JohnH
Well-Known Member
Background
Passive attenuators are wired between the amp output and the speakers. Their function is to absorb most of the output power of the amp, feeding a smaller amount to the speaker itself. This allows the amp output stage to run at higher power, letting the glorious tone of a good valve output stage develop, but without excessive volume.
The attenuator must present a load to the amp that is similar to a speaker and also maintain the tone as volume is reduced. It needs a consistent tonal and dynamic response from low attenuation, down to sub-bedroom level. This is where the simplest designs can be inferior, and the best commercial designs get expensive.
With feedback and testing by others I think we have a design that achieves this. For about $100-$120, anyone with workshop skills and the ability to follow a circuit schematic can build this. I want to thank everyone who has built one of these or contributed to this thread, with a special thankyou to @Gene Ballzz who was the first to see the potential and has been a great source of insight and practical help for everyone.
An important point:
Anyone who builds it does so at their own risk, and takes responsibility for working out their own wiring for their own private, non-commercial use, and completing it safely
Summary: November 2022
This thread focusses on the design of reactive attenuators for DIY building using simple construction and inexpensive parts. It started with just a few resistors and then developed into multistage resistive and reactive designs. The latest designs work much better than I’d first expected and have been built and tested successfully by many others. The design continues to evolve but the main principles have been constant for several years. This 1st post shows the current basic design M2, and a few possible additions to it.
It must be matched to the output tap of the amp, eg 8 Ohm or 16 Ohms. Component values for both are given, which differ by a factor of two:.
There are 4 attenuation stages, engaged or bypassed by switches.
Stage 1 is the key reactive stage and includes an inductor coil. This stage on its own, reduces power by a factor of 5, or -7db, reducing a 50W amp to 10W. The inductor coil is configured so that the impedance presented to the amp is similar to that of a real speaker (values based on various Celestions), particularly how impedance rises with frequency.
After Stage 1, three more stages are provided. These can be mixed and matched, but the design shown is based around additional -3.5db, -7db and -14db stages. By combining these switches in combination, and with Stage 1, reductions of up to -31.5db can be achieved in small, equal steps of -3.5db, at which point a 50W amp, at full power, is playing quietly at about 35mW.
On pages 111 and 112 are two construction layouts which may assist (designs are very close to above, not identical though). I recommend studying the diagrams above to understand the connections and then adapt the layouts to suit your needs. Further features can be added, discussed in the thread, such as bass resonance circuit, foot-switchable stage, variable input impedances, bypass switch etc.
If you'd like to see a schematic with most of these further add-ons, go to Page 158, post 3146 from 19/11/2022.
Component values and power ratings
The table above shows the recomended power ratings for each resistor, based on up to 50W amps. The component ratings need to have a good margin above the actual power. I use a factor of at least 3 for case-mounted aluminium resistors, bolted (using thermal grease) to a heavy metal chassis or heatsink , and a factor of 5 or more for air-cooled resistors. These values fit with the spec in the schematic diagram above and also allow for overdrive of the amp.
Wire for hookup and also the winding of air-cored inductors should be 18 gage for 50W attenuators, and this is also OK for a 100W one, if built to the 16 Ohm values. For switches, use at least 5A rating (at 125V ac) for a 50W 8 Ohm build. The best jacks are plastic Cliff jacks, TRS (ie stereo type) which grip the plugs better than mono jacks.
Cooling
With amps > 30W at high power, the unit will heat up as it dissipates power. A good size die-cast aluminium case is best. Once components are positioned, then a number of additional large vent holes should be drilled, in the top and in the base, with feet to raise up the base. This will help to promote good convective flow of air out through the top, replaced by cool air at the base. The best colours for cooling are dark. For amps more than 50W, a fan should be added
Mount the coil without using a ferrous bolt, with a few mm timber or plastic spacer off the case surface.
Thanks for reading. If you are interested, and since this is a long thread, I suggest to read this post, look at the layouts and the most recent few pages, then make a post yourself and we'll be happy to discuss what would work best for you.
For nice collection of completed builds, see this thread:
www.marshallforum.com
Attenuator M ( January 2019)
The following is the original reactive design M which I built myself, with sound samples. M and M2 are closely related and perform the same, although M has two coils. It also has a bypass switch and a -3.5dB setting.
The build was in a case 170 x 120 x 55mm of thick aluminium:
Simplified version M-Lite
This was the same design, omitting the bypass switching and the 3rd output. Minimum attenuation is -7db. Its an easier build and it should still meet most needs.
Performance
In the schematic above, there is a graph showing a calculated frequency response at each attenuation level from 0 to -31.5 db. These use a spreadsheet to calculate the signal at each stage of the circuit, as a series of voltage dividers, using complex number theory to assess magnitudes and phase angles. The speaker was represented, for analysis, by an equivalent load model, by Aiken:
http://www.aikenamps.com/index.php/designing-a-reactive-speaker-load-emulator
...adjusted to match the measured performance of a G12M 4x12 cab. The plots are based on small signals, with the amp output impedance assumed to be 20 Ohms, for an 8 Ohm tap, based on measurements of my VM2266C. These calcs were used to adjust the values in the design.
Sound Samples
The ideal is for volume to reduce, but with no change in tone or feel. This is best tested with a consistent loop, with attenuated sounds then normalised back to equal volume:
Attenuator M: Max attenuation to non-attenuated:
Attenuator M: Normalised:
It’s a simple looped riff, played twice at each attenuation setting from -31db up to full unattenuated in 3.5db steps. The second file is based on the same recording, with each stage normalised for volume so you can hear any differences in the tone.
My VM2266c amp was on LDR mode, body at 6, detail at 9, master vol at 6, tones and presence at 6, using my LP bridge pickup, miced off a speaker.
The plots are taken from the sound sample posted above. The lower set of data are the basic plots, from full volume down to -31db (db scale is arbitrary, but relative db's are right).
The upper plots are intended to show the differences between responses. I took the -7db recording as the base case, so this is shown as a flat line. The others are the various other settings, with the -7db trace subtracted. The ideal for these traces is therefore also a flat line. And for all the traces below -7db down to -31db, this is what is happening, there is virtually no further tonal change at all as you attenuate down as far as you want. It measures as consistent.
Passive attenuators are wired between the amp output and the speakers. Their function is to absorb most of the output power of the amp, feeding a smaller amount to the speaker itself. This allows the amp output stage to run at higher power, letting the glorious tone of a good valve output stage develop, but without excessive volume.
The attenuator must present a load to the amp that is similar to a speaker and also maintain the tone as volume is reduced. It needs a consistent tonal and dynamic response from low attenuation, down to sub-bedroom level. This is where the simplest designs can be inferior, and the best commercial designs get expensive.
With feedback and testing by others I think we have a design that achieves this. For about $100-$120, anyone with workshop skills and the ability to follow a circuit schematic can build this. I want to thank everyone who has built one of these or contributed to this thread, with a special thankyou to @Gene Ballzz who was the first to see the potential and has been a great source of insight and practical help for everyone.
An important point:
Anyone who builds it does so at their own risk, and takes responsibility for working out their own wiring for their own private, non-commercial use, and completing it safely
Summary: November 2022
This thread focusses on the design of reactive attenuators for DIY building using simple construction and inexpensive parts. It started with just a few resistors and then developed into multistage resistive and reactive designs. The latest designs work much better than I’d first expected and have been built and tested successfully by many others. The design continues to evolve but the main principles have been constant for several years. This 1st post shows the current basic design M2, and a few possible additions to it.
It must be matched to the output tap of the amp, eg 8 Ohm or 16 Ohms. Component values for both are given, which differ by a factor of two:.
There are 4 attenuation stages, engaged or bypassed by switches.
Stage 1 is the key reactive stage and includes an inductor coil. This stage on its own, reduces power by a factor of 5, or -7db, reducing a 50W amp to 10W. The inductor coil is configured so that the impedance presented to the amp is similar to that of a real speaker (values based on various Celestions), particularly how impedance rises with frequency.
After Stage 1, three more stages are provided. These can be mixed and matched, but the design shown is based around additional -3.5db, -7db and -14db stages. By combining these switches in combination, and with Stage 1, reductions of up to -31.5db can be achieved in small, equal steps of -3.5db, at which point a 50W amp, at full power, is playing quietly at about 35mW.
On pages 111 and 112 are two construction layouts which may assist (designs are very close to above, not identical though). I recommend studying the diagrams above to understand the connections and then adapt the layouts to suit your needs. Further features can be added, discussed in the thread, such as bass resonance circuit, foot-switchable stage, variable input impedances, bypass switch etc.
If you'd like to see a schematic with most of these further add-ons, go to Page 158, post 3146 from 19/11/2022.
Component values and power ratings
The table above shows the recomended power ratings for each resistor, based on up to 50W amps. The component ratings need to have a good margin above the actual power. I use a factor of at least 3 for case-mounted aluminium resistors, bolted (using thermal grease) to a heavy metal chassis or heatsink , and a factor of 5 or more for air-cooled resistors. These values fit with the spec in the schematic diagram above and also allow for overdrive of the amp.
Wire for hookup and also the winding of air-cored inductors should be 18 gage for 50W attenuators, and this is also OK for a 100W one, if built to the 16 Ohm values. For switches, use at least 5A rating (at 125V ac) for a 50W 8 Ohm build. The best jacks are plastic Cliff jacks, TRS (ie stereo type) which grip the plugs better than mono jacks.
Cooling
With amps > 30W at high power, the unit will heat up as it dissipates power. A good size die-cast aluminium case is best. Once components are positioned, then a number of additional large vent holes should be drilled, in the top and in the base, with feet to raise up the base. This will help to promote good convective flow of air out through the top, replaced by cool air at the base. The best colours for cooling are dark. For amps more than 50W, a fan should be added
Mount the coil without using a ferrous bolt, with a few mm timber or plastic spacer off the case surface.
Thanks for reading. If you are interested, and since this is a long thread, I suggest to read this post, look at the layouts and the most recent few pages, then make a post yourself and we'll be happy to discuss what would work best for you.
For nice collection of completed builds, see this thread:
Completed JohnH Attenuators?
Hey Fine Folks, The Simple Attenuator thread has become so lengthy that it is difficult to ascertain how many successfully completed builds there are of the fantastic @JohnH attenuator design. :thumbs: I'm starting this thread with the intention of showcasing all of these great builds. It is my...
www.marshallforum.com
Attenuator M ( January 2019)
The following is the original reactive design M which I built myself, with sound samples. M and M2 are closely related and perform the same, although M has two coils. It also has a bypass switch and a -3.5dB setting.
The build was in a case 170 x 120 x 55mm of thick aluminium:
Simplified version M-Lite
This was the same design, omitting the bypass switching and the 3rd output. Minimum attenuation is -7db. Its an easier build and it should still meet most needs.
Performance
In the schematic above, there is a graph showing a calculated frequency response at each attenuation level from 0 to -31.5 db. These use a spreadsheet to calculate the signal at each stage of the circuit, as a series of voltage dividers, using complex number theory to assess magnitudes and phase angles. The speaker was represented, for analysis, by an equivalent load model, by Aiken:
http://www.aikenamps.com/index.php/designing-a-reactive-speaker-load-emulator
...adjusted to match the measured performance of a G12M 4x12 cab. The plots are based on small signals, with the amp output impedance assumed to be 20 Ohms, for an 8 Ohm tap, based on measurements of my VM2266C. These calcs were used to adjust the values in the design.
Sound Samples
The ideal is for volume to reduce, but with no change in tone or feel. This is best tested with a consistent loop, with attenuated sounds then normalised back to equal volume:
Attenuator M: Max attenuation to non-attenuated:
Attenuator M: Normalised:
It’s a simple looped riff, played twice at each attenuation setting from -31db up to full unattenuated in 3.5db steps. The second file is based on the same recording, with each stage normalised for volume so you can hear any differences in the tone.
My VM2266c amp was on LDR mode, body at 6, detail at 9, master vol at 6, tones and presence at 6, using my LP bridge pickup, miced off a speaker.
The plots are taken from the sound sample posted above. The lower set of data are the basic plots, from full volume down to -31db (db scale is arbitrary, but relative db's are right).
The upper plots are intended to show the differences between responses. I took the -7db recording as the base case, so this is shown as a flat line. The others are the various other settings, with the -7db trace subtracted. The ideal for these traces is therefore also a flat line. And for all the traces below -7db down to -31db, this is what is happening, there is virtually no further tonal change at all as you attenuate down as far as you want. It measures as consistent.
Last edited:
