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 Post subject: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Fri 16, 2017 6:41 pm 
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hello folks,

hooking a few tube amplifiers up to line level sources has me wondering about eliminating some overload.

I have a big Motorola console with a wonderful PP amp in it and I have hooked it up to some line level sources.

if I wanted to attenuate the input of modern line level sources into this amp, and the amp uses a 1 meg volume control and a 6AV6 preamp tube, would I use a 1 meg linear to attenuate the input signal a little bit ? or a 500 ohm ? does it matter ?

I want to add a "volume control" before the amplifier without any impedance mismatch.

all I need to do is take the edge off a little bit without altering the amplifier or using a transformer.

help is appreciated b/c this is not my most fluent end of the hobby.
thanks.

see pic.

steve


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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Fri 16, 2017 7:20 pm 
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Hi Steve,

You can add a fixed resistor in series with the input.
That has the effect of limiting the maximum available volume, like turning the control down.

A value of 1 Megohm would limit the volume to "half".
But that's not really true since human hearing is not linear.

Experiment with values from 100,000 ohms to 1 Megohm and see what works best.
Or you could use the extra control, as you drew. It would cost more and add complexity.

- Leigh

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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Fri 16, 2017 7:36 pm 
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hi leigh and thanks for your reply.

if i were to go ahead and make a little box with a control and two rca female jacks to have volume flexibility, is using a 1 meg pot the way to go ? linear or log ?

i have a few high end vintage 70's-80's solid state tuners that have a volume control on their output.

this "volume control" is what i am trying to make.

this would be a "set it and forget it" type of thing for a few of my sets.

further thoughts ?

thanks.

steve

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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Fri 16, 2017 7:40 pm 
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Hi Steve,

Unless there was some degree of "standardization" of those output levels...
I suggest each such "level adjust" box would be mated with a particular tuner.

You can adjust each individually to present the same level to the amplifier.
That way you can switch inputs at will without a lot of adjustment hassle.

It does not appear there's any "standard impedance" involved in this interface.

- Leigh

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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Fri 16, 2017 7:45 pm 
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"no standard impedance", that is what i was wondering/thinking.

again, this is not in my realm of expertise, so thanks very much.

so, if i were going to go with making an attenuator, where would be a good place to start ?

250k, 500k, 1m ?

thanks again.

steve

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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Fri 16, 2017 7:45 pm 
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Impedance matching is usually not a concern unless you are trying to minimize losses and maximize the signal level to the amp and that's not your objective here.

Leigh's suggestions look like a good starting point to find the level of attenuation that best suits your needs.

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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Fri 16, 2017 8:01 pm 
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Given the amp uses a loudness tap on the volume control I dont know how a modern low impedance source will affect it.

That said your idea of a 1 Meg pot will work. Just be sure whatever box it is in I metal for shielding purposes.


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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Sat 17, 2017 2:51 am 
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thanks guys.

i will try both sometime.

thanks.

steve

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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Sat 17, 2017 3:23 am 
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You don't need to worry much about impedance matching when the source resistance is lower than the input resistance and there are no transformers involved.

Dave


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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Sat 17, 2017 3:30 am 
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thanks for all the info guys because this is not at all my level of expertise. i believe i will go with 1meg.

steve

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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Sat 17, 2017 6:10 am 
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One of the only times impedance matching is very important is if the tone control/controls are directly between the input and volume control.


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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Sat 17, 2017 7:03 am 
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I agree with Leigh's idea. Just make a small switch box that uses trimmer pots and adjust them for what looks best on a scope for each device that you are going to jack in, dude. As someone also mentioned, don't worry too much about the input impedance being changed, unless you are really worried about maximum power transfer, which shouldn't be a problem with this application. If I remember correctly, line-level input is actually an ANSI standard, and I believe that standard is 250mW's or so.

73,

Randy AB5NI

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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Sat 17, 2017 4:09 pm 
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thanks guys. i will give those ideas a try.

steve

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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Sun 18, 2017 12:54 pm 
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ab5ni wrote:
If I remember correctly, line-level input is actually an ANSI standard, and I believe that standard is 250mW's or so.
There are a bunch of different "line level" standards.

https://en.wikipedia.org/wiki/Line_level

Dave


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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Mon 19, 2017 1:49 am 
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Dave Doughty wrote:
ab5ni wrote:
If I remember correctly, line-level input is actually an ANSI standard, and I believe that standard is 250mW's or so.
There are a bunch of different "line level" standards.

https://en.wikipedia.org/wiki/Line_level

Dave


And there's also the level most tube operated devices with inputs required (AM radios and AM radios with an input for phonos with higher output cartridges) which is a bit higher than the normal consumer line level, although the +4dBu pro line level will drive them just fine.


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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Wed 21, 2017 3:44 am 
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I will advise in a long while. this project is tabled due to others that will be going on this summer.

steve

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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Thu 22, 2017 4:16 am 
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it will be done in a much shorter while, like a 36 hour "while".

https://www.amazon.com/PAC-LC-1-Remote- ... attenuator

:D

steve

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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Thu 22, 2017 11:35 am 
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That should work quite nicely.


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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Thu 22, 2017 12:55 pm 
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I couldn't find out what the device's input impedance is even after reading the Q/A section. I suspect it will probably work well with solid state sources but may affect the sound of tube equipment sources.

Dave


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 Post subject: Re: Attenuator to Match Line Level to Vintage Amps Question
PostPosted: Jun Thu 22, 2017 2:29 pm 
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thanks guys

two for $20, i'll try them and hope for the best.

all that is needed is to chalk off about 10% of the audio. there is no distortion, but a small edge of audio input needs to be removed.

we shall see...

steve

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