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Designing a universal diff-in/diff-out Head Amp
Designing a universal diff-in/diff-out Head Amp
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Old 14th January 2019, 09:12 PM   #111
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by Hans Polak View Post

But why then are you using a battery instead of the phantom power ?

Hans
For 1 meter connections the pseudo-balanced/single ended connection worked fine. Of course this is also the simplest way one FET and three resistors per channel and some bypass caps.
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Old 15th January 2019, 01:07 AM   #112
billshurv is offline billshurv  United Kingdom
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Designing a universal diff-in/diff-out Head Amp
Had to go off and bang my head on a wall for a bit as was being stupid as usual. I think that both a buffer and straight in should be tried. I might even for a giggle try my microtrackII if I can get it working mainly as this has no published specs* for the mic input and also has the least compliant phantom power known (about 30v apparently).



Based on what we have found from test records +/-1dB is as good as it gets and so worrying about the exact input impedance is a waste. OTOH phantom powered buffers will offend so many delicate souls it's worth doing just for the fun value



*The user manual has no figure, but on the web have found a couple of references to the mic input being '>5.3k'. That's sooo helpful.
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Old 15th January 2019, 04:39 AM   #113
nezbleu is offline nezbleu  Canada
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Designing a universal diff-in/diff-out Head Amp
Quote:
Originally Posted by billshurv View Post
OK, Thanks. Just to recap, the average bit of gear we are looking at has 3 types of input
1. Mic, balanced usually with Phantom power

2. Instrument (unbal)
3. line in (balanced)
I am watching this thread with much interest as I just took delivery of a Behringer UMC202HD. Promptly realized I have no 1/4" adapters and the local pro shop wants $15 per. I can wait a few more days to hook it to while some online-bought adapters make their way to me. I would rather buy local but not when they are ripping me off for cables and adapters.
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Old 15th January 2019, 10:31 AM   #114
billshurv is offline billshurv  United Kingdom
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Designing a universal diff-in/diff-out Head Amp
And today discover that the unit my guinea-pig friend has (Zedi-10FX) has no spec for input impedance on mic inputs as well. Does look like Scott's little buffer is the right way forwards.
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Old 15th January 2019, 01:39 PM   #115
scott wurcer is offline scott wurcer  United States
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Originally Posted by nezbleu View Post
I would rather buy local but not when they are ripping me off for cables and adapters.
I bit the bullet ten years ago and bought 4 of every adapter I could think of, even BNC to RCA in both flavors. Sometimes I need to series them like differential BNC to XLR.
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Old 22nd January 2019, 01:15 PM   #116
billshurv is offline billshurv  United Kingdom
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Designing a universal diff-in/diff-out Head Amp
Trying to pull some info back onto here that appeared on another thread.
PHLUX-II active phono cartridge is what appears to be a rebodied AT95 with a single ended amplifier board soldered onto the cartridge pins. This has encouraged me to think about Scott's balanced phantom powered concepts and if we could make a board small enough to go at the headshell. (Whether we should bother is a secondary question compared to the fun of trying). I rapidly realised that, for the balanced case this will be beyond the average DIYer in the first instance (inc me) but a board that goes inside the TT or just outside could be of interest.



To which point the first challenge is picking the FETs as many of us just don't have the sort of junk box Scott has and . LSK389/489 I just cannot find in small qty outside ebay in europe. So leaves it really to the onsemi products particularly the 2SK3557. Any reason not to baseline that for MM duties? Tad pricey at 23p each in 10 off but cheap enough so as not to worry hunting for anything else?
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Old 22nd January 2019, 01:53 PM   #117
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by billshurv View Post
Any reason not to baseline that for MM duties? Tad pricey at 23p each in 10 off but cheap enough so as not to worry hunting for anything else?
Looks like a BF862 substitute so it should be good here. I'll see what might work all SMT and tiny. I don't see the need to make it flea power as they seem to have.

EDIT - ~$16/100 at Mouser >6000 in stock.
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Last edited by scott wurcer; 22nd January 2019 at 01:55 PM.
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Old 22nd January 2019, 02:22 PM   #118
Hans Polak is offline Hans Polak  Netherlands
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Quote:
Originally Posted by billshurv View Post
Trying to pull some info back onto here that appeared on another thread.
PHLUX-II active phono cartridge is what appears to be a rebodied AT95 with a single ended amplifier board soldered onto the cartridge pins. This has encouraged me to think about Scott's balanced phantom powered concepts and if we could make a board small enough to go at the headshell. (Whether we should bother is a secondary question compared to the fun of trying). I rapidly realised that, for the balanced case this will be beyond the average DIYer in the first instance (inc me) but a board that goes inside the TT or just outside could be of interest.



To which point the first challenge is picking the FETs as many of us just don't have the sort of junk box Scott has and . LSK389/489 I just cannot find in small qty outside ebay in europe. So leaves it really to the onsemi products particularly the 2SK3557. Any reason not to baseline that for MM duties? Tad pricey at 23p each in 10 off but cheap enough so as not to worry hunting for anything else?
Hi Bill,

Farnell has a huge assortment of NFets, through hole and SMD.

https://nl.farnell.com/webapp/wcs/st...0004636&sf=721

However, when using the 47K termination, you really have to add a capacitance. This will be a bit more difficult when mounting a buffer on the Cart.
The other thing to realise is that the unit needs power. Depending on SE or Diff, 2 or 3 extra wires will have to go through your Arm to the Cart !!
You could use phantom power either from your 6i6 unit or from a 9 Volt battery, but in both cases you will need relatively bulky Electrolytic 47uF caps that could make your PCB quite heavy.
In that case you would need a second PCB to take care of the supply side and to connect to the cables from the 6i6.

All in all, a PCB in the TT Cabinet directly after the Arm seems to me quite a bit more attractive to realise.


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Old 22nd January 2019, 02:25 PM   #119
Hans Polak is offline Hans Polak  Netherlands
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Originally Posted by scott wurcer View Post
Looks like a BF862 substitute so it should be good here. I'll see what might work all SMT and tiny. I don't see the need to make it flea power as they seem to have.

EDIT - ~$16/100 at Mouser >6000 in stock.
Farnell ran out of stock and report that they are no longer in production.
A shame for this fantastic low noise NFet.


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Old 22nd January 2019, 02:31 PM   #120
billshurv is offline billshurv  United Kingdom
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Designing a universal diff-in/diff-out Head Amp
Hi Hans,


Yes it was the big caps that made me realise that phantom powered mini boards were, whilst possible NOT DIY. I do want to try your phantom MC headamp sometime as well.



On the 2SK3557 Digikey have 10k and farnell claim more stock in april so hopefully will remain supported for a while!
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