Variable Gain preamp?

Nisbeth

Member
2001-01-29 9:58 pm
Denmark
Hello!

I am presently planning a preamp design based mainly on Rod Elliots project 88. Rod recommends using potentiometers on the inputs to compensate for different input voltages. I´d very much like some form of matching, because my sources have very different output levels which can be annoying. What I am wondering though, is if it would be better to make the first gain stage variable instead? Specifically make a first gain stage for each input and placing these before the input switch (relays in this case). The gain would the be made variable by putting a multiturn trimpot in the feedback loop. Is this absolutely the worlds worst idea or...? Is there anything I should be careful about if/when I decide to try it?

I´m not concerned about the extra components and the increased power consumption, only if this would be a better (albeit more complex) solution to the problem?


Any help appreciated!
 
This idea pops up about once every three or four months.
Varying the feedback seems attractive on the face of it, but you can get into several different cans of worms. One is that the sound quality will change with volume, depending on the amount of NFB. Another is the sticky question of stability--some circuit topologies will get pretty radical if you're not careful.
But, that said, it will work within limits. You won't ever quite get to no output, but then again most people don't bother with stereos just for whisper-level background music.

Grey
 
Nisbeth

Since you are using relays for input switching, instead of providing multiple input stages with the associated rf components etc, why not increase the number of poles on each relay. A single input stage after the input switching could then have multiple adjustable feedback resistors which could be selected as appropriate for the input.

I either case you will need to heed Grey's warnings, though I doubt there will be a problem if you keep the input stage gain to between 0dB and +6dB.

Geoff
 
I used similar technique in my preamp based on A75 front end. I had 10 relays next to ea. preamp module (which were potted) and could switch gain from 10dB to 20dB by changing feedback resistors. It worked fine. I would not recommend more than one gain stage in any preamp.
 

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Nisbeth,

One solution.......

If you know what your sources are prior to design, you could use simple passive voltage dividers and attenuate inputs to match your sources. This would require a handful of resistors.

Example: CD -12dB, Tuner -7.5dB, VTR -3dB, Turntable, -0dB.

Great in theory. The real world is that within each of the above sources, the source material will vary. CDs are pressed at the same "level" Radio stations broadcast at different levels. This makes the attenuation level an in-exact science.

Buffering each input with it's own level of gain is not a bad idea. If the buffer/gain stage has a low enough output impedence, you could drive the volume control directly without any need for a second overall gain stage, Ala the original Pass Zen pre-amp.

All this really depends on your application, budget and desired level of audio purity.

Regards,

Aud_Mot
 
HPotter,

My bad. You are correct.

I ment to say they are NOT pressed at the same level. Some Cds are "louder" than others. Thus any choice of attuenation/matching will not completely solve the problem. There will still be bounce and bang from one source to the next.

The best that can be done is to minimise the problem and fix it on an average level basis.

For me, I expect that I am going to be turning the volume knob after I make a source selection. I do not channel surf on my audio equipment, so the different levels are not a problem.

Aud_Mot
 
variable gain

Rod Elliott's Project 01

http://sound.westhost.com/project01.htm

has a topology for an active volume control with high input impedance and positive as well as negative gain, that can be used as an input buffer, and here is one by Siegfried Linkwitz

http://www.linkwitzlab.com/images/graphics/preamp1.gif

I have buit the Linkwitz version, using it as a volume control in my preamp, and I like it, I have the impression to get better dynamics than with a passive volume pot in front of a gain stage or buffer. Also this is the only "gain stage" in my preamp.

An inverting stage as by schematics above can take a variable source impedance (to control gain in the feedback) without much penalties - the only one I can think of is that you need an inverting buffer at the other end, that can conveniently be your output buffer. I never quite understood why the topology of a passive divider (read, volume pot) in front of a non-inverting op amp is so popular when at the same time for any other non-inverting op amp stage you're supposed to match source impedance to the // Z of the feedback R's - so you do all this adjusting with your circuit and then at the input of it all there sits this volume control pot with variable Z(in) of say, 0-20k in front of a non-inverting buffer.

With an inverting stage, varying source impedance to adjust gain will get you less distortion and less bias current mismatch compared to a highly variable source impedance in front of a non-inverting stage. Or so I read in various application notes, op amp data sheets, and all.

These above topologies can be adjusted to your needs in terms of gain spread. Just recalculate. Other gain adjust circuits:

http://www.linkwitzlab.com/filters.htm

look up item 10.

The sound quality differences due to variable loop gains, at these gain spreads (say +/- 6 dB), and for an inverting stage, I would doubt you can hear it. I certainly can't hear a change other than volume ;-). I think you gain much (sic) by having one input buffer with a defined input impedance and a later, inverting active gain control for level matching. I think this is a much better trade-off than using a passive divider.

MBK