Hoffman Amplifiers Tube Amplifier Forum

Other Stuff => Effects => Topic started by: MORE_Guitar_Solos on May 14, 2020, 02:49:43 am

Title: High pass filter for attentuator/load box
Post by: MORE_Guitar_Solos on May 14, 2020, 02:49:43 am

Hi everyone,


Not sure where to put this since an attenuator is definitely not an amp but probably also not an effect box, so please kick me out of here if this is an inappropriate forum...


Anywho, I just purchased a Jettenuator to use with a home-built half-AC30 combo (based on Doug's AC30 layout). It works fine at delivering bedroom (or in my case ,dining room) volume at full tilt on the knobs. However, as expected it does trim some of that lovely Vox high end. Not as noticeable with my tele knock-off, but definitely sad face with my P-90 equipped Melody maker.


Has anyone either a) added a high-pass filter to one of these -or- b) built a resistive attenuator with a high-pass filter -or- c) can advise me how to proceed? I can't find a schematic for the Jettenuator online that seems to match what it looks like inside, but basically it seems to be a 100W L-pad in series with some wire wound resistors, the combination of which depends on whether you choose the 4, 8 or 16 ohm output.


Is it as simple as strapping a cap across the whole mess? There is a line out/cab simulator in there that I haven't experimented with but plan to- any considerations there?


Alternatively, anybody have a legit schematic for this box?


Thanks in advance!

Title: Re: High pass filter for attentuator/load box
Post by: SILVERGUN on May 14, 2020, 12:47:04 pm

You can experiment with a cap across the series resistance....I think I ended up with .01 on mine

Title: Re: High pass filter for attentuator/load box
Post by: MORE_Guitar_Solos on May 14, 2020, 06:13:26 pm

Thanks, Silvergun.


So a cap across the resistors but not across the L-pad? How come? How effective was this at bringing back to he highs? Also, did you make it switchable or on all the time?


Thanks!

Title: Re: High pass filter for attentuator/load box
Post by: SILVERGUN on May 19, 2020, 01:03:07 pm

Quote from: MORE_Guitar_Solos on May 14, 2020, 06:13:26 pm

Thanks, Silvergun.


So a cap across the resistors but not across the L-pad? How come? How effective was this at bringing back to he highs? Also, did you make it switchable or on all the time?


Thanks!

No, I meant a cap across any series resistance, so if the L-pad IS in series with the resistors then I would try it across the whole kit and caboodle.


Without an actual schematic it is just a generalized tip.


For me, it was effective enough to keep....on all the time

Title: Re: High pass filter for attentuator/load box
Post by: SILVERGUN on May 19, 2020, 01:19:55 pm

I took a look at a couple pics and it appears that it would be easiest to just place the cap from whichever input jack you are using (+) to the output jack (+)


Some of the additional resistor network is there to try to keep the impedance somewhat matched as you use different loads.

Title: Re: High pass filter for attentuator/load box
Post by: MORE_Guitar_Solos on May 19, 2020, 10:48:54 pm

Excellent, thank you Silvergun!


I will give this a shot, though I haven't tried the DI/speaker emulator yet, and I want to try that first to establish a baseline to see if the caps effect that. That will determine whether I make this switchable.


Thanks again!

Title: Re: High pass filter for attentuator/load box
Post by: MORE_Guitar_Solos on May 25, 2020, 11:59:48 pm

Finally got a minute to go in there with a soldering iron.


I had a couple of .1uF Orange drops laying around, so I threw one across the output jack and the wiper of the L-pad. That was better, but pretty subtle so I put the other one in parallel. That gave me a decent sound with the treble dimed and nothing taken off with the cut control, but with a Vox-ish amp I like to have some cut in to compress things a little, so I'm thinking a significant larger value will be the thing, maybe as big as 2.2uF - 4.7 uF?


When I can track some of these down I'll report back.

Title: Re: High pass filter for attentuator/load box
Post by: jjasilli on June 08, 2020, 10:09:28 am

This is what I did, after A/B'ing different treble bypass cap values.

Title: Re: High pass filter for attentuator/load box
Post by: MORE_Guitar_Solos on June 12, 2020, 12:27:37 am

Thanks for this jjasilli.


I'm not set up for .sch files and my attempts to find a free reader that works (for Mac) have failed. Would love to see this schematic, but could you let me know what cap values you found the best?

Title: Re: High pass filter for attentuator/load box
Post by: jjasilli on June 22, 2020, 02:18:06 pm

Sorry, missed this post.  GIF file attached.  But download Express Schematics.

Title: Re: High pass filter for attentuator/load box
Post by: MORE_Guitar_Solos on June 23, 2020, 05:52:01 pm

Thanks! the 2.2uf I have now does still seem a tad weak, 4uf it is then.


Yeah, I'm Mac only, so Express PCB won't work. I did download Eagle, which is supposed to be an acceptable Mac-alternative, but it couldn't open your file either.


Thanks again!

Title: Re: High pass filter for attentuator/load box
Post by: sluckey on June 23, 2020, 08:11:21 pm

Try this...

     http://jschem.bplaced.net/

Title: Re: High pass filter for attentuator/load box
Post by: jjasilli on June 24, 2020, 10:57:11 am

Quote from: MORE_Guitar_Solos on June 23, 2020, 05:52:01 pm

Thanks! the 2.2uf I have now does still seem a tad weak, 4uf it is then

 :thumbsup:


Don't remember how I arrived @ the 4uF value.  I probably looked at other schematics using variable L-pad pots (actually rheostats); then experimented with different cap values.  The cap value should correlate to the R values of the L-pad pot, which is supposed to maintain the speaker load,as the amount of attenuation is varied -- in my case 8 ohms.  I think the cap value should differ for an L-pad with a different load rating, per the R-C formula.