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 Post subject: VU Meters
PostPosted: 24 Feb 2013, 12:53 
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Joined: 19 Mar 2012, 19:22
Posts: 168
Location: Wales, UK
Hi,

I have a couple of Small round face Anders analogu VU meters going spare and would like to add them to my valve amp. I intend on designing a small circuit which has a buffer and a gain stage in order to drive the meters from the line in sockets. I was going to design a circuit using tubes but I will most probably use op-amps for simplicity.

Will adding the VU driver circuit and meters degrade the sound as it will add a small amount of load onto the source that is connected to the amp?

Many thanks

Phil


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 Post subject: Re: VU Meters
PostPosted: 24 Feb 2013, 13:16 
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Joined: 06 Apr 2009, 10:08
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Location: US Pacific Northwest
Properly designed, the buffer hanging off the inputs should have no effect on the amplifier. This is the definition of a buffer; to separate and isolate. Just remember to keep the input impedance of the buffer stage at least four times the input impedance of the first stage input and as non-reactive (i.e. resistive) as possible. This will prevent the buffer load from affecting the input of the amplifier.

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 Post subject: Re: VU Meters
PostPosted: 25 Feb 2013, 09:16 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Why not put them on the output as there is no chance that would mess up the signal. Same type circuit. My concerns with anything added to inputs is noise. I have discovered (sometimes the hard way) that anything and everything has noise and when connected to the input of a device that noise will be amplified by the device. Another possibility is to measure the signal at the cathode of the output tubes if they are unbypassed. Just remember that there is some DC component there.

Good listening
Bruce

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 Post subject: Re: VU Meters
PostPosted: 25 Feb 2013, 13:51 
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Joined: 19 Mar 2012, 19:22
Posts: 168
Location: Wales, UK
Hi,

Thanks you both for your suggestions. I had not thought about conecting the meter to the output of the amp. I suppose that if the circuit does generate a small amount of noise then it has no chance of being amplified when it's connected to the output terminals.

Will I still need to add a gain stage in order to get the meters to work? I know that the meters needs a bridge rectifier so I have signal diodes for that and also the meters need a series resistor of I think 3.9Kohms

Many thanks

Phil


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 Post subject: Re: VU Meters
PostPosted: 25 Feb 2013, 20:38 
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gofar99 wrote:
Why not put them on the output as there is no chance that would mess up the signal.
That's definitely an option. I guess it really depends on what you're using them for. On the input, they serve as an indicator of when you're pushing your amp into overdrive. They could do the same on the output but the complex frequency vs. impedance curve at the output of the amp is going to make tuning them the proper threshold a little more difficult.

Properly done it will take one dual triode per channel. One triode for a nice cathode follower buffer before the detector and one triode for the DC amplifier to drive the meter. The cathode follower shouldn't present much of a load and if you're worried about noise you could hang them on the output of the driver. Although you'll need grid stoppers on the power tubes then otherwise the meters will clamp as soon as the power tubes go into grid conduction. and you'll never get any overdrive indication.

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 Post subject: Re: VU Meters
PostPosted: 26 Feb 2013, 03:18 
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Joined: 19 Mar 2012, 19:22
Posts: 168
Location: Wales, UK
Hi,

I am hoping to have them on the input because I would like to monitor the input signal. I have been looking for a simple circuit using valves to drive the meter but I couldn't find anything on the internet, but that's not a problem. I'm sure I can make my own design using tubes. There are loads of diagrams on the imternet for solid state design using op-amps so I could always build one of them if I can't get good results using tubes.

Thanks again for your help

Phil


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