Class 5 Basement mod

[Don]

found that the bedroom mod (headphone output jack to internal speaker) attenuated too much and sounded thin and over distorted.

Looking at the schematic, leaving the original 15 ohm/25 watt resistor in parallel with the speaker and replacing R50 (100 ohm/2 watt resistor) with an 8 ohm resistor should reduce the attenuation to 6 dB and keep the load to approximately 15-16 ohms, I believe.

This would reduce the amp's output by 4x, down to 1-1/4 watts.

Please correct me if my math or thinking if either is wrong. I'm not an EE and would hate to blow up my amp!

I did this mod tonight and the amp sounds fantastic! The amount of attenuation is only enough to take the edge off. It's still fairly loud and sounds almost the same. Comparing back and forth, the attenuation is enough that it's very usable. It sounds good enough that I'd be happy to keep the switch in the headphone position and only switch it back when the volume is needed.

BTW, if you decide to do this mod, be very careful working on the PCB. The traces are easy to damage!

[Garrincha]

found that the bedroom mod (headphone output jack to internal speaker) attenuated too much and sounded thin and over distorted.

Looking at the schematic, leaving the original 15 ohm/25 watt resistor in parallel with the speaker and replacing R50 (100 ohm/2 watt resistor) with an 8 ohm resistor should reduce the attenuation to 6 dB and keep the load to approximately 15-16 ohms, I believe.

This would reduce the amp's output by 4x, down to 1-1/4 watts.

I wonder if you could hook up a pot instead of a fixed resistor. I'm no EE either but I figured the attenuation would be better controlable that way. Heck, I might just try that myself.

[SteveD]

found that the bedroom mod (headphone output jack to internal speaker) attenuated too much and sounded thin and over distorted.

Looking at the schematic, leaving the original 15 ohm/25 watt resistor in parallel with the speaker and replacing R50 (100 ohm/2 watt resistor) with an 8 ohm resistor should reduce the attenuation to 6 dB and keep the load to approximately 15-16 ohms, I believe.

This would reduce the amp's output by 4x, down to 1-1/4 watts.

I wonder if you could hook up a pot instead of a fixed resistor. I'm no EE either but I figured the attenuation would be better controlable that way. Heck, I might just try that myself.

Please don't!!

[Don]

found that the bedroom mod (headphone output jack to internal speaker) attenuated too much and sounded thin and over distorted.

Looking at the schematic, leaving the original 15 ohm/25 watt resistor in parallel with the speaker and replacing R50 (100 ohm/2 watt resistor) with an 8 ohm resistor should reduce the attenuation to 6 dB and keep the load to approximately 15-16 ohms, I believe.

This would reduce the amp's output by 4x, down to 1-1/4 watts.

I wonder if you could hook up a pot instead of a fixed resistor. I'm no EE either but I figured the attenuation would be better controlable that way. Heck, I might just try that myself.

I wouldn't. As I said, I'm not EE, but I believe that part of a normal pot's range would fall well outside what is acceptable to the amp's output transformer.

[Lowdown]

:scared I think that the old saying-"if it ain't broke don't fix it" should seriously be followed by some of you out there.

If you don't know what you're doing,leave it alone and walk away!

[Don]

There's nothing wrong with asking the question and performing some careful experiments.

[Beck-Ola]

There's nothing wrong with asking the question and performing some careful experiments.

You might put somebody'e eye out, lol.

[Don]

Someone else looked at my mod and pointed out that the amp is seeing about a 9 ohm load with the resistor that I'm using. That shouldn't be a problem. I think I'm going to try a 50 ohm resistor as a happy medium. The load will be higher and I think there will be little more attenuation.

Eventually, I'll stop monkeying around and build a proper attenuator!

[Lowdown]

[Don]

That's scary!

[KevinOConnor]

Hi Guys

To make the attenuator variable you need a rheostat rather than a pot. A pot is usually a carbon or plastic film element that cannot handle the currents even of a 5W speaker output. A rheostat is also called a "wirewound" potentiometer as the element is literally a coil of resistance wire.

You can buy generic rheostats from all the usual electronics vendors.

For speaker applications, you can also get an "L-pad". This is a control that does the same thing but is designed to present the same resistance to the source all the time.

Varying the load seen by the amp doesn't hurt the amp as long as the variation is slow. You cannot twist the rheostat shaft fast enough to be "too fast", or for the load to look discontinuous (see the other thread here about what things damage an OT).

A rheostat, L-pad, or fixed resistors will all work at attenuating the signal to the speaker. At high attenuations, you will want to add a small capacitance across the speaker to reduce fizz. This can be applied across the speaker output of the amp if a low-value resistor is placed in series with the cap; the resulting RC is a zobel network, also called a conjunctive filter. It simply rolls off treble. This can be applied to the head-phone jack used as an attenuator, as well. TUT4 explains how attenuators work and how to fix some of their sonic issues.

Have fun
Kevin O'Connor

[Don]

Thanks Kevin!

[explorer rob]

I've been happy with a Weber mini-mass attenuator won off of flea-bay for less than $50. It uses a speaker coil to burn off excess energy from the amp and sends a desired energy level to the speaker for low volume playing. This is a great topic for those using the amp for practice with families or other situations. Any home brew attenuators offered for sharing? And, I just can't resist because these are just too cool:
:moon :beatdeadhorse
Keep 'em coming!

[Lowdown]