F5 Turbo Builders Thread

[dazed2]

BigE,

On page 14 NP states

Turn this comment around and increased current increases the bandwidth to a point. Since the V2 is stated to have the same bandwidth as the F5 and two pair is the starting point in the comment about decreasing bandwidth it seems likely that a little more front end current will increase the bandwidth somewhat with 4 pairs. Will it be significant or more importantly, audible? That's up to you to decide. Build it both ways and report back. Those building 4 pair amps seem happy with a single pair, though.

So basically, in a 4pr setup with 1pr jfet you get lower bandwidth, and higher gain. However with 2pr jfet you get higher bandwidth, lower gain, but perhaps more punch?
Is that a correct assumption?

 

[buzzforb]

As i mentioned earlier there is perhaps a more important element than bandwidth that must be considered. Each gain stage creates a distortion profile and that distortion profile has a phase. Making alterations as you have suggested above will alter the final sound and distortion profile of the amp. I would suggest that there is more to be gained in this regard than in terms of overall bandwidth.

 

[BobEllis]

Oh, Geez! Another variable to evaluate. Thanks a lot Buzz.

 

[buzzforb]

Everybody knows that diyaudio is the most wicked rabbit hole you could possibly find.

 

[6L6]

Wicked? Nah... The mod team really is superb.

That said, I agree that it's an awfully deep hole...

 

[Triodethom]

There are many carrot to be found when you turn left at Albuquerque .

 

[BigE]

By distortion profile, do you mean Harmonic Distortion Spectrum? ie, the relationships between 2nd and 3rd harmonics?

I am very tempted to build the amp both ways. How close an Idss match to the pairs of jFETS must be present?

 

[buzzforb]

The irritating truth is that it all matters. To what degree it is discernible, is a question for your ears.

 

[BigE]

Should make a poll of 4 pair F5T builders -- one pair or two pair jFETs in front end?

 

[SRMcGee]

BigE:

In my case, one pair per amp.

Regards,
Scott

 

[buzzforb]

I think you might be surprised how few F5T's have been built.

 

[jaystone]

Thinking out loud here, hoping someone can confirm for me. I am cascoding the front end of a set of DIY audio f5t boards, I am trying to verify the values for R25,26,27 & 28. I will be running 32v secondaries, so figure 45v at the rail after rectification. Am I correct in trying to reduce that voltage down to 16v? I am figuring R25 & 27 values of 10k and R26 & 28 values of 5.6k. Is the formula 45v X (10000+5600)/5600 correct? What would the effects be of running the cascode voltage higher or lower? Stability?

 

[AudioSan]

the formula is correct. but your railvoltage will not be +/-45V. with 32V sec you will end up with around +/-41V under load.

 

[jaystone]

So is the 41v under load assuming for a couple volts loss through the circuit from the 32x1.414?

 

[AndrewT]

You can adjust the cascode voltage AFTER you measure to confirm the actual build voltages.
Then build the second channel to the new values.

 

[WalterW]

Under load it is more about 32 x 1.305...

@BigE: I used 1 pair of JFETs for my F5Tv3 with Toshiba J201 and K1530 outputs.

 

[AndrewT]

the peak emf from a 32Vac sinewave is sqrt(2) * 32
That value NEVER changes.

As you draw current the IR voltage losses increase as current increases, the emf NEVER changes.
The diode Vf changes as the current increases, the emf NEVER changes.

 

[AudioSan]

sec voltage*1.25-1.3 is a good roule of thumb to get an easy indication of the rail voltage.
this will get you in the ballpark. calculate exact rail voltage is hard to do, as there is a huge amount of factors to consider.

 

[BigE]

Funny that few should be built. I've just heard one, and it slays the F5 in every sonic attribute.... just my opinion of course.

AudioSan, cutting the heatsinks in half is a brilliant idea. This means you get 2 * the cooling capacity of one, as opposed to something like sqrt(2)* the capacity of one.

That *is* a significant difference.

RE: diodes. What do people use for their heat dissipation?

Thanks!

 

[buzzforb]

I have built 5 and working on number 6.