Yes, and you need the V2 with diodes bypassing the source resistors, for higher current peaks.
There's also the v3 with three pairs of output devices. According to the diystore,
one set of PCBs will make either a stereo F-5T v2, or a single monoblock F-5T v3.
But, you may not need the three pairs per amp, since two pairs will give 38A peak.
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Great - I need 2 monoblocs ... so I can use 1 board for each one if I build F5T v2. As you say, that current of 38a should be ample. 🙂
Andy
Yes, 1 audio board per amp, and of course you need a power supply board for each amp also.
Don't forget high current output binding posts. Only some, like Superior Electric, are rated for 30A.
Many are more like 10A.
That's a peak current test into an almost shorted output.
and is equivalent to 2.7Vac into that 0r1
A far cry from 20Vac to 30Vac into a sensible load.
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And so?
As I see it ... it shows that the amp doesn't blow up into an almost-shorted output. Which is pretty damn amazing! 😱
Not sure what you call a 'sensible load'. 😕 What I am interested in is an amp which can deliver flat to at least 50Khz into a 2ohm load. The F5T seems to meet this requirement.
Andy
Andyr,
sorry but I don't see a clear statement from you about the exact power requirements for your ribbon tweeter?
If it's a 3A into 2 Ohms (i.e. about 20W, as stated in post #18) it can be easily covered with a single pair of output devices, no need for heavy artillery (F5T). High damping factor is not critical since ribbons tend to have very flat impedance characteristic. Just go for an amp that sounds good to you - Aleph variations are often mentioned for nice upper range...
sorry but I don't see a clear statement from you about the exact power requirements for your ribbon tweeter?
If it's a 3A into 2 Ohms (i.e. about 20W, as stated in post #18) it can be easily covered with a single pair of output devices, no need for heavy artillery (F5T). High damping factor is not critical since ribbons tend to have very flat impedance characteristic. Just go for an amp that sounds good to you - Aleph variations are often mentioned for nice upper range...
Aah, juma ... herein lies the problem. 🙂
I am currently using a 70w-into-8-ohms Mosfet amp to drive my 2 ohm ribbons. A very nice-sounding amp. The designer told me "it would be able to handle a 2 ohm load". But he said it didn't double down into 4 ohms and double down again into 2 ohms - so maybe it can deliver 160w into 2 ohms?
Well the amp does "handle it" ... in the sense that it hasn't blown up! 😀 And for the last year, it has sounded pretty good.
But just recently I learned how to drive REW and so have been "measuring my room". The FR graph showed a 'cliff' at 16Khz ... but my Maggie ribbons are specced to 40Khz. So it's not the ribbons that are causing this problem!
So I wondered whether in fact my ribbon amps (monoblocs) were able to drive a 2 ohm load effectively. By "effectively" I mean deliver the same output voltage for a constant input voltage, from 6Khz up to 40Khz.
I then did some measurements which showed the output at 30Khz was only 40% of the output voltage at 6Khz ... which suggests to me that my amp is having difficulty driving a 2 ohm load. Hence my search for an amp which will be able to deliver a constant output voltage (for a constant input voltage) for the full FR of my 2 ohm ribbons.
In terms of the power requirement ... Maggie ribbons are delivered with a 2.5a fuse ... so I'm pretty sure they will blow if you supplied them with 3a continuous. (And at $500 a pop for a replacement, I'm not prepared to do this experiment! 😀)
Taking 3a current, max ... you get 20w rms power (well, actually 18w, for a 2 ohm ribbon). So you might think that a 70w-into-8-ohm amp that "can handle" a 2 ohm load should be just fine! But it's not! 😡
Andy
It's an unusual thing that frequency response of your amp falls with load's impedance. That needs some further examination...
an output that collapses down to 2.7Vac when presented with a high current demand is not much good.
that was effectively a peak current test, not a test of the ability of the amplifier to drive a demanding load
A sensible load for a 2ohms rated amplifier would be a 1r0 test load. The output voltage should not collapse and maximum power into 1r0 should be around 160% to 190% of the power delivered into the rated 2r0 load. R.Cordell states that a good indicator is >=180% of power delivery into the half rated load.
If you needed a more robust 2ohms amplifier to drive a highly reactive 2ohms speaker, then you could look at using a 0r67, or even 0r5
Then see how well the output voltage holds up when the current demand has increased.
But your ribbons are not a highly reactive speaker load, so I would tend towards the 1r0 as the sensible load for your situation, not 0r1.
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Hi Andy , I made the measurements :
load : 2.4 ohms non inductive resistor 10 w ( have only that on hand !!)
I used REW as generator , freq was steady , in voltage almost !!
10 hz in : 1.866 v out : 4.700 v
100 hz in : 1.974 v out : 5.000 v
1 khz in : 1.970 v out : 4.986 v
5 khz in :1.963 v out : 4.942 v
10 khz in : 1.930 v out : 4.778 v
15 khz in : 1.889 v out : 4.547 v
20 khz in : 1.842 v out : 4.232 v
REW is not able to go over 20 khz , what you see here is an 8.5% less voltage at 20 khz compare to 10 hz , not that bad 🙂
resistor temperature 28 / 30 ° Celsius steady .
my amp has 110.000 uf ( big Epcos caps ) per channel and 220 va trafo per channel .
and even some Wima MKP 10 , 10 uf per rail.
I am waiting for some 1.0 and 1.2 ohms Mills 10 w resistor next week , I will do some other test 😉
.
load : 2.4 ohms non inductive resistor 10 w ( have only that on hand !!)
I used REW as generator , freq was steady , in voltage almost !!
10 hz in : 1.866 v out : 4.700 v
100 hz in : 1.974 v out : 5.000 v
1 khz in : 1.970 v out : 4.986 v
5 khz in :1.963 v out : 4.942 v
10 khz in : 1.930 v out : 4.778 v
15 khz in : 1.889 v out : 4.547 v
20 khz in : 1.842 v out : 4.232 v
REW is not able to go over 20 khz , what you see here is an 8.5% less voltage at 20 khz compare to 10 hz , not that bad 🙂
resistor temperature 28 / 30 ° Celsius steady .
my amp has 110.000 uf ( big Epcos caps ) per channel and 220 va trafo per channel .
and even some Wima MKP 10 , 10 uf per rail.
I am waiting for some 1.0 and 1.2 ohms Mills 10 w resistor next week , I will do some other test 😉
.
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It's absolutely unacceptable! 😱
What it says to me is that the amp cannot drive a 2 ohm load effectively - now, the designer of the amp is admitting that this is the case (after selling me the amp, knowing I was going to drive 2 ohm ribbons! 😡 ).
Hence my search for an amp which will work properly into 2 ohms. 🙂
Andy
Excellent work, Fabrice!
Tomorrow (Sunday) I will do some more measurements - input to the amp and output.
Andy
Hi Andy
The F5 Turbo is an absolutely fine amplifier.
Since you've already purchased the pcbs, just proceed as planned.
The F5 Turbo is an absolutely fine amplifier.
Since you've already purchased the pcbs, just proceed as planned.
It's unacceptable in sense that it's not an easy thing to achieve - you need a special set of circumstances to provoke such an outcome. I suppose there's a major problem in your output stage like low ft BJTs, out of spec. or fake parts or some problematic compensation network.
Regular amp's frequency response doesn't depend on the load's resistance.
Schematic and good pics of the actual build would help to solve the mystery. Maybe you can still enjoy your amp if we find the error...
Hi Fabrice,
I've just finished making my repeat set of measurements - more extensive. See attached file.
Some notes on my test methodology:
* Output from the sig-gen was plugged into each amp, in turn.
* I had my CRO leads on the inside of each amp input RCA socket, so I could adjust the output of the sig-gen to be 60mV, for each frequency tested.
NB: 60mV is at the low end of my CRO's sensitivity range; it was difficult to make sure the output was reading the same at each frequency (the line on the CRO was a bit fuzzy) - so it is quite possible that the output of the sig-gen varied very slightly from 60mV at some frequencies ... which would cause the output to vary. So I am assuming anything from 97% to 100% ... is 100% - so flat.
* The bass amp is quite capable of handling 4.6 ohms; it just has a LF roll-off implemented in the circuit which even has an effect at 50hz. This is a different amp to the amp driving the mids & ribbons. It has a bit more gain - which I can take into account in my active system.
* The amp driving the mids & ribbons copes better with a 3.7 ohms load than a 2 ohm load. Output at the 4 measured frequencies which were common between mid & ribbon, shows the amp produces more output into 3.7 ohms. (My multimeter has an ancillary attachment for measuring resistances below 10 ohms - so I am confident the driver resistances are what I measured.)
* The mid amp performs flat in the frequency range which it operates in. Which is excellent for me! 🙂
* However the ribbon amp is not up to snuff! 🙁 It rolls off too much in the higher frequencies, with a 2 ohm load.
Regards,
Andy
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