Pad for guitar/bass amp to line/mic level

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shot

Well-known member
Joined
Nov 28, 2008
Messages
669
Location
Zagreb, Croatia
Hey guys,

On a recording session few days ago I tried using DI-Box with 40db pad in between bass heads (bassman and ashdown) and a mic pre. Even though we had to keep the output level of the amp very low, the sound was amazing. Just pure sound of the amp without room resonances that I usually pick up when miking bass cabinet.

I want to build a pad with 60db or maybe even 80db of attenuation to use for this purpose!
I've tried searching here for a similar pad schematic but I found none (or my search skills are limited).
I guess it is just a simple T-Pad but with very high power rating of resistors.

Can anyone point me to such schematic or any info?

Anyone tried or is using it?


:)

Luka
 
It would be more normal to feed such a pad into the line input which has a lot less gain than the mic input. However, if you want to use the mic input that is no problem. Back in the 70 when I was doing on location demo recordings I built just such a pad. It even had a couple of crocodile clips so you could connect it direct to a speaker terminals.

From the safety point of view you really need transformer isolation. I seem to remember working out that 100 watts into an 8 ohm speaker was about +29dBu so a 60dB attenuator will drop the level to about -30dBu. I think I used a 66dB balanced attenuator consisting of two 47K resistors and a 47 ohm resistor. Connect a good quality 600:600 transformer across the 47 ohms and feed the output to your mixer.

Cheers

Ian
 
Thanks Ian!

Yes, it makes sense to use transformer isolation here. I think I can dig some transformer for this purpose.

What about the power rating of those resistors? Would 2 watt be enough or I should get higher?
 
shot said:
Thanks Ian!

Yes, it makes sense to use transformer isolation here. I think I can dig some transformer for this purpose.

What about the power rating of those resistors? Would 2 watt be enough or I should get higher?
As Ian mentioned, a 100W amp delivers about 30 Vrms into 8 ohms. With his proposed values, the resistors dissipate about 10 mW, so you can use any standard 1/4W or 1/2W type.
I tend to think that 66dB attenuation may be excessive in some cases. I would suggest you use a potentiometer, that would allow optimizing the level.
 

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Brian Roth said:
Just be sure that you connect either a proper loudspeaker OR a dummy load across the speaker output of the amplifier.

Bri

The pad is sufficient of a dummy load? 

For the isolation, would a Triad A67j work here? 

Put the 47k resistors on a two pole rotary switch .  Along with other values so you could change the attenuation (would probably ONLY be able to switch when amp is off??) Put the pot across the output resistors and put the triad across the pot?  Might work?
 
MicDaddy said:
Brian Roth said:
Just be sure that you connect either a proper loudspeaker OR a dummy load across the speaker output of the amplifier.

Bri

The pad is sufficient of a dummy load? 
No. A tube amp needs to see close to its nominal Z. 20kohms is no match for 8 ohms.
For the isolation, would a Triad A67j work here? 
No reason why not. A benefit is that the primary can be wired as 600 (series) and the secondary as 150 (parallels) for another 6dB attenuation and better loading of the mic pre.
 
The guitar amp definitely needs to see it normal load. The attenuator is not sufficient for this. When I used this type of pad back in the 70s  most guitar bass amps had external speakers and a couple  of jacks to connect them. I often used  one of these jacks. Where this was not possible I connected directly across the speaker using crocodile clips.

Cheers

Ian
 
Ok, as far as I understand now, I should first put a loading resistor (4 ohm or 8 ohm high wattage) across the output terminals and follow that with Abbey's drawing (10k in series, pot in parallel, transformer...)
Right?
 
MicDaddy said:
ah ok, so it's fine to use the speaker as the load and just parallel from the speaker terminals?

Otherwise it would take one of those giant amplifier load resistors or similar?

Correct. As I was recording gigging bands, they wanted to play the way they normally did - with speakers connected so heavy load resistors were not needed.

Cheers

ian
 
speaker outputs can have some noise on them, not the cleanest output for a mixing board,

have you considered building a Reddi DI and using both a mic on the speaker combined with a DI signal? this works well as you can blend the two signals to get the tone you want from the bass,

 
CJ said:
speaker outputs can have some noise on them, not the cleanest output for a mixing board,

have you considered building a Reddi DI and using both a mic on the speaker combined with a DI signal? this works well as you can blend the two signals to get the tone you want from the bass,
Re-read the first post.
This guy has found serenity ("Just pure sound of the amp without room resonances that I usually pick up when miking bass cabinet") and you want to plunge him in the throes of doubt...?  ;D
 
CJ said:
have you considered building a Reddi DI and using both a mic on the speaker combined with a DI signal? this works well as you can blend the two signals to get the tone you want from the bass,

This was a way I did it for many years! And I won't stop doing it that way when doing clean (pop music) bass sounds.
The problem is when you have bass player who uses distortion pedals. DI signal taken after the pedal is nowhere near the sound they want, and I almost always end up using just miked amp sound. But when miking amp I find that when I put the mic too close to the speaker (one centimeter or two from the grill, the way it works with guitar cabinets) I end up with weird sub freq. response since some frequencies need distance to develop properly. Some notes tend to cancel themselves and it sounds inconsistent . So I need to put the mic more distant (at least 30cm) and that's where room modes come into play. And my room is not perfect.
This is why I find taking sound from the amp's output working best in this case!

:)
 
, actually some of the new amps are pretty quiet, like a Hartky mosfet job, but some of the  old tube amps have stuff you don't want to record, signal to noise ratio does not have to be that good when pete townsend is cranking hiwatts in a small room,
 
Harpo said:
abbey road d enfer said:
shot said:
The problem is when you have bass player who uses distortion pedals.
My recommendation: get a gun and shoot them! 8)
shot down distortion pedals don't sound much better ...
Lol! Assuming you're a good shot, they shouldn't sound at all  ;D
 
lately all we do is sell boss and digitech pedals, and a lot of ukes, i hope people are not playing the ukes thru the boxes,

there is a piezo- uke bass with rubber bands for strings,have you tried those?
Kala U Bass, fresh off the NAMM floor,
easy on the hands, amazing tone for rubber strings>

https://www.youtube.com/watch?v=9YbKCYKe4jA

 
CJ said:
lately all we do is sell boss and digitech pedals, and a lot of ukes, i hope people are not playing the ukes thru the boxes,

there is a piezo- uke bass with rubber bands for strings,have you tried those?
Kala U Bass, fresh off the NAMM floor,
easy on the hands, amazing tone for rubber strings>

https://www.youtube.com/watch?v=9YbKCYKe4jA
I've always been impressed by the Ashbory bass, that didn't really catch on. It seems the world is ready for the concept now.
 
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