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Old 30th December 2010, 03:48 PM   #1
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Default Could anyone opine on this circuit?

Hi Guys

I purchased a few of these pre-made 'pre-amp' boards to play with , as I'm trying to understand tube circuits. I've been using them as a gain stage for resistor i/v with TDA1541 dacs , as well as some PCM chips. The board comes with Chinese 6N1(1) tubes. I've also used 6N2 & 6N2P-EV tubes for higher gain. I'm able to use 25-33ohm i/v resistors with these.
I like the sound , I just don't know if i should . Does this circuit appear to be well designed for my (or any other) application? Also , any idea of the output impedence?

Thanks
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Last edited by homemade; 30th December 2010 at 03:55 PM.
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Old 30th December 2010, 04:43 PM   #2
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Default SRPP pre amp board

Sometimes a picture is worth a thousand words , and I hate typing
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Old 30th December 2010, 04:47 PM   #3
Zen Mod is offline Zen Mod  Serbia
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so do I - hate to chase datasheets for tubes used in project , just to answer on someone's question

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Old 30th December 2010, 05:15 PM   #4
Arnulf is offline Arnulf  Europe
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1: Your schematic must be incorrect, there is no way this circuit woudl ever sound like anything with 220R/2 anode load.

2: Whoever designed the circuit saved few cents on a couple of diodes and four capacitors, end result being that heater voltage is at the lowest end of specified range and that heater voltage regulators will have easier time oscillating, should they so desire.

3: With both halves of each tube paralleled I'd say this a waste of a perfectly good tube (which could instead have been used as cathode follower, presenting far lower impedance at the output).

4: I don't see grid stoppers anywhere.

My conclusion is this was put together by some kid for his elementary school project. Strip the board for bits, draw your own PCB and rebuild it properly.
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Old 30th December 2010, 05:27 PM   #5
Svein_B is offline Svein_B  Norway
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Quote:
Originally Posted by Arnulf View Post
1: Your schematic must be incorrect, there is no way this circuit woudl ever sound like anything with 220R/2 anode load.

3: With both halves of each tube paralleled I'd say this a waste of a perfectly good tube (which could instead have been used as cathode follower, presenting far lower impedance at the output).

The tube symbols are a bit unfamiliar, but after a close examination the topology appears to be SRPP.

Might not be completely hopeless, especially if it sounds good

... but this is a DIY forum; Of course it can be improved!

Svein.
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Old 30th December 2010, 05:46 PM   #6
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Originally Posted by Svein_B View Post
The tube symbols are a bit unfamiliar, but after a close examination the topology appears to be SRPP.

Might not be completely hopeless, especially if it sounds good

... but this is a DIY forum; Of course it can be improved!

Svein.

Yes , this is an 'SRPP PRE AMP KIT' , per the listing . Fully assembled. These are available from several Asian vendors , on ebay , for less than $35 shipped to the US. I purchased 4 of these for $100 delivered . The transformer is the more difficult part , as it can get expensive. I was looking for something to provide the gain needed. As long as I use MY pre amp after this board, I can even use a 10 ohm resistor for i/v , but I'm near maxed out on volume.

Thought these might be of use to someone else , if the design makes sense??
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Old 30th December 2010, 05:54 PM   #7
Arnulf is offline Arnulf  Europe
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Originally Posted by Svein_B View Post
The tube symbols are a bit unfamiliar, but after a close examination the topology appears to be SRPP.
Oh boy, I could never make this out by myself, I got lost during guesswork

Looks like they saved up a couple more components in this case, SRPP is a very fine stage ... for driving segments of telco cables of known, fixed impedance. Why do the Chinese fancy these for audio line duty ? They are bound to run into various different loads
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Old 30th December 2010, 06:13 PM   #8
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This is what the LAMPIZATOR site seems to love. Also the 6N2P-EV is his fav.
Since I have a large inventory of 80's cd players , mostly 1541 dacs , I was looking for something that would fit inside the players. It takes dome figuring , but it fit fairly well in Magnavox and lower end Sonys with the 1541 dac. Gain is about 100db , per the 6N2P data sheet. Not sure how SRPP affects the gain.

How does one determine output impedence?
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Old 30th December 2010, 07:03 PM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Arnulf View Post
Oh boy, I could never make this out by myself, I got lost during guesswork

Looks like they saved up a couple more components in this case, SRPP is a very fine stage ... for driving segments of telco cables of known, fixed impedance. Why do the Chinese fancy these for audio line duty ? They are bound to run into various different loads
It's trendy.. Think I've said that before.. What you say is true, but even so it performs at least adequately into a wide range of load impedances.

This board is crying for a conversion to mu-follower which provides a lot more gain for this I/V conversion application. Would require the addition of two resistors and a cap per channel - might be manageable with just a couple of etch cuts, and the components required could be installed on the bottom side of the pcb.
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Old 30th December 2010, 07:09 PM   #10
kevinkr is offline kevinkr  United States
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WRT the schematic, you can download LTSpiceIV (it's free!) from Linear Tech and use that to draw tube schematics. If you install some tube libraries as detailed in other threads you can even simulate the circuit.

Here is the link: http://www.linear.com/designtools/software/ltspice.jsp
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