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Old 15th November 2012, 04:28 PM   #141
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Originally Posted by homemodder View Post
Sakis I would not say its a bad practice at all, it depends on where the part is used and how. I always use fast parraleled drivers instead of one big slow one like mje15032 series. Harmon know exactly what they doing.

Those BDs are exceptional as drivers although limited by Vce and SOA, two of them can be used in parrallel and they peform better than those toshibas at their max Vce especially at frequencies higher than 1Khz and lower powers.

Lineup is simulating not measuring, reality could be very different. His spice models for the BDs could be incorrect as well as the other models too.
agree .... no problems with your approach also ... Point is that my POV is most of the time coming from repair so i have seen the burned pcb and the thermals around some HK models ...plus he difficulty to find proper parts and re-stabilze them back ...Thank god this happened to only a few models

Kind regards sakis
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Old 15th November 2012, 07:41 PM   #142
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Lineup,

If you don't have the capacity to physically test the design then I'll cobble one together on perfboard to see what it does. I don't know if it will start out as a BIGBT version right away, likely a triple EF since I have suitable parts for that handy. I will unfortunately be occupied with other obligations for a few days, hang tight unless someone else beats be to it.
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Old 15th November 2012, 08:11 PM   #143
lineup is offline lineup  Sweden
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Good News jkuetemann!
sakis and I are planing for a PCB.
He will make and order 4 PCB.
2 for himself and 2 for me.
But I tell him you may be interested in a PCB. Only if you want to, of course.
sakis has all the parts at home because he is an amplifier repairman.

So things progress.
When we have PCB we can see more guys come here and want to join the party.

Regards, Lineup

PS. The circuit will be Triple EF.
Like in the latest schematic I posted up here.
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Old 16th November 2012, 12:24 AM   #144
Struth is offline Struth  Canada
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Hi Guys

I have a bread board of the triple-EF with CSs running, but lo and behold my scope has died! So, I can measure DC voltages but not see wave forms for a little while.

DC offset is a little tweaky - seems very dependent on device parameters. This was a detail Leach mentioned regarding why he did not use CSs or even current mirrors in his design. Matching of devices would help here. And Leach had a point about turn-on thumps when CSs are used, and although his observations were based on late 1970s device performance, his observations are correct. Most people will tolerate turn-on/off thumps, but I do not. Potential damage to the speaker is only one issue; the sharp transient is simply unsettling to hear.

I never like designs that require an offset adjustment. What is the guarantee that the adjustment will be correct a year later? Ten years later? I prefer to make the design itself take care of the offset, and most methods require adding some more BJTs.

One approach is to use a second diamond buffer tied to the output but have its collectors tie into the current sources for the input stage. Depending on what compensation is used in the final circuit form, one may or may not need an RC to filter out the AC signal, between the speaker output and this buffer's input node.

Another approach uses complimentary diff-amps with one side grounded and the other with a filtered version of the speaker output. The appropriate collectors are tied in to the CSs for the input stage. This approach has a lot more gain to correct for larger offsets. However, the basic circuit does not seem to exhibit much offset except when mosfets are used. Still, that offset is higher than I like in an amp.

With the CSs overall response is wider and gain is higher, so layout becomes critical. The symmetry of the circuit suggests a very symmetrical and clean layout. The layout can become tricky with more pairs of outputs if you are going for widest bandwidth. For top performance, it is not just an issue to match devices, but also compensation caps. Cap tolerances are pretty wide, and even if you can get 1-2% discs, mica or polystyrene, you still need to select matched pairs for a given channel assembly.

Have fun
Kevin O'Connor
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Old 16th November 2012, 06:46 AM   #145
lineup is offline lineup  Sweden
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What do you think Struth,
about how to put transistors on the main heatsink.
Should the drivers be put there, too.
How did Mr Leach do with his triple follower.

As I told sakis is making PCB.
We need some advice.
I told him not to put the drivers onto main heatsink .. but I am not sure.
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Old 16th November 2012, 08:03 AM   #146
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U6,7 and U3,9 should be thermally coupled to prevent large DC offset

For pre-drivers depends on their heat dissipation value if they can stay away from the main heatsink
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Old 16th November 2012, 08:10 AM   #147
lineup is offline lineup  Sweden
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Quote:
Originally Posted by Lazy Cat View Post
U6,7 and U3,9 should be thermally coupled to prevent large DC offset

For pre-drivers depends on their heat dissipation value if they can stay away from the main heatsink
Yes, I told sakis those 4 transistors should be close, if not in touch with eachother.

For the 2SC4793/SA1837 drivers, they have like 50-55 mA running.
So they do get a bit hot.
But I have decided to put them on smaller separate heatsinks.

Only the Output transistors MJL3821/1302A together with the BD139 vbe multiplier on the Main heatsink.

Do you think this will be good?

My reference is the wellknown M. Leach lowtim amplifier
which uses Triple EF figuration of output stage.
The Leach Amp - Output Stage

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Old 16th November 2012, 08:37 AM   #148
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Quote:
Originally Posted by lineup View Post
Yes, I told sakis those 4 transistors should be close, if not in touch with eachother.
Not only close, pair's flats faces tightly bonded together.

For triple output Leach approach is just fine.
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Old 16th November 2012, 08:59 AM   #149
pergo is offline pergo  Italy
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can we use DC-servo?
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Old 16th November 2012, 09:41 AM   #150
lineup is offline lineup  Sweden
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Quote:
Originally Posted by Lazy Cat View Post
Not only close, pair's flats faces tightly bonded together.

For triple output Leach approach is just fine.
Thanks Lazy Cat.
I tell him.


Quote:
Originally Posted by pergo View Post
can we use DC-servo?
Of course one could add DC-Servo.
But I dont think we should.
There are other ways more simple.

If you look at my latest schematic in this post: New Lineup IDEA - Power Follower/Output stage
you see one trimpotentiometer on the input.
This is the way to adjust for the output offset.

Also one could adjust the current sources of the transistors.
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