F5 power amplifier

[Nelson Pass]

There's a reason why the thermistors are there. You don't have to
have them, but you'll spend a lot more time warming it up since the
bias starts out low.

[Zen Mod]

Originally posted by woodturner-fran
....

EDIT: I'm thinking strongly of reconnecting the thermistors and then also R15 IIRC. Might leave things a little more stable.

don't tell me that I didn't told ya ......

[Diomedian]

Originally posted by woodturner-fran
A question for those running without current control/ temp compensation....


Have you tracked voltage across R11 and 12 with case temp? I'm finding that voltage can rise by about 0.6-0.7V when my case is closed. Is this ok? R11/12 get to about 60 deg C.

Heatsinks are getting to 45-50 deg C


I have some imbalance between Q3 and 4 on one channel. What I think is wrong is that R11 and 12 on this channel got a roasting so maybe have drifted. If the resistance has drifted that would cause it right? This leads to higher offset naturally, but will it affect the sound?


Thanks,

Fran

With the case closed and the higher temp therein, your output Mosfets will pass more current (check the graphs on the data sheet - Vgs vs Id). This in turn causes more current across R11/R12, and more current with more or less constant resistance means higher voltage drop across them (V=IR).

As the temp goes up, the resistors only drift slightly in resistance, but when they reach their rated temp, their power handling drops (again, check the data sheets for temperature drift and power derating - I'm sure 3W is fine). As far as I know (someone correct me if I'm wrong), the resistor returns to normal when it's cooled.

What you'll have to do is go through the biasing procedure again, but with the case on... if you have MM leads with clips on the ends running through vent holes in the case, this might work. Or yeah, just replace the thermistors.

-j

[jackinnj]

Originally posted by Tea-Bag


I was playing with the F5 and wondered why the V+ and - was around 22.5v

Then I looked at my parts. For R5-R8, I used INDUCTIVE wirewounds. I think this may be the reason.

Inductive WWs shouldn't be an issue.

[woodturner-fran]

Originally posted by Nelson Pass
There's a reason why the thermistors are there. You don't have to
have them, but you'll spend a lot more time warming it up since the
bias starts out low.

Thanks for that. I don't mind a longer warm up time..... but will prob replace R15, 16 and the thermistors in time.

Originally posted by Zen Mod





don't tell me that I didn't told ya ......

Nope, you told me

Originally posted by Diomedian


With the case closed and the higher temp therein, your output Mosfets will pass more current (check the graphs on the data sheet - Vgs vs Id). This in turn causes more current across R11/R12, and more current with more or less constant resistance means higher voltage drop across them (V=IR).
.........
What you'll have to do is go through the biasing procedure again, but with the case on... if you have MM leads with clips on the ends running through vent holes in the case, this might work. Or yeah, just replace the thermistors.

-j

Actually this is exactly what I am going to do right now. Amp is warming as we speak and what I'll do is set bias a shade low, maybe 0.57-0.58 to start with to give a little bit of safe overhead. It will probably run a few degrees hotter when its in my fairly open shelving, but right now its in very open air on the table.

*******************************

As it stands right now, I'm running the IR mosfets and form teh above you know the set up. Now, I have a second set of boards, I have more SJ74/SK170 coming from jackiinj and I have a set of fairchild mosfets I got form mouser.

I'm strongly thinking of buying the few parts needed to populate the second set of boards the same way and after appropriate listening time swap in the fairchilds and see the difference. I haven't read all of this thread (about 15 pages in from each end), but I Nelson would say the fairchilds measure better, but the IRs are a little "warmer". Now from what I heard last night the IRs aren't lacking top end, they have a lovely unforced airiness that you hear from really good valve amps.

Would it be a worthwhile experiment? Has anyone in the pages I haven't read done it?


Fran

[Nelson Pass]

The "warmth" of the IR's is due to the transconductance shelf in
the IR P channel devices. They have higher transconductance at low
frequencies, and then go through what looks like "baffle step correction"
going into the midrange.

If you like it, it's probably because you like less feedback on the top
and more control on the bottom.

Or not.

[woodturner-fran]

Thats interesting. My speakers are quad 57s and as I understand it, they present a widely varying load - with the higher freqs demanding more current, opposite to the norm. I wonder if thats anything to do with it.

In any case, I need to "prove" this amp for a while, make sure its stable, live with it and get to know the sound before making any changes. I have a friend with some very nice marten design speakers that would give it a good run. I ran the mini-aleph there for a while so I kinda have a handle on "if it sounds like this with the quads it'll sound like that with the martens"...... that will be an interesting listen too.


Fran

[ichiban]

Originally posted by woodturner-fran

As it stands right now, I'm running the IR mosfets and form teh above you know the set up.
........Nelson would say the fairchilds measure better, but the IRs are a little "warmer". Now from what I heard last night the IRs aren't lacking top end, they have a lovely unforced airiness that you hear from really good valve amps.
Fran

Hi,
which IRs are you using, I have sets from a Borbely B60 and D100. 2SJ50/2SK135s and the others are higher power versions, I forget the numbers 2SJ56 & ??, all TO3s tho.
Your comment about them being warm but with airiness sounds interesting. I know these mosfets were designed for audio use and are fairly complementary!?

[woodturner-fran]

The IR ones I'm using are from jackinnj (ie tech-diy.com) and are the standard IRFP240/IRFP9240. The fairchild ones are the 19P20 and whatever the N-channel version is called (from mouser).


I left the amp run for a good 3 hours tonight and it seemed to play flawlessly for that time. I set the bias a little lower while on the open table, about 0.57 or so. Here the heatsinks ran to ~45 degC.

Moved into the rack and the heatsinks got a little warmer, taking numerous measurements over the whole area of the heatsinks, I get a range of temps from 45-50 deg C. So that would seem to be in the correct kind of area to be hitting (room temp 21-22 degC).

Another few successful cycles like that and I'll be a happy camper.

***********************

Definitely this is a better sounding amp than the mini-aleph (which is in itself a fine sounding amp). And where it seems to do best is from mids on upwards. Now bear in mind I listening thru quads so I don't have access to subterranean bass. Anyway, mids and highs are definitely improved. There seems to be a good resolution there and the soundstage is very good. I think this amp has hit a real sweet spot in that it gives a neutral sound, but still seems to have a sweetness higher up - so you don't seem to get any harshness - but yet is not too lean. I'm looking forward to it settling down and me getting to know it a bit better.

Hats off to you Nelson for this amp. You should be rightly proud.

************************


Fran

[Zen Mod]

Originally posted by ichiban


Hi,
which IRs are you using, I have sets from a Borbely B60 and D100. 2SJ50/2SK135s and the others are higher power versions, I forget the numbers 2SJ56 & ??, all TO3s tho.
Your comment about them being warm but with airiness sounds interesting. I know these mosfets were designed for audio use and are fairly complementary!?

you can try with them , but these have fairly low xconductance comparing to verticals ;and you need gain in F5 output stage ......

you'll have lower open loop gain , so less feedback ;

without any calculation - that amp should have smaller damping factor than F5 .......

second - they also have pretty smaller Ugs , so drain resistors in first stage must be smaller , further decreasing that stage gain ...... further decreasing OLG

don't think that you're first with these thoughts