F5 Turbo Builders Thread

[BigE]

This is what I am thinking of building:

1) 4 devices per channel
2) 0.63 amps/device (total 2.52 amp bias)
3) 36 VAC transformer (approx. 46 volt rails)

using a 1.5 KVA transformer. This gives about 118 wpc class A.

If you use a 24 VAC transformer, I figure that 1Kva ought to do a stereo amp at this bias current, and give you about 80 wpc.

[BigE]

I think my numbers show peak Class A watts. RMS would be about 60 and 40.

[sts9fan]

I am currently collecting parts for a V2. Could someone expand on the section of the F5T article below about the diodes? I am not 100% clear what is need to be done. I am thinking 32V rails and not sure of the bias.

We see that these devices will slowly start conducting at voltages just above the idle voltage across the 0.5 ohm Source resistors. It will be necessary to heat sink these diodes, remembering that their case is electrically connected.
The point at which the diodes conduct is temperature dependent, so you will want to set the bias so that it makes a nice transition above the bias point and doesn't run away when the amplifier gets hot.
If you are competent, fearless and also own a fire extinguisher, you can find this point. Just run the amplifier into a reasonably low impedance until it gets good and hot – as hot as you plan to let it get - ever. Then adjust the bias to a point below where the idle current starts to really take off. You should find that this point is around 0.4 volts across the 1 ohm resistors. If you are a fraidy-cat, then just set it at 0.3 volts, and conservatively fuse the AC line.

[BigE]

AndrewT, I have a spreadsheet for this now.

It says that an amp can be made with these parameters:

1) Vsec : 40 VAC
2) Vrail : 52 VAC (1.3* Vsec)
3) Ib : 0.7 A per device
4) Ibt : 5.6 A ( 8 devices * Ib )
5) VAbias : 582.4 ( (2 * Ibt) * ( 2* Vsec) )
6) VAcool: 1164.8 ( 50% transformer load at idle )
7) Wpeak: 291.2 ( Vrail * Ibt )
8) Wrms: 145.6 ( Wpeak/2 class A)
9) Wab: 291.2 (twice Wrms)
10) VAab: 582.4 (twice Wab, not 1.5)
11) Total VA: 1164.8 ( VAbias + VAab)

Note that this is the identical figure to VAcool -- that level of VA to allow the transformer to run cool. It is also exactly 8*Wrms.

So, you can make

A 145.6 Watt mono amp with 8 Fets, 52 volt rails, 40 volt secondaries biased to 0.7 A per device with a 1.2Kva transformer. Or this could be a stereo 72.8 watt class A amp. It dissipates 582 watts at idle.

If you have a 34 Volt secondary, and get 44.2 V rails, 0.7A, you can make a 123.76 watt monoblock or 62 watt stereo amp from a 1Kva transformer. It dissipates 500 watts at idle.

A 1A bias and 34 VAc secondary will need a 1414 VA transformer for a 176 W monoblock or 83 watt stereo amp. It dissipates 707 watts at idle.

A 1A bias and 36 Volt secondary will need a 1.5KVA transformer to produce a 98.6 watt stereo amp, or 187.2 watt monoblock. It dissipates 750 watts at idle.

The dissipation is VAbias.

I think these numbers are OK, but as usual, I will take NO responsibility for their accuracy.

[AudioSan]

0.7A bias pr device, and 4pairs = 2.8A bias. 52V*2.8A*2=291W dissipation.
this will give you about 125W Class A power at 8ohms. transformer need to be a minumum of 600VA pr ch.

[sangram]

Originally Posted by anilva

Sangram,

I am wiring up a V3 with exact same Toroid specs as yours, due for completion next week. I am quite unsure of the PSU caps, as I ran into ripple related hum problems on V2 with 8 x 10000 uF, 50V caps per rail. In V3, I am planning to use 16 caps of 15000uF, 63V CDE per channel. Hope they are sufficient.

Cheers.

Hi

I'll be using similar capacitance level (though my rails are lower) - 22000uF x 8 and 10000uF x 8. The first bank will be as per the V2 power supply recommendation, the second bank is twice as beefy. I am not sure that with the higher rails a similar capacitance will be sufficient.

Originally Posted by BigE

Is this a 1KVA, transformer with 24 Volt secondaries?

Yes. 25V minus the margin of error for mains voltage, which is never 230V.

Originally Posted by BigE

According to Andrews post -- ONE CHANNEL:

For a continuous draw of 4 amps, you need an 8 x 64 = 512 VA. Then multiply this by two for reliable operation, and you've got 1Kva for the single channel at this bias.

So 2KVA, is needed for a stereo amp with 4 amps bias per channel using 32 V rails. You are off by a factor of two.

I'm running two pair outputs for V2 (no diodes), not four pair. At 1A per device and two channels, that's 8 devices x 1A x 64 = 512VA dissipation and doubled it for safety (though I know 2x is a little skimpy). That's how I got the 1KVA figure.

Will be glad to see where I went wrong in this calculation.

[BigE]

According to this, with 2 devices ( 1 N and 1 P ) each device biased at 1.3 A ( so the pair has 2.6 amps) my spreadsheet shows 27 watts , 108 Watts dissipation and a 216 VA transformer ( For one channel ) 16 VAC secondary and 20.8 V rail.

http://www.firstwatt.com/pdf/prod_f5_man.pdf

In your example, if you calculate using 8 devices at 0.7 A each ( 5.6 A) you get a 250 Watt peak = 5.6 * 5.6 * 8 , which is 125 Watts RMS.

The dissipation is 582.4 watts which is 5.6 * Vrail_to_rail as in Andrew's example.

[BigE]

Sangram

According to the spreadsheet, you are exactly correct.
24 V secondaries
31.2 V rails
1 Amp/Device * 4 devices = 4 Amps draw.
Gives 64 Wrms per channel, using a 1Kva transformer. (1024 VA would be exact)
It will double in A/B and idle at 50%.

Ignore my previous post. It is exactly the right size.

[AudioSan]

1A pr device, 2pairs =2A bias. dissipation =2A*32V*2 = 128W pr ch. so minimum 250-300VA transformer pr ch.

[sangram]

:whew:

I am aiming for more voltage swing, so the 'stereo' is only indicative. In reality I will be running in balanced configuration (this 8device/1A bias/1KVA supply is for 1 channel) with another exact monoblock for the other channel. The power output target is ~80W in Class A and maybe 120-130W total.

Also, during summer as is my practice with the F5, the bias will be lowered to prevent me from bursting into flames.