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Old 10th March 2010, 04:16 AM   #21
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According to the IRF datasheet for the 640 there's a junction to heatsink thermal impedance of 1.5 K/W. The heatsink has a sink to ambiet impedance of 6.3 K/W best case with natural convection. So if you're dissipating 6.6W then you'll have a junction temperature of 51C above ambient.

If the heatsinks are in an inclosure, even one with ventilations holes above and below the heatsink, then the ambient will be some what higher than room temperature. Assuming 40C is usually a safe enough bet (30C on a hot day + 10C heating within the case). The in case heating will be much higher if you don't have adequate ventilation. This gives a junction temperature of 91C. Which is warm, but no where near the 120C maximum operating temp (The datasheet states 150C but it's reasonably widely accepted that 120 is as warm as you want silicon to get if you need long term reliability).

This is assuming you're using thermal paste, but no isolator. If you use a mica isolator you can be pretty sure you'll add around 1 K/W of thermal impedance, pushing your end junction temp up to around 98C.

Short answer is it'll work, but it'll get warm. I'd consider upgrading to the 2" ones if you have room in your case, though it's not necessary.

Mike.
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Old 11th March 2010, 03:49 AM   #22
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ok, that makes sense. but looking at the somewhat higher thermal resistance of the irf610 (3.5 K/W J-C), it looks like ambient temp is going to be around 108C... which is a bit high for my taste... (what do you think?) i would use the 2" sink, but it only gives 1 K/W less than the smaller one, and i'm trying to keep the case dimensions slim and trim.

there is also the possibility of milling or constructing (from milled parts) adequately sized heatsinks, or even integrating the case top panel (which will be 1/4" 6061 aluminum) into the solution, though it would require mica insulators, and i really would like to be able to test the circuit before marrying it to a chassis.

rod elliott has an interesting article on custom sinks made from milled parts, but using noncontiguous parts seems a bit too much like a hack. any opinions on methods or suppliers? or could i still safely stick with the wakefield sink?

~ brad.
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Old 13th March 2010, 03:54 AM   #23
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ok, i'm going with a different sink. (digikey part no. HS350-ND) interestingly enough, a sink with the same dimensions and material as the original, with a lower natural thermal resistance... wierd. oh well, as long as i sleep better at night.

i'm currently rechecking the BOM and trying to cross as many t's and dot as many i's (and j's) as possible before proceeding to the pcb design, which i should begin soon.

~ brad.
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Old 14th March 2010, 03:36 AM   #24
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Brad I might try something like this myself to compare it with a tube headphone amp.

When you next fire up your simulator perhaps you could see what the distortion figures are for your latest design when it is driving 50mW into 30 Ohms? (i.e. 1.225 V rms)
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Old 14th March 2010, 04:12 AM   #25
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gordy, from my experience, the szekeres is a very 'tubey' amp, so i think you'd be pleased if you chose to try it. it's more detailed than a tube amp without the harshness or overaggressiveness of a bjt amp. a good sound to complement sennheisers and to use for relaxed listening.

i'm on break for this week, but first thing monday i'll have that estimate for you. my guess would be in the ballpark of 0.018% THD, almost completely even harmonics and relatively nothing over the 4th harmonic or so. the latest series of modifications to the design have knocked THD down a bit from the time i posted originally.

i'm also planning on simulating various changes to the shunt regulators, including increasing the CCS current and increasing the CCS BJT base resistor a bit. results and a finalized first version BOM coming monday.

~ brad.
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Old 14th March 2010, 11:50 AM   #26
Gordy is offline Gordy  United Kingdom
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Thanks for the fast reply. I'll sit tight and look forward to your future post(s).
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Old 21st March 2010, 06:45 PM   #27
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Default more changes, results

alright, i've simulated 50mW into 30 ohms, and the results follow...

0.0488% THD:
2nd: -66dB
3rd: -107dB
4th: -116dB
5th: -119dB

past the 3rd harmonic, distortion values are roughly equivalent to (or lower than) power supply line rejection values.

on to the changes...

1. i changed R403,R503 to 0R82 vishay w/w resistors to increase ccs current to 800mA/ch. this means around 200mA/ch of constant dissipation through the regulator shunt mosfets.
2. i changed the heatsinks to HS350-ND parts from digikey and added symbols to the mosfets in the regulator schematic pages.

attached are the new bom and schematic pdf.

~ brad.
Attached Files
File Type: txt bom.txt (1.4 KB, 12 views)
File Type: pdf v1.pdf (30.5 KB, 53 views)
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Old 22nd March 2010, 10:42 AM   #28
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I'd not paid any attention to your power supply previously. I recently designed one almost identical to it for one of my amplifiers. Based on that there are two points I noticed:

1) The 1N3514 isn't sufficiently forward biased to be operating in constant current mode. According to the datasheet I would expect you to require at least 5V to ensure CC mode but you've clamped it to ~0.7V by the base-emitter junction of Q403 (Q503). At this voltage you'd save money by using a simple 140 ohm resistor with no performance hit.

2) The transformer mentioned (TE70064) isn't going to be large enough. It only has a current rating of 1.4A and you're talking of drawing 1.6A (I'm assuming from the schematic, that you only have one unregulated supply). Additionally while it will be possible to obtain 18VDC from an 18V transformer, in reality it's cutting things pretty fine. With 5400uF of cap and a 1.6A load your unregulated voltage will drop down to 20V in the troughs which only leaves two volts for your CCS to play with. This is assuming that your transformer is large enough that there is negligible voltage sag and that your mains is at or above nominal.

Hope this helps,


Mike
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Old 22nd March 2010, 02:09 PM   #29
Gordy is offline Gordy  United Kingdom
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brad, that's very intresting and thanks for posting your results.

You previously said that the circuit was very tube-like and I guess that the 2nd harmonic response reinforces that view. It's nice to see all of the other harmonics below 100dB down. I think that is good for 50mW into 30 R. I'm certainly encouraged enough to try a source follower headphone driver like this.

miket6000, I was contemplating a shunt reg and it's genuinely helpful to read of the real-world experiences of others. I think you have just helped brad, myself, and probably about two dozen lurkers! Thank you indeed.
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Old 22nd March 2010, 02:47 PM   #30
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Quote:
Originally Posted by miket6000 View Post
The 1N3514 isn't sufficiently forward biased to be operating in constant current mode...
thanks, anything to cut BOM cost. i'll try a 140R resistor and a JFET to see just what i can get away with in that position.

Quote:
Originally Posted by miket6000 View Post
The transformer mentioned (TE70064) isn't going to be large enough...
but the design uses a TE70084.

Quote:
Originally Posted by miket6000 View Post
Additionally while it will be possible to obtain 18VDC from an 18V transformer, in reality it's cutting things pretty fine...
do you think doubling the unregulated supply capacitance would sufficiently alleviate this problem?

Quote:
Originally Posted by Gordy View Post
brad, that's very intresting and thanks for posting your results...
glad they are encouraging.

~ brad.
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