Quick homebrew microphone preamp.

[bikerhifinut]

Hi folks, Thought i'd share this.
I needed to make a preamp for use with a dynamic Microphone into a line input only PA amp for the local minister.
after studying a 2 stage opamp based pre in my book of circuits i realised I could adapt one of the cheerful n cheap maplins stereo preamp PCBs.
I basically altered the input resistance from 1k to 10k, that gave a first stage gain of 10x with a feed back resistor of 100k. The circuit I used showed an intermediate input resistance to the 2nd stage of 47k, after a level setting pot, so thats what I used there along with a 470k feedback resistor, giving a stage gain 0f 10x again. I made the 2 stages simply by linking the L and R channels of the 5534 based circuit in series. I didnt do anything fancy with coupling caps but i did use bigger and better supply decoupling caps of 10uF instead of 100nF.
I put it all together in a small plastic box and powered it with a PP3.
I didnt expect it to be spot on on first switch on, but i was pleasantly surprised. I had expected to need a frequency EQ cap in the feedback loop but with the mic we use it seemed fine.
Anyway it shows that those cheap pcb based kits are quite flexible and adaptable.
I know I could have got a 100x gain off a single opamp stage but I think its pushing it a bit and this way I had a lot of control over the overall gain by use of the interstage level control., I also kept the 10K output pot in circuit and tweaked it up a hundred ohms or so as theres no resistor on the outputs to prevent oscillation.

It got me to thinking that the circuit might be adaptable to other uses, such as ceramic cartridge step ups. In that case I'd set the first stage as a unity gain buffer with a high input resistance, and then the second stage still as about 10x gain and adjust the output as needed.

I know there's tweaks and refinements but I just thought I'd share this as it worked rather well, to my surprise. And the board only cost me £6 which is not bad considering.

A.

[G6Tanuki]

A nice quick fix, and worth passing on.

A while back I repurposed a Maplin stereo mic-preamp - http://www.maplin.co.uk/p/microphone-preamplifier-qs41u - so I could use a moving-coil mic with an old 'KW Electronics' AM transmitter that expected to be driven from a high-output xtal mic. It worked remarkably well - giving full modulation with the transmitter's AF gain control only 1/3 of the way up. OK, we're not talking hi-fi here and I'm sure I was playing liberties with impedance matching, but hey! what works, works!

[bikerhifinut]

Found this one which I think in retrospect would have been even better.
http://www.maplin.co.uk/p/velleman-m...lder-kit-ve21x

data sheet:

https://maplindownloads.s3-eu-west-1...anual-3490.pdf

It looks like its got a buffer stage then the gain stage, and the clever bit is the gain is controlled by a trimpot in the feedback loop of the second stage.
It could be quite easily turned into a ceramic step up by setting the input resistors to an impedance of 1meg. this would also mean replacing the feedback resistor on the first stage to 1meg to maintain the unity gain of the buffer.

Andy.

[bikerhifinut]

Originally Posted by G6Tanuki

A nice quick fix, and worth passing on.

A while back I repurposed a Maplin stereo mic-preamp - http://www.maplin.co.uk/p/microphone-preamplifier-qs41u - so I could use a moving-coil mic with an old 'KW Electronics' AM transmitter that expected to be driven from a high-output xtal mic. It worked remarkably well - giving full modulation with the transmitter's AF gain control only 1/3 of the way up. OK, we're not talking hi-fi here and I'm sure I was playing liberties with impedance matching, but hey! what works, works!

Yes its a shame its discontinued. I would have bought one and saved myself the bother of building one!

Might be worth investigating if the amp is available elsewhere although i guess everything is on the well known online auction site.

A.

[bikerhifinut]

Originally Posted by bikerhifinut

Found this one which I think in retrospect would have been even better.
http://www.maplin.co.uk/p/velleman-m...lder-kit-ve21x

data sheet:

https://maplindownloads.s3-eu-west-1...anual-3490.pdf

It looks like its got a buffer stage then the gain stage, and the clever bit is the gain is controlled by a trimpot in the feedback loop of the second stage.
It could be quite easily turned into a ceramic step up by setting the input resistors to an impedance of 1meg. this would also mean replacing the feedback resistor on the first stage to 1meg to maintain the unity gain of the buffer.

Andy.

I think the velleman kits can be obtained cheaper than maplins from other vendors?

[bikerhifinut]

Just had another quick peruse of that Velleman circuit.
I think it would benefit from a series resistor on the output pin to help counteract any HF oscillations?

A.

[paulsherwin]

Originally Posted by bikerhifinut

Found this one which I think in retrospect would have been even better.
http://www.maplin.co.uk/p/velleman-m...lder-kit-ve21x

data sheet:

https://maplindownloads.s3-eu-west-1...anual-3490.pdf

It looks like its got a buffer stage then the gain stage, and the clever bit is the gain is controlled by a trimpot in the feedback loop of the second stage.
It could be quite easily turned into a ceramic step up by setting the input resistors to an impedance of 1meg. this would also mean replacing the feedback resistor on the first stage to 1meg to maintain the unity gain of the buffer.

Andy.

An LM358 wouldn't be my choice for audio work, though it would be easy enough to substitute something else.

[bikerhifinut]

I'll check next time I'm in Maplins Paul. I think they use a TL072 these days. That's certainly the chip you usually find in a Velleman Kit.
As its socket based it's dead easy to substitute.
My comment regarding a "stopper" resistor on the outputs was based on being told about chips like 5532/5534 being prone to HF oscillations by the way.

A.

[Diabolical Artificer]

Interesting Andy, you sound like you have an intuitive knack for opamps.

I was trying to knock up a similar simple circuit the other day. I was trying to fit an active tone control to a guitar. The circuit was a Baxandall using a TL072 as a voltage follower, I think, with feedback; it kind of worked, my opamp skills arn't quite as developed as yours.

One of those little boards would have been handy.

Andy.

[Radio Wrangler]

The common reason for putting a resistor on the output (and remembering to take the feedback from before the resistor) is to provide a little isolation from a capacitive load. Otherwise the internal resistance of the opamp's output forms a phase shift network with the capacitive load, and the amount of phase shift at high frequencies can wipe out the loop's stability margin.

The NE5534 and NE5532 are quite fast opamps and along with all the other types with higher-frequency capabilities, the networks around them have to be designed to stay good to those higher frequencies - even if you're only using them at DC.

This is a general feedback thing. If you look at the circuit of a well-designed audio power amplifier, you will see a series resistor in the output, with an inductor across t to reduce losses at low frequencies. Usually this is a big fat resistor with several turns of thick-ish wire wound on top of it, using the resistor body as a former. This added bit stops capacitive loads (like crossovers) making the amplifier unstable at high frequencies. This is also the reason why some boutique speaker cables which had high capacitance between conductors caused some rather expensive amplifiers to destroy themselves. Same problem, different scale.

The issue of phase margin also rears its head in valve amplifiers. The phase shifts due to capacitance in output transformers are difficult to minimise, and in the end they limit the amount of feedback which can be used without creating an oscillator.

If you want to get into opamps, Horowitz and Hill "The Art of Electronics" is a great book. It was written for students of subjects other than electronics, so it starts from square one and takes it gently without all the mathematical proofs you'd find in electronics books for electronics students. There's a third edition out recently, but a second-hand one of either of the two earlier ones will still do nicely and hit the wallet less.

David

[David G4EBT]

ESR Electronics sell the full range of Velleman kits. The 1803 pre-amp kit costs £3.25+£3.45 + VAT = £8.04, so if you have a Maplin nearby, it's cheaper to buy from Maplin.

https://www.esr.co.uk/velleman/products/frame_kits.htm

[bikerhifinut]

Originally Posted by Radio Wrangler

The common reason for putting a resistor on the output (and remembering to take the feedback from before the resistor) is to provide a little isolation from a capacitive load. Otherwise the internal resistance of the opamp's output forms a phase shift network with the capacitive load, and the amount of phase shift at high frequencies can wipe out the loop's stability margin

That's it David.
I forgot to put that bit about a capacitive load, i.e. most screened cables can have several hundred pF capacitance, especially if its being used with a microphone then a long lead to the main amp. I guess.
I'm no op Amp expert Andy (Dr Wobble), I just find them so very useful in all manner of Audio Preamp circuits. Generally speaking you know they are going to work, and work very well.
I have been finding it a bit of an education recently, with my foray into a scratch Valve power amp build and in the past I have experimented with various valve aspirated preamplifier and buffer circuits with varying degrees of success/satisfaction. It's been a good way to learn the practical aspects and pitfalls and many mistakes along the way.
However, and as a "dyed in the wool" valve head, it may come as a surprise to some to learn that My favoured front ends/preamps of late have been Solid state, admittedly of some pedigree but not strictly "audiophile". All have been based on operational amplifier design, the current and long standing version uses discrete transistors in a custom operational amplifier design, with some very neat tweaks in the way feedback is applied along with the volume control. I can best describe it as like "not being there" something I think david (Radio Wrangler) would understand? I must add this is a commercial preamplifier, not anything I made.
Anyway to cut the ramblings of a lunatic, I have found these cheap and cheerful designs and pcbs from maplins to be very useful bits of kit, especially if you treat them as a starting point for your own modifications in respect of feedback and input resistance etc. I'm mulling over the stereo design again as I think a pair would probably make a workmanlike 2 stage RIAA magnetic cartridge preamp with either a passive EQ between stages, or a more usual feedback based EQ around the second stage. Lets not get into arguments of passive vs Active EQ on this thread fascinating though they are. It's just me trying to illustrate how dam useful the humble opamp chip is.
Theres a good Australian website which a few members must already know, ESP i think its called (elliot sound Products?) some good ideas and circuits, and no "black magic"

Andy.

[Radio Wrangler]

There is a rather good RIAA stage for moving-magnet cartridges by Peter Baxendall. It uses a single NE5534 per channel and has a passive RC final pole on the output. If the subsequent stage isn't high impedance on its input, then an additional buffer stage will be needed. I think it was in Wireless World, late 70's to early 80's.

David