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Old 26th July 2010, 07:42 AM   #181
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Cool! Any pics of the bench? (to prove there are no burn marks )

- keantoken
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Old 26th July 2010, 09:52 AM   #182
godfrey is offline godfrey  South Africa
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Hi Ostripper

A while ago someone asked about the purpose of the 68pF capacitor which is labeled "C8 (optional)" in the latest schematic.

As far as I can see, it plays an important role in improving power supply rejection.

When the positive supply voltage is jumping up and down wrt earth, C9 injects junk current into the base of Q7. C8 compensates for this by injecting a similar junk current into the base of Q8.

Similarly, the junk current flowing out the bottom of C8 into the emitter of Q9 goes round the mirror to the VAS output, where it cancels out the junk current flowing out the bottom of C9 to the VAS output.

C8 and C9 have to be the same value for this to work but even if you have a 10% mismatch in their values due to component tolerance, you still get a 20dB improvement in PSRR.

Around that time, a couple of variations were proposed, most of which would have made PSRR worse. Eg: if the bottom of C8 is connected to the collector of Q10 instead of the emitter of Q9, PSRR of the positive rail will still be good, but PSRR of the negative rail will be screwed up.

I'm glad to see you stuck to the original.

Sorry about chipping in with this so late but my tatty old grey matter seems to spit out useful insights somewhat randomly as and when it feels like it, rather than when required.

Cheers - Godfrey
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Old 26th July 2010, 05:02 PM   #183
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Quote:
Originally Posted by godfrey View Post
Hi Ostripper

A while ago someone asked about the purpose of the 68pF capacitor which is labeled "C8 (optional)" in the latest schematic.

As far as I can see, it plays an important role in improving power supply rejection.

When the positive supply voltage is jumping up and down wrt earth, C9 injects junk current into the base of Q7. C8 compensates for this by injecting a similar junk current into the base of Q8.

Similarly, the junk current flowing out the bottom of C8 into the emitter of Q9 goes round the mirror to the VAS output, where it cancels out the junk current flowing out the bottom of C9 to the VAS output.

C8 and C9 have to be the same value for this to work but even if you have a 10% mismatch in their values due to component tolerance, you still get a 20dB improvement in PSRR.

Around that time, a couple of variations were proposed, most of which would have made PSRR worse. Eg: if the bottom of C8 is connected to the collector of Q10 instead of the emitter of Q9, PSRR of the positive rail will still be good, but PSRR of the negative rail will be screwed up.

I'm glad to see you stuck to the original.

Sorry about chipping in with this so late but my tatty old grey matter seems to spit out useful insights somewhat randomly as and when it feels like it, rather than when required.

Cheers - Godfrey

Thanx for input , I have not explored that aspect of C8 but I left the pads for it , just in case. What I did for PSRR is added the capacitance multipliers and referenced both the cascode and CCS to the rails . On the simulation I added 4V p-p @ 120 hz AC to the rails and measured PSRR ... --80DB + all across the audio band. On the amp in the picture I have driven full house shaking power and heard NOTHING on the undriven channel .... I tried this with and without the second 68pF cap.

As far as sticking to the original , "If it ain't broke , tweak it more" . I like the originals sound and it is 13 months old , no This one is just a pro replacement for it.

Quote:
By keentoken - Cool! Any pics of the bench? (to prove there are no burn marks )
My bench .. HA ! it is my desk .. I am poor. (see below) I was lucky to get my 20mhz scope and a amp. As far as funds , I have to wait till august to print the boards (plenty of time to check for errors. BTW , I have just burnt 1
njw0302 in 2 years and a dozen amps.

OS
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Old 27th July 2010, 12:57 AM   #184
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Just wanted to see if my signature was updated with my website on it.
All mongrel material and LT simulations are on it.

The root of the website has EVERYTHING (100,000 files)
The mongrel subfolder has all my current work. ENJOY
PDF 1 has all the electronics content.
OS
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Last edited by ostripper; 27th July 2010 at 01:10 AM.
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Old 27th July 2010, 03:57 AM   #185
Bonsai is offline Bonsai  Taiwan
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"When the positive supply voltage is jumping up and down wrt earth, C9 injects junk current into the base of Q7. C8 compensates for this by injecting a similar junk current into the base of Q8.

Similarly, the junk current flowing out the bottom of C8 into the emitter of Q9 goes round the mirror to the VAS output, where it cancels out the junk current flowing out the bottom of C9 to the VAS output."

The collector of Q8 is held at 1 diode drop above 0V by Q9. If you look at the base of Q8, it will only be moving around by a very small amount. On the Q9 side, C9 has very large voltage swings across it because one end is tied to the LTP collector load and the other to the VAs output, so the situation is completely different, and substantial capacitor currents therefore flow in C9, compared to C8.

I don't think C8 does anything here. As far as PSU noise injection goes, a diff pair is fairly immune to rail noise on the collector load (or mirror) side and especially where the diff amp is cascoded as is the case here. Besides, this design has a capacitance multiplier type rail filter - it should be fairly clean.
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Old 27th July 2010, 04:43 AM   #186
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Default All PDF's

I converted everything to PDF for smaller DL size.

Here : HFS /WEBSITE/MONGREL/

"overview parts" is a verbose placement map of both boards which coincides with the Mongrel V1.1 schematic. These should look REAL good when printed,etched and stuffed. LTP and cascode are "face to face" for heat-shrink thermal coupling. Power board marked for implementation of EF3 (2 - 3.3k 2w resistors).

To answer the question of "how the boards go together" ,
the voltage amp board will have:Click the image to open in full size.

Power board will have :

Click the image to open in full size.

PS. if you see any errors , please tell ..

Quote:
"When the positive supply voltage is jumping up and down wrt earth, C9 injects junk current into the base of Q7. C8 compensates for this by injecting a similar junk current into the base of Q8.
Yes , bonsai , I had a hard time verifying this both real world and simulation wise. I did leave the option open on the PCB.
OS
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File Type: jpg power_board_overview.jpg (259.9 KB, 258 views)
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Last edited by ostripper; 27th July 2010 at 04:53 AM.
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Old 1st August 2010, 06:59 AM   #187
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Default No more b_s..

Back at it !! This time , NO MORE B-S .... After looking at so many DIY and commercial amps , I think I have what I want , maybe even what others may want.

The previous layout was good and would of even worked well , but it was too specific to my purposes and had a tired, typical look. In order for my DREAM of the universal AB amp to be reality a more useful scheme had to be designed.
I ordered the parts ($171.00 for 6 amps), that is $28 an amp ... not bad.

THE CHANGES. The new board is the PB250 -8 output devices + Dual capacitance multipliers ... capable of 250W 8R /350 - 400W 4R depending on which devices are used. I kept the CX1.1 (symasym voltage amp), since it is perfect.

Here is what will be available. All have actually been built at one time or another (frugalamps - supersyms, etc.)

POWER BOARDS
PB-250 /250W/ biggest BJT board ,70-80V rails..... completed.
PB-70 /70W/ baby BJT board ,power supply built in... all in one!! (work in progress - 8/5/10)
PBM-200 / Vertical mosfet power board (still pondering this one )

VOLTAGE BOARDS
These are fun ...
AX1.1 - Your typical Doug- Self blameless amp , tweaked to the max ... .0002% at 1khz , .002% at 20khz , definitely the least distortion of them all.

BX1- The bootstrap amp , lousy .02%THD .. but ,when built, very musical indeed!! Mostly 2nd harmonic even as it is the "dirtiest amp".

CX1.1 -balanced "symasym" type amp , seems to have the best characteristics of the AX AND BX , 2nd harmonic dominant but with the super low distortion of the self amp.

NO DX ANYTHING !!

EX - complimentary balanced "leach" type amp (work in progress)
FX - Harmon Kardon "signature series" amp - very promising (also a work in progress).

One might think "why is he doing this ??" For one , I am selling these .. so I make my $$$'s back. Also , I learned just enough to be dangerous here at the forum , so I must recontribute .

Lastly , I am real disturbed by the direction the forum is taking. After reading though the DX BLAME ES and ST threads .. OH MY GOD !! The ego is through the roof , the engineering is buried beneath the floor. The AX1.1 would TOTALLY outperform the ST , better sound stage and much more stable (will run without grounded heatsinks, zoble, or a BIG STUPID 100PF miller cap !! ).

Below is examples of New board layout and the AX1.1 (I am building a pair of these, too).
ALL is available at : HFS /WEBSITE/MONGREL/



OS
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File Type: jpg better layout.jpg (323.5 KB, 295 views)
File Type: gif Mongrel_AX1.0VB.gif (27.5 KB, 409 views)
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Old 1st August 2010, 07:45 AM   #188
Rob_S is offline Rob_S  United Kingdom
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Congrats that is really looking good, I like the new layout. Will the other PBs follow the same layout?

Will you be selling PCB and/or Kits?
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Old 1st August 2010, 08:16 AM   #189
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This is a project I would recommend to others, mainly because of the different options.

I like the result,
- keantoken
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Old 1st August 2010, 09:11 AM   #190
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Quote:
Originally Posted by Rob_S View Post
Congrats that is really looking good, I like the new layout. Will the other PBs follow the same layout?

Will you be selling PCB and/or Kits?
Yes and Yes.

All will be the same standard.
I had to wrangle how to do the parts identification with a modular system. With most combo's I number the power boards R99,R98 / C99,C98 backwards. All voltage boards go up R1,R2... No conflicts any combination.

I am not sure about selling boards unless there is interest. Now that I know how to make them I might keep a couple handy for the mailbox.

Next is the AX1 and the PB70 , might have them done tomorrow. By friday the major 3 amps AX,BX,CX and the 2 power boards will be submitted.

Thank you , Kean..
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Last edited by ostripper; 1st August 2010 at 09:17 AM.
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