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Old 21st August 2006, 06:16 AM   #1
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Default Best way to match levels on sources

What is the best way to match gain on source components?

I use a remote controlled switch box to select AV sources, and the TV/VCR audio is a lot louder than CD. I'd like to reduce the gain on the VCR so the level is similar to CD level.

First I'd like to try passive - is this just a matter of using a pot?

1) Are there any special requirements for that pot?

2) I have 3 pins per gang on the pot and a + and -ve on the cables - how do I hook them up?

3) What value pot should I try. TV runs at 20% volume, CD 30% for about the same level. I want to match them.

Active

4) If this doesn't work, can anyone suggest options for an active circuit. What is the simplest option here?

5) What are the pros and cons of active vs passive in this situation?
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Old 21st August 2006, 07:17 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
passive will work just fine.
I would only go active if one needed gain on any input, or one already had a poor impedance match.

Similarly, I would avoid a pot as a permanent solution, although it will allow experimentation to find the correct attenuation for your average sound level from each source.

Buy a 10k pot insert it into your interconnect cable near the amplifier end.
Connect the screen on the input and output cable ends to the lower end connection on the pot (one of the outside pins).
Connect the input core to the other outside pin.
Connect the output core to the wiper (middle pin).
Plug in to your source and amp and switch on.
Adjust the pot until you have a match between sources.
You may want to do this to both the interconnects so that you can balance a stereo system.
Now, buy or borrow a digital multimeter (DMM) and set it to 20k resistance range.
Disconnect the interconnect at both ends but do not adjust the pot.
Measure from outside pin to outside pin. This should give a reading of about 10k.
Measure from wiper to bottom of pot (screen). As a check measure from wiper to top of pot. These last two readings should add up to about the first number you thought of.

Now, divide the second reading by the first reading (about 10k). The dividend (less than one) is your attenuation. We will not complicate this, today, by talking about decibels.

Make up a resistive divider for each channel to mimic the ratio you have just calculated. You need two 1% 250mW to 600mW resistors for each channel and they cost pence. The two values you need are the second reading and the third reading (not the 10k).

They are small enough to fit inside the interconnect RCA plug that goes into the amplifier input socket. You need to mark the plug so that it does not get swapped end for end or mixed up with a standard straight through interconnect.

Have you got access to a soldering iron?
Good, let's go on.
Attach one resistor (the second reading) to the interconnect core and solder the other end to the RCA pin. Solder the second resistor(the third reading) to the RCA pin and the other end to the RCA ground. Solder the interconnect screen to the RCA ground. You now have two connections to the RCA pin and two connections to the RCA ground. You may need to slip a short length of insulation tube over one of the resistor leads to prevent shorting.

Slide the RCA cover back on and screw up tight.
connect your cables and switch on.
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Old 23rd August 2006, 12:38 PM   #3
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Great answer! Thanks! Very very helpful.

I have tried the first part, using the pot. With a quick test it sounds decent. Now it's not so simple as I had thought.

The problem is when I watch a movie with the volume at 50% then switch to TV and it's blaringly loud. This also would happen with the tuner (even worse), and with CDs. I'd prefer to not have to always think to turn down the volume before switching. But suppose I reduce all of those sources to match the average level of a movie, then when I want to get a CD up loud, I will have to turn up the volume perhaps even full tilt. Is there any problem with doing this?

The problem I have then (potentially) is that if I go from a CD at full tilt to a movie, the system will then be pushed WAY too far if a loud part comes on first.

I think I might come up with a compromise and match the average levels of a movie at 30% - so everything else matches at this point. Then when I go from a movie at 50% volume to TV (which should be at 30%).

Does anyone have any ideas?

For the time being, I think I may delay wiring up those resistors until I have worked out what I think I will stick with.
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Old 23rd August 2006, 12:59 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
you can use a different attenuation from each source. They all have their own interconnects.

Some pre-amps have exactly this;- the ability to set sensitivity on each input. You can mimic this in the RCA plug. Just don't mix the modified cables with each other (good labelling).
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Old 23rd August 2006, 01:44 PM   #5
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Is there any reason why not to keep a pot rather than just use fixed resistors?
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Old 23rd August 2006, 01:59 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
accidental misadjustment.
lower quality sound output.
cost.
unmatched channels.
space utilised.

Pots seem to lose out in almost every respect.

Flexibility is a major advantage, but referring 1 and 4 from above this same flexibiltiy also introduces the risk of getting it wrong.

Buy one pot and use it to measure your nearly permanent requirements.

Each stereo attenuator needs only four resistors. Four 1% metal film resistors will cost you about 20p when bought in small quantities.
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Old 27th August 2006, 12:41 PM   #7
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Lets see if I understand this now ...

Attached image shows my interpretation.

R1 is the outside pins
R2 is between +in and +out
R3 is between +out and both -ve

Is this correct?
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Old 27th August 2006, 06:09 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
point us to the image.
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Old 28th August 2006, 01:34 AM   #9
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Argh ... forgot to include it!!!

Click the image to open in full size.
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Old 28th August 2006, 07:11 AM   #10
AndrewT is offline AndrewT  Scotland
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Hi your left diagram showing the pot is correct and it shows the two measurements you are going to take:- R2 & R3.

The right hand diagram is incorrect.
R2 is inline as you have shown (+).
R3 goes from line (pin +) to ground (-), not in the ground wire.

Try not to call AC signals + & -. Use hot and cold, or flow and return, or pin and ground (when on an RCA Phono plug). + & - are reserved for DC voltages.

The ground can be referred to as common, earth, 0V(zero volts), audio ground, depending on the context of the discussion. Each can have subtle differences in meaning and this changes with country of origin, so it's not in tablets of stone.
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