A basic Limiter

[mike123]

I changed the 3M9 resistor to 2M2. BIG difference as the release time is shorder is really kicks. Good for dance music with an eighties sound.
Satisfied with that, although I would like to get more grip on the spikes/peaks. The diodes solve that, but then the sound is to much distorted.

Here's my creation:

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[sinus trouble]

Excellent work Mike123!

Its good to see others experimenting with their own projects! As i mentioned before! There is NO perfect limiter settings!

You could play around with the attack and decay values?

The RC combinations are not that hard to determine, The higher the capacitor value, the longer the "Hold" duration!

The parallel resistor will control the discharge of that capacitor!

Infact , You could even add potentiometers to allow you to control the discharge! Once you have your happy range? A resistor or two in series with the potentiometer are easily included to limit range!

[Albert H]

You can increase the clipper threshold by introducing an extra pair of diodes in series between the 1k / 10k junction and the existing pair of diodes feeding the base of the clip indicator transistor, or you can change the pair of 1N4148 diodes for a pair of red LEDs - this will also increase the clip threshold. You'll probably find that the distortion introduced by the clipper is because the levels are a bit too high coming out of the limiter section, so increasing the clipper threshold voltage will raise the amount of headroom at that point.

Orban (who loves his clippers) uses a pair of back-to-back LEDS in the negative feedback loop of an op-amp. The op-amp tries to cancel the distortion caused by the clipping action, but the threshold is quite "soft" - as it goes into clipping, it's not adding harsh edges to the audio. I'll scribble some notes for you a bit later.

[reverend]

This thread led me to digging out my 3-band processor which was built about 25 years ago. It splits the audio into bass (< 200 Hz), then an 'everything else' channel and a third band whcih deals with the pre-emphasis (i.e. the treble boost above 3 kHz). All 3 bands are seperately compressed and then the bass is clipped (just enough to cut off any overshoots) before the whole ensemble is passed to an overall clipper. It then feeds an analogue stereo encoder (which, having re-connected the whole thing has a 19 kHz carrier level that's about twice what it should be )

This recording is taken off-air, and shows (I think) how effective analogue processing can be. 3 different tracks/styles of music and it seems to work OK with all of them. There are some mp3 artefacts in the file due to the low bit rate, and you can hear the noise from the sound card that was feeding the audio coming up in the quiet passages when the compressor gain increases.

3banddemo.zip

The sad thing is that I don't have the full schematic for the circuit any more I guess I could try and unpick the one remaining example I have left, but to be honest, with things such as StereoTool available, it seems kinda pointless.

[sinus trouble]

This thread led me to digging out my 3-band processor which was built about 25 years ago. It splits the audio into bass (< 200 Hz), then an 'everything else' channel and a third band whcih deals with the pre-emphasis (i.e. the treble boost above 3 kHz). All 3 bands are seperately compressed and then the bass is clipped (just enough to cut off any overshoots) before the whole ensemble is passed to an overall clipper. It then feeds an analogue stereo encoder (which, having re-connected the whole thing has a 19 kHz carrier level that's about twice what it should be )

This recording is taken off-air, and shows (I think) how effective analogue processing can be. 3 different tracks/styles of music and it seems to work OK with all of them. There are some mp3 artefacts in the file due to the low bit rate, and you can hear the noise from the sound card that was feeding the audio coming up in the quiet passages when the compressor gain increases.

3banddemo.zip

The sad thing is that I don't have the full schematic for the circuit any more I guess I could try and unpick the one remaining example I have left, but to be honest, with things such as StereoTool available, it seems kinda pointless.

Sounds really natural Rev!

No sign of Heavy clipping or distortion!

The recording with all its "Jingles" etc.... Would indicate that you put in some hours of preparation to produce an acceptable programme previous to (possible) broadcast!

If a DJ cannot distinguish between -6db and 0db! Maybe they need to find a more suitable hobby!

[rigmo]

is this heave any value?

[Albert H]

Unfortunately, the SSM2120 (if you can find one) tends to cost £80 - 100 these days! It was a brilliant chip at the time, and I used them in a few product designs.

Nowadays, the best simple ICs for gain control and compression / limiting are the "THAT" series of "Analog Engines". They're fairly expensive in small quantities, but they're the best things on the market. I designed (based on their datasheet) a nice stereo limiter for studio use, and have been considering design of a multiband, feedforward limiter using them.

I have designed PWM limiters in the past (for commercial manufacture). They tend to be complex, but use standard, widely available parts.

My "airchain in a box" has a slow-acting AGC first (to eliminate differences in source levels), followed by a gyrator-based 15kHz lowpass filter, three-band fast limiter, pre-emphasis, final clipper, then a stereo coder with 16X oversampling. It's an expensive box, but it works rather well.....!

[rigmo]

Unfortunately, the SSM2120 (if you can find one) tends to cost £80 - 100 these days! It was a brilliant chip at the time, and I used them in a few product designs.

I see cost 5 usd in china?
https://www.ebay.com/itm/271040582551

[Krakatoa]

Unfortunately, the SSM2120 (if you can find one) tends to cost £80 - 100 these days! It was a brilliant chip at the time, and I used them in a few product designs.

I see cost 5 usd in china?
https://www.ebay.com/itm/271040582551

From China? Beware of fakes!

I'd rather stick with analog engine ic from THAT corp, as Albert says.
But if you want to dig deeper, there is a brand called Alpha Rpar (a small company from Latvia) that makes VCAs at a more competitive price.

[Krakatoa]

Hi,

I build this limiter a while ago but have some questions.
If I'm informed correctly the 3k3 resistor (at the right site of the schematic) is for the release time?
I experimented with this resistor and increased the value to about 4k1.
I would like to change attack and release time.

I also disabled the two 1N4148 diodes because it's clipping way to much.
It might be they kick in too early, before the circuit starts limiting?

Also is there a way is disable the limiter function without bridging the circuit?
I would like to switch it on/off to check how much is actually limiting.

The circuit is also very picky which FET is used. I tried a 2N5458 because the 2N5459 was not available, but it did not work very well.

Going back a few posts, I remembered that I built this FET limiter circuit, and fixed some nasty positive feedback around the internal capacitances of the FET and to the Op-Amp, causing it to oscillate.

I took the time to draw it cleanly in LtSpice to share in the forum.
This limiter finally worked OK, and regained my faith in FET limiters. Details are given in the schematic itself:

[jvok]

But if you want to dig deeper, there is a brand called Alpha Rpar (a small company from Latvia) that makes VCAs at a more competitive price.

You got a source for those alfa rpar chips? Only one I can find is charging 6eur each. I can get a that2180 for less.

[LeeCavanagh]

Here's a Veroboard layout that I've used a couple of time when I haven't had a PCB available. The clipper diodes can be reduced to two pairs, or you can make one pair BAT41s (Schottky diodes) to raise the clip point by 0.2V. It's cheap and simple, includes a 19kHz filter to prevent the blinking stereo indicator problem, has a stage of 15kHz filtering just to shave the top end off a bit, and provides 50µs pre-emphasis. Considering its simplicity, it sounds pretty good on the air, and costs almost nothing to make. The only slightly unusual component is the 22mH coil.
Mono Broadcast Processor.png
This has been used by lots of stations, and all seem to like it!

Very difficult for my eyes to make out all the values, do you have a component list ? Thanks

[Albert H]

Lee

There's an improved Vero layout at download/file.php?id=1928 You should be able to read the values. I'll also dig out the Component List, and I'll put it up here later.

[rigmo]

Early 90s construction from magazine technical selection: compressor limiter designed by soulis. It is still a working one.

[Krakatoa]

Schematic is IDENTICAL to the Broadcast Wharehouse limiter that sold in the early 2000's.
But by the look of the PCB design and age of the components, that looks as the original source.
Good find!

[Albert H]

Nope - it's actually based on an ETI project from the May 1983 edition - page 32 (I was involved in its design).

Roger lifted the circuit from the ETI article, added a balanced input stage (but never bothered to balance the output), slightly simplified the sidechain (at the expense of attack speed!), added a little PSU module for the +/-15V supply from a single 12V DC input, and added the clipping LED (which is incorrectly drawn in his diagram). He also added an optional resistor in parallel with the one going into the transconductance amp (he called it "clarity" or "loud" modes). BW sold it as their "Limiter +".

His version of the circuit suffered from RF breakthrough problems and instability (unless you added extra filter caps beneath the board), and tended to overshoot horribly (which is why he introduced the LED clipper!). Incidentally, the LED is actually a pair of anti-parallel connected green LEDs in the same package (it's a Kingbright part), which conveniently just happen to go into conduction at PPM6 (1.55V RMS) (I suggested the mod to him when he couldn't get the limiter to stop allowing overdeviation on transients!).

Broadcast Warehouse also used the circuit in their "Combo Board" which included a rather poor stereo coder as well.

[yellowbeard]

Page 32 of this with the PCB drawing on page 85:
https://www.worldradiohistory.com/UK/El ... 983-05.pdf
The LM13600 will probably be difficult to get, Donberg have them for about €6 yoyos though.

[jvok]

LM13700 is virtually identical, only difference is the emitter follower which isn't used in that circuit anyway

[Albert H]

Nope. The LM13600 and 13700 are practically identical. The '600 had (slightly) tighter specifications, so was largely superceded by the '700 as production improved.

The OTA (operational transconductance amplifier) is quite a useful device, but suffers from a number of flaws - the current control isn't perfectly linear, and they're little better (in quality terms) than a 741 - they're noisy and distorted. In their early days, I designed a simple compressor / limiter, using one in the audio path. It could be built for mono using the CA3080 (an early, single OTA), or for stereo using the LM13600 / 700. The results were good enough for pirate broadcasting, but wouldn't be good enough for studio use. That's when we hit on the idea of using them to vary the negative feedback around a good quality op-amp. This has the advantage of reducing noise and distortion (it's mostly cancelled by the inherent "clean-up" characteristic of a feedback amplifier), but has the disadvantage of reducing the practically available range of control. Practically, you can squeeze ~24dB of gain reduction range....

The ETI circuit is a pretty good development of the feedback control topology, and the results from it are reasonably good,

[jvok]

Have a look at the equivalent circuits in the datasheets. The emitter follower buffer in the 13600 has its first stage current controlled by the bias pin. The 13700 has a true darlington pair with the emitter only connected to the base of the next transistor, no control from the bias pin.

Either way theres no difference in the actual ota circuit, only in the buffer which isn't used in the ETI circuit anyway