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Old 11th April 2013, 06:00 AM   #2611
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That is basically an Allison driver circuit, with emphasis on thermal matching. These require extreme care to stabilize, and it can be very difficult. Perhaps not as difficult when the outputs aren't included in the feedback loop. Furthermore aberrations in the circuit's response may limit the global stability. Refer to Ed Stuart's Super TIS page for the best way to stabilize Allison-related bias circuits.
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Old 11th April 2013, 06:10 AM   #2612
dadod is offline dadod  Croatia
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Originally Posted by Dave Zan View Post
The recent discussions over in the Self thread have made me revisit the Thermal Trak Vbe issue.

Bob posted thisThermal Trak Circuit before the book. It looks attractive but it never seems to have been followed up, I am curious to learn why.

Best wishes
David
I used this part in my TT amp:

"The two “extra” ThermalTrak diodes, D3-4, are used between the emitters of the driver transistors to establish the idle current of the drivers while keeping the impedance between the emitters very small, so that the drivers can operate in push-pull to provide turn-on and turn-off current to the output transistors. Keeping the impedance between the driver emitters very low at high frequencies is essentially what is often done with the speedup capacitor. The ThermalTrak diodes make it possible to do this in a d.c. fashion with the necessary precision because they track the output transistor Vbe’s."

BR Damir
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Old 11th April 2013, 06:45 AM   #2613
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IIRC the need for low impedance here was to prevent charge suckout from twanging the output bias.

In this case the diodes serve no purpose but to keep the driver idle more constant, because the Allison feedback loop lowers the driver output impedance greatly. This does not necessarily eliminate the need for the cap though, because regardless of any feedback loop, the drivers can't sink current in reverse. By the time the diodes are conducting the suckout current, the odd driver will already be in reverse saturation.

So a suckout cap here may still be needed. Unfortunately because the driver Zout has been reduced so much, you'll need a much larger cap in order to get the same level of drive sharing between the drivers.

So, actual dynamic ability of this output stage at high powers and high frequencies may be considerably less than that of an ordinary driver stage.

This is of course assuming that charge suckout is actually a problem. Is it really? The ONSemi devices are really fast, with very low Cbe. What does it take to saturate the drivers into these devices?
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Old 11th April 2013, 03:07 PM   #2614
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Originally Posted by dadod View Post
I used this part in my TT amp:
Yes, I noticed that, Ovidiu Popa also used it, but few others.
It makes sense to me and I wondered why Bob did not follow up with it in his book.
Actually it was your plan to use this with 4xxx transistors that made me re-check to see if it would still work with the different diode Vf. Then I became curious!

Quote:
The two “extra” ThermalTrak diodes, D3-4, are used between the emitters of the driver transistors to establish the idle current of the drivers while keeping the impedance between the emitters very small, so that the drivers can operate in push-pull to provide turn-on and turn-off current to the output transistors.
Yes, I understood Bob's explanation. It seems an excellent idea.
Unfortunately I see no obvious way to use this in a small amp with only one output pair. Damn.

Best wishes
David
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Old 11th April 2013, 04:50 PM   #2615
dadod is offline dadod  Croatia
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Quote:
Originally Posted by Dave Zan View Post
Yes, I noticed that, Ovidiu Popa also used it, but few others.
It makes sense to me and I wondered why Bob did not follow up with it in his book.
Actually it was your plan to use this with 4xxx transistors that made me re-check to see if it would still work with the different diode Vf. Then I became curious!



Yes, I understood Bob's explanation. It seems an excellent idea.
Unfortunately I see no obvious way to use this in a small amp with only one output pair. Damn.

Best wishes
David
You always can use capacitor parallel to the drivers emitter resistor, but then the bias spreader should be a bit different(different resistor values).
BR Damir
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Old 11th April 2013, 08:25 PM   #2616
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Originally Posted by keantoken View Post
That is basically an Allison driver circuit ...
Could you post a link to this please.
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Old 11th April 2013, 11:03 PM   #2617
forr is offline forr  France
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Quote:
Originally Posted by dadod View Post
I used this part in my TT amp:

"The two “extra” ThermalTrak diodes, D3-4, are used between the emitters of the driver transistors to establish the idle current of the drivers while keeping the impedance between the emitters very small, so that the drivers can operate in push-pull to provide turn-on and turn-off current to the output transistors. Keeping the impedance between the driver emitters very low at high frequencies is essentially what is often done with the speedup capacitor. The ThermalTrak diodes make it possible to do this in a d.c. fashion with the necessary precision because they track the output transistor Vbe’s."

BR Damir

Could you show the schematics and performances ?
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Old 12th April 2013, 03:25 AM   #2618
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Originally Posted by rsavas View Post
Hi Bob & others,

As we all know, writing something in a book/design spec. and getting it to work in practice, is not always the same. Lots of variables.
I see this in Doug Self's books too, okay I realize that you can buy the stuff from Signal Transfer company. Is there a way to make it more available/affordable to the masses?
How about saying that it is made in North America for a change Too bad lots of my parts are made/assembled in Asia, but that I have little control over and learn to live with.

My suggestion is to actually take one or more ckts as described in the book and make them work. I know Bob has done his own implementations, but they are not generally available to people who just want to buy the pcb(s), parts & solder them up. Most people on this site can figure it out. I know chassis are available. If not I'll do it for you, since I am looking for work
Not sure if this a proper thing to discuss on this site, since they offer there own stuff?

What do you think Bob, I can offer to do a layout or two for you for $0, maybe some things for your web site. Maybe like what you did with VinylTrack, still gotta buy that copy of LinAudioSanta did get me that one.
I have basically done this of late, just have to fab/test:
1) Input AC assy with auxillary power(+5Vunreg/3v3 lin reg, for logic, soft-start relay, remote on/off.
2) AC rectifier assy, big ecaps, fusing.
3) Linear regulators (+/- 65 -75V),+/-15V, +5/3V3, PA protection ckts
4) LME49830 (WireAmp) clone. (Could be convinced to make this as a super gain clone)
I always like to have others review my stuff since I have no trade secrets with this design. Since the AC stuff is very important, this is where I would like to have a second eye.
I am sure that there are many ways to implements this stuff, but the idea is to make it small, not overly complicated/expensive, use easy to get parts, Mouser, Digi-key, Newark. Make it of quality construction
As it is, I have already bought all of the expensive parts for this PA project. As well I have got the radio/media player/pre-amp designed, working/tested, so I am going through with my stuff.
Listening to a Stones LP on the FM (Q107/Toronto) radio, "Going Home", first album "After Math", the record crackle was a welcome change.

Nothing like this PA
Prototype power amplifier
I'd like to see his invoices!

Cheers
Rick
Hi Rick,

Sorry to be so slow getting back to you. These are great DIY ideas, and I have given thought to something like this, but just have not had the time. You're right - its one thing to breadboard a prototype and measure it, and yet another to create it in a way that others can readily duplicate. I generally do my prototyping on perf-board where the components are placed and routed in a way that is close to how the corresponding PCB would be laid out.

Send me a PM.

Cheers,
Bob
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Old 12th April 2013, 03:40 AM   #2619
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Quote:
Originally Posted by Dave Zan View Post
The recent discussions over in the Self thread have made me revisit the Thermal Trak Vbe issue.

Bob posted thisThermal Trak Circuit before the book. It looks attractive but it never seems to have been followed up, I am curious to learn why.

Best wishes
David
Hi David,

I am guilty as charged for not following up on it. I was writing my book at the time and was wrestling with the need to pull charge out of the power transistors to turn them off fast enough and mitigate dynamic crossover distortion. There is a section in my book where I discuss the problem, but did not follow up on this scheme. The usual capacitor from emitter to emitter of the drivers appeared to need to be quite large to be effective, and the charge being pulled out of the power transistors could charge up this capacitor and upset the bias shortly thereafter. I thought that the availability of the ThermalTrak diodes could provide an opportunity to achieve the function in a DC way that would not involve a charge storage device (i.e., the speedup capacitor).

The effective base-emitter capacitance of a power transistor that is conducting significant current is surprisingly large, even for a fairly fast power transistor. The hybrid-pi capacitance (gm/wt) gives a hint of this, although the effective capacitance changes with current.

I should have followed up on this idea. Indeed, there may be real-world practicalities that make it difficult to implement reliably.

Cheers,
Bob
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Old 12th April 2013, 03:50 AM   #2620
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by Bob Cordell View Post
Hi David,

The effective base-emitter capacitance of a power transistor that is conducting significant current is surprisingly large, even for a fairly fast power transistor. The hybrid-pi capacitance (gm/wt) gives a hint of this, although the effective capacitance changes with current.

Cheers,
Bob

can you give me a typical value/range of this effective C for significant current ??

Thx-RNMarsh
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