DIY attenuator

[J M Fahey]

So a year later I manage to get curious enough to start my own "crude attenuator" experiments. I have some Ohmite 270 series wirewound resistors (5 Ohms, 225W) laying around, so I took one and paralleled a 16 Ohm speaker to get just under 4 Ohms. I calculate about 12dB attenuation. Enough to notice, anyway. The treble seems to roll off a little, but it could be related to volume difference.

You are not attenuating a speaker, just overloading an amplifier the wrong way, please donīt.
Proper way is to add a parallel resistor with speaker, but also a series one with the couple so total impedance ramains roughly the same as original.

I'm wondering if the wirewound resistor has enough inductance to make a difference at higher frequencies? Anybody have experience with these large resistors?

Irrelevant, you are doing it the wrong way, sorry.

[eschertron]

You are not attenuating a speaker, just overloading an amplifier the wrong way, please donīt.
Proper way is to add a parallel resistor with speaker, but also a series one with the couple so total impedance ramains roughly the same as original.

I'm not trying to run an amp set for a 16 Ohm impedance into a 4 Ohm load. Sorry if I gave you that impression. My interest was piqued when I realized that nominal 16 ohm speaker parallel with a 5 ohm resistor (of which I have several, just cluttering up my workspace) would give me a reasonably-close-to-4 ohm load. I am driving it from the 4 ohm tap of my amp. I don't see that as being dangerous. Please tell me if this is not correct!

In the spirit of this thread "Crude Attenuator", I wondered if how close the impedance of the wirewound resistor would track that of the driver. At 4.8uH, not very well. I'm still looking for a crude and bone-simple way to bring my thunderous, whopping 15W amp's output down to levels that will play nice among church musicians.

I don't expect to build a power soak this way, certainly would not try to market such a thing, just use for my own amusement and sense of cleverness.

[John_H]

Check this out. https://www.tedweber.com/webervst/lpad.htm You just input the amount of attenuation you want along with the wattage, and impedance desired. It tells you the resistors that you need. Some are easy combinations such as a 4r series resistor, and an 8r parallel yields 6db attenuation at 8 ohms.

[catalin gramada]

In terms of loudness you.ll not feel something real happen if don.t start to cut from 15-18 db to become effective (I think also depends how big is the room you trying to excite...) Dummy load and reamp found is the way for low volume.

[Helmholtz]

Check this out. https://www.tedweber.com/webervst/lpad.htm You just input the amount of attenuation you want along with the wattage, and impedance desired. It tells you the resistors that you need. Some are easy combinations such as a 4r series resistor, and an 8r parallel yields 6db attenuation at 8 ohms.

[ATTACH=CONFIG]51296[/ATTACH]

An L-Pad is just a simple voltage divider with the output loaded by the speaker.

The attenuation values from the Weber calculator are only correct for amps with very low output impedance like SS amps. Most tube amps have output impedances > 5 Ohm and up to around 100 Ohm. These amp output impedances act in series with R1 and may strongly increase actual attenuation.

On the other hand attenuation also depends on the ratio of speaker impedance to R2. For a given value of R2 attenuation increases with higher speaker impedance. Above ca. 500Hz speaker impedance increases with frequency and at 3kHz the impedance is about twice its nominal value. Consequently treble frequencies get attenuated stronger.
This a simplified explanation but it shows the principle of treble loss with low value parallel resistors.

The frequency response published by speaker manufacturers is measured at a very low output impedance SS amp. When used with a high output impedance tube amp, speaker treble response and bass frequencies around the bass resonance are emphasized. This means an extended speaker bandwidth at the tube amp.

[eschertron]

The frequency response published by speaker manufacturers is measured at a very low output impedance SS amp. When used with a high output impedance tube amp, speaker treble response and bass frequencies around the bass resonance are emphasized. This means a wider speaker bandwidth at the tube amp.

Interesting. I'd never read that before.

[eschertron]

<post #21> I calculate about 12dB attenuation.

I was using the voltage gain equation. It should have been obvious to me that approx 1/4 the power would be closer to -6dB than -12dB. Still, a definitely sensible change in volume.

To make a 'proper' attenuator, I'll have to buy some stuff. Maybe soon.

[catalin gramada]

I was using the voltage gain equation. It should have been obvious to me that approx 1/4 the power would be closer to -6dB than -12dB. Still, a definitely sensible change in volume.

To make a 'proper' attenuator, I'll have to buy some stuff. Maybe soon.

You.ll be surprised, it is not. Consider -12db as startup. 15-18db to feel as effective reduction. I tell you by my experience with a cranked 50 watter into a band context with multimeter over outputs