Yeah, only one I played with was the old 1541 which did NOT like load resistances more than 50R or so.
Feed-through capacitors are usually low-loss, wide-band capacitors, and can be used in audio successfully. The common mistake is, people sometimes don't realize that the wire goes through them.
It's quite OK to use larger values for the I/V resistor with the AD1865. 200R is quite common. It's not like the 1541 (where even without being too dogmatic about the 25mV voltage compliance spec on the datasheet) it's wise to keep the I/V resistor pretty low. I'd say 32R is the highest I'm willing to tolerate with the 1541.
I'd be interested in knowing whether R4/R5 (the I/V resistor positions) are actually populated on that hui board, so the there is a resistor in parallel with the 1k4 (reflected by the 47k resistor that the transformer is loaded with).
I believe that older data sheets for this part stipulated a compliance voltage on the current output of the dac of 20mV or less in order to meet its published specifications. I used 10 - 20 ohm I/V resistors with this dac chip to stay below this number. Noting that the source impedance of the current output is 1.7K anything above a few ohms I suspect is going to create significant amplitude and linearity errors at the output..
Last known datasheet here: http://www.analog.com/static/imported-files/data_sheets/AD1865.pdf The specification I mentioned is not listed as far as I could ascertain in this version of the data sheet.
The AD1860, 1862 and 1865 IMHO were amongst the better sounding dac chips around 20yrs ago along with a couple of BB parts like the PCM63...
Last known datasheet here: http://www.analog.com/static/imported-files/data_sheets/AD1865.pdf The specification I mentioned is not listed as far as I could ascertain in this version of the data sheet.
The AD1860, 1862 and 1865 IMHO were amongst the better sounding dac chips around 20yrs ago along with a couple of BB parts like the PCM63...
OK - got the 4.7uF claritycap capacitors in and they are working fine.
The 1:6 ratio sure is correct!
These are the mods I have done so far:
-change the i/v resistors from 600R to 220R as per the schematic (they shipped with 600R for some reason). I got some 220R ZFOIL resistors on sale from partsconnexion
-change the tx from the stock old telephone transformers to Lundahl LL1530 wired in a 1:7 ratio (thanks to help from Kevin Carter!)
-change the input isolation transformer from PE-65612 to Lundahl LL1572 (there is no dc on the coaxial input so I have swapped 'like for like' in did not put a capacitor prior to the LL1572
-changed the AD1865 chip (may have been fake, unsure)
-changed the power supply caps to 22000uF and the tube psu cap to 470uF
-put 0.1uF MKT1813 capacitors across the rectifier diodes
-changed the 1uF capacitor - as per this thread!
Sound is great so far, much better than the stock unit, however obviously needs some run-in time first.
Anyone suggest the best way of wiring the feedthrough capacitor (if it is worwhile trying?)?
The 1:6 ratio sure is correct!
These are the mods I have done so far:
-change the i/v resistors from 600R to 220R as per the schematic (they shipped with 600R for some reason). I got some 220R ZFOIL resistors on sale from partsconnexion
-change the tx from the stock old telephone transformers to Lundahl LL1530 wired in a 1:7 ratio (thanks to help from Kevin Carter!)
-change the input isolation transformer from PE-65612 to Lundahl LL1572 (there is no dc on the coaxial input so I have swapped 'like for like' in did not put a capacitor prior to the LL1572
-changed the AD1865 chip (may have been fake, unsure)
-changed the power supply caps to 22000uF and the tube psu cap to 470uF
-put 0.1uF MKT1813 capacitors across the rectifier diodes
-changed the 1uF capacitor - as per this thread!
Sound is great so far, much better than the stock unit, however obviously needs some run-in time first.
Anyone suggest the best way of wiring the feedthrough capacitor (if it is worwhile trying?)?
You do know that the oil is PCB's. Not to mention the seals will leak from age. I have dozens of these setting on paper and every month I go look to see how much oil has stained the paper. It is disturbing...it takes that long for the leaks to show and that is at room temp. too.
So why would anyone pay such a high price for old ( questionable) stock when new stock such as Obbligato is so superior?
So why would anyone pay such a high price for old ( questionable) stock when new stock such as Obbligato is so superior?
Excellent point, and I am so paranoid I won't even allow people to bring these in my house for swaps during audio get togethers. I feel much the same way about MV rectifiers, although the very small ones aren't a particular problem - at least no worse than the few fluorescent lamps scattered about my house.
Surely, the Glass and Metal sealed PIO's are Ok with regards to --Non Leakage??
I would never trust a rubber sealed one, or those waxy covered things though....
I would never trust a rubber sealed one, or those waxy covered things though....
They should be fine, but many of them (although probably not all) seem to be pcb free.. I even had some that are clearly marked pcb free and they are at least 35yrs old.. Most of mine are PPIO or just PP or teflon types - I'm not really that big a fan of paper no matter what the pundits say about their DA..
well...I have at least 100 Gudeman, Sprague, Sangamo, and Seimens on the paper and 7 out of 10 show some kind of seepage. The ends are either glass or plastic molded into the metal before assembly and the metal "looks" clean, rust free, and dent free. I even have blackbeauty showing some leakage (they too are PCB oil so beware). There must be microscopic corrosion where the metal and eal meet, which does not show. When I finally narrow down the ones with no leaks, at room temp, I'll try them with hot sun and see if they pass muster. Otherwise I wasted my money on ebay, and they will go in the hazard recycle bin.
For me, no more old caps or resistors. They are a waste of money in my opinion.
For me, no more old caps or resistors. They are a waste of money in my opinion.
managed to locate the datasheet of the feedthrough capacitors, looks like they are PIO:
http://download.siliconexpert.com/pdfs/2005/10/11/semi_a/4/epco/capacitors/b85121_122.pdf
However, if these are used in the signal path, won't they act like unshielded cable in that the entire case is not shielded in any way?
http://download.siliconexpert.com/pdfs/2005/10/11/semi_a/4/epco/capacitors/b85121_122.pdf
However, if these are used in the signal path, won't they act like unshielded cable in that the entire case is not shielded in any way?
Feed through types should not be used in the signal path.. ever..
Feed through types are generally used for getting supply and control voltages into and out of RF sensitive/noisy environments. In this sort of application the chassis is the lowest impedance ground present and hence is used for RF grounding. The chassis is a shield at RF provided that it is not fully of holes that are significant relative to the wavelength of the interference you are trying to shield from.
Feed through types are generally used for getting supply and control voltages into and out of RF sensitive/noisy environments. In this sort of application the chassis is the lowest impedance ground present and hence is used for RF grounding. The chassis is a shield at RF provided that it is not fully of holes that are significant relative to the wavelength of the interference you are trying to shield from.
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