I'm unclear on a few concepts here..

Negative feedback in this circuit is provided by R2, correct? The output is phase inverted, so feeding it into the base through R2 subtracts from the input signal to some degree. What's the point of doing this? It would seem to attenuate the amplification, but why? This prevents distortion during signals that peak the transistor?

DC gain is the Hfe value, correct? So the transistors are widely variable in construction, so one may give much more gain than another out of the same box. That much I understand. What is DC operating point, then? I'm guessing this has something to do with the range of currents that the transistor behaves in a linear fashion? Assuming I know the measured Hfe of a particular transistor, could I calculate the expected voltage gain for a given signal?


I want to stop here for a second and say thank you to everyone who has taken the time to post here. This thread has already taught me quite a bit that I didn't previously understand. I'd also like to acknowledge that I understand the Op amp solution is obviously better, and I'll definitely build that as well. At this point, I'm simply playing with this circuit as a means to learn more and understand it may not give me a very usuable product. I did fix my wiring mistake, and I still don't get enough gain. Cranking up the soundcard's software gain control adds a lot of static that may well be from my circuit. Another thing I noticed is that I get some AM reception when I touch the mic casing. Sounded like talk radio. I'd imagine that's a grounding problem easy enough to chase down.

TJ

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KF4GAU

I haven't burned myself with the soldering iron in at least.. Ow that's hot!

 

Yes.

If the transistor's DC gain is low, then it won't have enough base current so its collector current will be too low, causing its collector voltage to be too high and the top of its signal will be clipped. Since R2 connects to the collector then when its voltage rises then the current in R2 will increase and provide more base current to compensate.
The opposite occurs if the transistor's DC gain is too high.
The signal in R2 cancels some of the input signal which reduces the circuit's AC gain and also cancels some of the non-linear distortion.

No. Hfe is AC current gain. hFE is DC current gain.

In theory, if the collector operating point DC voltage is halfway between the supply voltage and ground, then the collector signal voltage can swing the max amount possible. But as I showed, with high AC gain the top of the waveform is compressed with severe distortion. I tried it with lots of negative feedback and the AC gain was only 1, output voltage was nearly max and the distortion was barely visible.

No. Hfe hardly affects low level signal voltage gain. hFE also hardly affects low level voltage gain. The low level AC gain without negative feedback is the ratio of the collector resistor (and any load in parallel with it) to the internal resistance of the transistor's emitter to ground. A transistor's internal emitter resistance is about the same for all silicon transistors and is figured as 0.026/emitter current in mA. If the transistor has a resistor to ground in series with the emitter and the resistor doesn't have a capacitor bypassing its signal, then it enters into the AC gain calculation since it causes another type of negative feedback which reduces gain.

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Uncle $crooge

 

It's as I suggested earlier, it means you don't have to select the resistor values for every single one you build - the negative feedback is at DC as well as AC, so it stabilises the stage (to some extent). If you feed the base resistor from HT rather than the collector you don't lose the gain, but you do have to manually select the bias resistor for every single one you build.

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+++++++++++++++++++++++++++++++++++++
In order to get best S/N performance from a signal source operating at its the lowest level, impedance matching is the ONLY way to go.

I agree with your comments ....for a "majority of connections" which can be defined for high S/N levels, then maximum voltage transfer gives an advantage over
impedance matching.

BUT: Using your technique, a mike pickup would produce poorer S/N for low level signals.

hawk2eye

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hawk2eye

 

Why?, and what are you defining as 'low level'.

And it's not 'my' technique, it's EVERYONE'S technique - can you name a single audio input where impedance matching is used?. If you're thinking of suggesting microphone transformers?, they are simply used to change the impedance, which is then fed into a much higher impedance as normal.

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.................................................. ..................................
Nigel Goodwin

Apparently you have not been involved with low level, noisy signals very much.
I am correct and EVERYONE's technique is wrong.

I have emphasized the words "impedance matching"
with ...vvv... marks.

see:
http://www.cinemag.biz/mic_input/mic_input.html


.................||||||||||||||||||||||
.................vvvvvvvvvvvvvvvvvv............... ..
< Excellent impedance matching between the microphone and the pre-amplifier,
< excellent balance and common mode rejection ratios (CMRR), very low distortion,
< and superior bandwidth

see:
http://www.cinemag.biz/mic_input/mic_input.html


hawk2eye

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hawk2eye

 

Why would you have noisey signals?.

So you're basing your suggestion on an advertising flyer for microphone transformers?.

As I said previously, this ONLY matches the microphone to the transformer, and NOT the transformer to the preamp, not even back in the 1940's where that technology comes from!.

Presumably the advantages you're talking about are from using a BALANCED microphone output? - where any common mode 'noise' on the long cable is cancelled out in the transformer?. It's only an advantage for long cables in poor environments, and 'modern' (last 20-30 years?) electroniclly balanced inputs far out perform transformers. Transformers have too many drawbacks, and are far too expensive to be in common use - modern electronics have rendered them fairly obselete.

Almost any modern PA gear has electronically balanced inputs, and this helps to reduce any noise pickup on the cabling.

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[quote=Nigel Goodwin]Why would you have noisey signals?.
As I said previously, this ONLY matches the microphone to the transformer, and NOT the transformer to the preamp, not even back in the 1940's where that technology comes from!.
++++++++++++++++++++++++++++++++++
Nigel:
I repeat:
the mike impedance to the amplifier is (Npri/Nsec)^2*Rgen
as shown by ANY text book that describes impedance matching.

Look it up. there is nothing wrong in being wrong.
.................................................. ..........................
The input transformer serves as an impedance matching device.

mathematically, for the transformer

Nout*Iout = Nin*Iin
Eout/Ein = Nout/Nin
then
Eout = (Nout/Nin)Ein
Iout = (Nin/Nout)Iin

Rout = Eout/Iout
Rout =(Nout/Nin)^2 *Ein/Iin
Rout =(Nout/Nin)^2 *Rin

hawk2eye

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hawk2eye

 

I worked with very high quality PA amplifiers. They used an input transformer for dynamic microphones as a balanced connection for them. The transformers were used at impedances far higher than the mics and as stepup transformers to reduce the noise of the following opamp since then it didn't need such a high gain.

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Uncle $crooge

 

[quote=hawk2eye]And I repeat again as well, what has that got to do with the matching INTO the preamp - quoting formulas from a book doesn't help anything, do you have actual examples of microphone transformers being impedance matched to the preamp input impedance?.

No disrespect Audioguru, but how long ago was that?. Using a VERY high quality transformer on the input stage of a VERY expensive mike stage is probably still the best method available (although it still doesn't use impedance matching to the preamp). However, it would increase the costs VERY substantially for a very small level of improvement. Cheap, and medium price, mike transformers have performance levels below modern electronics.

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Hi Nigel,
I worked with TOA brand, a Japanese sound systems company, until 6 years ago. The expensive amplifiers have a high quality input transformer for perfect balancing and for a voltage stepup which feed a very low noise opamp. Although they were small, the transformers have a good low frequency response because they are used with their impedance unmatched.
Their cheaper amplifiers use transistors as an almost balanced input.

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Uncle $crooge

 

TOA? - no disrespect, but the TOA PA gear I've seen isn't anything wonderful (and I've repaired quite a lot of it!). Presumably you're talking about the same sort of gear?, vocal PA, and NOT quality audio for musical PA?. There's no need for any great quality from it, so a cheap transformer wouldn't be any drawback.

I suspect their only reason to use a mike transformer is so they can charge a higher price?, and it's certainly pretty pricey, but all that vocal PA gear is - I've never been able to understand why?, it's certainly NOT because of design or build quality - perhaps just small sales figures?, or because they are selling to industrial users?.

BTW, just checked the mike preamp spec on a cheap Behringer mixer:

Input impedance 2.6K balanced. (for 50, 150, and 600 ohm mikes)
Frequency response 10Hz to 200KHz -3dB. (why bother to 200KHz?)
Signal to noise ratio 110dB
Mic E.I.N. (20Hz to 20KHz)
@ 0 ohms, 134dB
@ 50 ohms, 131dB
@ 150 ohms, 129dB

Pretty impressive specs, although a GOOD transformer will beat it on CMR, which interestingly they don't quote!.

EDIT: BTW Audioguru, you mention the voltage step-up of the transformer, have you ever actually measured how much that is?, or the actual impedances of the transformer?, if not I suspect you will be quite suprised if you do!.

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The cheap TOA mixer has recently been discontinued. Its mic input S/N spec is 126dB @200 ohms. I don't know the ratio of the input transformer.

Their products were pretty reliable. Very few items were returned during the 2 years to 5 years warranty. Their best power amps had a "no questions asked" 5 years warranty and none came back for repairs. They had 1500 dealers in Canada.

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Uncle $crooge

 

You're a knowledgeable, experienced man - what is your personal opinion on the actual design quality, and build quality, of the TOA gear - assuming you're prepared to give it here?, no problem if you would rather not!.

None of it has ever impressed me, but the usage doesn't really impress me either, and it doesn't require any great specifications for it's intended use.

I've always been VERY interested in high power audio PA, for band and disco use - and much of that is very well designed and constructed, but it has a MUCH harder life than vocal announcement type PA.

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Hi Nigel,
TOA designs are conservative tried and proven ones, nothing new. The high end amplifiers and speakers are used in live show theaters and sound excellent. I heard some background music systems in stores that sound excellent.

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Uncle $crooge