If you are using the plastic TO3P style output devices, another technique for bias tracking is to use an SMD device and mount this close to the collector lead of one of the power devices on the PCB. The collector lead is part of the transistor heatsink tab and is in very good thermal contact with the die inside the package so the temperature of the collector lead closely tracks the die temp - and the thermal response is quick. If you use an SMD device it has a small thermal mass so it is quickly follows the die temp. I use this on my Ovation amp - works very well. You need to be pretty adept at soldering small devices, but its not insurmountable and quite easy to get a good solder joint. BTW, most suppliers will sell small qty's of SMD devices (SOT23) so you don't have to buy a whole reel.
Andrew,
photo attached. Search for the 'Ovation' amp on this site for pictures of final result. I use this technique in that amp (I'm listening to it now over my B&W 703's - heatsinks at approx 50C and nice and stable with Iq set at circa 500mA total across 5 pairs 21193/4, solid class AAB. Forgive the diversion, I've had a bottle of Shiraz to myself . . . warm and fuzzy vs the stuff from S.A. which is way older and heavier . . . )
I've been recently working on refining the overall design so the temp sensor has been moved slightly away from the power transistor to aid in assembly.
Re the Q about where the device is located. The main power devices sit between the PCB and the h/sink. The Vbe sensor transistor therefore sits underneath the PCB - i.e. between the PCB and the h/sink.
I used to design industrial instrumentation and one of the challenges was thermocouple cold junction compensation - this is where I got the idea from.
Hope this answers your question.
photo attached. Search for the 'Ovation' amp on this site for pictures of final result. I use this technique in that amp (I'm listening to it now over my B&W 703's - heatsinks at approx 50C and nice and stable with Iq set at circa 500mA total across 5 pairs 21193/4, solid class AAB. Forgive the diversion, I've had a bottle of Shiraz to myself . . . warm and fuzzy vs the stuff from S.A. which is way older and heavier . . . )
I've been recently working on refining the overall design so the temp sensor has been moved slightly away from the power transistor to aid in assembly.
Re the Q about where the device is located. The main power devices sit between the PCB and the h/sink. The Vbe sensor transistor therefore sits underneath the PCB - i.e. between the PCB and the h/sink.
I used to design industrial instrumentation and one of the challenges was thermocouple cold junction compensation - this is where I got the idea from.
Hope this answers your question.
Attachments
Would all this advice apply to a Leach amp VAS section? I'm just learning this stuff and in the process of building a Leach amp from the Prof's own PCB's. But I'd really like to replace any devices that would be better handled by something more modern -- especially in the case of those tophat packages.
I see Jens @ Delta used MJE340/350 pair, but it doesn't seem to offer any advantages that I can see other than the package type itself. And its datasheet doesn't list its FT or COB. TIP47 or MJF47 seem very similar; FT is 10 MHz but no COB is listed.
What are the volts/amps at work in the Leach VAS that I should be cognizant of when substituting? (I'm a little ohm's-law challenged still).
Any advice? Thanks.
...Todd
I see Jens @ Delta used MJE340/350 pair, but it doesn't seem to offer any advantages that I can see other than the package type itself. And its datasheet doesn't list its FT or COB. TIP47 or MJF47 seem very similar; FT is 10 MHz but no COB is listed.
What are the volts/amps at work in the Leach VAS that I should be cognizant of when substituting? (I'm a little ohm's-law challenged still).
Any advice? Thanks.
...Todd
VAS parameters for Leach
Hi,
for VAS and pre-driver the following are extracted from Leach v4.5 min spec.
Vce0>[+Vrail to -Vrail]
P>1W
Ic>50mA
hFE>50 @ 1mA to 10mA
fT>30MHz
Cob<10pF @10V
2sa1360/c3423 exceeds all these specs by a considerable margin.
All the small signal devices can be replaced with bc550c/560c
The drivers can be MJE15034/5 or 15032/3 or 2sb649/D669
The output devices can be replaced with any of the 200W+ devices from MJ21193/4 through to MJL4281/4302
Hi,
for VAS and pre-driver the following are extracted from Leach v4.5 min spec.
Vce0>[+Vrail to -Vrail]
P>1W
Ic>50mA
hFE>50 @ 1mA to 10mA
fT>30MHz
Cob<10pF @10V
2sa1360/c3423 exceeds all these specs by a considerable margin.
All the small signal devices can be replaced with bc550c/560c
The drivers can be MJE15034/5 or 15032/3 or 2sb649/D669
The output devices can be replaced with any of the 200W+ devices from MJ21193/4 through to MJL4281/4302
mlloyd1 said:
thanks mlloyd
I have never made a transistor spice model of my own.
I have just copied and pasted others.
Maybe time I study the howto of making models from curves ....
The 2SA1209/2SC2911 datasheet from Sanyo is very good.
With curves for most anything valid!
Download from my PDF web folder and have a look:
http://lineupaudio.freehostia.com/pdf/contents.php
Login. Exclusively for www.diyaudio.com members
Current Password: diyaudio
( notice, this password may I decide to change at any time,
like when I see those not not authorized by myself are downloading/copycat-ing my stuff )
Lineup
thanks, fotios
I look into it. I be back.
Lineup, regars
===================
edit:
The spice models for Onsemi MJD44H11 (npn) and also for MJD45H11 (pnp)
can be downloaded from this page:
http://www.onsemi.com/PowerSolutions/supportDoc.do?type=models&category=796
( .sp3 is the Level 3 .. sometimes not different from Level 1 )
thanks again
I look into it. I be back.
Lineup, regars
===================
edit:
The spice models for Onsemi MJD44H11 (npn) and also for MJD45H11 (pnp)
can be downloaded from this page:
http://www.onsemi.com/PowerSolutions/supportDoc.do?type=models&category=796
( .sp3 is the Level 3 .. sometimes not different from Level 1 )
thanks again
lineup said:
Hi Lineup, glad to meet you again after long time
These devices offered from ONsemi in two versions: MJD44-45H11 in D-Pack (in reel as prefer danville) and D44-D45H11 in TO-220 package. They are great devices with Vce=80V, Ic=8Amax, Pd=20W, ft=50MHz, hFE=250-350 for Ic=1A, the only drawback??? it is the Cob=130-230pF. I have implemented and checked those in two of my projects and their behaviour proved excellent.
The most important it is that we can obtain either of the two versions very easy. For Europe from Farnell and for USA from Digikey, in very low prices.
Regards
Fotios
fotios said:
yes.
absolutely.
BD139/140 are also 80 Volt devices.
Here we have a very good alternative to try.
I guess with a great deal of performance improvements.
As far I I can see, from a quick look at the curves in datasheet.
Lineup - from Sweden
--------------------------
PS.
Did you know, my friend,
that under the Junta military occupation of your country, Hellas
while sweden in the regime of Olof Palme
Mikis Theodorakis & Melina Mercoury often visited Sweden with musical conserts, under exile form home country,
for the support of renewal Democracy in Greece.
Finally was a success, after many & much sufferings from Ellas brave activists.
Today, Grekland is a better land, with fair elections to Governments
DS.
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