F5 variations for sub 4 ohm speakers? - diyAudio
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Old 3rd September 2014, 07:28 PM   #1
Jeffs is offline Jeffs  United States
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Default F5 variations for sub 4 ohm speakers?

Not sure where to post this but had been considering a F5 build some time ago. Last weekend I picked up some speakers I've been wanting for, oh, 30 years. Infinity RS-IIIb which are 4 ohm but known for dropping much below that, thus killing amplifiers. I believe efficiency is 89 db.
Can one of the F5 variations be done as to get 80 to 100w with the sub 4 ohm covered or do I need to look at another design?

I have an equally old NAD receiver rated at 20w, which is great for some acoustic recording and electronic but I'm afraid to turn it up and don't feel I am getting the low end like I should. It isn't giving me what i need for rock.
Spending half a day driving around and bothering amp repair people for recommendations and I keep on getting stirred toward McIntosh. I do not expect to be buying anything that expensive in this lifetime.
I don't have a pre-amp, and the NAD receiver does not have the capability to bypass the internal amp. Will be playing FLAC and would like ability to use them for a television when I get around to purchasing one.
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Old 3rd September 2014, 08:22 PM   #2
WalterW is online now WalterW  Netherlands
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Read the F5Turbo article from the firstwatt.com website and decide which to build.... But I think with a F5turbo V2 which delivers 38 Ampere peak into 0.1 Ohm you will be happy
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Old 3rd September 2014, 08:36 PM   #3
AndrewT is offline AndrewT  Scotland
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One of the F5t should be better than an F5 for driving very low impedance speakers.
But don't choose a version that uses a much higher supply voltage.
The standard F5 does ~27W into 8ohms and with a decent supply will do pretty close to 50W into 4ohms.

A 2pair or 3pair version will perform even better when the impedance drops well below that nominal 4ohms.

BTW
38Apk into 0r1 is only 3.8Vpk.
That has no relevance to driving a speaker.

19Vpk into 1r0 is very relevant to driving a widely varying 4ohms speaker, even though the raw number is only half that headline figure into an effective short circuit, i.e 19Apk vs 38Apk.
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Old 3rd September 2014, 09:22 PM   #4
Jeffs is offline Jeffs  United States
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What do you mean by 2 pair / 3 pair version? Based on memory, I would guess the 2 pair is what was being referred to as cascode? No recollection of a 3 pair version - if I have the concept correct.

I am rereading the types, but I remember a lot of evolution as those with understanding were able to change things. For some reason I thought I had my mind set on F5 V2 cascoded. Out of reading time for the day
My electrical engineering knowledge is only slightly above replacing a lightbulb with the help of ladder and two people to rotate the ladder while I hold the lightbulb in place.
All suggestions will be appreciated and I'll try not to dumb things down excessively.
To start with, the numbers you have given me are your way of showing a person that speaks electronics that you understand these speakers kill amplifiers and that something such as a F5 V2 cas design is highly resistant to death from being used with these speakers?
I do recognize the usernames of those that replied, I think I spent November to January obsessively reading threads.
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Old 3rd September 2014, 09:28 PM   #5
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i would have used a F5t v2 (+/-32V rails) with 4 output pairs and 3.5-4A bias.
but class A is a little waist for use with sub.
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Old 3rd September 2014, 09:29 PM   #6
Zen Mod is offline Zen Mod  Serbia
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same one , but with standard rails (+/-22Vdc)
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Old 4th September 2014, 12:58 AM   #7
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Check out TeaBag's layout/boards.
iirc, they take 6 or 8 pairs.

First step, imo, look up the reviews for the speaker. The one's in the mags, they generally do both a freq resp and an impedance curve. Don't guess, or go by what "people say". See what it actually is. You need to know for certain.

Then you want some margin anyhow.

Next step is to compare the current requirements vs. voltage through the actual MosFets that you are going to use. You want to look at the SOA graph on the mfrs spec sheets. Multiply current x the number of devices in parallel and de-rate. Now you know what you think the number of devices at what voltage (rail voltage) you will need to drive your speakers at what power resulting in what SPL at a distance of 1m. Then you drop that figure as you go back in space, unless the reverberant field is high (very high).

This tells you what you can expect as max SPL at your listening position with a given power amplifier (any given power amplifier).

This relates back to your build WRT the devices, the rail voltage (max swing, therefor max power into some Z - ohm's law tells all), and the number of devices used.

Remember now, more devices biased at a level equal to the baseline F5's bias point = much more heat = larger heatsinks. Manhattan size (not really, but a bit of exagerration helps here). Or a fan. I don' t like fans in my listening room.

In the case of a "power hog" speaker, it's probably ok to let the amp "switch " in to class B at higher output levels, and reduce the quiescent and average heat while playing...

This should give you something to do for a bit...

Oh, you MUST use genuine JFETS from Toshiba (not the asian fakes) OR the LSK / LSJ series sold here on DiyAudio 's store (or from other bonafide vendors with stock).
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Old 6th September 2014, 01:28 AM   #8
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Hi Jeffs. I worked in audio retail when the RSIIIb was current and did not have reliability problems with amplifiers. Over the course of several months, we used the Nakamichi PA-7, Haflers 220 & 500, NAD 2200, and larger NAD receivers with success. I would be confident with an F5T V2 or higher.
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Old 13th September 2014, 12:43 AM   #9
Jeffs is offline Jeffs  United States
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I'm backing off a bit to learn some basics, starting with an amp camp build. As far as a pre-amp stage for plugging my macbook into with 3.5mm jack considering an OB2, will the OB2 be sufficient for both an F5 t build and the amp camp? It's not a permanent solution, but don't want to build stuff that's not needed. I can add DAC in the future if I keep using it but a good chance it will go to my dad.
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Old 13th September 2014, 03:36 PM   #10
6L6 is offline 6L6  United States
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Do you mean the O2 headphone amp as a preamp? That will work beautifully.
The ODAC is also quite wonderful.
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