Cheap line amps for gain staging? - Home Theater Forum and Systems -

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post #1 of 5 Old 04-09-14, 11:16 AM Thread Starter
StanDingwave's Avatar
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Cheap line amps for gain staging?

Hi all, I don't usually post in this forum, but here's my situation and considering recommendations.

My system is basically a 2.1 audio-only. I run a source (DAC) to a stereo preamp (actually a mini headphone tube amp that works well for the purpose) -> MiniDSP 2x4 for highs/lows. In this case, "lows" are my DIY subwoofer of the month, effectively a mono (.1) set-up; the "highs" are actually two (or four) Bose-901 II that are EQ-ed via the MiniDSP. Amps are a pair of Behringer iNuke NU3000. I've tried various schemes over the months and I still cannot get enough gain at the amps. I am limited especially by the MiniDSP low in/out levels, around 1 V. I already push the hottest signal to the MiniDSP's inputs that I can, so any gain must be added after it.

So here is my scheme of the day, and I ask your input:

I need maybe 20 dB more gain before the power amp. Why can't I do this: take a "normal" stereo preamp and use it effectively as a unbalanced to balanced converter? For a single input, put it to a Y-splitter. I make a custom XLR cable for the RCA outs, common ground and separate "hots" for the balanced side. I need to invert one of the outs and in the MiniDSP I can do this. I already have done similar for the sub, to get a hotter signal. There should be llittle added noise because the balanced line should cancel out most of the added noise.

I'd like to avoid It seems any pre would work for this "West Virginia" (jerry-rig) scheme. The only sacrifice part is making the custom RCA pair -> single XLR cable(s). A used HT or (God forbid) a 1970s quadrophonic pre would be ideal, but I think those with RCA in/out are pretty rare.
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post #2 of 5 Old 04-11-14, 10:08 PM
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Re: Cheap line amps for gain staging?

Not sure I get the purpose for the XLR cable for RCA outputs (???). It sounds like you intend to take the hot side of two RCA outputs and wire them to the signal pins (2 and 3) of an XLR connector? Not sure if that would even work, but if it does it’s not a true balanced signal so there will be no noise cancellation.

A pre amp can certainly be an unbalanced-to-balanced converter - if it has balanced outputs. However, if the pre amp in question only has RCA outputs – naturally, that is not balanced. And you have to keep in mind that a pre amp is not a signal booster. Its volume control is merely an attenuator, meaning the maximum signal it passes will be no hotter than the input.

There is no free lunch; any signal boosting scheme will increase noise. The only question is whether or not the increase is enough to be objectionable. As explained my gain structure article, the best way to deal with inadequate signal issues is to simply get front end electronics with enough signal output to drive the amplifier in question. Of course this can be tough to figure out with the iNuke amps, since no one knows what their input sensitivity is.


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post #3 of 5 Old 04-12-14, 12:04 AM Thread Starter
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Re: Cheap line amps for gain staging?

I once emailed Behringer to ask the sensitivity. they told me and I ... forget! Anyway, I see the flaw in my preamp idea. What I did for the bass (mono) was to take the unbal outputs (1 and 3 in my case) of the MiniDSP and wire the hots from each channel to the hot, neutral of a XLR. Then I inverted polarity of Ch.1. Goal here was not noise cancellation but to give a hotter signal to the amp. I think that is a valid approach. While I would not get the noise cancellation (except, possibly any added by the preamp circuits) I see no reason I couldn't do the same with a "normal" preamp, as long as I have a way to invert polarity of one of the pair, something most pres don't offer.
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post #4 of 5 Old 04-13-14, 08:49 PM Thread Starter
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Re: Cheap line amps for gain staging?

I think I've found a good solution to my problems. After much surfing (and this isn't the first time) I glommed onto this:

Among other things, this caught my eye: "Usable as 6 in/6 out level matching amplifier". A quick check of the manual shows balanced or unbalanced ins/outs, up to +15 dB gain, a very high max in/out level (more than I need I'm sure). For $99 street price, 3-year warranty, I think I'll take it. While this isn't its intended use, I don't see why it couldn't serve as a stereo (or quad or hexaphonic?) line amp. I plan to use this to boost the paltry 1 V signal the MiniDSP puts out. If noise is an issue I may order a 2x4 Balanced model. This might be the opportunity to use the optional analog (?) level control the MiniDSP has and then I'd dispense with the "normal" stereo preamp that currently feeds it.

Now, I know that Behringer is not to everyone's liking. I know their stuff is designed in Former East Germany by a secret team of Neo-Nazi scientists, and rather than pay the high wage scale of the EU (the unit would probably cost $1000 otherwise) the units are made in a North Korean prison camp and then smuggled under cover of night to Red [oops, my age is showing] China where young orphans forced to work 72-hour weeks box them. Finally, they are shipped to the USA and the Chinese buy more of our National Debt Slightly more seriously, I've had good luck with three B products over the years (save a NU6000 that recently died but still sold for about $120 on Ebay.

And best of all, unlike their amps I am sure it has no fan for me to replace and void the warranty

Last edited by StanDingwave; 04-13-14 at 09:00 PM.
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post #5 of 5 Old 04-20-14, 09:23 PM Thread Starter
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Works well, looks like hell

A few days into using the MX882 as my preamp, it is working quite well. Noise doesn't seem to be an issue. It definitely is an unconventional line stage but it works well in that application. And I have three more channels if I need them, and a stereo pair additonal (I am unclear on that part, and have no reason to dig into the manual right now.) I think the MX882 can act as a stereo amp too. I am not clear if the 6 "mono" channels would mix into the stereo pair or not. Sometimes all you wanted is a tooth pick, and depending on the model, yes the Swiss Army Knife has one, if it hasn't gotten lost

Here is a photo of my current system for your amusement
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amps , cheap , gain , line , staging

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