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Old 28th January 2012, 05:57 AM   #1501
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by tvrgeek View Post
I have not posted a picture in so long, I have no clue what site I used to link to. Any advice? I have the cleaned up DH 120 drawing.
Best is to upload them right here. When you're on the "Reply" screen, scroll down a bit to the "Additional Options" section and click on the "Manage Attachments" button (or link).
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Old 28th January 2012, 06:00 AM   #1502
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I was using an EB design as an example of a 'problematic' approach IF one did not have complete control of the parts used, like many people here. Actually, in context, EB's design was OK, as he had control of the parts used.
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Old 28th January 2012, 03:10 PM   #1503
tvrgeek is offline tvrgeek  United States
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Cleaned up DH-120 with the bridge and dyna-qyad removed
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File Type: jpg DH-120-2.jpg (126.9 KB, 143 views)
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Old 28th January 2012, 03:17 PM   #1504
tvrgeek is offline tvrgeek  United States
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Quote:
Originally Posted by bcarso View Post
Finally seeing the schematic, that unballasted current mirror with the peculiar diode definitely got my attention There must have been a reason, although I guess EB has sworn off the forum.
I was not going to ask about that diode until I understood a lot more.
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Old 28th January 2012, 04:27 PM   #1505
DF96 is offline DF96  England
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Most of the 'extra' diodes seem to be there to stop BJT collector saturation. I guess that means they may be Schottkys.
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Old 28th January 2012, 04:54 PM   #1506
tvrgeek is offline tvrgeek  United States
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They are high speed, 4nS switching diodes, but not schottkey. D14 was the first in question, but 5, 6 and 15 I also do not understand yet. So much to learn.
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Old 28th January 2012, 05:11 PM   #1507
Previously: Kuei Yang Wang
 
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Hi,

Quote:
Originally Posted by tvrgeek View Post
I was not going to ask about that diode until I understood a lot more.
Looks to me as if it limits the voltage across the current mirror.

I would probably give the mirror some degeneration AND I would look at adding J-Fet buffers to the existing LTP. Use a 2SK389/LSK389 with a 56K resistive tail. In any case, I'd probably up the emitter degeneration resistors quite a bit. Especially with a CM load the degeneration only improves linearity, gain is only minimally reduced.'

Alternatively going for J-Fet inputs, at a minimum with zenner diodes in the drain leads to keep within the voltage limits, preferably fully cascoded may be better.

I'd also look at the VAS loading RC (C3/R11), it seems a bit brutal. It may be smarter to use a higher value and return it to the inverting input (See Bob Cordell).

C9 & C14 can probably use to fit the largest value you can physically fit (a few 1,000uF will do no harm and a Diode in series with R17/R22 may then be beneficial.

Q6, Q7 & Q9 are quite critical to sound quality, I think you can better than what is in place. Samuel Groner's commentary on Self's Power Amp Design Book can serve as beacon (another piece that deserves JAES publication and self publication). You could even use a J-Fet or Mosfet for Q6.

Q10 & Q11 are up next, Goldmund uses some very low Cob transistors there, for good reasons. These used to be "Video Transistors", with very high voltage ratings and very low Capacitances and high Ft, it seems most manufacturers obsoleted these parts with the demise of CRT TV's...

C1 and C4 would be next on my list, C1 for shorting out if a J-Fet (buffered) input is used, and with Fet buffered input you could use a "trick" circuit to deal with C4 - use a 1M resistor from the output to -Input and then a 1uF Film Cap of high quality to the point marked "B" on schematic, with the original connection disconnected.

Your DC Blocker capacitor is now inside your feedback loop and thus it's sonic impact is reduced and you can use a film cap. If you like a little "soft/warm" flavour for the Amp find Polycarbonate Cap's for this, I loved the old Siemens stacked film polycarbs for modding SS Gear, shame they are history.

If you use MKP you will have a bit more clarity, MKS will be even warmer but perchance a bit too soft. Inside the feedback loop the differences are quite a bit reduced.

My final point of attack would be to look at all these clamping diodes (D5, D6, D10, D13, D14, D15 - the others are voltage references) and look for the lowest reverse biased capacitance diodes you can fit.

Have a look at all these compensation cap's. If you can find them, my recommendation is non-magnetic mica of NP0/C0G Ceramic.

As a last item, the rail fuses may warrant attention, both fuse holders and fuses. Not that I'm suggesting expensive audiophile fuses there, but not all fuse holders and fuses are created equal. I would look for magnetic materials, good contact pressure etc. first.

Ciao T
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Old 28th January 2012, 06:24 PM   #1508
tvrgeek is offline tvrgeek  United States
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Notes taken. (5 pages from this thread) Thanks. I am avoiding big circuit changes until I understand it a lot better. Baby steps. When I get the Fairchild parts, I'll probably go ahead and put 100 Ohms in the CM, just because I'll have the transistors out. For now, study.

I also need to take time out and get the process for measuring IM down. I think I can do it with my existing PC tools.
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Old 28th January 2012, 07:09 PM   #1509
bcarso is offline bcarso  United States
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Thorsten makes many good points. I lament the passing of the CRT driver parts mentioned as well. John Addis told me about a current source he'd designed using a Philips (pre-NXP) 100V low-output-C part in a Larson* super-alpha pair with a 5GHz auxilliary transistor, with a 100V breakdown and a broadband output capacitance well below a pF, and sadly the main part is long gone. And Groner's articles are quite worthy of study.

But I would proceed with caution --- as you well know you're playing with fire here. I can't for the life of me see how that one particular diode helps, but EB surely put it in for a reason. And it seems the unballasted (no emitter resistors) current mirror's only virtues are pulling closer to the rails and being fast, but entail a noise disadvantage (see Groner for discussion).

Brad


*the circuit that feeds the main transistor base current back into its emitter via a complementary common-base stage, which I recently discovered dates back to at least 1957 as applied to common-base amplifiers for telephony by one Dr. Frank S. Boxall, work that apparently thereafter went mostly unnoticed. Larson at Tektronix filed a patent app in 1965, Baxandall and Swallow wrote about its application as a current source in 1966, and it's most-known today as the Baxandall super-pair (I'm sure none of them were aware of Boxall). Boxall called it the Feedback Compound Transistor (see for example Semiconductor Products magazine, Sep./Oct. 1958, especially Fig. 9(c) ).
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Old 28th January 2012, 07:20 PM   #1510
Previously: Kuei Yang Wang
 
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Hi,

Quote:
Originally Posted by bcarso View Post
I can't for the life of me see how that one particular diode helps
It is a single ended VAS. At clipping it can be driven to quite a decent current, more perchance than it likes.

The diode and other halve of the CM limit the base voltage of the VAS Transistor to two diode drops, say 1V, this limits the current. Nowadays often a protection transistor is employed instead...

I think I like Erno's diode more better. There are also baker clamps and other clamps, they may perhaps be not necessary and may omit them, but Erno was clearly going for "Belts, Braces, Flack Jacket, Ceramic Lined Helmet and a sandbag wall" in his fight to make sure the Amp stayed reliable (nothing for SQ) and recovered quickly and cleanly from any clipping (which does have an impact on SQ though only if we are clipping the amp).

Ciao T
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