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Old 12th December 2008, 04:34 AM   #1
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Default The new "My Ref" Rev C thread

Since there is an active group buy coming to fruition, interest in this topic will be high in the coming weeks. While the original thread contains a LOT of information, it is exhaustingly long. Hence, this new thread for those of us about to depart on the "my audiophile LM3886 approach" adventure.

I have a question regarding transformer selection because this is new territory for me.

Suggested xformer is 24v, but the widest VA selection seems to be among 20, 22, or 26v. I will use this amp in monoblock configuration to drive the highs and mids of a speaker system with active xover between bass and mids/highs. The impedance of these speakers is slightly more than 4 ohms at its minimum. Should I select transformers that are slightly higher or slightly lower than the suggested 24v?

I'm looking forward to learning a lot more about building this amplifier and its optimum configuration.

Peace,
Tom E
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Old 12th December 2008, 09:31 AM   #2
AndrewT is offline AndrewT  Scotland
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consider each amp and load separately.
What is the bass driver impedance (not Re nor minimum on the graph)?

What are the mid/treble driver impedances? Is either driver padded to equalise sensitivity?

Do you want to use independent supplies for each amplifier? or must you use a common transformer for a pair, or for all four amplifiers?

Now decide whether the voltage you require is suitable for each worst case load.
If there are any 4ohm drivers in your 3way then 24Vac is far too high.
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Old 12th December 2008, 04:13 PM   #3
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Andrew,

I appreciate your expertise in these matters, and I value your assistance. I'll try to not ask too many dumb questions, but I feel this issue is vital to success.

The amps will be run monoblock, with one transformer per channel (two xformers total for stereo). There will be no bass drivers on these amps, as the system is actively filtered between bass section and mids/highs section, with a bigger amp for the bass.

In each mids/highs section, there are two midranges in parallel. The minimum impedance of each driver is 5.6 ohms, with 0.4 ohms DCR in the xover inductors. In addition, I might need to pad the mids a bit, but I haven't determined that yet. The single tweeter is 5 ohms minimum.

Yes, I am afraid 24v is too high, and that's why I was asking for advice. Perhaps this amp isn't suitable at all? Now would be the best time to find that out!

Peace,
Tom E
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Old 12th December 2008, 07:54 PM   #4
frog993 is offline frog993  Germany
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Hello guys

I have the same question for my Ref-C amp because my speakers have a small dip to 3,5 ohm between 50-100hz.
I have attached a pic with the simulatet resitance.

I hope this is not a no go for this amp.
If this so, i change the crossover. I dont want to make partys with this amp, its only for my workingroom so i dont need much power.
But the question is what is the best secondary rating for those speakers with this amp.

Hope for answers,

greets Udo

Edit: Sorry, wrong picture. This is the right one.
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File Type: jpg imp_skyline.jpg (87.9 KB, 9330 views)
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Old 13th December 2008, 08:16 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi Frog,
I think that impedance plot is telling us that the manufacturer used a 4ohm bass/mid and an 8ohm treble driver to create a 4 to 8ohm speaker.
You should design the amp to drive the 4ohm part of the loading.
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Old 13th December 2008, 01:56 PM   #6
frog993 is offline frog993  Germany
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Quote:
I think that impedance plot is telling us that the manufacturer used a 4ohm bass/mid and an 8ohm treble driver to create a 4 to 8ohm speaker.
The manufactor are myself, the chassis from Visaton. Bass are two 13cm 8ohm in parallel,mid one 13cm 8ohm(the same as in the bass) and treble are 8ohm too.
The crossover createt by myself with Boxsim(simulationtool for Visatonchassis).

Quote:
You should design the amp to drive the 4ohm part of the loading.
That means for me a transformer with 18V secondarys but works a RefC with 18V secondarys ? Recommend for RefC is 20-24V.
Russ White wrote that the RefC have no problem to drive 4ohm speakers but with wich transformer ?

Greets Udo
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Old 13th December 2008, 02:27 PM   #7
AndrewT is offline AndrewT  Scotland
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to design a 4 to 8ohm speaker to be driven specifically by a chipamp is a bad design decision.
But, having made that poor decision, you must now design the chipamp to drive the 4ohm part of your complex load. i.e. 18 to 20Vac.

There is an alternative.
Split your passive crossover into three sections. Two identical bass/mid sections each driven by their own chip amp and a treble section driven by it's own chipamp. Each of the three chipamps is driving an 8ohm load and thus can be designed for that duty. i.e. 24 to 26Vac
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Old 13th December 2008, 03:13 PM   #8
frog993 is offline frog993  Germany
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Quote:
Two identical bass/mid sections each driven by their own chip amp and a treble section driven by it's own chipamp. Each of the three chipamps is driving an 8ohm load and thus can be designed for that duty. i.e. 24 to 26Vac
If i understand you correct, that means i need 6 mono amps for stereo ?

And i cantt split the bass-mid section with 3 amps, if so i will need 4 amps for one speaker for an 8 ohm load. In the bass section are two 13cm chassis parallel and for the mid is another 13cm chassi. Its a 3-way speaker/crossover with four chassis.

Here a pic from the crossover.

Udo

P.S. The speakers not finished yet, and changes in crossover are no problem.
The orginal speakers are found here:Skyline
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File Type: jpg skylineweiche_mk_2.1.jpg (37.2 KB, 9210 views)
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Old 13th December 2008, 03:51 PM   #9
frog993 is offline frog993  Germany
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I have thing about your post, i was a little bit confused.

You mean to drive every speaker of the bass-section with one amp and the mid-high part with another amp ?.

Another solution where to drive the two bassdrivers with one amp at 20vac and the mid-high section with one amp at 25vac. Then i will only need two amps for one channel.
Or i try to drive the complete speaker with one amp at 20vac secondary.

But when i read this from Russ White, Im a little bit confused.

Quote:
Protos, I am driving 4 ohm 87db speakers very very hard right this moment, and have been for the last 2 hours or so. My IR thermometer registers 47C at the case. My heatsink is not huge, 4" x 5-3/8" x 1.5" for two channels. If yours are getting very hot I wonder if your HS are too small, or your amp is oscillating or something.
Its from page 91 at the orginal MyRef thread.

I think i will try it with a 20-22vac transformer.

Greets Udo
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Old 13th December 2008, 04:01 PM   #10
AndrewT is offline AndrewT  Scotland
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the two bass/mid drivers in parallel is the problem. They are an effective 4ohm load.

If you drive the whole crossover with that 4ohm load on the other side then the amp must be designed to drive a 4ohm load.

If you can remove one of the bass/mid drivers then the crossover has two 8ohm drivers on the other side and the amp can be designed to drive an 8ohm load. But you still need an amp to drive the extra 8ohm bass/mid driver. This can be added to make a 2.5way speaker or it can be added as an identical bass/mid as the first with an identical low pass crossover driven by it's own amp.

There is yet another alternative.
Keep the parallel bass/mids and design a parallel chipamp to drive the combined 4ohm load. Each chipamp sees an effective 8ohm load and is designed as such. This is my least favoured solution.

Are the 3 BF8 drivers identical? Why roll off one of them to roll back in two drivers lower down? Throw away on BF8. Keep one as a bass only and the other as a bass/mid, this gives you that 2.5way mentioned earlier.
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