Attenuator for solid state amp

[utas5]

I'm new to building amps and pedals but I'm currently working on my first amp project. For the preamp I'm using Rod Elliott's project 215 (40W version of project 27: https://sound-au.com/project215-p27-revisit.htm ), for the power amp I'm using a TPA3118 class D amplifier and for the speakers I'm using a 10" Celestion Ten 30. I was thinking to add a switch where I can switch between 30W and 1W and after some digging and asking, I found out I need an attenuator, but after looking into the forums here, I've seen some people say that you don't need an attenuator for a solid state amp, so I'm a bit clueless on what to do now.

The use for the amp would be for home practice, pedal making and testing (main reason) and also occasionally use it for jam sessions with a friend of mine.

[phatt]

Yes you can use an attenuator on SS Power Amp but there is little benefit as most of them run much cleaner that a Valve Pwramp.
With SS rigs 90% of the OD effect is done in the preamp stages.
whereas with Valve rigs you can get extra sag and compression just from driving the Amp LOUD. Hence they use Attenuators on Valve amps to get the pwr section in sweet Spot But at reduced Volume.
The only time I use an Attenuator on SS amp is for bench testing a circuit under load, to make sure it can actually handle sustained full power.
You have a Master Volume on Rods Circuit and that will do the same as an Attenuator.

The diode Clipper before the Master Volume in Rods circuit is unlikely to give you full bore Screaming OD/Distortion.(If that is what you are chasing) You would need to drive it all with pedals if you want more Screaming OD.
Phil.

[tonyharker]

What impedance is your speaker? Check the data sheet for the 3118, It will work down to about 5v supply when the output will be about 2w into 4 or 8 ohms.

[J M Fahey]

What both said above.

In SS amps, power amp clipping means symmetrically hitting power rails, say +/- 28 Volts or whatever you have, which gives you a sinewave with clipped tips; nothing more, nothing less.

Which is not _bad_ as tube heads claim 🙄 , it is actually usable, but not too "interesting" either.

You can get exactly same waveform and sound, on a way more controllable way, by adding diodes between preamp output and power amp input, adding a volume pot for Master control.

Exactly what Rod's preamp does.

Sound is exactly same as if you clip your TPA3118 or any other SS power amp, but at any volume ... including bedroom or workbench level.

It's a "polite" distortion.

If you want high gain, raunchy sound, chainsaw buzz, etc. , add it with pedals.

Mind you, most high gain "tube amp players" do the same! 😲

They add pedals *or* use a very nasty preamp which does the job (think Dual Rectifier, Soldano and their clones)

Small World, just today I was watching a Gary Moore video, killer sound and player.
Comments below stated that he used a Marshal tube amp, a JCM2000, ... pushed by a Marshall The Guv'nor distortion pedal 😲
Yes, an SS pedal (Op Amps inside) with diode clipping 😄😄😄

Revered Steve Ray Vaughan pushed his various Fender tube amps with a SS Tubes Screamer.
Again Op Amps and diodes.

Only extra detail I strongly suggest is: use an Audio/Log potentiometer as Master Volume for better control at low levels (as you demand), a Linear pot will be "too loud too soon" and trickier to handle.

[utas5]

My speaker's impedance is 8 ohm. The reason that I thought to add a attenuator so I can use the whole volume pot, because how I know if the volume pot is only at 25%, then it won't have that good of a tone like at 75%. I know that this is kinda true at valve amps, but I don't know if it's true at SS amps.

[Jazz P Bass]

Which amplifier are we talking about?

[J M Fahey]

My speaker's impedance is 8 ohm. The reason that I thought to add a attenuator so I can use the whole volume pot, because how I know if the volume pot is only at 25%, then it won't have that good of a tone like at 75%. I know that this is kinda true at valve amps, but I don't know if it's true at SS amps.

*DO* use your volume pot as high as you want, even 100% for quite raunchy sound if you wish, but then attenuate _output_ power to desired level using the Master Volume pot, which is its intended use.
Tube amps with Master Volume do exact same.

@Jazz P Bass: read first post: a Rodd Elliott P27 preamp driving a TPA3118 power board.

[Tassieviking]

I am pretty sure that any distortion or growl in Rods preamp is produced before the master pot, you can just turn the master down and crank the volume pot and you should be able to vary the signal with the master pot.
If you want to add a switch for a lower setting place a resistor in series with the master pot,  between M1 and the master pot and use a switch to short the resistor out for full volume.
I would try a 50k resistor to start with and then increase it later if you want less output.

[J M Fahey]

Yes.

By using a Log/Audio pót you have a finer control at lower settings.

[joecool85]

Ironic timing, but I'm actually playing with an attenuator for my K-20X.  I could have altered the circuit (more) but decided to just try an attenuator.  In my case I am using a 25ohm resistor in series with the positive lead and an 11ohm resistor in parallel with the speaker.  Since I have a 4ohm speaker, this will have the amp seeing 8ohms.  Overall output will be dropped to about 0.6w rather than the full 20w.  Switching will be done by an on/on spdt switch.  I have the parts, once I get around to actually doing it, I'll be sure to update my K-20X thread.

The K-20X is bothersome in that I need to use it on volume 2-3 at most and it's very particular about where I set it.  This should allow me to use around 5 on the volume at home and be more finite with adjustment.  Again, I could have done this on the preamp side, but this was easier for me and lets me have a feature currently seen on the "big boy" new amps :-)