attenuator direct out clipping my buffer only when set to 16ohms?

[ode2no1]

Hey all,

Haven't posted in years, but have finally been getting back into circuit building and I have a kind of random, yet specific question that I wonder if anyone might have any insight into.

I guess first things first: I have a Weber Mass Lite attenuator that I've been using with a Fender Bassman 50 for almost 20 years. More recently I've been using it as a load box for recording, running the line out into a Torpedo Cab M+ to use for the speaker emulation. All good there.

I more recently got a Mass Mini 50w to use with my DRRI and my vintage Bronco (Vibro Champ), due to it having switchable impedance and not wanting to blow up yet another champ like I have in the past. I noticed that even though the Bronco is only 5w, using the Mass Mini as a load box, the direct out would slightly clip the Cab M+ for some reason.

Even more recently I picked up a used AC15C1 at a good price, and was stoked that I could use the external speaker out into the Mass Mini at 16ohms into the Cab M+ for recording. But again...even though the amp is only 15w, it was clipping the Cab M+.

So I decided to build a buffered trim box for the direct out so that I could slightly lower the volume into the Cab M without dulling the signal with a passive volume control. I had run a test using a simple opamp buffer at both 9 and 18v with the Bronco and it seemed that 18v wasn't compromising the input signal at all, whereas 9v was squashing the input signal (although not audibly clipping in a bad way).

Today I built up this simple opamp buffer using an NE555 voltage doubler to power the buffer and tested it with the Vox at 16ohm and it was clipping like mad. I was going nuts thinking it was a mistake in my layout. It wasn't even the Cab M clipping this time, it was the actual buffer circuit. I gave it a rest for a few hours and when I got back to it I decided to see if switching the amp and the attenuator to 8 ohms instead of 16 would make a difference, and BOOM. There it was. For whatever reason running at 8 ohms the buffer works perfectly. I can now slightly trim the input signal to the Cab M and avoid clipping it, and I'm getting no clipping from the buffer itself. Switch back to 16 ohms and it's all garbage.

Is there a simple explanation as to why this is? I'm so happy I decided to switch the impedance before I ripped the circuit apart and threw it away. The AC15 is also the only amp I own that can run at 16 ohms, and it can switch to 8, so not a big deal either way, but I'm just trying to understand why the huge difference coming from the direct out of an attenuator.

Thanks ya'll!

[antonis]

Hey there..

A schematic of buffer and connection (including attenuator) should be much helpful..

[r080]

There is a schematic of the mini mass on the other forum (replace asterisks in link below). The line out looks pretty simple. Without doing the math, I have a feeling that clipping is exactly what you would expect with the 16 Ohm setting.

How concerned are you about dulling the signal to the Cab M+, given the line out is already a passive volume control? It looks like the Cab M+ has a speaker in and out. Wouldn't it make more sense to go from the amp into the Cab M+ Amp In, and from the Cab M+ Speaker Out to the mini mass?

https://www.****stompboxes.org/members/soulsonic/schematic/WeberMiniMASS.gif
https://wiki.two-notes.com/doku.php?id=torpedo_cab_mplus:torpedo_c.a.b._mplus_user_s_manual

[anotherjim]

I'd expect a tube amp to output a higher speaker drive voltage at 16ohm than 8 or even 4 ohms. If power = amps x volts and you expect the same power for a 16ohm speaker on a 16ohm tap as an 8ohm speaker on an 8ohm tap, the voltage must be higher at 16ohm since the higher speaker impedance results in reduced current.
This rule doesn't work for solid-state amps, since you don't have a transformer to match the speaker impedance.

[ode2no1]

How concerned are you about dulling the signal to the Cab M+, given the line out is already a passive volume control? It looks like the Cab M+ has a speaker in and out. Wouldn't it make more sense to go from the amp into the Cab M+ Amp In, and from the Cab M+ Speaker Out to the mini mass?

In doing some A/B testing, the buffer between the line out and the Cab M sounded better to me, so I figured I'd just run with it. And yeah, I know I can run the actual amp speaker out to the Cab M and then that into the Mass, but with the way my room is set up it would be kind of a nightmare. I'd have to move whatever amp I want to record with that day right next to my computer desk, and it would just be in the way and not as easy/fun as being able to run a cable from across the room and switch between amps easily. I've actually never tried running it that way, specifically because of not wanting to move amps, but I should give it a test just to see if any of the amps with clip the Cab M that way. I've definitely read about it being an issue with certain amps.


I guess I never thought about 16 ohms literally running higher speaker drive voltage. Makes sense. I wonder if running the buffer at 24-36v would work. I'd have to replace one of the caps that is only rated at 25v, but certainly something that might be worth checking out for fun.