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Sonic Impact bi-amping
Sonic Impact bi-amping
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Old 2nd August 2007, 11:43 AM   #1
mustik is offline mustik  Finland
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Default Sonic Impact bi-amping


I'm quite a newbie in audio world and having recently purchased what I considered pretty good pair of speakers (Quad 21L), I was looking for
and have couple of questions. I recently purchased the SI t-amp (actually two of them). I'm planning to do the basic mods later. Meanwhile, I was thinking of bi-amping my speakers with these. From what I've gathered, there's basically two way of doing this: 1) As my speakers allow bi-wiring, I could run tweeters from one amp and drivers from the other or 2) wire each speaker from it's own amp, f.ex. using L for tweeter and R for driver (see picture).
Now, would option 1) be better in terms of sound quality: in case the amps sound different in some way, the difference would be equally distributed among the speakers?
Should I power each of the amps with dedicated power supplies? What are the potential problems I should address before trying this?

In the attached picture I have put a preamp, which I don't have yet, so at this point volume would be controlled indepently using t-amps' original pot.
Later, when I actually have a preamp, t-amps' pot would be by-passed and pre-amp's volume control used instead.

Which brings me to my final question for the time being. Do you think a tube pre-amp would work nicely with the t-amp(s)? As I'm on a very limited budget right now, the options would be something like TCC TC-754 (cheap passive preamp) or TubeHead (cheap simple 12VDC tube stereo pre-amp).

Btw, my SI t-amp and Quad 21Ls (88db/w, IIRC) sound very nice together most of the time and I can get decent amount of decibels from it. I can't wait what the input cap mod will do to it, as the bass is quite limited.
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Old 2nd August 2007, 11:54 AM   #2
Saturnus is offline Saturnus  Denmark
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You should use one T-amp for each speaker thus elimination the quite significant channel crosstalk in the T-amps and at the same time load balancing the PSUs if you use 2 which I'd recommend.
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Old 2nd August 2007, 12:00 PM   #3
sreten is offline sreten  United Kingdom
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I would drive each speaker with its its own amplifier, this shares
power supply juice more evenly than amplifiers for bass and treble.

On a budget I'd always go for a passive pre-amp and short cables.

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Old 2nd August 2007, 12:03 PM   #4
atleer is offline atleer  United States
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I too am interested in hearing some info back on your questions, I have an almost identical scenario going...
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Old 2nd August 2007, 12:03 PM   #5
atleer is offline atleer  United States
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I've been looking at that Tube Pre and others on there site, but a little leery of purchasing from an overseas website...

Has anyone had luck with that?
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Old 2nd August 2007, 12:16 PM   #6
AndrewT is offline AndrewT  Scotland
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I have gone for a stereo amp on each speaker.
I think it works well for passive bi-amping.
It allows very short cables into the crossover, saving cost and voltage drop.
I think one of the big benefits is the reduced load on each amp channel. This allows each amp to deliver better performance.
I also think that using a single transformer supplying two channels of amplification works better if the demand from each amplifier is different. i.e. feed a treble driver and a bass/mid driver from each stereo amplifier.
Thus, I recommend diagram B.

This is not a cost saving exercise.
More costly is a monoblock amplifier to every driver. Some do that and report improvements.
regards Andrew T.
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Old 2nd August 2007, 05:42 PM   #7
adason is offline adason  United States
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Hi Mustik,

in my opinion it does not matter too much since both of your stereo amplifiers are the same. You should have got one low power highly refined stereo amp (or better two monoblocks) for mids/tweets and one big high current stereo amp for woofers (crosstalk is less important here).

Since you have already got two of the same amplifiers, try both versions and report back. You are the judge.

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Old 2nd August 2007, 06:13 PM   #8
gofar99 is offline gofar99  United States
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Greetings everyone, No where in the discussion do I see any crossovers. I have a pair of Super Ts that get used on occasion (usually I use tube gear). A set up I have found that works well is to use a preamp (your choice of SS or tube) driving an external crossover and then the outputs going to the T amps and to your speakers. I found an electronic crossover that is in kit form and works well and is cheap to boot. Try Marchand Electronics in New York (they are on the web). Their cheap kit is a 24 db/oct hi and low. If I recall correctly it's about $25us. Well made and only needs a case and power supply. Others will certainly work as well. The set up allows for single volume control with the woofers and tweeters separately adjustable. To do this you use one T amp for tweeters the other for the woofers. I have not found crosstalk to be an issue.

good Listening
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Old 2nd August 2007, 09:10 PM   #9
Pano is offline Pano  United States
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Sonic Impact bi-amping
Ahhh.... the joys of biamping/multiamping.

For your setup I'd go with what sreten and others have mentioned, use one amp per speaker. Use separate power supplies. (There are ways to optimize this, see below).

When you start to multi-amp there are soooo many ways to do it. What we are recommending to you is the "Vertical Passive" method.

Why Vertical, why passive? Let's look at a typical professional sound system. It will be a horizontal active system. It works like this:
  • Source signal goes to active crossovers. Signal is split into Hi/Mid/Lo for both left and right.
  • These 6 signals (H/M/L -right H/M/L - left) go to 3 stereo amps. Thus 6 channels of amplification.
  • The Lo signal goes to a big stereo amp (say 1200 W) to drive the left and right woofers.
  • The mid signal goes to a 500W amp to drive the left and right mids.
  • The Hi signal goes to a 120W amp for the tweeters.

The advantages of a system like this is that you can choose an amplifier that best suits the driver it is connected to. The active crossover divides the signal into parts, so each amp get only that part. There is no crossover in the speaker, because it's been crossed before the amp. You will typically see a rack like this with the big bass amp at the bottom, then mid amp above, then hi- with the crossover at the top. It's called horizontal because the layout is across the amps.

In smaller home systems an active crossover is often not used, because the speaker already has a built in, passive crossover. So you simply split the full signal into each amp channel and let the passive crossover do its work.

By going "vertical" you will use one amp per speaker. So that amp does not do left and right, instead it does Hi/Lo. Its brother on the other side does Hi/Lo for the other channel. Of course, you will also see active systems that are vertical.

Advantages? Since the amp channels all have the same gain and power, you don't need to worry about gains or volume matching. The signal going to the passive crossovers will be the same volume for Hi/Lo, which is what the crossover was designed for. No worries about turning the tweeter up too high or down too low, it is automatically matched to the woofer. You also get plenty of left/right channel separation by using separate amps per side..
Take the Speaker Voltage Test!
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Old 2nd August 2007, 10:07 PM   #10
Pano is offline Pano  United States
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Sonic Impact bi-amping
Default Multi-amping part 2

Before getting into active crossovers, there is still a lot that can be done with multiple amps and passive crossovers.

Let us suppose that you have a 2-way speaker and two stereo amps. One amp is a big powerful brute, the other is a sweet little T-AMP (tube or Tripath).

You might want to run your woofers with the brute and use the more "refined" amp on the top. Can you? Should you? Yes. Here's how:

You need to split your signal somehow. A preamp with 2 outputs, or a simple splitter cable, perhaps. Then feed the split signal to the big amp for your woofers, small amp for your tweeters.

The problem here is that the gain of the amps will not be the same. The big amp is likely to have much more gain, i.e., it will put out a much higher level than the small amp for the same signal in. What can you do about it? You can turn the tweeter amp volume up all the way and the bass amp down all the way. Control the overall volume with your preamp. Now slowly increase the volume of the bass amp until you have a level that brings things back into balance. You can do that by ear, but it would be better to do it with test tones and measurements.

Now you have a horizontal, passive, bi-amped system.
Can you make it better? Yes!

If your speaker has a level control for the tweeter, you're in luck. In a typical system, the tweeter (or midrange) is much more efficient than the woofer. So the passive crossover will have to throw away some voltage (and power) to bring the tweeter down to the level of the woofer. Usually this is done with a resistor divider network. If the divider is adjustable you can turn up the tweeter volume. That way, you are not throwing away power. Your tweeter amp does not have to work as hard and will now have more headroom and will be less likely to clip. A good thing!

Proceed as before with the tweeter amp wide open (or almost) and bring in the bass amp to match. I often use this system at home with very good results.

Can you make it even better? Yes!
You can do a bit of passive filtering in the amp. For the moment will will just do the tweeter amp.

The stock Sonic Impact amplifier has small input caps that limit the bass. Replacing them with larger caps for better bass extension is one of the basic mods. But what if the amp isn't running bass? Why amplify it if you don't need it? Stay with the small caps, or use ones that are even smaller. You will be creating a 1st order high pass filter at the input of your amp. In fact, it's already there, you can just move the frequency up.

As a rule of thumb, the T-AMPs use a 2.2uF input cap for full bass extension. Don't need 20Hz? Divide by 10. A 0.22uF cap will be good to below 200Hz. A 0.02uF for 2Khz, and so on. (These are not the actual cutoff frequencies, but are a good bit higher so as not to interfere with your passive crossover slope.)

So now your tweeter amp is not amplifying the bass section of the music, which it does not need. Result? More headroom. You can get more level out of the mids and highs because the bass paart of the music isn't driving the tweeter amp to clipping. A good thing!

That may all be as clear as mud, but don't hesitate to ask if you have any questions.
Take the Speaker Voltage Test!
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