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WDYSUN 18th January 2012 01:36 PM

AD797 Preamp based on stolen trademark
 
Dear Forumers

I had the chance to listen to the MBL 6010D, an expensive machine and I was truly impressed how it compared with my references in my primary system (Threshold Fet10, and Counterpoint SA5000).

Finally I was amazed to learn that the MBL preamp has its own heart base on AD797s opamps.... This op must be really good, Oh Well MBL mates must also be very smart for getting all that sound out of it!

Now I am putting up a second system with on all-DIY-chain. I was thinking to build a preamp based on the AD797.


I found a Chinese kit:

6010D Preamp PCB Board MBL Circuit DIY Dual to Mono P006 | eBay

Often these Chinese kits are overlooked and sloppy thought. Can somebody tell me whether the circuit above is any reasonable? Do you find any mistakes? Do you think I can get something out of it?

Best Wishes
Pierre

analog_sa 18th January 2012 01:58 PM

There is certainly at least one more Chinese interpretation of the 6010D circuit floating around. The ebay circuit does not show where the volume pot connects. Is it between the stages? If not, then what's the point of two amplification stages?

Other than that the circuit will certainly work but i fail to notice any magical properties. No output resistor does seem a bit risky too. Even if the original MBL shares the same circuit, power supply design, regulation (none on board???) and parts choice can make all the difference.

WDYSUN 18th January 2012 02:05 PM

Quote:

Originally Posted by analog_sa (Post 2867628)
There is certainly at least one more Chinese interpretation of the 6010D circuit floating around. The ebay circuit does not show where the volume pot connects. Is it between the stages? If not, then what's the point of two amplification stages?
...

Are you aware of any AD797 preamp desing for DIY?

Pierre

SoNic_real_one 18th January 2012 03:02 PM

There are two stages because first is 5x and second 10x amplification - which is perfectly acceptable.
Going all the way to 50x with just one stage limits the output top frequency limit (due to BW product being limited) and also increases the distortions (due to reduced feedback).
Sure, if you don't need that level of gain, just ommit first stage or adjust the feedback network to suit.
No output resistor increses the capability to drive capacitive loads (next stage input).
PS: Looks like the pot is connected at the input of first stage.

analog_sa 18th January 2012 03:28 PM

Quote:

Originally Posted by SoNic_real_one (Post 2867712)
There are two stages because first is 5x and second 10x amplification - which is perfectly acceptable.



Are we looking at the same circuit? Gain of x50 perfectly acceptable for a line stage???

First stage gain seems to be over x6 and second - x2 which is a bit more acceptable. In the "other" Chinese circuit i've seen the pot is sandwiched between the stages which makes a lot more sense. And input stage is a buffer which also makes more sense.

The 797 is a really nice sounding opamp. Layout and decoupling are absolutely critical though. I guess an output resistor is also essential for use with most cables. Other than that even a textbook circuit can sound great with it.

WDYSUN 18th January 2012 04:07 PM

Quote:

Originally Posted by analog_sa (Post 2867628)
There is certainly at least one more Chinese interpretation of the 6010D circuit floating around. The ebay circuit does not show where the volume pot connects. Is it between the stages? If not, then what's the point of two amplification stages?
...

Are you aware of any AD797 preamp desing for DIY?

Pierre

analog_sa 18th January 2012 04:24 PM

Never really looked for one. At the time i built a truly text-book, single stage preamp on a protoboard and probably even used 3-terminal regs. Just keep in mind it's prone to oscillations with poor layout and insufficient decoupling.

The ebay board may be a good starting point as the layout is probably good enough and decoupling appears to be ok. Up to you whether to use it as a single stage or as a buffered pot. I would probably do some surgery and fit a daughter board with some nice regulators close to the opamps. Maybe order more than one board :)

Ken Newton 18th January 2012 06:16 PM

Unless my eyes deceive me, I calculate a net gain of about 10.6x or 20.5dB. This is a common amount of gain for a line stage. This figure includes the small attenuation due to the two low-pass R.F. filters at the input to each op-amp. Not too many interesting design details are evidenced in the schematic. I've already mentioned the R.C. input filters. Some audio engineers feel that bandlimiting the input to an op-amp is among the most importants design details. Perhaps, what's more interesting are the details not in evidence.

Using a second stage to deliver another 6dB in gain appears unecessary. It looks as though the output (second) op-amp could have been wrapped inside the feedback-loop of the first, ala' Walt Jung. Such as, matching the source impedance as seen by the op-amp input terminals.

apoopoo999 9th February 2012 07:36 AM

here's another one
Balanced preamplifier PCB w/parts, ref MBL 6010D /A-11M

WDYSUN 9th February 2012 09:32 AM

Quote:

Originally Posted by apoopoo999 (Post 2898541)

I wonder whether it is too complicated. I thought a simple two stage AD797 could do the job anyway.

P


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