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Old 11th April 2013, 04:44 PM   #8831
dvv is offline dvv  Serbia
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Quote:
Originally Posted by nigel pearson View Post
Has anyone tried a very long tail on a long tail pair ? If could be zener protected for start up . Put it up at 300 V . Only problem I can see is hiss . 150 K if 2 mA . Without doing the maths I guess the usual CCS to be superior ? The zener would be miles below operating in usual running .
What is a "very long tail pair"?

Could I trouble you for a jotted schematic?
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Old 11th April 2013, 04:59 PM   #8832
dvv is offline dvv  Serbia
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Here's the three types of CCSs I have been using for the last 20+ years.

Frankly, I never could detect any sonic difference between B.) and C.), but Perhaps I never made an amp good enough to convey any possible differences. It's not like I built hundreds of them, less than 10 over the last 20 years.

I'm not saying these are the best there are, but I am saying that I reckon they are very good for the money.
Attached Files
File Type: pdf CCS circuits.pdf (33.0 KB, 28 views)
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Old 11th April 2013, 06:26 PM   #8833
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Quote:
Originally Posted by dvv View Post
What is a "very long tail pair"?

Could I trouble you for a jotted schematic?
Will do it in the morning . Basically a long tail pair circa 1935 had the cathodes of two valves joined together and the connection between them would be the tail ( Alan Blunlein and others ) . The concept was to take the tail to infinity . Thus the tail would see an infinite resistance . The higher the resistance the longer the tail ? Often in valve circuits the tail was very short . With transistors we can have artificially long tails using CCS . My idea is make it long in reality .
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Old 12th April 2013, 08:17 AM   #8834
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by nigel pearson View Post
Will do it in the morning . Basically a long tail pair circa 1935 had the cathodes of two valves joined together and the connection between them would be the tail ( Alan Blunlein and others ) . The concept was to take the tail to infinity . Thus the tail would see an infinite resistance . The higher the resistance the longer the tail ? Often in valve circuits the tail was very short . With transistors we can have artificially long tails using CCS . My idea is make it long in reality .
With due respect to precise technical definitions as per Amos' Electronics Dictionary, if the general discussion has been in relation to the input stages of a transistor amplifier then a Differential Amplifier will mean a long-tail pair arrangement with a single output.

In valve(vacuum-tube) days where the term long tail pair originated, this would mean a phase splitter with two outputs working in push-pull.

You would find this for example V2 in the second stage of the Leak TL12 amplifier - Schematic - Leak TL-12 Plus Tube Amplifier @ AmpsLab.com

Not withstanding that you have used the term long-tail pair in it's correct context I take it you are theorising over an input stage involving a Differential Amplifier.



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Old 12th April 2013, 09:15 AM   #8835
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Click the image to open in full size.

I know of no amplifier better than the Hitachi to show some variations . I didn't spend much time on this and even less thought . I choose 430 V as being cheaply available using a valve HT transformer . For fun did the maths for supplying the VAS . Now that's as long a resistor tail as we might get .

The other example is showing that there is no great voltage swing so equally possible .

Harley Davidson I suggest would never have been made if designed by simulator . How say you about Long Tail Pairs ? Not interested in right or wrong , just why .

Last edited by nigel pearson; 12th April 2013 at 09:18 AM.
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Old 12th April 2013, 05:11 PM   #8836
dvv is offline dvv  Serbia
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With all due respect, Nige, what I see on your illustration is what I have seen countless times, and what I, all by myself, own in two versions, once from HK 680, and once from Toshiba SB-45, both integrated amps. The HK was relatively expensive, the Toshiba was cheap.

In a highly modified mode, I have a third sample, HK 6550, but to even recognize it as a later evolution of the basic circuit, you have to know them all and have some imagination.

But the first two are immediate hits everyone can easily recognize.

All that said, I would now need to be told what is it that makes their tails so long, because as far as I can see, in all cases a differential amp is driving another differential amp, in an attempt to make a single ended input differential amp appear more symmetrical to the output stage.

In all cases, i am very znhappy with the input stage CCS, batteries and all. I have for years maintained - and still do - that instead of fancy input circuits and so forth, the single most beneficial thing you can do for your amp is to separate the power supplies for the VAS from those of the output stage, run them at somewhat higher voltages to compensate for drops across transistors, and allow your current stage to work at slightly lower voltages, so as to move more to the left of the SOAR curve of the output devices.

Not even to mention the fact that full electronic voltage regulation (assuming it's properly done) gets rid of much line noise before it ever reaches the circuits.
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Old 12th April 2013, 05:55 PM   #8837
bcarso is offline bcarso  United States
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Quote:
Originally Posted by dvv View Post
I have for years maintained - and still do - that instead of fancy input circuits and so forth, the single most beneficial thing you can do for your amp is to separate the power supplies for the VAS from those of the output stage, run them at somewhat higher voltages to compensate for drops across transistors, and allow your current stage to work at slightly lower voltages, so as to move more to the left of the SOAR curve of the output devices.
A good policy, only caveat being to ensure, by some means, preventing the output devices, if bipolars, from entering hard saturation at clipping.
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Old 12th April 2013, 06:12 PM   #8838
dvv is offline dvv  Serbia
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A good policy, only caveat being to ensure, by some means, preventing the output devices, if bipolars, from entering hard saturation at clipping.
Spot on Brad, that IS a serious concern.

Now that you mentioned it, how would you go about it, in principle?
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Old 12th April 2013, 06:29 PM   #8839
bcarso is offline bcarso  United States
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Originally Posted by dvv View Post
Spot on Brad, that IS a serious concern.

Now that you mentioned it, how would you go about it, in principle?
I haven't seen a single "best" approach. Schottky clamping is impractical because those diodes are generally too-low reverse breakdown voltage. Having diif amps sensing each collector-emitter voltage, comparing that to a reference, and rapidly limiting base drive is cumbersome. Sensing rails and clamping input signals is difficult to do without introducing other distortions below clipping levels.

This storage time problem with bipolars is one of the reasons DMOS appeals, despite other suboptimal attributes depending on configurations.
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Old 12th April 2013, 06:42 PM   #8840
benb is offline benb  United States
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The problem of keeping a bipolar transistor out of saturation in digital logic was solved with a Shottky diode as used in Shottky TTL:

Schottky transistor - Wikipedia, the free encyclopedia

But power transistors tend to have higher saturation voltage, so this won't work. Surely there's something not much more complicated that will. Following a link from the above article, maybe the Baker Clamp could be used - if this isn't enough to keep it out of saturation, make D1 a Shottky and/or add diodes in series with D2 until it is:

Baker clamp - Wikipedia, the free encyclopedia
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