Running a solid-state amp without a load (facepalm content)

[petejt]

N:o I haven't done this yet.......................

 

 

 

I have a small compression driver speaker to use in a rotating Leslie system, y'know the swirly Hammond organ thing. Think Eric Clapton or Cold Shot. It's 200 watts RMS so I definitely won't blow it up with most guitar amps.

I can't find the impedance rating of it.

It has an 800Hz crossover.

 

 

A few months back I was writing to a bloke that used to fix old Leslies, well he reckoned he did. I don't think he took me seriously about my enquiry, and thought I was going to blow the amp up.

 

So he recommended to use an attenuator with the amp, set to LOAD. Then he said to run the speaker in parallel with the attenuator, so it gets the signal from the amp. He said to do it "just to try", thinking that this was just a toy.

Yes it is an amateur custom-made home project, but I want it to be practical, like the real thing.

 

 

I personally don't see much in what he recommended.

 

 

So, I'm thinking of using a solid-state amp, such as a Crate Powerblock, to run this Leslie. I'll just run the Crate as a poweramp, the preamp signal will be slaved out from my MarkIV. I'm not too worried about the sound, as I want the poweramp to be very clean and high wattage. The Crate puts out 150 watts mono, @ 16 ohms I think (hope).

 

Is this feasable? Will the Crate Powerblock freak out if it doesn't think there is a 'load' attached to it?

[newholland]

i'm confused pete-- there's no reason to worry about a solid state amp load. they don't care in the least if there's a speaker attached.

 

meanwhile-- is your horn/driver on the compression driver matched to ALLOW it to go to 800 hz? it would have to be a BIG horn to get you to 800 hz. and is your crossover purchased, or did you run sims on the horn/driver to find out what it's REALLY doing.. horns are tricky business...

 

seems like what you're doing is basically running a slave off the mark 4 to a solid state amp-- which runs completely in the realm of 'line level'.. so it's just like running a discrete guitar amp off of your mark 4's line out. you set your level like you would the gain pot on a mixer, and then turn the amp up to the level you want. since it's solid state-- no worries if there's a 'load' or not. compression drivers are SO efficient, you won't need much...

 

but seriously-- you won't get to 800 hz with that driver unless your horn is fairly large-- to the order of 2 feet wide and a foot tall at the mouth, and probably 14-16 inches deep, not including the driver. typically, for good sound, you'd need the horns 'natural' response to go roughly an octave LOWER than your crossover, or it'll sound 'honky'... and compression drivers SELDOM respond well to prefab crossovers... because horn response plays a HUGE part in the loading of the driver as well....

[petejt]

meanwhile-- is your horn/driver on the compression driver matched to ALLOW it to go to 800 hz? it would have to be a BIG horn to get you to 800 hz. and is your crossover purchased, or did you run sims on the horn/driver to find out what it's REALLY doing.. horns are tricky business...

 

The driver has an 800Hz crossover built into it. Basically it only puts out sound from 800Hz to 30kHz. I'll have to use an EQ unit prior to the poweramp that will be running it, to block off frequencies above 10kHz otherwise it will shriek too much.

 

Here's a picture. It's a Motorola/CTS KSN1188a.

 

 

from http://www.adelcom.net/MOTOROLA_ksn1188a.htm

 

PRODUCT DESCRIPTION:

 

POWERLINE COMPRESSION DRIVER WITH 800 Hz CROSSOVER AND BUILT-IN ELECTRONIC OVERLOAD PROTECTION. PIEZO ELEMENT REQUIRES NO ADDITIONAL CROSSOVER. Driver Only.

 

PRODUCT SPECIFICATIONS:

 

* POWERLINE Power Handling: 200 watts RMS / 400 watts MAXIMUM.

* Thread: Standard 1-3/4"-18 TPI throat for a variety of lens designs.

* Frequency Response: 0.8 kHz. - 30 kHz.

* Impedance: Appears as a 0.13 uF capacitor.

* Efficiency: 93 dB SPL (Sound Pressure Level) 2.83V Input @ 1Meter.

* fs: 0.8 kHz..

* Crossover Required: None. The Piezo Element Requires No Additional Crossover. However, a 20 ohm, 30 ?att square ohm resistor should be used In SERIES when adding an L-Pad control.

* Protection: Protected to 400 Watts.

* Physical Dimensions: Diameter: 3.85", Depth: 3.06".

* Net Weight: 10.0 Ounces.

 

[newholland]

gotcha-- it's a piezo element. i'm still a little worried about your horn loading to GET it to 800 hz-- but i reckon it's not really a big deal, as it's not running anywhere NEAR full range anyhow- just a limited bandwidth.. is there a woofer UNDER it in this cab? if so, you're gonna need to attenuate it pretty hard with an LPAD to get it to match.

 

i wouldn't worry about eq above 10k-- chances are VERY slim it'll get there anyhow-- your main worry is gonna be 1-4k being WAY too hot... and the horn could very well aggravate that if it's too small!

 

EDIT: oh... {censored}-- i just saw the efficiency rating! wow that's LOW for a piezo driver... usually they're way over 100db/w!

[petejt]

 

seems like what you're doing is basically running a slave off the mark 4 to a solid state amp-- which runs completely in the realm of 'line level'.. so it's just like running a discrete guitar amp off of your mark 4's line out. you set your level like you would the gain pot on a mixer, and then turn the amp up to the level you want. since it's solid state-- no worries if there's a 'load' or not. compression drivers are SO efficient, you won't need much...

 

 

 

The MarkIV's Slave Out is providing the tone. Because I want to be able to "switch on/off" the Leslie, I'll use a loop on my GCX switcher that is able to handle line-level signals. If I plug the amp into it, it'll blow up.

 

I'll run the GCX send to the Powerblock's fx return, and from there to the driver.

 

I'm glad that both the driver & solid-state amp would be fine, I just wanted to know for sure. I knew that dickhead was away with the fairies and didn't take me seriously.

[newholland]

GOOD info on piezos...

 

Using piezo tweeters wisely: a "how to"

The only piezo tweeters worthy of high fidelity or pro audio use are the Motorola/CTS units, and of those, only the Powerline Series are suited for pro audio. The ones made by other companies are junk. I have seen them selling for as low as 75c each to dealers. If you pay more than $2 for one made by anyone but CTS, you are being ripped off.

 

Sometimes it is said that piezo tweeters are "harsh" or "bright". First, if they aren't a real Motorola/CTS unit, they are junk. But with the Motorola/CTS units I have come to see (and others agree) that they have higher sensitivity than the published 92-93 db/wt SPL ratings. They seem to be 3-5 db higher.

 

I have used the KSN1165a, rated at SPL = 92 db @ 1w @ 1m, with woofers in the 96-97 db range and they seem to have excellent balance. Using the KSN1165a with a 92 db SPL woofer, the tweeter does sound very bright. By reducing the tweeters output by 4 db it matches the woofer very well and makes for an excellent speaker. (more on this later)

 

Ignore everything you have read about resistors and piezos. Most all are quoting each other and MISQUOTING the manufacturer. Some will tell you to place a resistor in series with the piezo tweeter, some say parallel. And both are wrong.

 

Years ago with some of the Motorola line, not all, just some, and I have the paper from them about this, Motorola recommended a 20-50 ohm resistor in series with SOME of the tweeters. This gave protection to the amp from high freq (100khz or so) oscillations as well as some protection to some of their tweeters. They did not recommend using resistors for any of the Powerline series, which include the KSN1142a, KSN1165a, KSN1188a, and a few other in the series.

 

(The Motorola piezo speaker division was purchased by CTS some years ago)

 

The CTS/Motorola Powerlines were the very high quality ones for pro use, and contained built in protection circuits, quite sophisticated. This protection circuit uses a resistor and thermal breaker paralleled with a tiny light bulb. At about 100 watts the thermal breaker heats up, opens and now the power goes through the light bulb. As the bulb heats up it increases resistance. What this does is give "soft clipping", that is, a sort of compression of the highs. This is very mild and almost inaudible, and certainly not noticable. They burn out about 400 watts. These are once again available through Martin Sound.

 

http://www.martinsoundpro.com/item.asp?id=340

 

Anyway, don't use a resistor with the Powerline Series piezos for power protection purposes; it is simply not needed. If you do put a resistor in series (and even Martin Sound says for Lpad use a 20-30 ohm resistor) I say, go ahead, but it will NOT make a difference in sound. Do you know why? Because a piezo tweeter has an impedance of 1000 ohms or greater. If you put a 20 or 50 ohm resistor in series with a 1000 ohm speaker, well, it is a miniscule change in power to the speaker. Duh!

 

So, how DO you pad down a piezo?

 

The CTS/Motorola paper says you can put a small CAPACITOR in series with the piezo to reduce the level, and that can work. But if you put an 8 ohm resistor across the terminals (follow this whole discussion now...) now it looks like an 8 ohm load. The piezo will not know the difference, it works off voltage only, drawing almost no power.

 

 

 

Now, put an L-pad to this 8-ohm load. Make sure you use an 8 ohm L-pad. (or if you use a 16 ohm L-pad, use a 16 ohm resistor) An L-pad is actually two rheostats, one in series with load, one in parallel, and the taper of each is such that the load presented back to the crossover is constant. Now this piezo, plus 8 ohm resistor, plus 8 ohm L-pad looks like 8 ohms to the crossover.

 

 

 

What crossover? Well, now that it is 8 ohms, it needs a crossover to keep from putting unnecessary load on the amp below the frequency produced by the piezo tweeter, power that would be wasted in the L-pad and resistor. A simple cap calculated for an 8 ohm load (or 16 ohm load if 16 ohm L-pad and resistor are used) is all that is needed. For example, for the KSN1165a or KSN1142a, which roll in at 1800 hz, if these were to be crossed in at 2000 hz it works out to 9.9 uf, which is a non standard value. But 10 uf is standard and plenty close enough. If you wanted to use all of the 1800 hz-20 khz available from that piezo tweeter use 11 uf (10 uf + 1 uf in parallel). If you wanted to cross this at 3000 hz use 6.6 uf (5.6 uf + 1 uf in parallel). Etc. Just remember, you can't go lower than the piezo tweeter is capable of going, but you can go higher. Calculate the capacitor using an 8 ohm impedance.

 

 

 

So now with this setup you can not only control the level, but control the crossover point, too.

 

You may also use a "fixed L-Pad" like this:

 

 

 

-3 db R1 = 2 ohms, R2 = 6 ohms

-4 db R1 = 3 ohms, R2 = 5 ohms

-6 db R1 = 4 ohms, R2 = 4 ohms

 

These are approximate, as in, "close enough for jazz".

 

One caution--Do not try to use a steeper crossover, 12 db/oct, or higher. This is not needed for tweeter protection, and the piezo can react with the inductor in these and produce chirps on occasion.

[petejt]

gotcha-- it's a piezo element. i'm still a little worried about your horn loading to GET it to 800 hz-- but i reckon it's not really a big deal, as it's not running anywhere NEAR full range anyhow- just a limited bandwidth.. is there a woofer UNDER it in this cab? if so, you're gonna need to attenuate it pretty hard with an LPAD to get it to match.

 

There are two Electro Voice 200watt 8 ohm 12" speakers underneath, putting out the signal from my Engl Fireball. Just a regular guitar sound, although will be set rather dark.

 

 

I'm basically building my own version of this

 

 

 

 

It will be built into a 4x12 cab, with the two 12" speakers beneath it. In a way it's a 2x12 cab with the Leslie thingamajig above it, all enclosed inside a 4x12 cab.

[newholland]

ok-- i gotcha.. this is interesting stuff! i never knew how leslies worked!

 

if you're working with EV's.. you're probably gonna be fine. the piezos really dont add any load to the total impedance if it's split off the main signal-- which is WHY they don't list an impedance-- which in your case is GOOD! you don't need to factor it in to the cab impedance at all-- just tap it off your input.

 

i thought you were running it SOLO... which woulda bee BRUTAL on the ears!

 

now i see what yer doing here.. very interested in seeing how this turns out-- don't bother with that post above, UNLESS you want to attenuate the horns, which i highly doubt you'll need to with 2 EVs underneath!

[petejt]

ok-- i gotcha.. this is interesting stuff! i never knew how leslies worked! if you're working with EV's.. you're probably gonna be fine. the piezos really dont add any load to the total impedance if it's split off the main signal-- which is WHY they don't list an impedance-- which in your case is GOOD! you don't need to factor it in to the cab impedance at all-- just tap it off your input. i thought you were running it SOLO... which woulda bee BRUTAL on the ears! now i see what yer doing here.. very interested in seeing how this turns out-- don't bother with that post above, UNLESS you want to attenuate the horns, which i highly doubt you'll need to with 2 EVs underneath!

 

Yeah I hear ya on the BRUTAL! way too harsh on its own!

 

 

I'm basically turning my rig into a wet/dry setup, with a few differences so to speak.

 

After a bunch of pedals (patched in/out/around with a Switchblade8), the guitar signal gets split into two. One signal to the MarkIV, the other to the Fireball.

 

 

My MarkIV, with its Stiletto 4x12 cab, is the 'dry' sound.

 

The Marshall 4x12 cab is becoming the 'wet' cab. Two EV speakers firing out the effected sound from the Fireball (I'm currently deciding what rack effect unit to get), and then the Leslie swirling away above it, whose signal is fed from the MarkIV's slave-out and Crate Powerblock's poweramp.

 

I can mute the input of the Powerblock/Leslie with the DMC GCX Switcher, so I can turn it on when I want, via MIDI .