F6 Illustrated Build Guide

[mlwebb]

I'd take Anand up on his offer, I also suspect your transformer wiring is mixed up, it is not obvious.

diyAudio Power Supply Circuit Board v3 illustrated build guide

Basic transformer connection question.

Michael

 

[Chiptech]

Well, I have the same problem.

From post #1: Power Up "If you start with the pot in it’s default position, it will most likely have too much bias initially. Turn off the power, turn down P2, and try again." That didn't sink in.

I tested the R9 resistor and it still works. I replaced it.

I now think I understand that while I thought I had both trimpots in the "middle position'" but both were set too high. So how to set trimpots at start?

Using your guidance I measured the pots and made adjustments:

P1 measures 1.98K high and 82.9 ohms in the low position.

P2 measures 10.15K high and 131.0 ohms in the low position, using your directions.

I'm assuming that 82.9 and 131.0 ohms are viable targets to use.

Net, net, if startup bias target is 500mV, set trimpots below that target.

If I get this, since we want 500mV to start, both trimpots should be set below that target.

If this is correct, I'm a little better educated. If not, I'll need further guidance.

I turned the amp on with the dim bulb. It glowed and then subsided. Good.

My DMM on the .47 source resistor showed 0.0 mV.

But the R4 resistor started to smoke.

So if my learning from yesterday is correct, I have the trimpots dialed way down, as evidenced my the 0.0 mV reading, so my problem resides somewhere else.

I will recheck that all parts are correct.

Other than that, I'd appreciate help sorting this out.

Thanks

 

[Dennis Hui]

Did you measure the voltage across R1 (0.56 ohm)? Also, what was your dc offset?

 

[henryve]

You have checked that the JFETs are fine and that you do not have a solder bridge on one of their pins?

R4 is shortcut to power ground only from a few places.

Also, check your R6. It seems that you have used 47R in stead of 47K there. It won't cause R4 to smoke though so you have some other problem besides R6.

 

[angrypat]

when you realize that you're moron, there is start of unspoiled happiness and prosperity

no obstacles in feeling bad because you don't know something , just realization and then learning it in best way possible

I'm happy all the time

Words to live by, the zen of Zenmod.

 

[Chiptech]

Did you measure the voltage across R1 (0.56 ohm)? Also, what was your dc offset?

I did not measure R1 or the DC offset.

Once I replace R4 and R6 I will take new measurements.

Thanks

 

[Chiptech]

You have checked that the JFETs are fine and that you do not have a solder bridge on one of their pins?

R4 is shortcut to power ground only from a few places.

Also, check your R6. It seems that you have used 47R in stead of 47K there. It won't cause R4 to smoke though so you have some other problem besides R6.

You are right. R6 is 47R.

I will check the JFETs when I replace the resistors.

Thanks

 

[ChrisM91]

Been using my F6 for a week now. The amp has CL-60s in the transformer primary, Antek 400VA.

Is a soft start circuit worth it?
Was looking at the design from the store.
Soft Start & Speaker Turn-On Delay / DC Protector Combo – diyAudio Store

 

[myleftear]

It depends on what you're looking for, and what you get with the current configuration.

A cl-60 is an as easy as it gets inrush limiter that comes with a price: 1. the cl-60 gets warm, and 2nd it drops some voltage. (Don't know the figures, though...)

I‘d say if you're not missing something, the chassis is big enough to take that little more temperature, and the voltage is good enough, then leave it.

Otoh, if you're very curious, the chassis gets rather warm and the PSU delivers just enough power, go for it.

PS/edit: I use it in my F-4 and am very happy. But I don't know how it would be with CL60. And one of the master-builders was mocking it as one of my „whatnots“ [emoji4]. The build isn't as straightforward as we're used to, because the project evolved and not everything is as clear as, say, 6L6's build guides. (But you’ll find everything you need to get through)

 

[henryve]

I see Tomasz at Toroidy now sells soft start modules as a sealed unit.

 

[myleftear]

I saw that too, about a year ago—he said I wouldn't need one with a 500VA audio-grade... (so I built the one from the store )

(And so I have no Idea what’s in it)

 

[Monk55]

I use the toroidy 500va 2x18 with CL60 with no issues at all.

 

[ChrisM91]

Tempted to just add a soft start circuit, just for the hell of it! Not aware with any issues so far with the CL-60s.

I guess with no CL-60s that would slightly alter my voltages and require resetting the bias/offset.

 

[Zen Mod]

nope

difference is negligible

 

[jameshillj]

Just curious about the difference between the standard, and 'higher' quality classes of transformers from Toroidy - I see the very attractive metal case surround and assume this means lower radiating field but apart from that, how do the different versions effect the sound?

I have the 'standard' TS300VA ones (built in 2013) and they function really well

 

[myleftear]

I _believe_ the audiogrades and the supremes are the same transformer, the supreme being potted in a mumetal shield.
Toroidy is a bit tight-lipped on that topic.
Sound-differences? They’re both very nice... (my f-4 has an audio-grade 500VA, with circuits very close on the sides and top, and it's just perfect although I still couldn’t measure it)

 

[jameshillj]

Thanks David, much as I thought.

My old torroids from England (40+ years ald) have a different metal type core and do behave slightly differently and I wondered if the different versions from Torroidy did the same - interesting, eh, but good transformers whichever version IMO.

We have a local manufacturer that'll build high quality EI and C-core transformers in the 100 - 500va sizes, but unsure of the cost - probably not cheap!

 

[henryve]

My only issue with Toroidy about their Supreme Audio Grade versions are that they only have the pots in a few sizes. The result is that you get a 200VA in a massive 150mm diameter pot, which is just as big as a 400VA Supreme Audio Grade transformer. So if you are thinking of getting a 200VA Supreme Audio, don't. Rather get the 300VA or even 400VA, unless your funds are severely limited. You are not going to save on space by going with the 200VA.

I did not notice that issue at the time that I ordered my 200VA Supreme Audio transformers. With the amount of shipping I have to pay anyway, I would for sure have opted for the 400VA versions.

In my case in the end I did not mind as I am going to use these for my M2 with dual PSU setup, so I have some nice screening plus a smaller magnetic field inside there.

 

[poseidonsvoice]

This is precisely the thought process that went through my mind. I was looking for a single 600VA for these Class A Pass designs but then I noticed the diameter was the same as an 800VA. An 800VA will run cooler (remember it’s encased) and have less voltage sag at higher bias currents. It’s also going in a 5U/400 so loads of space available. And mounted near the center so it doesn’t feel unwieldy on the occasional pickup.

Think of it as insurance. Go big or go home !

I also ordered it with extra secondaries since my build has (2) SLB PSU’s. Softstart from Neurochrome and Guardian 86’s on the outputs of course.

Best,
Anand.

 

[Chiptech]

Still smoking R4

I turned the amp on with the dim bulb. It glowed and then subsided. Good.

My DMM on the .47 source resistor showed 0.0 mV.

But the R4 resistor started to smoke.

So if my learning from yesterday is correct, I have the trimpots dialed way down, as evidenced my the 0.0 mV reading, so my problem resides somewhere else.

I will recheck that all parts are correct.

Other than that, I'd appreciate help sorting this out.

Thanks

Per the input I received I:

Changed R6 to 47K
Checked all parts
No solder issues with JFets
Checked Mosfets for continuity

Turned on with dim bulb, it faded, but R4 smoked

My readings today:

Bias 0.00 mV
Offset 223.0 mV
R1 46.5 mV

Researching this forum I turned up PJN who reported a smoked R4 in one channel after using the amp for five months. He ended up replacing the JFets as the consensus was they were blown due to high offset voltage. [#500 thru #521].

ZenMod concluded high output offset and suggested to check the JFets out of circuit. It could be that when I first turned on the amp the output trimmer was set way too high and that is when I toasted the JFets? Working hypothesis given the 223.0 mV output reading today the pot is still set way too high?

I can test the JFets and replace as needed.

Is there anything else I should consider?

Thanks for the help as always.