A basic Limiter

[Albert H]

Unlike most limiters based on operational transconductance amplifiers, I don't pu the OTA in the audio path. They tend to have inferior audio qualities compared to even quite cheap op-amps. Instead, I use them in the negative feedback path of a good quality op-amp. To reduce the through gain, I increase the gain of the OTA, thereby applying more negative feedback, and so reducing the gain of the op-amp.

Level sensing is done after the gain control cell, and the channel outputs are resistively combined so that both channels operate in tandem.

It is possible to do the level sensing before the gain control circuit, but this tends to give coarser control and is prone to overshoots.

None of the components are special - you should even be able to get them (at an outrageous price) from Maplin!

AH-FBLimiter.png

It needs a +/-15V supply (I use 78L15 and 79L15 regulators) and it has a high input impedance. The output impedance is of the order of 200Ω (which is why the crude resistive combination of the channels for the sidechain doesn't compromise the stereo separation).

The dual audio op-amp can be the LM833 (as shown) or the more common NE5532 without affecting performance. The PNP transistors can be 2SA1015, BC213L, BC560C, 2N3906 etc - just be aware of the pin-out!

[Banus_radio]

Thanks albert, looks good, thats build for this weekend defo

One question though, How do I get a negative supply. Id like to mount this into my transmitter which has 24v switchmode meanwell supply, can i make a circuit to generate the negative voltage?

[Albert H]

It's not trivial, since you'll need the same current on the -15V rail as on the +15V rail. There are DC - DC converters available that will give you -15V from a +15V supply. One 15V converter I've used is

https://www.rapidonline.com/hn-power-si ... ma-59-2377

which gives both +15V and -15V at 100 mA each side from a 24V single supply. If you're going to use this, make sure that the "limit" LED is a high-efficiency / low current one, and increase the 390Ω current limiting resistor to 10k or so. I use

http://cpc.farnell.com/broadcom-limited ... dp/SC11586.

It'll still blink away OK, but won't draw enough current to put clicks on to the audio as the converter tries to keep up with the changing current! You have to buy the LEDs in at least 5-off, but at 15p each, that's not going to break the bank! You'll find that CPC have loads of other stuff that you'll want to buy, so you'll end up avoiding their postal charges.

[MiXiN]

Albert,

What's your thoughts on this please? http://www.ebay.co.uk/itm/182365803223?redirect=mobile

[Albert H]

I hope that he's not passing the audio through that 741 op-amp - it'll hiss! Also, the operational transconductance amp (the 13700) isn't a good thing to put directly in the audio path, Lastly, it's mono, and pairing them up isn't going to work too well unless there's provision to link the control paths.

The basic limiter I've put up here is low noise, low distortion and will keep most levels in check. You can also build it for <£20. Ever used "Veroboard"? I'll put a Veroboard layout up here shortly. There's also a nice PCB layout for it too, which I'll put up here.

[MiXiN]

I'll leave it then Albert, and thanks for your response.

I've got a load of Veroboard here, so will have a go at the limiter you posted earlier.

[ragga]

Albert could you use your stereo limiter in mono until you had a stereo coder to add to the signal path or is dedicated mono limiter needed?

[Banus_radio]

I bought that limiter last week, it was a little slow on postage, took about a week to arrive.
It sounds awesome shuffy, I highly recommend it for £13.
There is definetly no hiss and no hum and its in a 350w box unshielded and on air now. Wink wink
I can't see anywhere to link audio paths if you was to run 2 separate circuits for stereo but I can tell you the audio doesn't go through the 741, it looks like it's used to amplify and rectify the audio for the LED and to feed the gain pin of the lm13700n.
For £13 it's a steal. I will put it on the oscilloscope if you like? This is the limiter I was talking about in the other thread, I will try and copy the circuit if you like?

[Banus_radio]

It's not trivial, since you'll need the same current on the -15V rail as on the +15V rail. There are DC - DC converters available that will give you -15V from a +15V supply. One 15V converter I've used is

https://www.rapidonline.com/hn-power-si ... ma-59-2377

which gives both +15V and -15V at 100 mA each side from a 24V single supply. If you're going to use this, make sure that the "limit" LED is a high-efficiency / low current one, and increase the 390Ω current limiting resistor to 10k or so. I use

http://cpc.farnell.com/broadcom-limited ... dp/SC11586.

It'll still blink away OK, but won't draw enough current to put clicks on to the audio as the converter tries to keep up with the changing current! You have to buy the LEDs in at least 5-off, but at 15p each, that's not going to break the bank! You'll find that CPC have loads of other stuff that you'll want to buy, so you'll end up avoiding their postal charges.

Albert is there not an easier way than buying this from rs? It sort of defeats the object of building a cheap limiter. I'd love to build your circuit and try it but not will to buy a negative 15v thingy from rs at a tenner a pop each time.
Surley there's a simple circuit I can build

[ragga]

Banus if its the outlay there are dual 15v psus on ebay for a couple of quid that will give enough current , I dont know how clean they are but there is obviously some filtering. If its a diy thing then there are plenty of examples of circuits on the net , you just need a transformer rectifier and + and - 15v voltage regulators ....plus some smoothing with caps etc... ( i made one for the experience from a circuit i found and it worked fine ...if i can do it anyone can )

[Banus_radio]

I get what your saying but to me that's hassle.
I simply buy a rig of my builder and like to fit my own Rds and limiter. I don't run stereo, I don't know of many pirates that do.
To have to fit a power supply just to power the limiter is hassle and its tight on room inside the boxes I buy as it is.
That £13 limiter is working a dream, it would probably cost nearer a tenner to build anyway with the pcb and connectors.
I tried the pira limiter and wasn't impressed, I bought the newer limiter off pcb wizard and it sounded wank. The only one if had any luck with is the one you mentioned above on eBay. I'm going to copy the circuit and post it up here.

[ragga]

I simply buy a rig of my builder and like to fit my own Rds and limiter. I don't run stereo, I don't know of many pirates that do.

I'm with you, the only bits of kit I have made and use myself are the 10ghz tx/rx link circuits and only really because i coudnt buy anything, or if i found something it seemed a bit dear or of dubious design.
Thats why I asked Albert if the stereo limiter was any good at mono, I'd build one now if I could use it in my current setup and if I did eventually experiment with stereo it was ready to go.

It seems you can buy relatively cheap modules (tx,pa's, stereo encoders etc) that if designed and made with a bit of care will give performance close to what I hear from pro fm but when It comes to signal processing the diy or cheap ebay stuff is a long way from the pro dsp.
A 1u rack that would sit in a small fm station seems to need multiband compression and complex limiting/clipping to work well and would perhaps cost 1k and up, this is why I am keen to try Alberts circuit. If he can come anywhere near a basic rack, with his very affordable design then I'm well interested.

I bought that limiter last week.It sounds awesome shuffy, I highly recommend it for £13

And this sounds like a bargain if the sq is as good as you say ! was it easy to set up once you'd slotted it in with your other gear?

[shuffy]

It sounds awesome shuffy, I highly recommend it for £13.

For the price, and for mono, it's probably fine. Did you get me mixed up with Albert though?
Some years ago now there was a limiter design based around the LM13600 which was used by some of the London builders. It was a bit noisy, but OK. The LM13700 used in your board had its output buffers further isolated from the rest of the device which in theory should improve things but if I remember correctly that was noisy too, although I was using that in a non-broadcast application (music production) where it was a bit more noticeable than on FM radio!

[Albert H]

OK - here's a cheap and simple audio processor for FM mono. This has been used by dozens of stations, and allows direct feed of a link rig (or main rig if you're bold) without any further hardware. It's cheap, mains powered, and can be set up to simply hit 75kHz deviation at clip point and it goes no further!

AH_Mono_Processor.png

You'll also see a cheap and simple + / - linear mains power supply.

The first section gives 50µs pre-emphasis and some lowpass filtering. The L-C filter gets rid of any content at 19kHz, to prevent the "blinking stereo light" problem. The limiter section is fast attack / slow decay, and the time constants were chosen after a lot of listening tests - you can change them if you like, but I'm confident that you'd come back to the values we chose. There's an LED that indicates the action of the limiter. The clipper drives an LED and handles transients and overshoots that the limiter is too slow to deal with.

I have a nice PCB layout (and a reasonably good Veroboard layout) which I'll put up on here shortly.

[Albert H]

I bought that limiter last week, it was a little slow on postage, took about a week to arrive. It sounds awesome Shuffy, I highly recommend it for £13.

For £13 it's a steal. I will put it on the oscilloscope if you like? This is the limiter I was talking about in the other thread, I will try and copy the circuit if you like?

I've since found out that the 741 on that £13 limiter is used for the sidechain - it's NOT in the audio path - so it should be pretty low noise. As a cheap solution to mono processing in a hurry, it's probably ideal.

[Albert H]

The L-C filter gets rid of any content at 19kHz, to prevent the "blinking stereo light" problem.

I've since found that it's a good idea to reduce the "Q" of the L-C filter by putting 47Ω in series with the inductor. This slightly reduces the steepness of the 19kHz notch, widening it a bit to account for inaccurate inductors and capacitors. I got one batch of inductors that gave me a notch at 20.7kHz - not much use. In that case, I had to tack extra capacitors underneath the board to get the frequency to exactly 19kHz.

Calibrating that notch is easy if you have an audio generator that goes up there or a stereo coder that you can steal a 19kHz signal from - connect your 'scope to the output of the second op-amp and fit padding capacitors until the 19kHz disappears!

[mike123]

I want to build the mono version.
Does it work on +/-12V also?
Because all my PS are 12v (13v max).

I could make +-12v from this module with a standard 12v DC power supply?

eBay item number:252049612127

[Maximus]

Albert- I remember seeing your design on the first site and more recently on other European sites. It looks proper sound and keep up the good work.
Thank you


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[Albert H]

Mike

+/- 12V will be OK. The headroom will be slightly reduced, but this shouldn't matter, since the clipping prevents any signal bigger than +/- 1.25V leaving the circuit. We found that 2.5V p-p was an ideal level to drive down a piece of audio coax at pretty low impedance, without picking up noise and RF.

This is a very simple processor, but it has all the basic elements that you need to sound good. It won't be as loud as an Optimod, but it also doesn't cost thousands!

[Albert H]

One further (minor) tweak - put a 47 or 56pf capacitor in parallel with the 12k feedback resistor in the last stage - it improves stability when there's RF around and gets rid of the worst excesses if you go heavily into clipping!