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Old 20th July 2013, 10:34 PM   #621
sipo75 is offline sipo75  Europe
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Quote:
Originally Posted by TarnishedEars View Post
Joe:Stupid question: What precisely does the acronym "OTA" stand for in your posts above? I'm racking my brain on this one, my best guesses are clearly missing the mark (Output Transformer Amperage, or Output Topology Amperage, or Output Transistor Amperage, or ??? ).
See post #601
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Old 21st July 2013, 12:16 AM   #622
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Quote:
Originally Posted by TarnishedEars View Post
Joe:Stupid question: What precisely does the acronym "OTA" stand for
It's not a stupid question. There was a fair amount of detail to table, so I could have given an explanation. It stands for Operational Transconductance Amplifier, but it also has a few other labels such as Diamond Transistor. For reading an article that also discusses the history, click here.

So an OTA means voltage in and current output amplifier. But we won't be using it as an O for 'operational' because we won't be using feedback back to the "-" of the OTA. So we will be using the OTA as a Diamond Transistor stage with no feedback.

For example, "opamp" is short for 'operational amplifier where the 'operational' is a reference to feedback setting the operational point of the amplifier.

Most OTAs like OPA660, AD844 and the latest OPA860 (available in SOIC and hence the focus on it for our purposes) all includes another optional Buffer stage. So the 'Operational' part of OTA assumes that feedback will be taken back (so-called current feedback) to the "-" and that is something we won't be doing.

Hope that helps a bit, but Google "Diamond Transistor" and you will find quite a bit of info. They are NOT normal opamps even though they are opamps, some of them can be used in non-opamp modes (no feedback) and that makes them such interesting devices, so much so than ho-hum opamps. They are in fact fascinating and more flexible.

You can even use it to make a Phono Stage Preamp where the current is pushed through an RIAA network, creating the signal across the RIAA which is then buffered. No negative loop feedback at all, 1MHz bandwidth, low distortion, low noise, amazing rise and settling times (incredibly fast circuit). This was a design, MC capable, that I did with the late Allen Wright:

Click the image to open in full size.

Yep, OTAs and Diamond Transistors, I like them, I am familiar with working with them and they appeal to me.

Cheers, Joe

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Last edited by Joe Rasmussen; 21st July 2013 at 12:32 AM.
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Old 29th July 2013, 05:51 PM   #623
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I wonder whether some one can identify the plastic connector that carries the power directly from the IEC to the circuit board. I might like to try bypassing the IEC, or getting a better IEC.

Thanks,

Len
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Old 30th July 2013, 01:20 AM   #624
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Originally Posted by MLM View Post
I might like to try bypassing the IEC, or getting a better IEC.
Just be aware of the transformer switching which has Primary windings in either parallel (115V) or series (230V) - the SMPS board can handle both 115V and 230V (and everything in between), but don't overlook the transformer.
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Old 30th July 2013, 01:44 AM   #625
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THIS IS GEETING SERIOUS NOW!

I thought about it and made a decision...

I have just brought in an Oppo 105 for one purpose only - I am going to do a player as per earlier discussions here and post everything I do, everything will be documented.

What will be done to this '105 would be - stereo channels only - what Oppo could have done at no extra cost if done at the manufacturing stage:

1. Post-DAC circuit using OPA-860 for stereo channels - and regulators powered from the existing 15B rails, down to 5-6V.

2. Murata 10uF/25V SMD (6) on the DAC in place of electrolytics

3. Clocking 54MHz with a SAW oscillator, then deriving 27MHz from that via 4040 (NXP 98MHZ version) divider.

4. Still retain the Silma output caps, but perhaps give them a small positive

5. Instructions how to null the output, so that Silmics are not required. Both 4 & 5 are documented as we have the 'Oppo with' option and the "DIY" option where caps can be dispensed with. It would be interesting to get feedback, to what extent is the Silmics audible (to put it bluntly, what is lost?).

I have all the bits in now except one. The adaptors for fitting SOIC into DIL8 sockets - got some, but were too wide - have ordered correct size and should be here shortly.

ANYBODY CAN REPLICATE ABOVE.

The one component hard to source may be the 54MHz SAW oscillator - but Coris has some and while I do too, I use them at a fair rate and four months delivery time means I have to keep a lot in hand.

Please wait a little longer... only a little time...

Cheers, Joe
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Old 30th July 2013, 04:50 AM   #626
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Quote:
Originally Posted by Joe Rasmussen View Post
Just be aware of the transformer switching which has Primary windings in either parallel (115V) or series (230V) - the SMPS board can handle both 115V and 230V (and everything in between), but don't overlook the transformer.
Not looking to do anything fancy here. Can you identify the connector that connects to the board?

Len
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Old 31st July 2013, 07:21 AM   #627
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Quote:
Originally Posted by MLM View Post
I wonder whether some one can identify the plastic connector that carries the power directly from the IEC to the circuit board. I might like to try bypassing the IEC, or getting a better IEC.

Thanks,

Len
I`m not very sure what you refer to about IEC... It seems to me that you think about the power in (from outlet) stage.
From my part, I will recommend to everybody to do not touch this power in (to switching PSU) stage of the player. There is not any improvements to be done in this area. Main power input and AC filtering is definitely not something one may work around on this device. The important improvements come from special care in better filtering and noise removing from the DC power resulting from player`s switching PSU (+5v), and from the analogue PSU.
Is to be named also here that huge improvements come from modifying crucial circuits in audio/video signal processing. The areas one may focus on are: clock system for whole player, the power system for the clocks, and post DAC analogue signal processing (special for stereo stage).

Last edited by Coris; 31st July 2013 at 07:23 AM.
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Old 3rd August 2013, 01:36 PM   #628
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I'm simply trying to identify the white connector at the lower left of this picture. I don't care about the color, though. Perhaps it's a specific Molex. Thanks.
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Old 3rd August 2013, 09:36 PM   #629
Coris is offline Coris  Norway
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Yes, it is a Molex...
(Farnell - MOLEX - 09-50-3031 - CRIMP HOUSING, 3.96MM, 3WAY)
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Old 4th August 2013, 02:57 PM   #630
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Just a quick question - the Oppo 105 uses a 54MHz oscillator (X1) directly, as far as I can tell, directly into the Stereo Sabre DAC. But the clock signal to the Multi-Channel Board/Sabre DAC seems to be going through a Buffer (clock Buffer U7). I think it is a SOT353 (or perhaps SOT555) package.

Something interesting happened, when increasing 54MHz to 100 MHz, the Stereo Sabre DAC works fine all the time, but there is a chance that the Multi-Channel becomes intermittent because it does not see the clock signal directly in its Sabre DAC. The Buffer may not be rated to handle 100MHz.

I just thought others may need to be aware of this when modifying the 105 - that increasing clock frequency may mean a problem on the Multi-Channel side, where it just drops out, period.

I tried to read the number on that U7 Buffer. It is very faint, but it looks something like "NO10" - but searching has not shown up anything.

I would love to know the spec of "NO10" - there is a similar Toshiba Clock Buffer that was rated to 80MHz only. So using 100MHz and we are likely to loose clock signal.

Cheers, Joe

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