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Old 18th April 2012, 06:15 AM   #11
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Originally Posted by h_a View Post
Are you sure that this is the entire phono part?

Using only 2 transistors for a full RIAA-circuit is possible, but untypical due to the many compromises to be made.

Hannes

PS: no need to worry much about the supply voltage; simply use 15-20V and be set. Any average TO-92 transistor takes 100mW power dissipation (rather some more), which means 5mA bias current which is much larger than the typically used values are.
Yes that's it. It was taken out of a very ordinary little receiver built in the early 70s....I believe there are a total of four transistors on the board....two per channel...does that sound a little more par for the course?
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Old 18th April 2012, 07:17 AM   #12
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Ok...I see..they are marked C1571 G-E7
In that case I assume they are BC157, a PNP BJT. If so the leg nearest the front edge in phot 1 is the collector, with the opposite leg the emitter.

The ideal supply voltage will be the value at which the collector DC value on the output transistor (the one on the left side of the board in the phot) is half way between the emitter DC value and ground.

I'd start at 12V and go up until you reach this value. If it doesn't occur before 24 V then I'm wrong.
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Old 19th April 2012, 12:30 AM   #13
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Thanks Mark and all....I'll let you know if I am successful.

One question however. ( I know eyes may roll in some quarters) but hey, I'm still learning

The LO and RO (hot leads) and the E's earths are destined for R & L input RCAs of my amplifier but where does the negative from my DC power supply go to on this board?....to one of the screws that hold down the board?
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Old 19th April 2012, 01:29 AM   #14
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I'm thinking they're probably 2SC1571's.

2SC1571 pdf


I'd say 15-20Vdc is a good starting point for B+,it was likely in that ballpark originally.
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Old 19th April 2012, 02:39 AM   #15
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Yes, good call, that's much more likely in a Japanese component.

In that case ignore what I said, it's an NPN transistor and the pinout is different.

The principle, however is the same - you want the collector sitting halfway between the emitter and the power supply rail (in the case of NPN it's the positive rail)
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Old 19th April 2012, 02:49 AM   #16
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Once again thanks Digital Junkie and Mark,,,,hmmmm...you guys do realize you're dealing with someone who still doesn't know where I would connect the negative lead from my DC power supply to on this board....so I could even begin to try any of this out.
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Old 19th April 2012, 04:09 AM   #17
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I think you negative supply can go on the "E" connections along with your signal ground.
I have a Dynaco amp that also only has two transistors per channel in the phono pre amp.
l
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Old 6th May 2012, 12:03 AM   #18
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Finally got around to applying some DCV to this little board. Although my voltage options were limited I managed to get the circuit energized at about 16VDC via an old HO Train Transformer (my dedicated adjustable DC voltage source only goes from 0-10VDC).

While the phono-signal amplification seemed spot on there was a fair amount of hum...however, I noticed that the moment I disconnected either one of the + or - DC leads the hum disappeared and the signal was clean as the circuit de-energized and then collapsed.

I checked the ground on my turntable connection firm, and the continuity connections on my cartridge/headshell are also a-ok, as is the wiring of the arm.

As a test, I disconnected the ground wire from my turntable and noted that this hum was a different frequency than that of noted above. So I am assuming that the intitial hum I heard is a product of the DC power supply?

Any ideas on how I can eliminate this? I connected the negative lead from my DC voltage source to several negative spots on the board with no change in the hum.

Would the use of a 16VDC battery pack help to eliminate this?
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Old 6th May 2012, 09:35 AM   #19
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Your old HO train transformer likely has no output smoothing at all , you would probably get less hum by using a DC Wallwart , but this also falls way short of what you require . You could try adding a pi type smoothing filter network to the output of the "HO"........1000uF in parallel,100ohms in series , then another 1000uF in parallel......then connect to the pcb .

Powering from a battery is going to be the quietest.

PS the HO train transformer may be totally inadequate......it could be 1/2 wave rectified....making it even harder to smooth.

Last edited by epicyclic; 6th May 2012 at 09:46 AM.
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Old 6th May 2012, 04:31 PM   #20
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Originally Posted by epicyclic View Post
Your old HO train transformer likely has no output smoothing at all , you would probably get less hum by using a DC Wallwart , but this also falls way short of what you require . You could try adding a pi type smoothing filter network to the output of the "HO"........1000uF in parallel,100ohms in series , then another 1000uF in parallel......then connect to the pcb .

Powering from a battery is going to be the quietest.

PS the HO train transformer may be totally inadequate......it could be 1/2 wave rectified....making it even harder to smooth.
Thanks for answering epi...will give it a go...you've confirmed a few of my hunches but wanted to see if I was on the right track.
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