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Old 2nd August 2007, 08:51 AM   #1
Dr_EM is offline Dr_EM  United Kingdom
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Default Different models of amp in bi/tri amp systems

There are certainly times when using different models of amp for the woofer, tweeter or mid is desirable. However, do certain models exhibit different phase shifts? By looking at the circuits how would you figure out the overall phase shift of an amplifier? For instance I may want to use:

http://sound.westhost.com/project19.htm

For the tweeter and:

http://sound.westhost.com/project101.htm

For the mid+bass. Would that potentially cause issue, or is the phase shift at these frequencies only ever going to be 180, 0 or 360? As far as I know it is only capacitive coupling that can cause a phase shift and usually this is only at much lower frequencies with the size of coupling capacitor concerned.
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Old 2nd August 2007, 09:49 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
arrange for both amplifiers to be either both non-inverting or both inverting (not recommended). That gets rid of the phase inversion problem.

There is a small amount of phase shift in all amplifiers. But it usually only becomes serious as the frequency approaches the F-3db cut-off points. We can usually ignore this.

Check the phase coming from the filters. They are usually non-inverting, although a few topologies are inverting.

Now the big Q.
What are the impedances and sensitivities of your drivers?

I am surprised at the difference in voltage drive you are planning to build into the two amplifiers.
50W into 8r0 is 28.3Vpk.
180W into 8r0 is 53.7Vpk.
That is quite a difference in voltage, 5.6dbV!
To clarify why I'm prompting you to think about sensitivity and voltage drive, I believe that the peak SPL from all the speakers in a music playing system (and this applies to film sound tracks as well) should be similar.
If the drivers all have the same sensitivity and the same impedance, then they should each be capable of being driven to the same peak voltage.
If the sensitivities are different then this must be taken into account when selecting the power amplifier voltage output.
If the impedances are different then the effective load determines how robust the output stages need to be (to be reliable).
Forget all those do-gooders that talk enormous power for bass. The best that can achieve is disco sound, unless that's your target.
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Old 2nd August 2007, 10:12 PM   #3
Dr_EM is offline Dr_EM  United Kingdom
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Thanks for your helpful answer

If I ever did do this I planned to use an XG18 mid and MDT33 tweeter with 86 and 92.5 db sensitivities respectively. With both at 8 ohm the tweeter theoretically only requires roughly one quarter of the power to produce the same output (I think). Rod himself also noted that average tweeter power was much lower than the mid or bass with normal content, although it clipping would be pretty awful so the headroom is worthwhile.

I should have mentioned I planned on using the P101 at a lower voltage to produce around 100w for the mid and at around +/-63v for the bass unit; although this is a more sensitive driver, (Monacor SPH-275C) at 90db so theoretically requires less power than the mid I have heard that bass (sub 250hz content) recorded in music and film soundtracks is around 10db louder than the rest in peak output. This is also something I've noticed when producing my own recordings, although the "trend" now is often to limit everything totally flat .

The filter would be the Linkwitz Riley 24db topology so phase shifts at filter level should not be a concern.
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Old 3rd August 2007, 04:27 AM   #4
anatech is offline anatech  Canada
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Hi Dr.EM,
Normally one would use at least the same make of amp, just different power ratings. I don't think it's that important to most people on average. The important thing is that you have a way to measure the output of the amp to be able to match sensitivities of the amps, or correct for your speaker mismatch there.

If one amplifier inverts and the other doesn't, who cares? What you then do is reverse your speaker polarity on that amp. All fixed.

One more point. Do not attempt to use the power meters on the amplifiers. They are not normally accurate, not even close.

-Chris
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Old 3rd August 2007, 06:14 AM   #5
suzyj is offline suzyj  Australia-Aboriginal
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It's a really good idea to use different models of amps to drive the different speakers in a biamp setup. Biamping allows you to tailor the performance of the amplifier to each driver, rather than having a compromise.

For example, you could design your tweeter amp for lower power, but faster slew rate, than your woofer amp.

Phase shifts should be negligible for most any audio amp in the audio band, so don't worry about that.

I'm currently building an amp to biamp my monitor speakers in my study. I'm using a home-built 50W mosfet amp based on the AEM6000 for the mid-woofer (a Vifa P13WH), and an amp using eight paralleled TPA6120 headphone driver ICs for the Vifa D27TG-35. The tweeter amp is only good for 10W or so, but this application uses a crossover frequency of 3300Hz (or thereabouts) so that's oodles.

The nice thing about the TPA6120 is that it's got a really stupendous slew rate. It'll do a 10V step in 5ns, so its intermods are practically non-existant, and it's got gain out past 100MHz. With the exception of being designed for 64 Ohm loads (and paralleling a bunch fixes that) it's the perfect tweeter amp.

One thing to be careful of is that the levels are set correctly. My tweeters are 6 Ohm, with 91dB sensitivity. My mid-woofers are 8 Ohm, with 88dB sensitivity. Taking the different impedences into account, and using my mid-woofer amp gain of 16 as the reference, I calculated that I'd need to set the tweeter amp gain to 9.8.

Then if I put 1V RMS in, I get 9.8V at the tweeter, and 16V at the mid-woofer. That equates to 16W into the tweeter, and 32W into the woofer, a difference of 3dB, so after their difference in sensitivity is adding in, it should be nicely flat.
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Old 3rd August 2007, 09:25 AM   #6
Dr_EM is offline Dr_EM  United Kingdom
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I had considered the different amps strengths and weaknesses to some extent but perhaps not as fully as I could have.

The P101 is perfect for bass with its very high damping factor and high current delivery, i'm using it in a sub with good results. I hadn't put much consideration into its use for mids beyond that it would be easy to make a second similar amplifier unit. By all reports its a good amp accross the board and has a decent output to get that low sensitivity mid going.

The Gainclone style amp specs with lower noise which I felt may be an advantage with the tweeter being most sensitive and also the typical "hissing" noise being most prominent at tweeter frequencies. Compared to your TPA6120 its slew rate isn't so impressive but looks pretty fast in general terms. I take it 50W is possibly excessive at above 2khz or so? The MDT33 supposedly handles 200W, thats an awful lot of treble power it seems

For calibrating I was actually considering mics. Presumably pointing a flat response mic at each driver while it is playing a tone in the middle of its range and adjusting each ones level will achieve a fairly flat response? I suppose variations in thier frequency response may upset this though. Either way, final adjustment will likely be made by ear.

I own a basic oscilloscope so I can check things like phase, amplitude, oscillations, power output etc on that to confirm everything is at it should be. Built in 1984, bought for 30 second hand; works fine though
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Old 3rd August 2007, 10:52 AM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
The MDT33 supposedly handles 200W,
I have never seen this tweeter, but I guarantee that it cannot handle 200W.

I suspect the spec said it is suitable for systems upto 200W.
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Old 3rd August 2007, 11:11 AM   #8
Dr_EM is offline Dr_EM  United Kingdom
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It certainly seems unlikely. It's not a spec I chose it for, its just a well regerded tweeter with a smooth sound:

http://www.morelhifi.com/products/pdf/mdt%2033.pdf

I don't like the overbright, exaggerated sibilence sound I seem to be getting woth my DM602's. I use the treble control to tame it but it's still excessive. I'm not sure how much of this is down to the way things are recorded now though but so long as the speaker handles it without getting all piercing/resonant sounding it should be ok.
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Old 3rd August 2007, 02:35 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Dr.EM
I don't like the overbright, exaggerated sibilence sound I seem to be getting woth my DM602's. I use the treble control to tame it but it's still excessive. I'm not sure how much of this is down to the way things are recorded now though but so long as the speaker handles it without getting all piercing/resonant sounding it should be ok.
it's more likely to be the treble driver or the treble amplifier than the source.
Could it be slight clipping of the treble frequencies? Again that would be down to the amp and too little voltage drive.
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Old 3rd August 2007, 03:04 PM   #10
Dr_EM is offline Dr_EM  United Kingdom
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Its good news that you also think its the system responsible, otherwise i'd never be very satisfied with a lot of music. The system can always be changed however.

I would be putting this all together right now if it wasn't for one thing. That is the space I have available. There really isn't room in my bedroom for it and the small space will not help the sound at all either way. It would be possible to accomodate by changing a lot of whats in there and moving things around but it really isn't ideal
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