woodturner-fran said:
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
Nelson Pass said: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.
Zen Mod said:
don't tell me that I didn't told ya ......
Nope, you told me
Diomedian said:
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
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.
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.
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
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
which IRs
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 said:
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!?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
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
Re: which IRs
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
ichiban said:
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.
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
> 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 ;
True. But you can lower the source resistor from 0R47 to 0R22 to compensate for that, as the source resistor is actually dominating in the original circuit.
> second - they also have pretty smaller Ugs , so drain resistors in first stage must be smaller , further decreasing that stage gain ...... further decreasing OLG
Not true. You can keep the drain resistor values by lowering the first stage bias by say 2mA or so. No problems for the JFETs.
> don't think that you're first with these thoughts
Of course not. Been there before, umteen pages back.
Jackinnj also have a layout with Hitachi laterals.
Patrick
True. But you can lower the source resistor from 0R47 to 0R22 to compensate for that, as the source resistor is actually dominating in the original circuit.
> second - they also have pretty smaller Ugs , so drain resistors in first stage must be smaller , further decreasing that stage gain ...... further decreasing OLG
Not true. You can keep the drain resistor values by lowering the first stage bias by say 2mA or so. No problems for the JFETs.
> don't think that you're first with these thoughts
Of course not. Been there before, umteen pages back.
Jackinnj also have a layout with Hitachi laterals.
Patrick
EUVL said:
true ...... but even in stock F5 current through first (Jfet) stage is already on lowish side ........
I know that Jackinnj tried them ( and like them ) ...... but 2mA is sooo low ..... and that wasn't F5 anymore ;
you can do that , Jack too ..... but not everyone , knowing what's needed to make it work .
> Any particular reason that feedback resistors are listed as 3W, they seem to run rather cool?
The theoretical max swing is say -4V from rail.
Even in my balanced version, that would be 12V. Which means 10V across the feedback resistor.
10*10/50/2 = 1W. Safety factor 3 -> 3W.
I actually use 4x PRP 1W in parallel.
Actual signal level is of course normally much lower.
So in theory you might get away with 0.5W or so.
Patrick
The theoretical max swing is say -4V from rail.
Even in my balanced version, that would be 12V. Which means 10V across the feedback resistor.
10*10/50/2 = 1W. Safety factor 3 -> 3W.
I actually use 4x PRP 1W in parallel.
Actual signal level is of course normally much lower.
So in theory you might get away with 0.5W or so.
Patrick
Oh man, I am in trouble now.
I am using 4 F5's -- with Fairchild devices driving these Pioneer (TAD) S-1eX spxrs. I have a bunch of the Renesas (Hitachi) devices but haven't used them since last year.
On the bench is an NP/aX design from the AX article -- and we are are gonna look at the THD% figures for various combinations of JFET with Idss all over the lot. I know that THD% is not the ne plus ultra stat for the definition of quality, but it's the onlu completely reliable and reproducible stat that I can produce.
I am using 4 F5's -- with Fairchild devices driving these Pioneer (TAD) S-1eX spxrs. I have a bunch of the Renesas (Hitachi) devices but haven't used them since last year.
On the bench is an NP/aX design from the AX article -- and we are are gonna look at the THD% figures for various combinations of JFET with Idss all over the lot. I know that THD% is not the ne plus ultra stat for the definition of quality, but it's the onlu completely reliable and reproducible stat that I can produce.
Fran,
Try a new pot and also reflow your solder connections to the pot on the board and on the pot. If that doesnt do it then try reflowing the rest of your soldering. I find that usually when I have crackling due to change in volume that there is a screwed up solder joint somewhere and I have also had that happen with junk pots or nice pots with bad solder joints. Never know, if you are using a stepped pot you could have a nasty joint somewhere on there as well.
Uriah
Try a new pot and also reflow your solder connections to the pot on the board and on the pot. If that doesnt do it then try reflowing the rest of your soldering. I find that usually when I have crackling due to change in volume that there is a screwed up solder joint somewhere and I have also had that happen with junk pots or nice pots with bad solder joints. Never know, if you are using a stepped pot you could have a nasty joint somewhere on there as well.
Uriah