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Need help with a preamp board I bought on eBay
Need help with a preamp board I bought on eBay
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Old 21st March 2015, 06:48 AM   #1
escott31 is offline escott31  United States
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Default Need help with a preamp board I bought on eBay

Hi guys, I need a little help. I purchased this board on eBay

Tube SRPP RIAA and Line Preamplifier Stereo PCB | eBay

I received the board in the mail today, and still haven't received any help from the seller. This is my first venture in tubes, and I thought this was a good idea to start with a prebuilt board, but now I'm having second thoughts. I just don't have enough info on the proper components needed. Should I just assume 1 watt for all the resistors? Then some capacitors like C16 have no voltage rating, only 10U. And then in the middle of the board, with caps c2, c4, c12, c3, c11, c13, I don't know what to make about any of those. and where does the low voltage go for the filaments? Is that was connections 4, 5, and 9 are for? Then there is the whole source / srpp thing. Does that mean I can select between a phono input and a line input? which would be great because I would like to be about to do that. Anyways, sorry for being a tube noob and asking all these questions, I was hoping this circuit board would be a little easier to figure out than it is. Thanks.
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Old 21st March 2015, 07:35 AM   #2
mdamp is offline mdamp  Australia
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R1,2,3,4,5,6,7,8,9,10,11,12,13 ALL 1W Resistors will be fine R 14,15 2W RESISTORS. C7,8,9 350v+ C1,2,3,4,5,6 ALL 630V
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Old 21st March 2015, 09:18 AM   #3
mdamp is offline mdamp  Australia
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Quote:
Originally Posted by mdamp View Post
R1,2,3,4,5,6,7,8,9,10,11,12,13 ALL 1W Resistors will be fine R 14,15 2W RESISTORS. C7,8,9 350v+ C1,2,3,4,5,6 ALL 630V
Ops just had a look at board those resistors may be only 1/4w or 1/2 watt.

Last edited by mdamp; 21st March 2015 at 09:23 AM.
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Old 21st March 2015, 04:39 PM   #4
Palustris is offline Palustris  United States
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Are we to understand that the board came with no documentation other than a schematic? I would definitely leave that information in the ebay feedback. This is not really a beginner project.

There is only one input J4 and one output J5. The power supply voltage is 280V; so 350V or 400V caps should be OK. Filament voltage can be 12V or 6V so pins will be either 4 and 5 for 12V or 4 and 5 parallel and 9 for 6V.
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Old 21st March 2015, 05:08 PM   #5
spendorite is offline spendorite  Canada
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Quote:
Originally Posted by Palustris View Post
Are we to understand that the board came with no documentation other than a schematic? I would definitely leave that information in the ebay feedback. This is not really a beginner project.
That is usually the case with Chinese audio electronics sold on Ebay.
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Old 21st March 2015, 06:25 PM   #6
escott31 is offline escott31  United States
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Thank you guys for the help, much appreciated. I finally received an email of a schematic with the values on it. But I'm still confused about the source/srpp switch and what that does. will that switch the input to accept a line input instead of a phono? If not, is there any way to modify this so I can use a line level input and switch between that and phono? Thanks guys, much appreciated.
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Old 21st March 2015, 08:03 PM   #7
rayma is offline rayma  United States
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Originally Posted by escott31 View Post
I'm still confused about the source/srpp switch and what that does. will that switch the input to accept a line input instead of a phono?
If not, is there any way to modify this so I can use a line level input and switch between that and phono? Thanks guys, much appreciated.
That switch selects a circuit topology, either SRPP or active loading, for the phono stage. Only one of these positions could give accurate RIAA
response however, not both, because of the differing output impedances. Use the position that you like the best and hard wire it.

Instead, you could connect the switch at the phono circuit's output (after C5/C14 and before the volume controls) to select between phono and line.
That's a much more useful feature. Add a 1M resistor to ground between C5/C14 and the switches, to prevent pops when switching.
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Old 21st March 2015, 08:18 PM   #8
escott31 is offline escott31  United States
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ok, great that makes a lot of sense. Thank you so much for you help. So I should hardwire either the active loading or the srpp option? I do want the correct riaa standard for the phono for sure. Should I just leave that as a switch? and the switch for line or phono makes perfect sence. thank you.
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