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Author Topic: Attenuator question  (Read 11688 times)

will316

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Attenuator question
« on: January 06, 2011, 11:17:12 PM »
I don't recall the site I downloaded it from but there's a schematic for a low powered amp speaker attenuator. The schematic is hand drawn (if that helps anybody to remember) It shows 2 10 ohm 10 watt resistors wired in series going to a 25ohm 3watt wirewound pot. Now here's my confusion- it appears that it calls for another resistor to go over to ground? The pic is labeled 4.7 watt? (I'm not sure because it's hand drawn)I'm also confused as to how to wire the pot. Do I ground the body like on a guitar? My thinking is that the two resistors and the pot plus 2 jacks and I'm good or am I missing something here? This is my 1st project and I found this schematic to be the simplest. I have a decent understanding of electronics and circuits from my work as a hvac technician but we don't repair boards, we replace them. I'd post the schematic but I don't think my phone's capable. Thanks in advance for any help.
« Last Edit: January 07, 2011, 12:22:08 AM by will316 »
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joecool85

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Re: Attenuator question
« Reply #1 on: January 07, 2011, 05:22:44 AM »
Could you post a link to the schematic at least?  That would help a lot.  Also, most of the attenuators I have seen have been built with light bulbs instead of resistors.  Why do you want an attenuator?  Have a tube amp that's too loud?
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phatt

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Re: Attenuator question
« Reply #2 on: January 07, 2011, 09:16:07 AM »
Need more info,
Is it a Valve Amp?
If so how many Watts?
Or What type of power Valves? (i.e. 2x EL84)

If so a 50 Watt HiFi L-pad will do that trick you and save a lot of messing about.
Not the best way but does work for small amps.
Phil.

will316

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Re: Attenuator question
« Reply #3 on: January 07, 2011, 10:20:24 AM »
Blueguitar.org appears to be the site.  I got to it via a link on diystompboxes. To answer the questions-yes, it"s a valve amp. It's a crate vc508. 5 watts. single el84 power tube. I know you're laughing but I am getting complaints from family to "turn it down!" so I must quiet it down a bit. She really comes ti life when the master's cranked so I'd like to keep that tone. I'm having a bit of luck running a processor (digitech rp200a or maudio black box reloaded) in front of her and using the level control to turn it down but I really think the attenuator sounds like a better idea. I also own a Radio Shack attenuator but the impedance is too low(varies from 3ohm to 1 ohm throughout its range). I also wonder if I could wire a resistor in series with this? I'm wondering if I need a resistor just in the + wire or both + and -? Just wondering if I'm understanding resistors right or not. I understand that they convert eklectricity to heat, my question I guess is this: can I just run them in series in the + line or do I need something in the - line also?
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J M Fahey

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Re: Attenuator question
« Reply #4 on: January 07, 2011, 12:35:13 PM »
You can make a *fixed* "keep the Family happy" attenuator with an 8 or 10 ohm, 10 watt resistor, plus a 1 ohm or 0.47ohm, 2 W one.
Wire both in series; connect the free end of the 10 ohm one to the amplifier output hot lead, the free end of the 0.5 or 1 ohm to amp out ground and speaker ground, the midpoint to speaker hot.
Instant bedroom amp.
You can build it into a box with a switch to bypass it, or just with an input and output jack, or on a board bolted to your Crate inside, on a side or bottom.
Anyway you do not need to switch it quickly, it's not an effect, plugging it in/out is quite acceptable.
If you need a drawing, I'll do that later, now i'm sort of busy.
Good luck.

will316

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Re: Attenuator question
« Reply #5 on: January 07, 2011, 12:49:07 PM »
I believe I've got you. Hot lead>>larger resistor>>+ terminal on speaker. Ground lead>>smaller resistor>>- terminal on speaker. I think you can decipher that. Now is it possible to wire a pot in? Or would this be "pissing in the wind" so to speak? As far as switches a dpst should do it I'm presuming? Just can't afford to blow the amp up! If I've got the scheme correct then all I can say is: thanks! I'm looking forward to learning alot here and you my friend, are a very good teacher.
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joecool85

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Re: Attenuator question
« Reply #6 on: January 07, 2011, 12:58:14 PM »
I believe I've got you. Hot lead>>larger resistor>>+ terminal on speaker. Ground lead>>smaller resistor>>- terminal on speaker. I think you can decipher that. Now is it possible to wire a pot in? Or would this be "pissing in the wind" so to speak? As far as switches a dpst should do it I'm presuming? Just can't afford to blow the amp up! If I've got the scheme correct then all I can say is: thanks! I'm looking forward to learning alot here and you my friend, are a very good teacher.

Not quite.  Connect the hot lead to the 10 ohm resistor, connect the other end of the 10 ohm resistor to the + on the speaker.  Then connect one end of the 1 ohm resistor also to the + on the speaker and the other end to ground.
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will316

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Re: Attenuator question
« Reply #7 on: January 07, 2011, 03:17:26 PM »
By "ground" you mean to the - side of the speaker or straight to ground(side of enclosure)? I'm just trying to be sure. I know a bit about electronics but also realize I've got a bit of learning ahead of me. I also plan to look back through the archives here because this site is very informative and everybody seems very knowledgeable. Refreshing to say the least! I am a fast learner so my questions will become less and less in the future(I hope!) Anyway, once again, thank you for sharing your knowledge and answering my noob questions.(you can't learn if you don't ask.)
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joecool85

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Re: Attenuator question
« Reply #8 on: January 07, 2011, 03:48:22 PM »
By "ground" you mean to the - side of the speaker or straight to ground(side of enclosure)? I'm just trying to be sure. I know a bit about electronics but also realize I've got a bit of learning ahead of me. I also plan to look back through the archives here because this site is very informative and everybody seems very knowledgeable. Refreshing to say the least! I am a fast learner so my questions will become less and less in the future(I hope!) Anyway, once again, thank you for sharing your knowledge and answering my noob questions.(you can't learn if you don't ask.)

The negative side of the speaker wire should go to chassis ground.  Chassis ground is where you want the resistor heading to, doesn't matter how it gets there.

And you're right about asking questions.  When I started doing this stuff I had no idea either, so feel free to ask as many questions as you need to and we will do our best to help.
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J M Fahey

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Re: Attenuator question
« Reply #9 on: January 07, 2011, 04:31:53 PM »

rowdy_riemer

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Re: Attenuator question
« Reply #10 on: January 07, 2011, 05:25:43 PM »
For a very small amp, like the ruby or a small 5w tube amp, the 25 ohm pot sold at radio shack isn't too bad.

will316

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Re: Attenuator question
« Reply #11 on: January 07, 2011, 10:14:19 PM »
I picked up a pot at Raduio Shack. I would imagine this does have to be used with the resistors, correct? You can't just use the pot alone, can you? I know I'm full of questions, but that's how I learn. My idea was to mount it all in one of their "project enclosures" (metal not plastic). Soon I hope to build a Lead 12 pre amp and mate it with a 50-100 watt power section. *baby steps* lol
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rowdy_riemer

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Re: Attenuator question
« Reply #12 on: January 08, 2011, 02:44:05 AM »
In my case, I put a couple of 10 ohm power resistors (rated for 5W each, I think) in series, and put that series in parallel across the outer terminals of the pot. The input jack (coming from the amp) is connected across the outer terminals of the pot as well. So with no speaker plugged into the attenuator, the amp "sees" about 11 ohms of resistance. The output jack (going to the speaker) is connected to the wiper terminal and which ever outer terminal is connected to ground. With an 8 ohm speaker plugged in and the pot turned all the way up, there will be about a 5 or 6 ohm impedance. As you turn the pot down, the impedance "seen" by the amp approaches the 11 ohm resistance seen without the speaker plugged in. And, of course, as you turn down the pot, the current through the speaker is reduced. There might be a better way of doing this, but it is an easy attenuator to make, and allows you to adjust for a trade off between attenuation and good tone. It sounds good with my ruby. Sounds like crap through another homemade amp. Haven't tried it yet with my 5W ValveKing royal 8 yet. You might do better wiring the pot as a rheostat and use it as R2 in JM's schematic.

rowdy_riemer

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Re: Attenuator question
« Reply #13 on: January 08, 2011, 02:46:39 AM »
Or, just wire it like with JM's schematic, where the resistance between the hot terminal and the wiper terminal is basically R1 and the resistance between the wiper and ground terminals is R2. But, of course, the resistance across the outer terminals is more than double the 8 or 10 ohms in JM's schematic.

will316

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Re: Attenuator question
« Reply #14 on: January 08, 2011, 04:38:22 AM »
I believe I've got it. How does this scheme sound: input jack coming from amp>1 10 watt resistor in series to outer leg of pot. Wiper leg of pot to hot side of speaker. Ground leg from amp to opposite outer leg of pot to ground terminal on speaker. If my thinking is correct, this in effect makes the pot r2 as in the schematic shown above. Please show me my error if I'm wrong. Now I also bought one of their in wall speaker faders. The impedence varies from 0 to about 3 ohms when I ohm it out. Could I just wire a resistor across the ter minals on one side of this to also have an attenuator suitable for my needs? I'm also wondering if you can go with "too much" resistor? Example: 20 watts of resistor for a 5 watt amp? I know it's rated at 5 watts but seems to be able to put out closer to 15. It's just as loud as a 20 watt ss amp I also use and itzms quite a bit louder than a 15 watt ss amp I also use.
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