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Prefix - Increase Output?

P

Paul3

pfm Member
Hi

I have just fitted a Prefix to my LP12 / Ekos / Troika and I need to increase the output to better match the other sources into my 32.5 with 821 cards.

When I listen to my DAC, the vol is between 9 and 11 o'clock, but with the Prefix I need to go to 4 o'clock to get a reasonable level. At this setting, there is a lot of noise present, but below this, LPs sound dead.

Is it possible to do this? How?

Pic - http://picasaweb.google.com/paul31154/DropBox?authkey=Gv1sRgCNiQitjWlfG9Ag&pli=1#5558807081454858514

The prefix is fed 25v x 2 from 2 Teddy Regs in the 32.5, and I was wondering if increasing the supply volts would help. How high should be safe?
I understand that there is voltage regulation in the Prefix. How can this be disabled?

Should I just remove the prefix, and fit a higher output cartridge?

Thanks in advance
Paul
 
Paul3 said:
Hi


The prefix is fed 25v x 2 from 2 Teddy Regs in the 32.5, and I was wondering if increasing the supply volts would help. How high should be safe?
I understand that there is voltage regulation in the Prefix. How can this be disabled?

Thanks in advance
Paul
Click to expand...

Extra voltage will not change the prefix gain.
 
Have a look at the circuit diagram on Neil's site. Key items are the 47uf caps. On the prefix these are the big flat gold ones in the middle of the board. One is for the part of the circuit near the power input (RHS on Neil's circuit - once you rotate it so that the writing makes sense). The other is for the feedback part of the circuit (in the middle on Neil's diagram) This is what we're interested in. Try to find a 220r resistor that connects between the + of the 47uF cap and one leg of a transistor and also to a 100k resistor and a 4k7 resistor. To identify the resistors have a look here http://masc2279.no-ip.org/Plone/best-diy/diy-knowledge-articles/resistor-color-code-guide Trace it all through slowly and keep posting your findings here

Tim
 
Hi Tim,
I have found the 220r, 100k and 4k7 close to each other, near the middle of the board.

It must be the correct 220r, as there is only 1 on the board. I guess that this is the one to replace with a lower value?

While on the subject, is it worth me buying a mixed pack resistors off ebay, or are they not good enough quality? eg http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=160527756133&ssPageName=STRK:MEWAX:IT

If not, where is the best place to buy in small quantities?

Thanks
Paul
 
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Paul3 said:
I have found the 220r, 100k and 4k7 close to each other, near the middle of the board.

It must be the correct 220r, as there is only 1 on the board. I guess that this is the one to replace with a lower value?

While on the subject, is it worth me buying a mixed pack resistors off ebay, or are they not good enough quality? eg http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=160527756133&ssPageName=STRK:MEWAX:IT

If not, where is the best place to buy in small quantities?
Click to expand...

Those eBay resistors are carbon film, you'll want metal film, E96 or better.
 
This site contains affiliate links for which pink fish media may be compensated.
If I'm right then yes it's the 220r resistor you need to change. Putting in a 150r will increase the gain from 23 to about 31. I would like someone else to confirm this though. Anyway if you want a couple of 150r's just pm me your address and I'll put them in the post
 
Watching this with interest as I have the same problem Prefix K/Klyde/Troika into my Chevron Audio pre.
Please be sure to post your results.
 
timH said:
If I'm right then yes it's the 220r resistor you need to change. Putting in a 150r will increase the gain from 23 to about 31. I would like someone else to confirm this though. Anyway if you want a couple of 150r's just pm me your address and I'll put them in the post
Click to expand...

Hi Tim

Thanks for your offer. I am trying to buy a stock of resistors, for future use, but I don't know what to buy, any advice?

I have just received my stock order for my B4 build, so I will rob one from there.
Neil has just informed me that he halved the value, as per his site, so I will try this first.

Thanks again
 
Paul a handful of DALE rc55 or similar .1% will come in handy. I have a selection box of these just for phonostage fiddlings.
 
Paul.

Just to throw a spanner in the works I had a similar issue using 821s, the gain is too low for applications with NAIM phonostages IMHO. That's why I sent them back, like you I had to have the volume at 4 o'clock to get enough gain.

I have 321/729s in my 72 and 4 o'clock would have the bass cones sat on my lap using my prefix.

Andrew.

NB. This is NOT an observation about the quality or otherwise of the 821s, just that the gain is insuffcient for use with NAIM based phono cards, so no PMs please...............
 
Can someone clarify if there are one or two resistors to change in the Prefix.
Just thinking of 323 boards there would be one on each board.
 
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