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F5 Turbo Builders Thread
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Old 8th January 2019, 08:59 AM   #5321
BigE is offline BigE  Canada
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That is not my understanding. According to kirchoffs laws the current flowing through the mosfet is shared between the parallel resistors and the diodes. The mur3020 datasheet tells you how much current will flow through the diode pair for any given voltage drop across the diode at any junction temperature.

The issue is that as the diode conducts more current, less current flows across the resistor pair. This means that all current will flow across the diodes once their effective resistance becomes so low as to be negligible, causing one of the mosfets to begin current hogging and explode. You will never see multiple mosfets blow on a single rail because of this.

Last edited by BigE; 8th January 2019 at 09:07 AM.
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Old 8th January 2019, 04:50 PM   #5322
Bfpca is offline Bfpca  Canada
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F5 Turbo Builders Thread
Yes, You can think of it in terms of currents. I tend to view it from a temperature compensation point of view. Once the diodes conduct you loose the compensation provided by Rs.
If one of the diodes begins to conduct during idle conditions that mosfet will heat up and the heat will spread across the heat sink. How fast this happens depends on the thermal conditions. Who knows what will happen when a mosfet shorts, putting dc on the output. With feedback the rest of the circuit will try to correct that dc output voltage. With the huge power supplies built into some of the F5T builds I've seen, there is a lot of energy storage in the supply and also with big transformers and main fuses a lot of power is available from the line until the fuse or something else in the power supply blows.
I have about 1/2 farad of C in my monoblock supplies and 800va transformers. I don't want to see that supply shorted out. I decided against the diodes in my build. Not worth the risk to my amps or speakers.
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Old 8th January 2019, 05:24 PM   #5323
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Quote:
Originally Posted by BigE View Post
Bear in mind that NP says that the V2/3 really starts to sound better when the diodes are conducting. In my system, with nominal 4 ohm voltage, that is around 1 watt:

P = i^2 * R = 0.5^2*4 = 0.25*4 = 1
V = I*R = 0.5*4 = 2 Volts.

This will drive my speakers to 93 dB, which I find too loud for regular listening.

This implies that the F5T would be a better match with less efficient speakers and/or a much larger room, as the diodes would be conducting at normal listening levels.
I can't follow your calculus concering as to when the diodes start conducting to be honest, the O.5R in your formula is the paralled source resistors ?
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Old 9th January 2019, 01:20 AM   #5324
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R is 4 ..the speaker load. I is 0.5 the current needed to drive the speaker to 93 db. Because that is 1 watt per 93 dB at one meter.

NP said the F5T starts flexing its muscles when 2 amps are flowing. Which through my speakers would be at 8 watts output.

But bias current could decrease this ?
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Old 11th January 2019, 07:03 PM   #5325
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Clarify:

The F5T starts flexing it's muscles with 2 amps *going to the load*.

This is independent of the bias.

If you want those 2 amps to be Class A amps, then the total bias current must be at minimum 1 amp.

With 4 F5 pairs in parallel, that's 0.25 amps per device, which occurs at 0.3V for junction temperature of 150 deg C.
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Old 14th January 2019, 02:10 PM   #5326
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Quote:
Originally Posted by coolnose View Post
I can't follow your calculus concering as to when the diodes start conducting to be honest, the O.5R in your formula is the paralled source resistors ?
0.5 is I. Note that it is squared.
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Old 20th January 2019, 11:34 PM   #5327
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Default F5turbo v2

I want to build an f5 turbo v2. I've built an F5 I'd like to recycle some parts.
Can I still use the 35 v bridge rectifiers or do I need to get the Vishay 3020
Diodes. I plan on another transformer with 24v secondaries, getting the store transistors and following 6l6's guide and the diy BOM. I'd appreciate any advice
Thank you
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Old 29th January 2019, 09:05 AM   #5328
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Default F5T V2 - More Gain ?

Hi all,

I have fired up my F5 Turbo with 45V rails feedback resistors are 220R in parallel as per schematic.

I would like some more gain and was wondering if I could increase the feedback resistors some more ? I understand it will decrease feedback at the same time.

Anyone out here has an idea how far to go or not to go ?

Any help is appreciated.

Max
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Old 29th January 2019, 10:40 AM   #5329
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Using the search function showed some comments on using 380R resistors in parallel for R7 - R10 increasing the gain to 26db instead of the 22db.

I was aiming for +6dB over the V2 turbo (21.6dB) could I use 500R resistors in parallel or is that too much decreasing the feedback ?

Thanks,

Max
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Old 29th January 2019, 11:16 AM   #5330
coolnose is offline coolnose  Europe
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Using the search function showed some comments on using 380R resistors in parallel for R7 - R10 increasing the gain to 26db instead of the 22db.

I was aiming for +6dB over the V2 turbo (21.6dB) could I use 500R resistors in parallel or is that too much decreasing the feedback ?

Thanks,

Max
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