PASS DIY F4 beaten hands down by my class D amp, what's wrong?

[mr_push_pull]

I just reminded about one issue I've forgotten about: I couldn't get more that 5 something volts across the 5K trimmer in the front end, even when turned all way up. but that would not indicate too high a Idss but the opposite? anyway this is weird. I feel embarrassed about forgetting this.

the front end is powered from the same PS as the power stage which may not be a good thing either.

all in all I think it's time to short the front end and use my 6N1P heaphone amp as a gain stage and do some more listening to see how things change/improve.

well

 

[Zen Mod]

I just reminded about one issue I've forgotten about: I couldn't get more that 5 something volts across the 5K trimmer in the front end, even when turned all way up. but that would not indicate too high a Idss but the opposite? anyway this is weird. I feel embarrassed about forgetting this.

...

what's important is :

- current through LTP ( voltage across R205) in 10mA ballpark
- current through Q203 ( voltage across R207) in 15mA ballpark

voltage across P201 is just informative

....

the front end is powered from the same PS as the power stage which may not be a good thing either.

.....

in your boots , I would place at least cap multipliers in both rails for FE

 

[mr_push_pull]

9.5, 1.27 V respectively.

 

[Zen Mod]

decrease R205

even if you're almost there , so I can't expect miracles

culprit is in reference speaker choice

 

[generg]

all in all I think it's time to short the front end and use my 6N1P heaphone amp as a gain stage and do some more listening to see how things change/improve

well

Very good idea! It is difficult to get the SJ109 out and measure the Idss.

And can you drive both, your class D and F4 with this headphone part?

And when the F4 still is not fulfilling your wishes, you buy two F5 boards, good Dales, the few FETs, the rest you take from F4 and you continue the Pass line..

 

[generg]

And you connected really C208 to the +output of speakers and C 203 to the input?

And there was a little uncertain point about the polarity of c203. As far as I remember it is different when using the front end with BA-1 or BA-2 but it no problem when you take at this place a foil Cap without polarization.
But F4 has the JFets at the input, so I suppose +of the elco is directed to ......
hmm.. I am just not quite sure... Can anybody help?

 

[JZatopa]

I'm surprised that nobody asked what class D amp he is comparing this amp to.

 

[generg]

And my last idea, I hope I do not bore you,

The input FETs of F4, are they o.k.? And no fakes ( easier to control, than SJ109)

Now I go to sleep and dream of your class D amp....

 

[Zen Mod]

Generg - you must remember one thing - LTP , as any simple diff gain stage , will do anything possible with it's output(s) - to achieve and maintain equal condition on both inputs

so - if left input is on gnd potential , you can expect that right one is on same level/potential .

so - you can expect that left part of Q201 will steer Q203 (via P201) to maintain 0 Volts at Q203 drain .......... just to have that 0 Volts at Q201 right part's gate

so - it's best to have C203 bipolar , just because it's right leg is flapping up and down

 

[Zen Mod]

I'm surprised that nobody asked what class D amp he is comparing this amp to.

we already agree that D class amp is irrelevant here , same as F4 is irrelevant with those hungry Dynaudios .

well - two balanced F4s will wipe the floor with D class ......

 

[vdi_nenna]

Plus the low impedance of the Confidence-3, definitely.
Unfortunate choice from the start, a flea amp requires a flea loudspeaker.

I agree.

 

[tinitus]

I remember reading about someone building a big array, with 'flea drivers'
to be driven with a flea tube amp

unfortunately the flea tube amp didn't like all those drivers, even if small

simple things are not always simple at all

I expect it would drive a big coax with ease
whether it sounds good or not, different matter

 

[tinitus]

another different scenario

maybe your F4 just leave all 'reproducing' to your speakers
what you hear is your speakers

due to my Mirand amps, I know its that way with amps working like 'wire with gain'
sounds absolutely wonderful, but only with really good speakers

and your classD doesn't do that, but instead reproduce a sound of its own

your F4 ought to sound good with any properly working speaker
but with low to moderate SPL, ofcourse

 

[mr_push_pull]

first of all, I have no intent of revealing the make of the class D amp, that is not the subject of the discussion. being on the "Pass Labs" section I kind of imagine why some may feel somehow offended by the comparison but I've always thought of the class D as a compromise and a good class A as the true reference and all I'm trying to do is track down the problem.
currently I'm almost sure that the issue is related to the front end and that's why I'll give the thing another listen with the FE disconnected and using another gain stage (my tube headphone amp). with one hour of warm up, like mr. Pass advizes.

or maybe it's simply bad matching with the 86 dB speakers.

I'm considering building another gain stage that uses easier to source parts, any recommendations are welcome.

 

[FATHER]

The power source seems too small to me...

 

[mr_push_pull]

seems small as in looks small? tranny size is 6x15 cm, I would estimate 400-500VA?

 

[FATHER]

I'm planning to build a BA-1 with 2 X 7-800 VA trannies and at least 200.000 uF caps per channel, because i got hungry speakers too... Sper ca n-am ajuns sa gandesc ruseste...
That toroid looks very friendly to me but you need a beast. That EMI filter is OK?

 

[generg]

Try to get original SK170 and J74 for your F4 and replace your SJ109 in the frontend by two closely matched J74.

May be Bill Fuss here is still selling some..

When you have done this and it does not help, sell your F4....... Maybe some in the forum wants it......

Good luck with your headphone amp test....

 

[buzzforb]

OR....build another F4, run both in mono-parallel from new front end or preamp capable of swinging 20V. You will have all the power you need. Just a question, have you read through the F4 article at Firswatt? Six moons review. The Firstwatt article will tell all about the F4's capabilities and needs and the Six moons review will talk about the importance of the preamp.