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SX-727 head-amp (phono) W21-002 - transistors?

E

Ecaroh

Member
I'm hoping to get more life out of the phono section of my fabulous SX-727.

Recently had occasion to plug in a splendid free-standing phono stage (into the Pioneer line-stage) and the difference was night-and-day.

I've already replaced the electrolytic caps. I thought I might replace the six transistors now.

The FSM states:

2SC871 or
2SC458

However, the ones on my actual board are:

2SC1312

Experts here say the SC871 and SC458 are noisy and not great.

My SC1312s are not noisy. However, they have a much lower collector current spec than the two named in the FSM. I'm wondering -- could I not get more punch with higher-current transistors like those two? Punch is what I lack right now.

Fred Longworth and others here have recommended the 2240 as a (low-noise) NPN transistor for phono stages. But the current spec there seems to be rather low too?

Would love some hands-on advice about good choices. Thanks!

Ecaroh.
 
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low noise, use them a lot:
512-KSC1845UBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea

collector current spec has nothing really to do with "punch"

this is an old design. don't expect miracles.

You could try bypassing all the electrolytic caps with metalized poly caps for hi-frequency performance. As the impedance rises with frequency in the electrolytics (parasitic inductance) , the poly films would become the dominant element.

These are small sized caps, and relatively inexpensive for experimenting.
(mouser.com)
598-DSF050J224 0.22uf 50v (non-polarized)

you could also bypass your main electrolytic caps with:
5989-100V1.0-F 1uf 100v
 
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Sounds like your handy. If so, think about building a Bugle DIY phono stage. I think they are still under a $100 and perform well beyond that.
 
Thanks to both Fixer and Slowhead.

Will go with those transistors. I wasn't gonna bypass those caps, but maybe I will. Do you suppose it is really necessary to go 10 uF on the output caps? It might be nice to just get those down to a value where I can use film.... The caps on the In side are 22 uF....

Ecaroh.
 
You can't decrease capacitance without sacrificing the low frequency response.

It has to do with internal impedance. There are circuits that work with 0.47uf and a high enough input impedance.
 
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