Changing output or input signals to match impedance.

[treitz3]

Hello folks, treitz3 here and I have stumbled upon a brick wall that I need some assistance on. If anybody either has information on where to go or information that can help me into what I would like to accomplish, it will be appreciated as this is the last step in designing my entire rig.

Here's the deal, I have a Carver C-16 preamplifier that has two audio outputs. One of them is emulated to sound like a tube output stage and one of them is used as an SS output. Currently, I have the tube emulated output running my 8 ohm Focal 807v's and the SS output running my 4 ohm Tyler acoustics Taylos 7U's in a dual amped, dual speaker'd stereo system. Unconventional, I know, I know but it works for me.

What I would like to do is reduce the signal output on the tube emulated out to best give the same [end result] volume out of both speakers. Unfortunately, the schem's and service manuals are part of the library that the Carver forum does not have in their possession at this time which definitely throws a wrench into the project.

The end result can be a fixed output or variable output**, but I would like the signal to have the best resolution as possible as the end result. All work will be performed by me and I will provide pic's along the way. Does anybody have any suggestions as to where to start knowing what you know now?

Or, would it be easier/wiser to just adjust the input of the amp powering the Focal's [Carver M4.0t] without degradation of the signal itself?

**Variable output preferred if possible.

[treitz3]

What I mean by variable output is that I would be able to adjust the line level to a certain point and once the speakers match DB levels, then the volume would adjust both outputs like normal.

[Face]

Have you measured both output voltages? What makes you believe the impedances are different?
As for matching impedances: http://www.mitcables.com/?option=com...=406&Itemid=29

[ShinAce]

I don't get it. If each pre-amp output goes to a dedicated amp, why is there a volume problem?

If you just want to lower a line output, a volume pot will do the trick.

[treitz3]

No problem with the pre or the amp. The problem stems from the 8 ohm Focal's having a louder volume than the 4 ohm Tyler's. All I want to do is equal the volume so they are at the same level.

Any suggestions other than what I'm asking?

[GV#27]

If you just want to lower a line output, a volume pot will do the trick.

Yeah,a good quality dual ganged pot before the inputs on the amp driving the Focals should do it. try 10-25k.

[shadowofnight]

Damn I actually have a C-16 here AND the owners manual and I didnt know the 2 outputs ( Main1 and Main2 ) were different. I cant remember which one I used to drive my Silver 7t mono amps with ( All are disconnected now waiting for my new HT to be built this summer )

Since the Silver 7t's already have a vacuum transfer function inherently built in would I use the Main 2 output to drive them ?

I can play with the C-16 tonight and see the differences in output voltage in relation to volume control inputs.

Reading the manual it states the output impedance is 1K ohm , and that you can use the pair of main outputs to drive several amplifiers at once.

[treitz3]

Yeah,a good quality dual ganged pot before the inputs on the amp driving the Focals should do it. try 10-25k.

That's what I was thinking, but placing it right at the output of the C-16 and placing the pot on the back plate in a convenient area for adjustment. Do you guys see an issue with this?

Would it degrade the sound in any way? That's my main concern.

[treitz3]

Damn I actually have a C-16 here AND the owners manual and I didnt know the 2 outputs ( Main1 and Main2 ) were different.

Main 1 is tube, Main 2 is SS FYI.

[ShinAce]

What I would like to do is reduce the signal output on the tube emulated out to best give the same [end result] volume out of both speakers.

1) If there are 2 amps, use the volume controls on the amp to compensate. Volume at max is a nice ideal, but not always appropriate.

2) If you insist on max gain amp volume, you can install a volume pot to the tube pre-amp output. Another option would be an op-amp with less than unity gain. This would be nice because of the load buffering it provides(which is useless here), but complex because you need to measure the SPL difference between the speakers and design the amp circuit. You'll also have to tap into the Carver's power supply and add a circuit board in there.

Just drill a hole on the carver, mount the pot. Hopefully, the RCA outputs are wired with leads that can be cut for you to add extensions and wire the pot.

[GV#27]

placing the pot on the back plate in a convenient area for adjustment. Do you guys see an issue with this?

You could mount the pot into one of the nice Hammond cases( http://www.hammondmfg.com/1455.htm ) and use 4 high quality RCA's.Ofcoarse thise would mean extra pair of IC's.

Would it degrade the sound in any way? That's my main concern.[/QUOTE]Not if you use a good conductive plastic pot like an Alps or Noble.Even better would be a stepped attenuator.With it you would only be adding one resistor in series with the signal path.

[treitz3]

1) If there are 2 amps, use the volume controls on the amp to compensate. Volume at max is a nice ideal, but not always appropriate.

The amps have no controls. One is a Carver M4.0t and the other is an Odyssey Stratos amp. In theory, I don't want to add gain....just adjust the input signal down on the M4.0t [the one that feeds the Focal 8 ohm speak's].

[treitz3]

You could mount the pot into one of the nice Hammond cases( http://www.hammondmfg.com/1455.htm ) and use 4 high quality RCA's.Of coarse this would mean extra pair of IC's.

I'm trying my best to avoid more IC's to the signal path. K.I.S.S. is what I'm attempting to keep this mod to achieve no degradation or alteration in sound beyond the volume adjustment going to the Focal's.

[ShinAce]

I'm not sure if this is true for that amp, but there might be a gain adjustement trimpot(screwdirver adjustable) hidden on the back. Usually used to balance left and right, but usable here.

Less than unity gain means an attenuation. Even with an op-amp, you can make it attenuate the signal.

[treitz3]

It would have to be hidden within the amp. I do believe I have the schem's for the Carver amp. I'll have to check. Thanks for the help fellas, BTW.

[GV#27]

I'm trying my best to avoid more IC's to the signal path. K.I.S.S. is what I'm attempting to keep this mod to achieve no degradation or alteration in sound beyond the volume adjustment going to the Focal's.

Yep I hear yah,this could be an option if find it impossible to install within the pre amp.Installing a pair of small Bourns pots into the amp is also an option.

[bikerboy]

Wouldnt he be able to use one resister per side soldered to the + side of an IC and not have to buy a volume control? Start at 100 ohm and go from there. Or is there more to it?

[GV#27]

Wouldnt he be able to use one resister per side soldered to the + side of an IC and not have to buy a volume control? Start at 100 ohm and go from there. Or is there more to it?

The series resistor would have to be many times the 100 ohms which will increase the output impedance and would require having a lot of different values and experimenting to get the desired attenuation.Using a pot or stepped attenuator (which is really a voltage divide)makes for easy adjustment and results in less series resistance so output impedance is not raised as much, although it does change with pot position.

[treitz3]

Had another thought. What about an autoformer? Does anybody know where to get one at a reasonable price?

[treitz3]

Ok, how about something like this.....Niles Speaker Volume Control?

Anybody with experience?

[GV#27]

Ok, how about something like this.....Niles Speaker Volume Control?

Anybody with experience?

But they are for hi power low impedance usage not line level.(Unless ofcourse you meant for use on the speakers).I think the pot or stepped attenuator is your best option.

[jakelm]

In-Line attenuator. 3dbs should do the trick.

[treitz3]

http://www.partsexpress.com/pe/showd...number=266-240

I like the reviews of the Rothwell attenuators but from what I can see, they are only offered in a 10db drop.

After further thought on this, wouldn't a variable attenuator be a wiser choice of all the options given due to.......and I'm thinking out loud here......an ohm rating for the Focal's and the Tyler's that may not be exactly 4 and 8 ohm, the different sensitivity between the two speakers and the difference [if any] between the input sensitivity of the Carver 4.0t and The Odyssey Stratos? Or am I off base?

[ShinAce]

Once the difference in loudness is measured, it doesn't need to be variable.

In this case, the best bet is a variable solution in case the speakers change.

Just because it's called a pot doesn't make you a stoner. Now get to soldering.