airbrake and impedance matching...

[riscado]

Could someone explain to me why an airbrake doesn't really require impedance matching much like most attenuators on the market?

thanks

[geetarpicker]

Truly I believe the Airbrake is around 8 ohms. Since it's in between 4 and 16 ohms, it's probably a decent compromise value in the middle range to fit most applications without worry. As in, within 4, 8 or 16 ohms it's never more than a 100% missmatch so for most amps it's plug and play. The unit does NO automatic impedance switching or something. Others may have better info than I, but I just treat the unit as an 8 ohm device and all has worked out fine.

[riscado]

tks for the input Glen, I did realize the unit would not switch impedance, but I guess what you said makes sense, it's probably confortable working around those impedances.

Thanks

[poipounder57]

I just finished putting together a Airbrake. I plugged the Airbrake into a 8 ohm cab and the speaker cable the plugs into the head is where I took my readings. I tested it with my multimeter...here are the reading I got. No attenuation I got a 8 ohm reading. As I increased the attenuator setting the ohm readings got higher. When I reached lowest position in the bedroom level the multimeter read 14 ohms and in the max 22 ohms.
I'm no electronic wiz...but others say thats how it should be.
It works and sounds great!!! Best thing....nothing blew up !!!
If that doesn't sound right PLEASE let me know...I'd hate to screw up my amp.
Hey Glen...great CD!!!

[RJ Guitars]

I found similar values with poipounder57 on those impedances... so I made some further mods and by changing the value of both big resistors simultaneously, I managed to keep the impedance right on or at least very near 8 ohms no matter how much attenuation I had...

In the end, I cannot hear any noticeable difference. I had hoped that this might explain why the sound wasn't so great at bedroom level, but it was just the same as before I made this mod.

I like the airbrake on the first couple notches but on the rigs I tried it with, when I get it dialed down to lower levels there isn't much tone quality.

just my 2 cents... some guys are having great success all across the volume range with these units. I think it's a bit amp dependent.

rj

[nickt]

just my 2 cents... some guys are having great success all across the volume range with these units. I think it's a bit amp dependent.

rj

Hi RJ - just a thought - could it be taste/expectation/ear/listener dependent?

Personally be very interested to actually *hear* a good sounding attenuator - but never have.

Have you heard such a thing? Serious question (not having a dig ).

cheers
Nick

[novosibir]

To make it clear: The Airbrake is designed for use with ONLY 8 ohms, despite they're claiming different!

When you're using it with cab & amp set at 16 ohms, you're stressing the output tube's plates and the OT due to enhanced current.

When you're using it with cab & amp set at 4 ohms, you're stressing the output tube's screen grids and may support arcing at the tube's base/tube socket.

Keep in mind, that all the others are offering different attenuators for 4, 8 & 16 ohms - and there must be a reason! And I can't imagine, that you'd be refunded from your Airbrake dealer, after you have dead output tubes or a toasted OT

Larry

[nickt]

Hi Larry,

With respect - I think you're mistaken. The AirBrake is kinda like a 16R speaker. With the two 25R's in parallel you get 12.5R which is about the DC resistance you get from a 16R speaker (BTW I've never seen a speaker measure it's rated ohmage!!)

There's a KF article in the "DIY airbrake" thread where he talks about handling different impedances.

I recall he makes the point that speaker impedance varies all over the place with frequency. I think he also talks about the effect of the speaker's movement on the impedance (but that might have been another article - can't remember off-hand).

My own "cheapskates airbrake" (one fixed setting at 10 o'clock on the bedroom setting) had my tubes flashing on 8R but was ok on 16R. So a sample of x 1 says its ok on 16R

Anyway the point of all this is that I don't think the DC ohmage is that critical for tube amps. So long as you're not turning your OT into an spark coil (ie open circuit) you should be ok!

cheers
Nick

[Fischerman]

The impedance changes with different settings. If you set it to no attenuation then it's just like a cable and your amp is connected directly to the speaker cab (so a 16ohm cabinet is 16ohms). If you set it one click down then there is a 24ohm resistor in parallel with the load...so if you had a 16ohm cabinet connected you would actually have something closer to 8 ohms. As you increase attenuation from there it adds series resistance but still keeps the 24ohm resistor in parallel with the load. At full attenuation it's back up to about 24ohms total.

I'm not sure what difference it makes that the Airbrake is purely resistive and the speaker is reactive. I do know that at least one of my amps sounds better with the Airbrake in series and set one click down (must be one click down).

[nickt]

I'm not sure what difference it makes that the Airbrake is purely resistive and the speaker is reactive. I do know that at least one of my amps sounds better with the Airbrake in series and set one click down (must be one click down).

Really interesting - I wonder (in you amp's case) if it's anything to do with the trannie ratio? For instance perhaps your amp is usually 4k - 8R/16R but gets shifted with the brake in circuit say so its the equivalent of 5k to 8R/16R or 4k to 6R/10R? Anyway just a thought...

[novosibir]

In the 1-st attenuating step, the Airbrake puts a 24R in parallel to the load, which still goes straight through the box.

At step 2, 3, & 4 the Airbrake keeps the parallel 24R, but puts each step a serial resistance of about 5 ohms in series to the cab.

At step 5 is again the 24R in parallel and the other 24R completely in series to the cab, additional running through the Rheostat, bypassed with a 715P .1/400V

Since a 16 ohms cab is a resistive load of about 13 ohms, we get a resistive total, when using a 16 ohms cab:

step 1: 8.4 ohms
step 2: 10.3 ohms
step 3: 11.7 ohms
step 4: 12.9 ohms
step 5: 14.6 ohms or higher, depending on the setting of the Rheostat

Reason enough for me, never to use it with amp & cab @ 16 ohms

Larry

[novosibir]

Oh yes, forgot to mention:

The Rheostat is a Ohmite 150R/50W

Larry

[Noel Grassy]

Oh yes, forgot to mention:

The Rheostat is a Ohmite 150R/50W

Larry

I would like to use a 400R/50W that I already have. Does anyone see a problem or should I limit the rheostat to "halt" at 150R's just to be safe? Thanks for the help, NG

[Noel Grassy]

Oh yes, forgot to mention:

The Rheostat is a Ohmite 150R/50W

Larry

I would like to use a 400R/50W that I already have. Does anyone see a problem or should I limit the rheostat to "halt" at 150R's just to be safe? Thanks for the help, NG

Er, anyone care to comment?

[novosibir]

A 400R instead of the 150R would still more dullen the sound and ripping off some more of the remaining and anyway not very much dynamics, when set to a lower range.

That's saving bucks & efforts on the wrong end IMO!

Larry