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Old 11th April 2014, 06:37 PM   #5621
keantoken is offline keantoken  United States
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I think use of ground planes and inert local decoupling will allow us to make the most of any output stage.
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Old 11th April 2014, 07:14 PM   #5622
grhughes is offline grhughes  United States
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I ran into this on the web. I'm not an engineer just an audiophile so I'm taking a big chance in posting this, so be kind. But I have never seen this topology used anywhere except here. I'm referring to the complementary long tail pair TR1 and TR2 used in this schematic. Why is this config not used more and can it be used as a constant current output stage?
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Old 11th April 2014, 07:16 PM   #5623
grhughes is offline grhughes  United States
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Default Complementary long tail pair (tall less pair)

I ran into this on the web. I'm not an engineer just an audiophile so I'm taking a big chance in posting this, so be kind. But I have never seen this topology used anywhere except here. I'm referring to the complementary long tail pair TR1 and TR2 used in this schematic. Why is this config not used more and can it be used as a constant current output stage?
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Old 11th April 2014, 07:45 PM   #5624
keantoken is offline keantoken  United States
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I've used it a few times. It works, but you have to dance around it if you want good overload behavior. I never experienced it to cause bad sound. It doesn't cancel Early voltage effects like the LTP so it can be a source of load-independent distortion. I used it here:

Rush Cascode Headphone Amp + JLH Output Stage

Keep in mind I was 16 or so then, so a lot of things have changed. I still want to revisit the circuit but I'm not sure what direction to take yet.
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Old 11th April 2014, 09:51 PM   #5625
mcd99 is offline mcd99  United Kingdom
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Quote:
Originally Posted by keantoken View Post
I think use of ground planes and inert local decoupling will allow us to make the most of any output stage.

Sorry another beginner question...
Please could you explain what is meant by "inert" decoupling?

Paul
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Old 11th April 2014, 10:20 PM   #5626
keantoken is offline keantoken  United States
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When you parallel a 100n cap with a reservoir lytic, the small cap resonates with the loop inductance through the lytic. This is a parallel resonance, meaning a severe impedance spike. At RF, low PSRR combines with rail impedance to make output phase dependent on rail impedance, so when your ULGF is high relative to PSRR, resonant rails can cause your amp to oscillate. There is also the effect of lead inductance resonating with power transistor capacitances, and this can also disrupt phase at high frequencies.
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Old 12th April 2014, 01:59 AM   #5627
Bob Cordell is offline Bob Cordell  United States
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Quote:
Originally Posted by keantoken View Post
When you parallel a 100n cap with a reservoir lytic, the small cap resonates with the loop inductance through the lytic. This is a parallel resonance, meaning a severe impedance spike. At RF, low PSRR combines with rail impedance to make output phase dependent on rail impedance, so when your ULGF is high relative to PSRR, resonant rails can cause your amp to oscillate. There is also the effect of lead inductance resonating with power transistor capacitances, and this can also disrupt phase at high frequencies.
Good point. I've investigated the problem and it can be a serious concern. See page 350 in my book.

Cheers,
Bob
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Old 12th April 2014, 08:13 AM   #5628
danielwritesbac is offline danielwritesbac  United States
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I've also been investigating, a great deal more crudely, sure, but the soldering iron and much effort was involved.

Most of us are familiar with using 1 of 250v capacitor sized at 1% of the decoupler cap size like this: 470u decouplers and then there is 1 (one) of Cornell Dublier's Mallory SEK 250v 4.7uF capacitor from V+ to V- at the amplifier board.
*common name is rail to rail cap
Benefit: slightly tamer midrange, and slightly cooler amplifier.


HOWEVER, last year I removed that sort of thing and replaced it. . .

More effective alternative:
A diode series with V+ and a diode series with V- and the locale is where the DC cable attaches to the amplifier board. Sizing the diode forward voltage drop depends on the decoupler capacitance, sort of like this:
If decouplers are 100u, then use MBR730 for series elements
If decouplers are 220u, then use MBR1645 for series elements
If decouplers are 330u, then use MUR820 for series elements
If decouplers are 220u||220u (440u), then use 6A05 for series elements
If decouplers are 470u||470u (940u), then use 10A1 for series elements
*If the power supply is also onboard with the amp, so that the DC cable doesn't exist, then as replacement, ferrite beads can be placed over the pin of the series element diodes at whichever side faces away from the amplifier.
*It takes 4 diodes for stereo split rail amplifier and it can have enhanced stereo separation as well.
*common name is virtual dual mono, "VDM"


Anyway, I bought a lovely 4 pack of 6a05's (used for dc power series elements located at the amplifier board) fetched from the local shopping mall, overpaid radioshack for the 6a05's, paralleled up some 220u caps for my decouplers to be low loss 440u per each rail, and had a most excellent experience with the audio quality.
I got a more open sound, tone problems vanished, I was able to set the amplifier gain *slightly* lower, the usable amount of peak dynamic power increased, the stereo separation worked better, and the audio seemed much clearer, which was a nice surprise considering the highly palatable tone.

This bit of simplicity with inexpensive diodes worked ever so much better than excessive cap and cable selection labor.

P.S.
Weirdly, the diodes mentioned can work great on DC power, or very poorly on AC power (doesn't work for rectifier). So, indeed I was using a DC power board which has FairChild Stealth that work great on AC but terrible on DC (doesn't work for the filter). That's just funny.
SO, if you have some diodes that made the worst bridge rectifier ever, you can try re-employing them for my DC filter since it won't be doing any switching.
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Old 12th April 2014, 08:32 AM   #5629
RNMarsh is offline RNMarsh  Thailand
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The diode technique has been used in the past to improve channel to channel separation when single transformer/rect supplies are used. Forgot all about it. THx -Richard
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Old 12th April 2014, 08:44 AM   #5630
danielwritesbac is offline danielwritesbac  United States
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Quote:
Originally Posted by RNMarsh View Post
The diode technique has been used in the past to improve channel to channel separation when single transformer/rect supplies are used. Forgot all about it. THx -Richard
Yes, the separation is noteworthy but not the star of the show.
Given the series diode's forward voltage drop (prior to the amp board decoupling caps), that is also a cousin of a CRC filter, except that the locale is much different so it doesn't hinder charging your power supply reservoir (aka doesn't hinder bass nor cost a larger transformer).
I believe that it was the filtering that helped the amplifier stability and thereby allowed me to set the amplifier gain lower.
P.S.
Due to bizzare furniture arrangement, I was actually using Monophonic, so in my case, I didn't install the filter for stereo separation.
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