exactly how this amp works, and I'm trying to learn the concept of impedance matching. I'm also trying to build and use this circuit in practical application.

My first problem is that the BC547B isn't readily available at the RadioShack down the street. Rather than order it, I'd like to substitute it and learn more about the parameters that govern transistor selection. Here's the BC546B data sheet, as well as the substitute I selected: the 2N4401. This substitution yielded a circuit that doesn't really provide sufficient gain. I'm guessing by looking at the datasheets that the 2N4401 was designed to function at higher currents and doesn't have a high enough hFE at low base current.

Other possible substitutions are the 2N3903, and the 2N2222. I'm guessing by the spec that the 2N222 would be the best choice as it is designed to work at roughly the same currents as the BC546, correct?

For the sake of completeness, here is the data sheet for the MPS2222A, which is Radio Shack's equivelent to the BC546, but not available to me currently.

My next question is in relation to how this circuit works. My understanding is that the signal first enters R1. R1 is present because the base impedance of the transistor is low. R1 increases the impedance presented to the mic and prevents it from being loaded down. C1 prevents any DC current from passing back to the microphone. R2 is the bias resistor which adds voltage to the incoming signal so that the signal is no longer alternating but always above ground voltage. This is characteristic of a Class A amp. I don't understand how to calculate the proper amount of bias here, and I read that this affects the amount of amplification. I'm guessing that with my higher power 2N4401, I would have to decrease the value of R2 to increase bias voltage to get higher gain out of the transistor, but this would also waste current from the +5V source. As for the output, C2 prevents DC from flowing out the output. Since this is electrolytic, though, I would think that it would be reverse biased during the negative part of the audio wave and not work correctly. Wouldn't this be better as a thin film cap? C3 is just a filter to provide cleaner voltage from Vin, and R3 is to knock down the voltage to the appropriate level to power the transistor. I'm not sure how the value for R3 is chosen, either. Using ohm's law, if the transistor is putting up no resistance, 1mA is flowing through R3. So, I'm guessing that 1mA is desired on the emitter, so the R3 value was chosen accordingly. Would this need to be modified for use with the 2N4401?

Finally, how would you determine the output impedance? I'm guessing that this would be a characteristic of the transistor. I know that the soundcard input is high impedance. My understanding is that a low impedance output into a high impedance input offers optimum voltage transfer, however high-impedance in and out offers optimum current transfer. Which provides more gain to the soundcard? I read that with modern equipment, low-Z into a high-Z input is normal and desireable.

Thank you very much for any input!!!
TJ Harrell

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KF4GAU

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I expect a 2N3904 would work quite well, and naturally 549 etc. As far as I know the 2N2222 is noisy and probably isn't desirable in this application.

In this case, you are after voltage/signal transfer, so a low output impedance is desired.

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What you want is for the collector of Q1 to be at 1/2 the supply voltage or 2.5Vdc. Choosing the value of R2 requires you to know the Beta (gain) of the Q1 and the value of R3. We know what R3 is, but the transistors Beta will vary between different ones of the same part number. The easiest way is just to sub different resistors until you see 2.5V at the collector of Q1.
An simple formula if you know the Beta of Q1:
R2 = Beta * R3
On a commom emitter amplifier such as this, it is aprox equal to R3

No, because the input to the audio card should already go to ground through a pulldown resistor etc.... There should be 2.5V collector of Q1 so an electrolytic is OK.

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I simulated it with a "typical" 2N3904 that has spec's about the same as a BC547B. A 2N4401 has spec's that are very broad, all over the map.

Single transistor circuits with high gain like this one produce horrible distortion at high levels. This circuit clips off the bottom because the base current is too high:

Attached Images

preamp with 220k.PNG (21.0 KB, 19 views)

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Here is the same circuit but with the value of the base bias resistor increased to 1M so the transistor doesn't have too much base current. The distortion compresses the top so much that the voltage gain can't be counted.

Attached Images

preamp with 1M.PNG (21.5 KB, 20 views)

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Uncle $crooge

 

< I'm trying to understand exactly how this amp works,
< and I'm trying to learn the concept of impedance
< matching. I'm also trying to build and use this circuit in < practical application.
.................................................. ...................................
IMPEDANCE MATCHING
The math indicates that if R_signal_source = R load
then the maximum Power, (NOT voltage ) is transferred to the load.

An important consideration for a signal amplifier S/N
……………………………………………………….
Using R_signal_source = R load in a power supply design would be wrong , WRONG.
In a power supply R_out should be close to R_out =0 ohm to improve efficiency.
………………………………………………………………………………………..
Operation of the circuit shown:
Q1 is connected in the same configuration as an IC op-amp.

C1 blocks +Vbe from biasing the mike

R1 acts as the input resistor of th e “op-amp”

R2 acts as the feedback resistor of the “op-amp”

R2/R1 gives gain of x40 as mentioned in < Picture Source >
note that gain varies with the transistor chosen and
distortion is much worse than any op amp

C2 blocks +Vce from the load connected to the output
…………………………………………………………………………………………
In fact Q1 could be replaced by an op-amp IF < RING , Bias from SB > can provide suitable +Vcc.
- for op -amp:
R2 value changed to 39K , to maintain gain of R2/R1 = x40

-C3 can be used to insure low ripple on the +Vcc connection

-R3 is not used by the IC op-amp,

hawk2eye

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hawk2eye

 

Except you don't want impedance matching, it's a BAD idea for audio (or for the majority of connections) - by definition impedance matching is very inefficient, only 50% at the very best.

You want to feed a low impedance source into a higher impedance input, for maximum VOLTAGE transfer, you should be looking for 5 to 10 times higher impedance on the input.

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After reading some replies and thinking about this, I've come up with a few more thoughts/questions.

As Dr. EM and Nigel Goodwin both point out, the hi-Z input of the sound card should be fed with a low-Z output from this circuit to achieve optimum voltage (signal) transfer. The goal of this circuit is to increase the gain of the signal going into the soundcard. Since the soundcard has a hi-Z input, we aren't really trying to feed more current, but higher voltage peaks than the mic outputs. A transistor is a current device, though. My understanding is that the gain it is providing would be current, and it would not increase the voltage peaks. Thus, this circuit really isn't amplifying voltage, which is what I think I need.

So, can someone clarify whether I'm looking for voltage or current gain? In the case of this circuit, is it providing current gain as I guessed, and what specifications should I look at on the data sheet to determine the gain? If I am indeed looking for voltage gain, shouldn't I be using a FET?

Another thought is that kchriste says that R3 sets output impedance. R3 is 4.7K, not what I would call low impedance! In order to provide the low-Z output, I would need a circuit with an R3 of 600 ohms or so, right? I'm not sure of how the selection of R3 relates to Q1. Can I choose any R3 I like as long as I adjust R2 according to the formula:
R2 = Beta * R3

On the subject of impedance, is impedance similar in concept as internal resistance? For example, a mic outputting a 5V peak-to-peak sine continuous sine wave with a low impedance might be equivalent to 5VAC with a 50 ohm resistor in series with it's output, while a high impedance mic with the same output might be equivalent to 5VAC with a 50k ohm resistor in series with it's output. This explains why it is very bad to connect a hi-Z mic to a low-Z input. The low-Z input would have low resistance drawing near maximum current from the hi-Z mic, therefore it would load it down because the mic would have such a high load and the mic would not be able to produce a well formed wave.

In response to hawk2eye, would an op-amp be a better choice for this circuit, and if so, for any reason besides producing a cleaner signal? Also, what requirements does a typical op-amp have? Can it run on 5VDC, and how much current would it require? I did test this circuit with the 2N4401, which was mentioned to be very noisy and it didn't introduce a terrible amount of noise, however it didn't provide much gain, as previously mentioned.

TJ

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PC mike inputs are really strange?, I've never seen one yet that works by plugging a mike in it?. I have read in the past that the original Sound Blaster card (which most are based on) was designed to use a carbon mike (as in antique telephone handsets).

If you're looking for a decent quality mike input, it's probably best to use an opamp preamp and feed the line input instead of the mike input.

It provides both current gain and voltage gain, in fact I thought it was a really nifty little circuit - should do all that's needed, use a high gain, low noise audio transistor - personally I'd probably use a BC109, the original metal canned version of the BC159.

The output impedance will be considerably lower than R3, you would probably need to measure it to find out what it was.

Yes, that explains why it's bad, but the reason isn't so much a 'well formed wave', it's mostly that the internal impedance of the mike and the input impedance of the preamp form a potential divider - reducing the already tiny output from the mike.

I built one (very quickly) using an opamp fed from a 9V battery (for an on-line training course with Sony).

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The voltage gain of the original single transistor circuit is 80, and it has severe distortion when its input voltage exceeds only 20mV p-p which is only 7.1mV RMS. If its bias current is reduced to make it more symmetrical then its distortion is much worse. An opamp is much better.

A transistor's collector is a constant current source or sink so the output impedance is its collector resistor.

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Ack! I totally hosed my implementation of the circuit. I reversed the emitter and collector. Not sure what that even does. I'm thinking it functioned, but with no gain. I'm going to fix the problem with the 2N4401 I have in hand and try it out.

I'm still curious to know what would happen if I used a 2N3903 instead. I'm also still unclear as to what criteria to use to select a transistor in the first place. I'm guessing hFE and noise?

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KF4GAU

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Basically noise really, as it has negative feedback the gain isn't as critical as it might be.

Try different transistors and see what happens?.

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A transistor with its collector and emitter reversed will have a very low AC and DC gain.

These simple high gain transistor amplifiers hardly have any negative feedback. Therefore their DC operating point varies considerably due to the wide range of a transistor's DC gain. Also there is hardly any negative feedback to reduce distortion.

The DC gain of a 2N3903 is from 40 to 120 at the current it is used at in this circuit, a 2N4401 has a wide DC gain range that is about the same, although has a much higher DC gain at higher currents. The range of gain is so wide that you won't know which transistor has the correct amount of DC gain.

Try a 2N3903 and it works well. Try another and it works lousy. The same for a 2N4401.

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Undoubteble the suggestion to use an op-amp preamp system powered exteranally by a 9v battery (or 2) into the line input will yield the best results. I have microphones made with electret cardtridges, such as the ones used in PC mics, and connected via a mixer into the line in. The sound quality is massively better with much lower noise and higher headroom than using the onboard mic preamp, and it's not a rubbish sound card either.

It may be possible to use the 5v bias into a DC-DC convertor to give a high enough supply to power the preamp system, though i'm not sure about available current (most soundblaster models have the bias supplied via 2.2k).

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The simple transistor preamp circuit goesn't have a gain control. So loud sounds will be terribly distorted. An opamp preamp circuit can have a gain control as its negative feedback resistor so it won't overload when it is turned down.

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