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Old 21st February 2010, 11:28 PM   #1
sganti is offline sganti  United States
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Default 811A Class A2 with Power Drive

Hi
Having finished 845 based monoblocks, I wanted to jump into another project, namely, 811A based Class A2 amp with PowerDrive. Thanks to tubelab's excellent website, I tried to understand the concept.

I am attaching a pdf of the complete amp including power supply and the +ve/-ve supply. I did some search at Digikey for mosfets, and found the following two mosfets to have, what I think to be good characteristics for power drive -- Vishay -- IRF820A, and IXYS -- IXTY 1R6N50P.

I am planning on using 56 as the driver for the 811A. My thoughts are, since the MOSFET handles the current requirements for 811A grid, I could get away with a low current driver such as 56 runing 5mA thru it. Just not sure if this OK to overcome the input capacitance of the MOSFET.

Would appreciate any comments or suggestions, especially if the above listed MOSFETS make good candidates for power drive application.

Thanks in advance for all the help.

Sridhar
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Old 21st February 2010, 11:39 PM   #2
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PDF is missing :-(

Davide
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Old 21st February 2010, 11:41 PM   #3
sganti is offline sganti  United States
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Default Did not upload the file

Hi
Looks like what I tried to do for upload did not work, will try again.

Succedded in uploading the files, but I am not sure about the resolution.

Thanks
Sridhar
Attached Images
File Type: jpg 811AAmpWithPowerDrive.JPG (38.1 KB, 420 views)
File Type: jpg 811A-Mosfet-Grid-Supply.JPG (54.7 KB, 410 views)
File Type: jpg 811A-PowerSupply.JPG (46.9 KB, 403 views)
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Old 21st February 2010, 11:48 PM   #4
sganti is offline sganti  United States
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Default One more try for posting pdfs of the circuits

Hi All
Looked at the jpegs I uploaded, and they are not clear, will try to load multiple pdf files.

My apologies on the mess up with loading, this is the first time I am trying to load..

thanks
Sridhar
Attached Files
File Type: pdf 811A-AmplifierCircuit.pdf (61.0 KB, 138 views)
File Type: pdf 811A-PowerSupply.pdf (172.0 KB, 65 views)
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Old 22nd February 2010, 12:46 AM   #5
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Transistors typically are such high gain devices that it hardly matters which one you use as the behavior is largely independent of device characteristics. The IRF820A will do nicely: it has a rather low Ci which will be reduced below its static value since it will see very little current, and the Crt is also quite low, and this is the most important one since it will make the lion's share of input capacitance. A 56 running 5.0mA should drive it without a slew rate problem.

Just make sure that you give it enough headroom; don't let Vds drop below 50V or so, otherwise Crt will start to rise rapidly. That's not good for sonic performance.

The main difficulty with working with the 811 is the high r(p). It will probably require some NFB (local and/or global) to get that resistance down. Otherwise, you just might have the same problems that running a pentode final would have: poor woofer damping and more than the usual amount of h3 and higher order harmonics.
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Old 22nd February 2010, 01:08 AM   #6
sganti is offline sganti  United States
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Default Feedback on 811A A2 amp

Hi Miles
Thank you for the advise. I was planning on adding local negative feedback, i.e., making a connection from the o/p to the cathode.

I had previously built 811A A2, but used an Interstage technique like Shishido San. However, instead of doing cancelling of currents like he proposed, I just had Jack from Electra Print wind me an IT that takes 60mA of current.

This amp schematic can be found at the one electron web site
Ganti's 811-A class A2 Amp

This amp sounded good on vandersteen 1B's, but when I upgraded to Zu Druids, it sounded mediocre, the highs were shrill, the lows were missing, and one could not listen to it for more than 20 minutes :-). So I dismantled that amp, and bought a Yamamoto A-08S.

The bug of 811A has returned thanks to the powerdrive concept that Tubelab posted.

I will use the IRF820, as I have them on hand, and I will make sure I have local feedback.

Thanks
Sridhar
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Old 22nd February 2010, 01:50 AM   #7
sganti is offline sganti  United States
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Default Headroom question

Hi Miles
In my thoughts about feedback I forgot to ask about headroom issue you mentioned.

This sounds stupid, how should I make sure that Vds does not fall below 50V. I am totally new to the world of transistors, and my basic knowledge is ohms law, and some understanding of impedances with capacitors and inductors.

Thanks much!

Sridhar
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Old 22nd February 2010, 06:39 AM   #8
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Quote:
Originally Posted by sganti View Post
Hi
Having finished 845 based monoblocks,
Hi, I haven't been following what you did with the 845's but I'm interested in what you did and what you thought of the final product, there doesn't seem to be many that build them over this neck of the woods...

Apologies for hijacking the thread!
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Old 22nd February 2010, 02:04 PM   #9
sganti is offline sganti  United States
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Default 845 Monoblocks

Hi
Attached is the circuit of the 845 monoblocks. The driver is triode connected D3a, and the coupling is Hashimoto 5k IT. Hashimoto's like the driving tube's Rp to match the primary impedance specified, so I used a small unbypassed cathode to increase the effective Rp to come close to 5K. There is a CCS-Shunt Reg that feeds the B+ to the D3a's.

The sound is pure magic, its big, smooth, and has an effortless character to it. It brings out strong emotions conveyed via the music.

I recently ordered Obbligato Gold Premium caps -- 47uF + 15uF, and I will be replacing the Nichicon Muse bypass capacitor with those and see if it makes a difference to the final sound.

Sridhar
Attached Files
File Type: pdf 845-Amplifier.pdf (94.2 KB, 143 views)
File Type: pdf 845-PowerSupply.pdf (108.8 KB, 63 views)
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