Preamp doesn't work?

[Gaston]

i'm not an expert on preamps but I now a little about troubleshooting. mabey this will help. what i would do is hook up a steady signal to the input, then take your scope and probe the original signal were it enters, the probe the other side if the first capacitor and make sure the signnal gets through. if it doesn't you know the problem is there, if not got to the other input of the next component, if the signal is there you know the trace up to that point is good and not open or shorted. then go to the other side of the component and see what you have ....and on down the line untill you don't get the signal. then at least you know where to look for the problem.

 

[audioguru]

I think L. Chung's simulation has an extended high frequency response because his input signal is 10mV peak, the gain is more than 200 and the amplifier is clipping. Its max output is only 1.5V peak.
I simulated it with a steady sine-wave and it is clipping badly.

A dynamic mic has a coil with one side grounded. It can't produce a DC voltage.

Headphones at the output would lower the max output level to nearly nothing since the circuit has a fairly high output impedance.
Its 4.7uF output capacitor would cut frequencies below 4.3kHz into an 8 ohm earphone or cut frequencies below 1.1kHz into a 32 ohm earphone.

 

[Ventura]

Yes, audioguru, as you said, the dynamic mic is unpowered and produces no DC. With loud noise I'm showing about 50mVAC output from the mic on a meter.
I'd like to make this little beastie work, and am paying attention to suggestions that folks are posting here. I will keep fiddling with it, but if you're aware of a better, good-but-not-too-fancy, op-amp-type circuit that would work with a dynamic mic, unbalanced output, 12VDC power, I'd love to see the schematic. Sooner or later I'll probably want to step up to something like that.

Thanks, again.

 

[Nigel Goodwin]

Yes, audioguru, as you said, the dynamic mic is unpowered and produces no DC. With loud noise I'm showing about 50mVAC output from the mic on a meter.
I'd like to make this little beastie work, and am paying attention to suggestions that folks are posting here. I will keep fiddling with it, but if you're aware of a better, good-but-not-too-fancy, op-amp-type circuit that would work with a dynamic mic, unbalanced output, 12VDC power, I'd love to see the schematic. Sooner or later I'll probably want to step up to something like that.

It's trivial to make a preamp with an opamp, check the 'sticky' at the top of the 'General Electronics Chat' forum, which gives the circuit configurations.

You could use either a balanced or unbalanced configuration, it makes very little difference - just decide what gain you want, and what input impedance.

 

[eblc1388]

I think L. Chung's simulation has an extended high frequency response because his input signal is 10mV peak, the gain is more than 200 and the amplifier is clipping. Its max output is only 1.5V peak.
I simulated it with a steady sine-wave and it is clipping badly.

Thanks Audioguru. That's exactly what happened when SPICE get into the wrong hands.

The output waveform is so badly clipped when I done a transient analysis which I have not done previously. But after I reduced the input signal amplitude to 1mV, the output does not clip anymore. I'm surprised that the frequency response plot still looks exactly like the one before. I was hoping it will be a little better off when it is not clipping but no.

It shows how easy one can fall into these traps if one is not careful in choosing the signals. No excuse for me this time. I'm learning as this is only my third or fourth simulation trials with SPICE. Usually I would just build the circuit and test it but I'm getting lazy. It is so easy using SPICE but what you see is often what you don't get.

The quick rolloff in high frequency response in ronsimpson's simulation is due to alternate feedback capacitor connection in his circuit.

 

[Roff]

Thanks Audioguru. That's exactly what happened when SPICE get into the wrong hands.

The output waveform is so badly clipped when I done a transient analysis which I have not done previously. But after I reduced the input signal amplitude to 1mV, the output does not clip anymore. I'm surprised that the frequency response plot still looks exactly like the one before. I was hoping it will be a little better off when it is not clipping but no.

It shows how easy one can fall into these traps if one is not careful in choosing the signals. No excuse for me this time. I'm learning as this is only my third or fourth simulation trials with SPICE. Usually I would just build the circuit and test it but I'm getting lazy. It is so easy using SPICE but what you see is often what you don't get.

The quick rolloff in high frequency response in ronsimpson's simulation is due to alternate feedback capacitor connection in his circuit.

No, this is what happens when you have two people who don't understand Spice AC simulation. The simulator linearizes all components after computing the DC operating point, so it is analyzing a linear network. The AC input level makes absolutely no difference. This is why your frequency response plot looks exactly the same. I almost always use 1V, because the gain reported is relative to 1V. You could use a zillion volts, and the Bode plot would have the same shape.
If you want to see distortion, you have to do transient simulations.

 

[Ventura]

Just want to update re my adventures with the “Smart Kit No. 1024” microphone preamp kit that I couldn't get to work despite my very careful assembly:
I ordered this kit from "Information Unlimited" in New Hampshire, USA nearly two months ago. I've since then seen a video about the fellow I was (politely) dealing with via e-mail, owner, Robert "Bob" Iannini. He's quite a character, who is into building magnetic pulse weapons (projectile and wave), blowing things up, and generally having a good time while making money. This is all well and good (I'm actually jealous) but it should be known that, since he GRUDGINGLY allowed me to send the preamp back so they could take a look at it well over a month ago, they are "shining me on." Prior to my sending it to them, Bob bluntly informed me that I had done something wrong. Maybe so, maybe not, but one has to marvel at the attitude and what passes for "customer service."

 

[Roff]

I sent the guy an email, suggesting that it might be in his best interest to read this thread, and especially your most recent post.

 

[mvs sarma]

The whole thing should cost around $2/- as BOM, and sending and getting back would cost you a moon.
I only wonder whether the OP used BC547 by himself or the supplier sent him like this. The circuit appears a close simulation of Tape head preamplifier. The first transistor could be BC549, but condition that the input signal should be very low.

 

[McAmp]

Non functioning mic amp

I looked over the circuit and the resistor values don't work to bias this amp correctly. I've dug about in my old files and found a simular circuit which a friend of mine built and used many years ago for a mic amp on his disco unit.
Firstly you will have to supply the circuit with a 9volt stablized supply.
Modification as follows.
R1 = zero ohm link. R2 = 5k6, R3 = 68k R4 = 47k, R5 = 1k0, R6 = 1k2,
R7 Remove, R8 = 220k, R9 Remove. C7 to be resistor R10 = 18k
C4 to be two cap in series C4a to be 4300pF C4b to be 15nF
C4a to be above R10 and C4b to be above R8 on the diagram.
Make link between junction R8,R10 and C4a and C4b.
C1,2,3,5,& 6 are close to the values in my circuit so should be ok.
I have not built this circuit myself and send you this info in good faith as I said the circuit has been with me for many years.

Kind Regards
McAmp

 

[audioguru]

Many words but no schematic.

 

[Ventura]

The “Smart Kit” preamp under discussion was sent to me with BC547 transistors, but the accompanying info/instruction sheet noted that they could be also be BC548 or BC549.

Smart Kit Electronics is a Greek company with distributors all over the world. Robert Iannini’s “Information Unlimited” company is the U.S. distributor. I’m going on the assumption that they wouldn’t be selling this preamp if it didn’t work, but I sure can’t figure out what the problem is. I make mistakes like anyone else, but I’m generally pretty good at building this sort of thing. I really was careful during assembly.
I’m probably going to end up building one of the more modern balanced output preamp circuits I’ve seen online, and buying a suitable microphone to go with it.

Thanks, guys.

 

[mvs sarma]

Please see the below link
The circuit is sameand the input signal is not to exceed 1 or 2 mV. Only for the T1 you may change to BC549 if you have one.


https://www.electro-tech-online.com/custompdfs/2008/03/1024.pdf

 

[techmanx]

Hi take a further look ?

The attached preamp circuit is a reproduction of what came with the “Smart Kit No. 1024” microphone preamplifier kit that. It’s purported to be suitable for dynamic and condenser mics. I carefully soldered it together and put it in an enclosure with appropriate jacks. It didn’t work.
An oscilloscope (that I built from a kit) shows my dynamic mic putting out a signal, but the preamp’s output is nearly flat-line.

The outfit in New Hampshire that sold me the kit has been, frankly, a bit rude, but they eventually, grudgingly agreed to take a look at the silly thing. They said it will be low priority (nice folks, eh?), so I don’t expect a response for some time. In the meantime, I bought all new components from DigiKey, including good quality metal film resistors, and built another preamp from scratch, on perfboard, following the same circuit. It doesn’t work either.

I’m no electronics wiz, but I’ve put together a number of projects over the years, some from scratch. They all work. I can’t figure out what’s going on with this little beast. Do you guys see anything odd about this preamp? Does it look like it should work?

Thanks.

HI, I hope you will have some luck by now, but I cannot see a good reason why the shown circuit will not work. I used to sell electronic components and teach electronics, and whenever I heard that a known and tested circuit did not work, I asked the person to go back and very, very, carefully check every part of the circuit assembly. The result useually was a solder short, a short caused by a sliver of copper, an incorrectly inserted component, etc.etc. Please take another carefull and if possible using a magnifying glass, look at your circuit, I am sure that you will find the error. Be patient, best wishes to you Brian

 

[Bob Scott]

Yes it does seem odd that two voltages in the same transistor are exactly identical to 3 digits:

TR1: Collector: 1.84
Base: 1.84
Emitter: 1.24

How close CAN you get to saturation? There might be a copper short or solder blob between base and collector.

 

[Nigel Goodwin]

Yes it does seem odd that two voltages in the same transistor are exactly identical to 3 digits:

TR1: Collector: 1.84
Base: 1.84
Emitter: 1.24

How close CAN you get to saturation? There might be a copper short or solder blob between base and collector.

Except that's not saturated, if it was the collector voltage would be close to the emitter, and less than the base - as you say, it sounds like a short between base and collector.

 

[Roff]

One of the guys who ran sims could try shorting base to collector and see if the results are similar to the measured values.

 

[eblc1388]

The OP must be lucky then to have solder bridge on both a finished PCB and an another perfboard.

I bought all new components from DigiKey, including good quality metal film resistors, and built another preamp from scratch, on perfboard, following the same circuit. It doesn’t work either.

Edit: I did tried a simulation with collector/emitter swapped on both transistors and the preamp is still giving out amplification, albeit a bit less. It is certainly not dead either.

 

[Nigel Goodwin]

Edit: I did tried a simulation with collector/emitter swapped on both transistors and the preamp is still giving out amplification, albeit a bit less. It is certainly not dead either.

Using transistors with the emitter and collector reversed is a standard technique, you get less gain from them, but the CE saturation voltage is lower. It's often used for the mute transistors in CD Players.

 

[Ventura]

Yes, Sarma, that Quasar kit is indeed the same circuit as the Smart Kit I built. The Smart Kit is sold as appropriate for use with both dynamic and condenser mics.
I can't find specs on my "Ampex 2001" dynamic microphone, but I believe this mic is a rather ordinary, mid-level-quality item. It appears somewhat similar to a Shure SM57 dynamic which has "sensitivity" (output?) of 1.6mV. The Shure SM58, another dynamic, has a sensitivity of 1.85mV. I see no reason to assume that the Ampex is wildly different. And if mine did have a little too much output in loud conditions, would a functioning preamp respond with nearly flat-line output, even with the most faint whisper of sound into the Ampex? Doesn't seem reasonable.

Techmanx/Brian, I'll say again, it may very well be that I messed up. If I were anyone but me, I'd be mighty suspicious about the fact that the second version of this preamp, built on perfboard, seems to act just like the first (kit) one. This is why I originally asked for opinions about the viability of the circuit itself. All I can say is, I’ve built a number of projects from scratch without problems. I'm pretty careful. I used a low-wattage soldering iron and a heat-sink clip on nearly every lead. I peered at this preamp for an hour with magnifier glasses on and couldn't see anything wrong.

I don't know if I was out of line asking Information Unlimited to take a look at it, but it seems pretty rude and unprofessional that they agreed to do so and have ignored me for well over a month since then. My particular home security system relies on the ability to record noise connected with events outside my home, and it's been out of commission the entire time I’ve been trying to work this out. Very frustrating….