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Old 28th March 2017, 09:07 PM   #261
PB2 is offline PB2  United States
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Quote:
Originally Posted by ostripper View Post
Typical 3 stage gain "Sansui type" High Ft classic amp.

What is weird , they use a phase inverted driver stage (diamond like)
and then use positive feedback.
(below)

The NFB version of this amp (sansui) . is typically 20ppm-20k.
That is , with faster 35mhz sanken outputs.
Click the image to open in full size.
OS
This draws very much from the Matti Otala Low TIM amp.
Not the subject of this thread but thought I'd mention it.
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Old 2nd April 2017, 09:12 AM   #262
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Default RA-820AX peculiar fault

well it's peculiar to me anyway. I am recapping one of these and there is a hum that ONLY occurs when the balance control is half way left or right, NO hum when it is in the middle or fully right or left. The hum is on the opposite speaker to the balance setting. It is there when the volume is fully off and does not increase with volume, the music will drown out the hum! any ideas as to were to look? I've tried resoldering, it also gets slightly louder when I touch the chassis, but this does not have an earth wire to the mains
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Old 2nd April 2017, 02:15 PM   #263
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posted a new thread on this:

ROTEL RA-820AX peculiar fault
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Old 17th June 2017, 03:27 PM   #264
AngelP is offline AngelP  United Kingdom
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Default Hello again y'all!

After a long hiatus due to work, the row of Rotels waiting for me to upgrade is now getting rather embarrassing - and their owners increasingly (albeit understandable) impatient.

So, I decided to finally stop doing the very time consuming fumble-boarding of the CM and VAS circuits, and instead design and order proper double sided SMT boards from PCBway - which I must say I have had very good experiences with in the past.

At first I decided to also ask PCBway to populate the pcb's for me, but the initial web price quote for this simple job suddenly almost doubled and it appeared that someone in their procurement considered a 100k 1% resistor chip as a good match for the specified 100E 0.1% (!)
So, I sort of lost confidence and thought "Ok, I can do this better and cheaper". Well.....

After some experimenting and getting to a decent solder paste stencil printing setup result, I first tried to solder the small pcb's with my trusted SMD hot air gun.
I still have to locate some of the the boards and most of the tiny 0603 components somewhere on the floor .

So, I bought a cheap T-962 IR reflow oven, which, after having done all the essential safety repairs and firmware upgrades as recommended on the web, now performs very nicely indeed.

My first finished miniature boards of the CM and VAS are pictured under the original components for which they will be a direct drop-in replacement.

But a DYI'ers troubles never stop there. Stay tuned!
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Old 17th June 2017, 05:48 PM   #265
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The Pioneer SX780 provides an excellent platform for improvement as the tuner can be modded with an op amp output instead of the 2SA7xx transsitors, it regulates the amplifier VAS stage and double regulates the Phono and preamp sections.
I am adding in the Lichstark-x mosfet amp , Volume preamp ,6922 tube preamp and new phono stage. This unit is easy to work on as it opens up easily.
new amp puts out 75w peak before clipping with 22000uf Caps
early tests indicate phenomenal sound quality.

Last edited by stocktrader200; 17th June 2017 at 05:50 PM.
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Old 23rd June 2017, 05:39 PM   #266
memofer is offline memofer  United States
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Hi AngelP,
I was wondering if your mod would apply to a Rotel Rx-950ax? the amp section does look close to yours but not exactly the same.
Thanks
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Old Yesterday, 09:07 AM   #267
AngelP is offline AngelP  United Kingdom
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Quote:
Originally Posted by memofer View Post
Hi AngelP,
I was wondering if your mod would apply to a Rotel Rx-950ax? the amp section does look close to yours but not exactly the same.
Thanks
Hi memofer,
I have never been under the bonnet of a Rx-950ax, but had a brief look at the schematic of the power amp stage. The diff PNP input could definitely be improved with properly paired 2SA992's and a current mirror instead of their two collector resistors. I would also short out the strange single 47k emitter resistor that clearly unbalances the input and is a proper noise source.

For the VAS section, my small pbc module contains all the 950ax circuitry, but also includes the important pre-VAS emitter follower. It will not quite be a direct drop-in replacement as in the RA-930 series, but the mod should be doable with a bit of tweaking of the pcb's "collector" pin connection.

Maybe you should attach the RX-950ax amp section schematic for other DIYers to be able to follow?

Cheers,
Per
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Old Yesterday, 09:44 AM   #268
AngelP is offline AngelP  United Kingdom
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Quote:
Originally Posted by AngelP View Post
But a DYI'ers troubles never stop there. Stay tuned!
As readers of this thread may know, I have a setup of a dummy load box with a built-in XonarU7 audio to USB unit which makes RTA of amps very quick and easy.

Due to other work I had it stacked away for quite some time, but having finished the new pcb modules I was very eager to set it back up and do pre- and post mod measurements. Too eager.

I forgot to attach my trusted USB isolator (pictured) to the cable, and the first RA-931 patient I connected - had 120Vac stray voltage on the speaker outputs. Not much energy in it, but enough to feel a small tingle when touching the outputs and the PC enclosure.

And enough to show the weakest link in the setup - the Xonar's USB com circuitry, which apparently does not have a proper overvoltage protection.

I now have an invisible XonarU7 box - at least to any USB port.
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