limiter design

[catalin gramada]

Hello. can someone io explain the basics how to build a limiter with diodes to put somewhere in signal path to not overload too much the powerstage,please?
I have no ideea to work with ss diodes. I just saw some ideeas about how to use it regular in series or zeners back to back. I can imagine how works but I have no ideea which is a best solution for and why. I talk about 5-6 V peak limiter. Thanks.

[Jazz P Bass]

Here is a good read : Limiters & clipers

[nickb]

It sounds like you are thinking of a simple diode clipper.

The easiest is two back to back zeners. The peak to peak clipping voltage is about 2 x (Vzener+0.7). The other thing you need is for the source impedance not to be too low. When the the zener conducts it has a fairly low dynamic impedance and that acts as a potential divider with the source impedance.

I think the main differences between switching diodes vs zeners are
(a) breakdown voltage
(b) Dynamic impedance ( smaller for a switching diode)
(c) Junction capacitance - you'd need to look at the datasheet to find that out. The point is that is exists and if the source impedance is high it might roll off HF
(d) Switching speed - zener will be slower than the diodes but I don't it will matter much at these sorts of frequencies.

A different kind of limiter gently varies the gain of a preceding stage my monitoring the signal level. This is very different in that it prevents the later stages from clipping and does not of itself cause any clipping. The begs the question what is your aim, reduced clean or dirty power output?

[catalin gramada]

double post, sorry

[catalin gramada]

Good infos. Thanks gents
Nickb :I feed the powerstage through a driver wich start to clip at 3.5V The signal at output start to flatten at this value. it goes to hardly overdrive till 7v arround but further the output signal start to flick , sign of critical discharge somewhere. I want to limit the signal before this amount to be sure I cannot blow something from accident. The OT is somehow protected by series diodes but think is more safe to limit the signal amount somewhere

[R.G.]

As I think I've mentioned before, the much-maligned Thomas Organ Vox amps came with a secret weapon: an adjustable limiter that does exactly what catlalin is asking for here. They did it to keep their output stages from dying on massive overdrives, but it also had the effect of converting the hard-edged clipping of a semiconductor output stage into a softer clipping and much better sounding output under overdrive.

One schemo is here: voxtmixlmt.gif gif by rickydon | Photobucket It will probably need some work to adapt to other amps, as I've found recently, but it's certainly possible

[nickb]

Good infos. Thanks gents
Nickb :I feed the powerstage through a driver wich start to clip at 3.5V The signal at output start to flatten at this value. it goes to hardly overdrive till 7v arround but further the output signal start to flick , sign of critical discharge somewhere. I want to limit the signal before this amount to be sure I cannot blow something from accident. The OT is somehow protected by series diodes but think is more safe to limit the signal amount somewhere

By 'flick' you you mean overshoot?

If so I expect that comes froms the various inductances that exist in the circuit. The energy in those depends on how long the voltage is applied, in other words a square wave will build up more energy that a sine wave. So let's suppose that when driven hard you get 700Vpp square wave on the plates. If you add your clipper you will restrict it to something less say 650Vpp and so the energy will be less but not by much and the problem is not solved. Part of the problem is the high rate of change of the clipped waveform causes the spikes as the inductance resists the change. I suspect that if you clip enough prior to the power out you still have fast edges and the problem will still be there.

If you are really concerned then think about MOV's or snubbers across the OPT ( much discussed elsewhere on this forum). OTOH there are a zillion tube amps out there working happily for decades without such protection so maybe it's not something to cause too much concern.

[catalin gramada]

By 'flick' you you mean overshoot?

Hi Nick. Yes, I think so. I have a huge headroom in previously stages and current capability so I can push the powerstage twice if I wish but at certain amount the powerstage overshoot somewhere. I not realized yet if is happen in OT or internaly in power tube. I have to do a plan to not destroy some in a test. Thanks

Edit: maybe limiting the output tubes grid current fit better, please ? Or simply to limit max voltage amplification in signal chain with a potential divider...just thinking what is better solution to not blow some but also to not get an anemic sound...I like the ideea to cut the powerstage in heavy overdrive so easy as knife in butter but in my case is too much...still, Have to limit somewhere,somehow...limiter diodes comes quick through my mind,but maybe is not optimal solution ..perhaps

[Enzo]

Blocking distortion?

[catalin gramada]

Blocking distortion?

Hi Enzo. Never thinking about but I.ll give a try to see what.s happen if I raise grid stopper value. Thanks for tip

[vintagekiki]

Watchdog (peak) limiter

http://www.voxshowroom.com/us/amp/v1143_preamp_hood.html ----> Watchdog

http://music-electronics-forum.com/attachments/26053d1383058505-vox-westminster-v118-service-manual.pdf ---> Schematics

http://www.voxshowroom.com/us/amp/watchdog.html ---> Calibration procedure

http://www.powersoft-audio.com/en/downloads/technical-notes/804-how-to-setup-limiters/file
How to setup limiters

[catalin gramada]

Thank You.

[trobbins]

Some vintage PA amps had limiters based on light bulbs or neons driving an LDR to attenuate an input stages signal. The bulbs were driven off 100V line or output stage anodes.

Eg.
http://dalmura.com.au/projects/Phili...%20EV4437a.pdf

[catalin gramada]

Hi. Thanks. Do You have any ideea how input capacitance of pentode is affected by ultra linear mode? Can I consider as same as regular pentode connected, please? Thanks

[trobbins]

Do You have any ideea how input capacitance of pentode is affected by ultra linear mode? Can I consider as same as regular pentode connected, please?

Not sure I understand where your query is coming from?
Is it related to miller capacitance, and how that may affect the high frequency roll off of the signal getting in to a push-pull output stage?
Normal pentode mode buffers the miller capacitance affect, as the G1 is 'screened' from seeing the anode voltage varying. Whereas UL doesn't 'screen' the anode, but is not quite as bad as triode mode.