Attenuator Build

[Littlewyan]

Hi Guys

I've designed an attenuator which will have 3 stages of attenuation. The first 2 stages will essentially be voltage dividers that will attenuate 6dB each and the 3rd will be an LPad which will bring the volume easily down to bedroom level. It will also have a 2nd LPad which can be switched to using a footswitch that hooks up to a relay.

The first issue I've come across is what resistors to use. I wanted to use the nice big 100W Ohmite resistors but they're 16.5cm long which means the case will have to be huge! I've looked at chassis mount resistors and for the 100W they recommend a heatsink that is 995cm2 in size. Now, correct me if I'm wrong, but thats massive isn't it?!!

Anyone had any experience with these 100W chassis mount resistors?

[Structo]

Are you using the two stage attenuator by Gerald Weber as a starting point?

I bought a 8 ohm 100w resistor as a dummy load.
I use it when working on tube amps.

If it is mounted to a chassis it would dissipate even more power.

http://www.parts-express.com/8-ohm-100w ... r--019-020

Looks to be 6.5 inches long.

More:
http://www.parts-express.com/cat/dummy- ... stors/1535


I didn't think L-Pads came in very high power configs.
Some wire wound rheostats may be able to handle it.
Isn't that what the Air Brake used?

[Littlewyan]

Dammit! Its 16.5cm not 165cm! Sorry .

I'm using Gerald's attenuator for the 1st stage. 2nd Stage is a bit different as it works with the LPad.

Basically my current attenuator has one stage of 6dB attenuation before the the LPad. However the LPad gets far too hot when turned down low, even though its rated for 100W. So I thought if I stick another attenuation stage in there before the LPad then it will keep the LPad cool and help me control the volume better.

I did measure the power after the 1st attenuation stage and it was 18W when using my Trainwreck Express. Way below the 100W rating for the LPad so it shouldn't be getting hot at all!

Yes the Airbrake used a Rheostat but I don't like the way the Airbrake doesn't match the impedance very well.

[Smokebreak]

I did measure the power after the 1st attenuation stage and it was 18W when using my Trainwreck Express. Way below the 100W rating for the LPad so it shouldn't be getting hot at all!

Keep in mind that just because a resistor has a huge wattage rating(100W), doesn't mean it won't get hot. 18W is still 18W , and that's a lot of heat!

[Structo]

I use the above resistor for simple things like scoping or to set the bias.

[RockinRocket]

Is there a schematic available for this "two stage attenuator by Gerald Weber"?

[Structo]

Here

[Littlewyan]

Ok what about the chassis mount resistors? 995cm2 seems like a very large heatsink.

[sliberty]

Although your design may be different, your attenuated project sounds conceptually similar to the Airbrake. Have you heard the Airbrake? Personally, I didn't find it very satisfying. With the exception of very minor power reduction, it changed the sound of my amps in unpleasant ways, and I gave up on it. Frankly, I've never heard an attenuators that didn't do this. Maybe yours will be better, and if so, I'll be very interested. But given the general similarities, I am thinking the results could be disappointing.

[Littlewyan]

My design is quite different to the Airbrake. It is a LOT better at matching the impedance and sounds ok as well to my ears. Obviously as with any attenuator when it gets turned down low it sucks the high end out but I don't think you can really avoid that.

I've attached a schematic I quickly made in mspaint. I missed off the relay switch and the 2nd LPad and the bypass switch as I'm sure there is no need to see them.

I'll attach a schematic of my current attenuator which only uses 1 stage of attenuation before the LPad but the resistors are hooked up differently.

[Littlewyan]

Here is my current attenuator. This also matches the impedance perfectly but the LPad gets way too hot. I know the LPad may be designed to get hot but twice now I've had to take it apart and clean it using switch cleaner as the contacts inside become so dirty that they create additional resistance. There is no actual dirt on them that I can see but after I wipe it over with a cotton bud and cleaner it goes back to normal. I only assume the heat may cause this.

[vibratoking]

Here is my current attenuator. This also matches the impedance perfectly but the LPad gets way too hot. I know the LPad may be designed to get hot but twice now I've had to take it apart and clean it using switch cleaner as the contacts inside become so dirty that they create additional resistance. There is no actual dirt on them that I can see but after I wipe it over with a cotton bud and cleaner it goes back to normal. I only assume the heat may cause this.

Neither of those schemes matches the impedance perfectly in all cases. That is one drawback of most of these type attenuators. If you want true matching, which I don't think is worth it, you need a much more complex circuit than you have drawn. Scholz did a better job of it and it is much more complex and it is full of power resistors. Too expensive and I don't like the way it sounds.

[Leo_Gnardo]

Obviously as with any attenuator when it gets turned down low it sucks the high end out but I don't think you can really avoid that.

"Bright" cap as on a guitar volume control, only scaled to your attenuator. I've used 200V or higher film caps, 1 to 5 uF. I see there are brand name atten's that do the same, THD for one. You can select a value that will work with your amp, your speaker, & your ears.

[Littlewyan]

Well they both match the resistance measured which I know isn't the actual impedance but its much closer than the Airbrake.

Edit: I know the impedance of the speaker changes depending on the frequency fed into it, do you mean the attenuator won't match the impedance very well then? I've just been going by the impedance (resistance really) measured by a ohmmeter. That gets matched perfectly.

Edit2: I forgot to point out the 2nd design is the attenuator I currently have and it works great. It does however tone suck when the LPad is turned down.....I'd say below half way. My issue is I need an attenuator with a switchable boost for solos.

[Leo_Gnardo]

Edit: I know the impedance of the speaker changes depending on the frequency fed into it, do you mean the attenuator won't match the impedance very well then? I've just been going by the impedance (resistance really) measured by a ohmmeter. That gets matched perfectly.

Close enough for rock n roll... One part of the impedance curve you may not want to follow perfectly is the enormous increase in impedance right at the speaker's resonance point. I expect that would be heavy & costly as well as a headache to design in.

Edit2: I forgot to point out the 2nd design is the attenuator I currently have and it works great. It does however tone suck when the LPad is turned down.....I'd say below half way. My issue is I need an attenuator with a switchable boost for solos.

Try 2 or 3 uF across the second 16 ohm resistor, that should brighten your day. Adjust value to taste.

Switchable boost for solos? Now yer' thinkin!